# AC Vs DC



## DrGee (Aug 22, 2018)

Is there a post on this wiki or in the main forum comparing the pros and cons of AC motors vs DC motors?
Thanks everyone!


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## MattsAwesomeStuff (Aug 10, 2017)

This should be of assistance while researching:


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## DrGee (Aug 22, 2018)

MattsAwesomeStuff said:


> This should be of assistance while researching:


Okay.. Point taken 😏😏 I guess I walked into that one!..


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## Duncan (Dec 8, 2008)

Hi DrGee

DC means series wound with brushes (there are some sepex but very rare)
Old school technology - you can get oodles of power cheap but you can't get re-gen
Cheap and cheerful and powerful - but unsophisticated

AC - as in new AC - expensive and wimpy 

AC as in taken from a crashed EV
Definitely the best - sophisticated powerful and you get re-gen
BUT you either have to nut out all the CANBUS stuff - or else replace the "electronic brain"


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## MattsAwesomeStuff (Aug 10, 2017)

Specifically, what type of AC motors are you considering.

Like, if you want to do basic research, you can probably do that on your own.

If you're having trouble deciding between a few things you've looked into, that's where the forums are helpful.

I presume you don't want a technological review, you just want to know from a DIY perspective, why would you choose one over the other?

AC:
- More expensive.
- More expensive controllers.
- More complicated.
- Can use regen braking (adds 5% to your range, saves you from a $50 brake job every 5 years).
- Easily reversed/transmissionless.
- Generally, OEM salvage are fairly beefy motors and a complete solution.

DC:
- Dirt cheap, used you pay $200 scrap value.
- Cheaper controllers.
- Dead simple.
- Can't use regen braking (technically can, but no one does and no controller uses it).
- Brushes eventually wear out after years and need replacing.
- Reversing is a bit of a chore mechanically with most controllers, usually you'd keep the car's original transmission to do that.

I wouldn't go purchasing a new AC motor or anything other than an AC motor from a wrecked OEM vehicle.


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## DrGee (Aug 22, 2018)

Great stuff guys, thanks so much!
A bit disappointed that DC motors don't do reverse easily - I was hoping to get rid of the transmission in my proposed ev build. 
I can do without regen. I love the concept, but it's a bit too much complexity for a little gain.
Once again, this forum is the best!


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## Duncan (Dec 8, 2008)

Hi
DC motors do do reverse - that is how my car works

But it's not as simple as reversing the wires - you need to reverse either the field coils or the armature

The simple way is to use a "reversing contactor" - every forklift has one!


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## DrGee (Aug 22, 2018)

Aha.. a DC motor with a reversing contactor - this looks like the way forward for me ( excuse the pun)


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## zapyourrideguy (Oct 25, 2012)

just a few comments after going through several DC motors, They can be gotten cheap, great power capability and they work well but make sure that you don't:

design poorly or gear poorly as you will overload/heat the motor (AC motor is sophisticated enough to protect itself from overheating.)

ever hill hold or accelerate while creeping backward or have a less knowledgable driver drive the car ever. (AC motor is more bullet-proof, no brushes to burn up)

I have Hpevs AC motor that I really like. not much increase in range from regen but saves the brakes and the resulting brake dust pollution.

The Orion BMS talks to the 1238 AC controller and when a cell gets too low, power is cut slowly to the motor ensuring that you don't over discharge cells. (This feature is probably available for dc controllers with a BMS so equipped.)

I am not an engineer but have driven and upgraded EVs for 20+ years.


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## Duncan (Dec 8, 2008)

zapyourrideguy said:


> design poorly or gear poorly as you will overload/heat the motor (AC motor is sophisticated enough to protect itself from overheating.)
> 
> .


As a cynical engineer I wonder if the aftermarket AC motors protect themselves from being overheated by "sophisticated" means 
Or just by being so wimpy that they are protected that way!

If I limited the current on my forklift motor to match the power on the Hpevs AC motor then it would never overheat!


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## DrGee (Aug 22, 2018)

Leaving the motor aside for a moment and shifting to the gearbox.. Is keeping a 4x4 automatic transmission feasible in an ev conversion? Emphasis on "automatic". If it's a sensible option, I'll keep the gearbox & use only one large motor instead of my plan to use two smaller ones.


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## brian_ (Feb 7, 2017)

DrGee said:


> Leaving the motor aside for a moment and shifting to the gearbox.. Is keeping a 4x4 automatic transmission feasible in an ev conversion? Emphasis on "automatic". If it's a sensible option, I'll keep the gearbox & use only one large motor instead of my plan to use two smaller ones.


That really doesn't have much to do with AC versus DC... why not put everything about your build in one build thread in All EV Conversions and Builds?

A reference source (not a place for discussion) from the _DIY EV Wiki_ > _EV Information_ section:
Automatic Transmissions in EVs


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## DrGee (Aug 22, 2018)

brian_ said:


> DrGee said:
> 
> 
> > Leaving the motor aside for a moment and shifting to the gearbox.. Is keeping a 4x4 automatic transmission feasible in an ev conversion? Emphasis on "automatic". If it's a sensible option, I'll keep the gearbox & use only one large motor instead of my plan to use two smaller ones.
> ...


Thanks so much Brian!


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## Sunking (Aug 10, 2009)

Duncan said:


> Hi DrGee
> 
> DC means series wound with brushes (there are some sepex but very rare)
> Old school technology - you can get oodles of power cheap but you can't get re-gen
> Cheap and cheerful and powerful - but unsophisticated



That is not entirely accurate. I am no fan of DC motors, but SEPEX motors are found in all three the major golf cart manufactures, and crosses over into their PTV and NEV lines as well. Secondly every one of those SEPEX do regen. In the golf cart world Series use to be default, but not today. EZGO is th eonly one who uses AC motors for their high end stuff, but Fleet line and consumer carts are SEPEX.






Duncan said:


> AC - as in new AC - expensive and wimpy



Expensive yes, wimpy not so much IMO. Again my experience is with racing golf carts, but HPEV AC15 motor smokes any DC motors has to offer. The AC15 develops 70 foot-pounds of Torque from 0 to 5500 RPM (71 HP Peak) and rated 16 HP continuous at 8000 RPM. Not wimpy in golf cart world and is the most powerful motor available for golf carts and NEVs. None of the DC motors made for golf carts can even come close. With a 17:1 transmission is roughly 1200 foot-pounds of torque applied to the wheels on a vehicle that weighs less than 800 pounds. Not many ICE vehicles have that power to weight ratio.


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## brian_ (Feb 7, 2017)

Duncan said:


> DC means series wound with brushes (there are some sepex but very rare)
> Old school technology - you can get oodles of power cheap but you can't get re-gen





Sunking said:


> I am no fan of DC motors, but SEPEX motors are found in all three the major golf cart manufactures, and crosses over into their PTV and NEV lines as well. Secondly every one of those SEPEX do regen.


There seems to be an assumption in this forum that all DC motors are salvaged from old forklift trucks or purchased from a supplier or two who specialize in motors that look like refurbished old forklift truck motors. If you assume that, then they nearly all have series field windings. The commonly used controllers are for these motors, so they cannot handle SepEx motors.

Since the series winding configuration is the only reason that "forklift" motors are not suitable for regen (because the field cannot be controlled), it makes sense that motors with separately excited field windings are routinely used for regenerative braking.


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## Duncan (Dec 8, 2008)

brian_ said:


> There seems to be an assumption in this forum that all DC motors are salvaged from old forklift trucks or purchased from a supplier or two who specialize in motors that look like refurbished old forklift truck motors.


It's the size - for a car you want at least a 9 inch motor which will be about 60 kg

I'm using an 11 inch 102 kg motor

As far as I know Golf Carts all use much smaller motors

Which makes perfect sense as they are intermittent use in lightweight vehicles at low speeds while Fork lifts are heavier with much more intensive use


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## Sunking (Aug 10, 2009)

brian_ said:


> There seems to be an assumption in this forum that all DC motors are salvaged from old forklift trucks or purchased from a supplier or two who specialize in motors that look like refurbished old forklift truck motors. If you assume that, then they nearly all have series field windings. The commonly used controllers are for these motors, so they cannot handle SepEx motors.



Brian I do not disagree with anything you are saying. Series wound is the right choice for a Fork Lift where low RPM torque is priority over speed. Speed is priority for today golf carts because the three major manufactures also off basically the same vehicle for NEV, LSV and PTV options which means lift the speed limits in the controller along with added DOT requirements to make the Licensed Street Legal. They cannot do that with Series motors. 

OTOH there is a crowd in the custom Golf Cart World that insist on using Series wound motors, and there is a large after market motor offerings like from D&D Motors, Admiral, and FSIP to name a few. This crowd is the work horse crowd where the cart is used to haul heavy material, pull trailers, rock climbers, hunting, and swamp buggies. Series motors work best for that where Torque at low rpms are required and speed is not an issue as 10 to 20 mph is more than fast enough for them. Nor do they need a cart to hold speed going up hills. Then there is the other crowd that wants speed and that crowd uses either SEPEX, or if it can be afforded the best of both worlds torque and speed using AC induction motors. 

As a fun factoid the worlds fastest golf cart is faster than most ICE manufactured vehicles including sports model. Plum quick motors holds the World Record fastest golf cart. Clocked the 1/4 mile in 12.241 seconds @ 118.76 mph.


*Video Link Here*


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## MattsAwesomeStuff (Aug 10, 2017)

> Since the series winding configuration is the only reason that "forklift" motors are not suitable for regen (because the field cannot be controlled), it makes sense that motors with separately excited field windings are routinely used for regenerative braking.


Except that, mechanically speaking, 99% of forklift motors actually are SepEx, in that, they have all 4 terminals (both field and armature winding ends) independent and accessible. Because almost all forklifts are electrically reversed and have emergency "plug" breaking by reversing the field.

A true Series-Wound motor would only have 2 terminals.

...

That said, to be usefully controlled SepEx usually have thinner wire gauge on the field windings IIRC because they're not simply forcefed the same current as the armature, so, everyone's right in that sense.

Also true that almost no series-wound controllers would ever make any use of the 4 accessible terminals except to mechanically switch them for reversing. They never vary the power levels to them independently, even though they could.


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## DrGee (Aug 22, 2018)

Is there relatively simple formula for calculating what type of forklift motor to convert to power a car?
For example, if the motor is rated at 6 hp & 36V DC, can you expect 24hp from it if you power it up to 144V?
Also, when it comes to range, is there any advantage in AC over DC apart from the ability to use regen? 
Thanks guys!


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## MattsAwesomeStuff (Aug 10, 2017)

> Is there relatively simple formula for calculating what type of forklift motor to convert to power a car?


Almost all electrical limits are thermal.

As in, what you have to worry about is heat buildup. If you add fan or liquid cooling, you can demand far more from the motors. I'm sure there's a theoretical maximum, or at least massively diminishing returns for magnetic reasons eventually, but not in the ranges you'd see people using.

Go look up other builds, but, general rule of thumb, I don't think I've seen anyone even flinch at doing 2x the voltage, 3x is common. Duncan's running his at like, 8x for sprints.



> For example, if the motor is rated at 6 hp & 36V DC, can you expect 24hp from it if you power it up to 144V?


Power is the product of Voltage x Amps.

But Amps is the result of Voltage / Resistance, and resistance in the motor isn't changing.

So when you double the Voltage you double the power for that reason, but you also double it again because the Voltage doubles the Amps that will flow.

As you quadrupled the Voltage from 36v to 144v, whatever the Amps that were flowing would (or could) also be quadroupled. 4x from Volts and 4x from Amps is 16x total, so, 96 horsepower.

This of course, as long as you pick a controller that can handle that Voltage, and, not safety limit or blow up at that much current.

750 watts per horsepower so that 6hp motor is actually just rated for 125 amps. You'll be shoving 500 amps through it.


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> Power is the product of Voltage x Amps.
> 
> But Amps is the result of Voltage / Resistance, and resistance in the motor isn't changing...


But the motor is not a resistor. The windings have resistance, but most of the voltage isn't just going to overcome resistance, so it just doesn't work that neatly.


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## MattsAwesomeStuff (Aug 10, 2017)

brian_ said:


> But the motor is not a resistor. The windings have resistance, but most of the voltage isn't just going to overcome resistance, so it just doesn't work that neatly.


Well, inductance and etc. But, regardless, Power is proportional V^2 no?


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> Well, inductance and etc. But, regardless, Power is proportional V^2 no?


Through a resistor, yes, but in general, no... because power proportional to the square of voltage is based on the assumption that the load is a constant resistor so current is proportional to voltage. Power is still the voltage multiplied by current... but with current not simply dependent on voltage and resistance, and a power factor included to account for voltage and current being out of phase.


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## Sunking (Aug 10, 2009)

MattsAwesomeStuff said:


> Power is proportional V^2 no?


Correct if we were discussing a passive pure resistance circuits. Throw in inductance and Back EMF and things change. 



If you double the voltage on say a Series wound motor would only increase motor RPM, but Back EMEF pushes back more as RPM increases and results in lowering current.


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## Sunking (Aug 10, 2009)

MattsAwesomeStuff said:


> Power is proportional V^2 no?


Correct if we were discussing a passive pure resistance circuits. Throw in inductance and Back EMF and things change. 



If you double the voltage on say a Series wound motor would only increase motor RPM, but Back EMEF pushes back more as RPM increases and results in lowering current.


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## DrGee (Aug 22, 2018)

I'd hoped there was a simple formula. From what all you knowledgeable folk are saying, It seems that there isn't one.
I'm going to look for a really large forklift motor with a relatively high voltage rating to begin with. (13 inch, 76V, DC). I know they're out there...
I see that a high commutator count is preferable for a motor that's going to be overvolted. What's a good count?


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## Duncan (Dec 8, 2008)

Hi Dr Gee

It's actually quite simple - the current controls the Torque

Current x voltage = mechanical power + a bit for heat

It takes (roughly) 5 v to push 1000 amps through a 9 or 11 inch motor - 5 Kw of heat

At zero rpm - that's all it takes - ZERO actual Power

As the revs rise the required voltage rises - and the power rises

This keeps going until your controller is maxed out - 100% PWM
Then the voltage can't go any higher and the current (and torque) must drop as the rpm increases

This gives the usual DC motor torque and power curve

Torque starts high and stays there until the controller maxes out and then drops 
Power starts at zero climbs in a straight line until the controler point then drops as the current drops

I "model" it as resister - plus a power converter


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## MattsAwesomeStuff (Aug 10, 2017)

So, to separate this to be useful to the person asking the question...

What he asked was: "For example, if the motor is rated at 6 hp & 36V DC, can you expect 24hp from it if you power it up to 144V?"

And I contended no, you can expect 96 horsepower at that voltage. But it's going to get a lot hotter than it originally did.

Is that wrong?

He is looking for a ballpark motor size he needs. He wants to know generally how hard you can push the motors and what you can expect out of them power-wise.


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## brian_ (Feb 7, 2017)

Duncan said:


> It takes (roughly) 5 v to push 1000 amps through a 9 or 11 inch motor - 5 Kw of heat
> 
> At zero rpm - that's all it takes - ZERO actual Power


This is a great illustration of the resistance in a motor, since with a DC or synchronous AC motor at stall (zero rpm) everything is direct current, with no induction or other magnetic effects. Once the motor starts turning, other effects become important, and at high speeds the resistance (which is still there) is a relatively small part of what is going on.

It also shows why very low speeds are inefficient, since the power lost to heat because of resistance is substantial compared to the actual work being done.


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## major (Apr 4, 2008)

MattsAwesomeStuff said:


> So, to separate this to be useful to the person asking the question...
> 
> What he asked was: "For example, if the motor is rated at 6 hp & 36V DC, can you expect 24hp from it if you power it up to 144V?"
> 
> ...


Yes, that is wrong. And demonstrates a lack of knowledge concerning electric motor fundamental theory on your part. I think I've mentioned to you previously that you need to study and learn motor basics before handing out advice to others.

Regards,

major


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## MattsAwesomeStuff (Aug 10, 2017)

major said:


> Yes, that is wrong. And demonstrates a lack of knowledge concerning electric motor fundamental theory on your part. I think I've mentioned to you previously that you need to study and learn motor basics before handing out advice to others.


Maybe it does, or maybe we're making different presumptions or talking about slightly different things.

But go on, tell someone they're wrong without, y'know, saying why or saying what's correct in its place. /s

...

When he turns the motor on, with 4x the voltage, it's going to be drawing 16x as much power. As the motor spins up and generates its back-EMF counter-opposing voltage and all that will come into play.

What Duncan is saying about the controller limiting the current by limiting the voltage until it's applying the max voltage of the pack and all that, okay, also true.

The guy just wants to know how big of a motor to use and what kind of performance he'll expect out of motor if he overvolts it, and no one gave him a simple enough answer for him to use so he gave up.

If that's the best help the forum can be, okay. Great. What's the point of being knowledgeable and helping people if... you don't?


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## major (Apr 4, 2008)

MattsAwesomeStuff said:


> Maybe it does, or maybe we're making different presumptions or talking about slightly different things.
> 
> But go on, tell someone they're wrong without, y'know, saying why or saying what's correct in its place. /s
> 
> ...


Both brian_ and Sunking have given you explanation. Have you bothered to research and learn what they're talking about? I don't think so. I'm willing to help a guy who has a good attitude and does his homework, sotospeak. Over 7000 times I've done so on this board alone. 

Remember this?


MattsAwesomeStuff said:


> ...
> most people, when they find themselves on the side where they might be wrong, don't investigate. They double-down, they get stubborn, they get defensive, and they pretend like they don't need to change. ...


You and Functional Artist showed up about the same time. Just taking a passing interest I followed posting from both of you. At times I confused you two with each other, mainly from similar attitudes and inexperience. 

It's easier to think you know than to learn what you don't know, isn't it? Good luck with your EV projects.

major


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> What he asked was: "For example, if the motor is rated at 6 hp & 36V DC, can you expect 24hp from it if you power it up to 144V?"
> 
> And I contended no, you can expect 96 horsepower at that voltage. But it's going to get a lot hotter than it originally did.
> 
> Is that wrong?


Yes, it is wrong, as explained earlier, in post #21, post #23 and post #24. If you don't understand those statements please ask for clarification, but just ignoring them isn't productive.



MattsAwesomeStuff said:


> When he turns the motor on, with 4x the voltage, it's going to be drawing 16x as much power...


No, it won't. The explanation of why current is not proportional to applied voltage, provided earlier, still applies. So power is not proportional to the square of voltage.


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> What he asked was: "For example, if the motor is rated at 6 hp & 36V DC, can you expect 24hp from it if you power it up to 144V?"
> ...
> He is looking for a ballpark motor size he needs. He wants to know generally how hard you can push the motors and what you can expect out of them power-wise.


The desire is reasonable, but the expectation is not. The description of the motor by a power rating and operating voltage is simply not enough information to predict performance at a much higher voltage.



MattsAwesomeStuff said:


> The guy just wants to know how big of a motor to use and what kind of performance he'll expect out of motor if he overvolts it, and no one gave him a simple enough answer for him to use so he gave up.
> 
> If that's the best help the forum can be, okay. Great. What's the point of being knowledgeable and helping people if... you don't?


No simple and correct answer was provided because there is no simple answer which is correct. An overly simplistic and incorrect answer is not helpful, no matter how well-intentioned it is.


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## MattsAwesomeStuff (Aug 10, 2017)

major said:


> Both brian_ and Sunking have given you explanation. Have you bothered to research and learn what they're talking about? I don't think so.


In detail, enough to say, calculate something? No. Enough to understand the concept? I thought I understood it at, maybe, 20% and then a little bit more of reading, maybe 30%? One can always learn more, but, I thought I had layman's grasp of it.



> It's easier to think you know than to learn what you don't know, isn't it?


Y'know, it's hard to learn someone tells you your questions aren't intelligent enough and you're not worth explaining to. Or smugly mocks you rather than educates you. Which are both things you've done to me.

*shrugs*, I'll just participate less on these forums. Partly because it's disparaging to keep trying, and party because I'm giving bad or useless advice anyways (no argument there). It seems like there's some elitism from those that have a high level understanding and, it's impossible to make progress in that direction.


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## major (Apr 4, 2008)

MattsAwesomeStuff said:


> In detail, enough to say, calculate something? No. Enough to understand the concept? I thought I understood it at, maybe, 20% and then a little bit more of reading, maybe 30%? One can always learn more, but, I thought I had layman's grasp of it.
> 
> 
> 
> ...


Have you read a text book on basic electric motor theory? Have you followed through and done the motor equivalent circuit problems and examples in the books or websites? Do you even know how to read and apply a motor characteristic performance curve? It is obvious. And if you're unwilling to attempt to gain the fundamentals from authors who are better teachers than myself, what chance is there you will understand what I tell you? Just like my last three posts here. You learn nothing from what I say. 

D. I. Y. Do It Yourself. Applies to learning. We don't sugarcoat knowledge and serve it to you. Get it yourself. We can, and, in most cases will, help you.

And yes, please stop giving bad advice. 

Regards,

major


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## brian_ (Feb 7, 2017)

major said:


> Have you read a text book on basic electric motor theory?


I would suggest going back further, and learning the difference between a resistor and an inductor. That's a level of electrical understanding well beyond the average person, and even beyond what many people who safely and productively wire buildings for power have, but it is fundamental to understanding motors (or anything with a coil of wire and changing current in the wire).


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## Sunking (Aug 10, 2009)

Matt what you are missing and not understanding is BACK EMF. Once you get your noodle wrapped around Back EMF, then you will understand the answers you have been given. 



Motor manufactures are stingy with technical details you need to design with. So for example lets say we have a 36 volt 4 hp motor. That does not tell a designer anything other than a 4 HP using 36 volts. There are a few more specs you need like LRA or Locked Rotor Current. The Locked Rotor Current determines the maximum current that can flow with out damage.


So you a got motor and trying to figure out what it DOES. Well all you have is 4 Hp at 36 volts.What is missing is RPM. I can only tell iat 36 volts making 3000 watts


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## Duncan (Dec 8, 2008)

Sunking said:


> Matt what you are missing and not understanding is BACK EMF. Once you get your noodle wrapped around Back EMF, then you will understand the answers you have been given.


 Yep - I think "back EMF" is like centrifugal force - the purists don't like it but it is the key to understanding what is happening


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## DrGee (Aug 22, 2018)

Major, Sunking, Brian and of course Duncan & your wonderful Device... You guys are great and I really admire your expertise. I fully understand and agree with your point about people posting misinformation on a technical forum like this. However, have some mercy on Matt. He's really getting the short end of the stick here and I think he just wants to help..
Now, back to the second part of my question - what's a "good" commutator count on a motor you intend overvolt by factor of 4? How about a factor of 2?


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## MattsAwesomeStuff (Aug 10, 2017)

major said:


> Have you read a text book on basic electric motor theory?


Years ago, but yeah, passed the class and got the credit too for what that's worth.



> Do you even know how to read and apply a motor characteristic performance curve?


Yes, actually, obviously.



> What chance is there you will understand what I tell you? Just like my last three posts here. You learn nothing from what I say.


You haven't said anything other than that I'm wrong and need to learn, without providing any discourse. I wouldn't call that "teaching".

You literally told me to enroll in an electrical engineer degree program or I wasn't worth having my questions answered. That the questions I was asking, to figure out where I was mistaken, weren't "intelligent" enough to warrant answering. So, you're correct, I will nothing from what you say.

I don't think I would ever treat anyone that way, no matter how knowledgeable I was on a subject or how little I felt like giving them my time. It's not necessary to talk down to someone for being curious.



> We don't sugarcoat knowledge and serve it to you. Get it yourself. We can, and, in most cases will, help you.


I've said a few times, by no means am I entitled to your, or anyone else's advice, and certainly not a specific way. But I will say that the way you've treated me has not been effective. I'm not challenging you or saying you're wrong. I'm sure you're right, I'm sure you're smarter than me, you're certainly more knowledgeable about this than me. But "teaching" someone by telling them they're wrong, no explanation, moving on, didn't work, sorry.

Philosophical tangent. To me, teaching isn't just throwing facts at someone or giving the same explanation you'd give someone who already understands it at a high level, or, showing off that you know the answer. Effective teaching is, a little bit trying to sympathize and get into the head of the person who isn't knowledgeable, trying to figure out what about the way they're thinking about it is wrong, and then giving them the explanation that is most useful to them. Not the way that states things the way _you_ understand it, the way that _helps them_ understand it. Sometimes it's difficult to find someone close enough to yourself, to get that perspective. This is especially true in technical circles dominated by a group of people who all have similar ways of thinking, who talk amongst themselves easily. Perhaps so easily that they forget there are different ways that people think about things.

There are people who have the bizarre opinion that "the" "right" way to learn is to hand-write your notes out 2 or 3 times. Somehow being a Xerox machine will make you understand something, because it worked for them. Others think "the" "right" way to learn is to go through math because "it's right here, see?". Others through analogy. Etc. They're all wrong, the right way is whatever works for the person learning, not the way that was perfect for them.

Often the people who are best at teaching something, are those that struggled with the material, rather than those that had an easy time with the textbook explanations written by people who intuitively understood it also.

...

Just speaking of me personally, I have some formal education on the matter, and have picked up tidbits of other things over the years. That's true of my knowledgebase in general. On many subjects I have what many people think are awkward gaps in my understanding. I will understand A, B, and D, but will somehow never have picked up C along the way.

In this case I don't think there's an outright gap in my knowledge. I know all the pieces, I'm perhaps poor at putting them together. That's why I, at times, have what might seem to be weird questions, as I try to narrow in on what I might be mistaken about and how. Because I didn't learn 0-100% linearly in a textbook in a planned curriculum for 4 years.


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## MattsAwesomeStuff (Aug 10, 2017)

Brian said:


> as explained earlier, in post #21, post #23 and post #24. If you don't understand those statements...


I do, at least, I think I do. There was nothing in there I didn't already know and no statements I didn't understand. So if I'm wrong, I don't know how I'm wrong, or how to tweak what I know.

As a collection of facts, I understand torque curves. I understand induction (not proficiently, but, sufficiently). I understand power factor under inductive loads with current's waveform lagging voltage's. I understand back-emf. I understand those things in isolation. That might seem weird, to know those things and still be wrong, but, it is what is.

...



> The explanation of why current is not proportional to applied voltage, provided earlier, still applies. So power is not proportional to the square of voltage.


Of the voltage applied, no. But the voltage difference between the applied voltage and the back-emf (which increases with speed), still holds the relationship true, doesn't it?

I'm probably putting the pieces together wrong. Here's how I've understood it:

You feed a motor a flat, oh, 24v. At first it'll pull whatever current, let's say 100 amps. As it spins up the motor generates back emf and, at some speed that back-emf crosses a point where it cancels out, say, 12v. Now you've got only got a 12 volt difference. Shouldn't the current pulled from the battery be at 50 amps at this point?

How fast the motor is spinning depends on how hard the motor is being loaded, so the RPM doesn't tell you anything about that itself.

So, whatever the voltage difference is, still has a squared impact on power at whatever point along the curve, no? Or am I missing a whole extra variable that changes the relationship?

I dunno, maybe give a counter-example. Take his motor, give it 4x the voltage as he asked. What is your opinion on how many horsepower he could get out of it? Or, would you say 4x is too much? Or is a non-sensical question that can't have an answer?



> No simple and correct answer was provided because there is no simple answer which is correct. An overly simplistic and incorrect answer is not helpful, no matter how well-intentioned it is.


Backing way up...

The guy is asking roughly how big of a motor he needs to use, and roughly what he can get away with in terms of overvolting and what he could expect power-wise.

It is probably the single most common beginner question asked in this community.

Answers have been given far, far more generally and less technically than that.

Heck, answers often tell people how many horsepower they can get based on the diameter of the motor, which is like, 3 tiers of ballparking quality less than this conversation, and those answers worked just fine. So, I don't think it's true to say that there's no answer one can give him.


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## Sunking (Aug 10, 2009)

MattsAwesomeStuff said:


> I understand back-emf. I understand those things in isolation. That might seem weird, to know those things and still be wrong, but, it is what is.


OK Matt I am not trying to beat up on you. What you know is Academic and correct, but you do not know how to apply the knowledge. So perhaps this may help.

Let's say we have a 100 volt motor and looking for a controller. First thing you want to know is Locked Rotor Current (LRA) so you can select the right size controller. You want a Controller that will limit current equal to or less than max LRA of the motor. Controllers limit current (Torque) and regulate voltage (RPM)

LRA = Voltage Supply / Motor Coil Resistance. 

So we hit the Motor with say 100 Volt battery and the motors coil resistance = 1 Ohm. So:

100 volts / 1 Ohm = 100 amps. 

So 100 Amps @ 100 volts is your peak input power of the motor or 10,000 watts, roughly 10 to 13 Hp Peak. You are shopping for a 100 amp or less controller that is rated in the voltage range of your battery.

The only time the motor would ever see 100 amps is at 0 RPM if you applied 100 Volts instantaneously which you would never likely do.

Next piece of info you need and what you are missing is what is max continuous current at operating speed. Once a motor spools up in RPM, the motor acts like a generator, thus produces a voltage aka Back EMF expressed as Eb.

So current at operational speed = Battery Voltage - Eb / Motor Coil Resistance.

So lets say the Eb = 76 Volts @ 5220 RPM's

100 volts - 76 volts / 1 Ohm = 24 amps

Power = 100 volts x 24 amps = 2400 watts or roughly 3 hp with 3 pounds of torque. 

So is 10 hp peak and 3 hp continuous enough for an application? BTF out of me, I have no clue what the parameters are. What I can tell you if you want say make a golf cart to go 60 mph scary fast I plug in the weight, rolling resistance, coefficient of drag and it tells me I need 13 to 14 hp. So I am looking for a motor with a continuous hp of at least 14 hp continuous and as much peak hp I can find at the RPM required to turn the drive wheels. I ended up with a HPEV AC15 motor which is actually an AC9 motor operating at 96 volts. Peak Hp = 60 Hp @ 4800 RPM with 67 ft-lbs of torque, and max Continuous hp = 16 @ 7000 RPM with 12 ft-lbs of torque. 

My cart has 22-inch tires with a 6:1 Differential Ratio that goes from 0 to 60 mph faster than a Corvette as I am putting roughly 3600 ft-lbs of torque on the pavement.

*
EDIT NOTE:*


Question was asked how high of a Voltage can you apply to a Motor? The theoretical answer lies in max LRA and Motor Resistance. Once you have that it is child's play, Ohm's Law; *Voltage = Current x Resistance.* In my example 100 amps x 1 ohm = 100 volts. That does not mean you should use 100 volts.


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> I understand induction (not proficiently, but, sufficiently). I understand power factor under inductive loads with current's waveform lagging voltage's. I understand back-emf. I understand those things in isolation.
> ...
> But the voltage difference between the applied voltage and the back-emf (which increases with speed), still holds the relationship true, doesn't it?


Okay, so you just completely forgot to consider anything but resistance until we reminded you (for the fifth time or so) that there back EMF induced. 

Matt, you're closer to the right track now, and no longer trying to apply a pure-resistance model to a motor, so I'll leave deeper details to others.



MattsAwesomeStuff said:


> How fast the motor is spinning depends on how hard the motor is being loaded, so the RPM doesn't tell you anything about that itself.


True, which is why a no-load speed is informative.


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## DrGee (Aug 22, 2018)

Sunking said:


> MattsAwesomeStuff said:
> 
> 
> > I understand back-emf. I understand those things in isolation. That might seem weird, to know those things and still be wrong, but, it is what is.
> ...


Thanks Sunking, 
This last note is particularly useful. Most folk with a basic technical background know Ohm's law, but I had never heard of max LRA 'till I saw it in 
this string of posts. Something to check on any motor I look at.


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## Sunking (Aug 10, 2009)

DrGee said:


> Thanks Sunking,


You are welcome. Allow me to make two points.

1. Max current on a motor is at ZERO RPM or a Locked Rotor for a given voltage. As RPM increases, current decreases due to Back EMF pushing the voltage down. In theory for s Series wound motor, at some RPM current is Zero amps. In theory that occurs when Back EMF and Supply Voltage is equal. In practice never happens and physically impossible. 

2. LRA on its own is maximum torque, but that does not mean maximum torque occurs at LRA. In fact with most motors, peak torque is developed lower than LRA because saturation current is less than the thermal limits of the wire. Any current above Saturation is just waste heat robbing your battery of energy to make waste heat. 

Having said all that, motors are not my specialty. So I may have over simplified things. I am an EE but an EE is a lot like a doctor in that we specialize in certain areas. Example you would not go see a Proctologist for brain cancer. I take that back, there are some people that should see a Protologist for their headache. My area of expertise is Power Generation, Battery Plants, and Communications. I come from Telecom and Electric Utility sectors. My username comes from dabbling in off-grid solar for the last 15 years doing side jobs.


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## MattsAwesomeStuff (Aug 10, 2017)

Sunking said:


> OK Matt I am not trying to beat up on you. What you know is Academic and correct, but you do not know how to apply the knowledge. So perhaps this may help.


Helps to see someone walk through it. I don't think there's anything new in here to me. This is how I have understood things since before this thread began. I don't see where it makes things I said earlier incorrect.



> Let's say we have a 100 volt motor and looking for a controller. First thing you want to know is Locked Rotor Current (LRA) so you can select the right size controller.


Okay, so, in EV context, right off the bat, almost every DIY EVer runs their motors way above their rated voltage. "Spec" doesn't mean much to most of us.

Without knowing the max voltage you can't say what the LRA would be, because, if you feed it higher voltage you're going to get a higher LRA, right?

But I do get that, the current will never be higher than this, it would be the absolute peak amount.



> You want a Controller that will limit current equal to or less than max LRA of the motor.


That makes sense, as there's no point in overbuilding the controller for the worst-case scenario for its current draw. Or at least, whatever the current draw would be at the voltage you chose for the motor (probably won't be the factor LRA spec, but you can extrapolate).



> Next piece of info you need and what you are missing is what is max continuous current at operating speed. Once a motor spools up in RPM, the motor acts like a generator, thus produces a voltage aka Back EMF expressed as Eb.


Sure, still following along. You might not know the "operating speed" as, it'll depend on how fast you want to go.



> So is 10 hp peak and 3 hp continuous enough for an application? BTF out of me, I have no clue what the parameters are.


In an EV context, most people want to be able to go highway speed, few people are all that interested in performance (which would of course change their numbers). But, you want to have some torque left to get to highway speed, especially with a bit of hill climb. A mild incline is going to add ~100% to your power requirements on a normal vehicle. 

Most people would only be interested in not being sluggish to come up to speed, not necessarily care about those power requirements indefinitely.

That is, even though it only takes ~12hp to maintain highway speed on a Honda Civic, Maybe double that to climb a hill at that speed (24hp), they're not built with engines that can only handle 24hp. They're built much more powerful than that, even though no one needs that. One would presumably want similar performance out of an EV, even though you don't need the sustained power draw.

And motors, similar but better than engines, can be abused if you can manage the heat. Some motors are rated that way, for example the last forklift motor I looked at had the 100%, 50% and 15% duty cycle ratings in the specs, in forklift duty (no high speed airflow or fan cooling I presume).

So, all this spec stuff is nice, but if you build something to most specs motors will have listed in most use cases, you'd never go anywhere. Or you'd have to do a bunch of air calculations for cooling and so on. Which, it the number of people who have done that for their EV is above zero, it's certainly not much above zero. Everyone's winging it.



> What I can tell you if you want say make a golf cart to go 60 mph scary fast I plug in the weight, rolling resistance, coefficient of drag and it tells me I need 13 to 14 hp.


Sure, this page is my best friend here: http://www.enginuitysystems.com/EVCalculator.htm

It helps you figure out what you'd need for a given situation.



> Question was asked how high of a Voltage can you apply to a Motor? The theoretical answer lies in max LRA and Motor Resistance. Once you have that it is child's play, Ohm's Law; *Voltage = Current x Resistance.* In my example 100 amps x 1 ohm = 100 volts. That does not mean you should use 100 volts.


I lose you a bit here.

If in your example the maximum voltage you should use is 100 volts, then that means that you have nothing left at the 5220 RPM you wanted. No ability to climb hills, etc. You have conservatively sized your controller for the momentary starting surge and turned a 13hp motor into a 3hp motor.

You could keep that same motor, and get a controller that can handle 176 volts and 176v-76v = 100 amps at 5220 RPM. Which is the 100 amps that the motor wiring is rated for (wires aren't getting any thicker).

At low speed, that motor would be pulling 176v / 1ohm = 176 amps. Or 176v * 176a = *31,000 watts*. If your controller is sensitive to overpower, it would have to be sure to modulate that voltage as it brought up to speed so as to never exceed 100 amps (motor can almost certainly take more heat abuse than transistors).

In this case you increased the voltage from 100 to 176v, (1.76x). And increased the power at that RPM from 24amps to 100 amps (~4x). 

176v * 100a = 17.6kw = 23.5hp (I'm fuzzy if you're saying that that doesn't still hold true, I'm not confident it does).

If so you took a situation that at 5220 RPM was providing you 3hp, and you've increased it to 23.5hp (7.8x) by only increasing the voltage 1.76x.

Is that what you guys are getting at? That 1.76*1.76 = 3.1x, and that's less than 7.8x?

If so, would it make everyone happier to say that in DrGee's original example, when feeding a 36v motor with 144v (4x) that he would get *at minimum* 16x the power as before, *but even higher proportional performance increase at any speed above zero*?


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## major (Apr 4, 2008)

DrGee said:


> Major, ... - what's a "good" commutator count on a motor you intend overvolt by factor of 4? How about a factor of 2?


Hi DrGee,

A shame this thread has gone off topic. 

Comm bar count. Just a interesting design feature. All else being equal, more comm segments are used for higher voltage, but reduced current. More comm segments can also be used for lower design speed at same voltage but that will yield lower power.

However, ignore all that when re-purposing (using it in an application different than it was intended) the motor and running at higher than design voltage. Undoubtedly you'll need to advance timing (adjust brush position relative to field) to get acceptable commutation. Once that is done correctly, I've seen 29 bars work as well as 99 bars.

A lot depends on your application. Drag racing... Low bar count, big copper current path, willing to tolerate arcing and wear for ET & trophy. Long lasting commuter car... Higher bar count, lower current, less arcing.

The PWM controller should allow you to use relatively low bar counts without going drag strip nuts and see reasonable performance and life. Beware that such motors can have low inductance and play havoc with some controllers.

Comm bar count. Nice to note. But no big deal.

major


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## DrGee (Aug 22, 2018)

major said:


> DrGee said:
> 
> 
> > Major, ... - what's a "good" commutator count on a motor you intend overvolt by factor of 4? How about a factor of 2?
> ...


Nice one Major! 
That clarifies alot. 
Cheers! 
Greg.


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## Sunking (Aug 10, 2009)

MattsAwesomeStuff said:


> Helps to see someone walk through it. I don't think there's anything new in here to me. This is how I have understood things since before this thread began. I don't see where it makes things I said earlier incorrect.



Nope you still don't get it, what you know is Academic. 




MattsAwesomeStuff said:


> Okay, so, in EV context, right off the bat, almost every DIY EVer runs their motors way above their rated voltage. "Spec" doesn't mean much to most of us.



Al that means is your dangerous and do not know what you are doing. 




MattsAwesomeStuff said:


> Without knowing the max voltage you can't say what the LRA would be, because, if you feed it higher voltage you're going to get a higher LRA, right?



There is where you get of track. Its a Thermal limit. If you ignore it like a DIY as you suggest, you are playing with fire and burn things up. LRA is a maximum thermal limit a DIY would push. As long as you do not exceed thermal limits, it works sort of for a while. But there is huge flaw with that approach. All you are doing is making a lot of heat that is wasted and burning up a motor and wasting energy you do not have to spare



All electromagnets, transformers and like passive devices has a saturation limit where pumping more current does not increase magnetic flux, just makes more heat to burn the insulation off your motor windings. You can certainly push more current and go all the way to thermal limits, but it is all wasted as heat burning up the motor winding and shortening motor life. a properly designed system would not do that. DIY, who knows what you get? 



You are right, you design a vehicle for speed. That is a function of RPM not current.


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## Sunking (Aug 10, 2009)

DrGee said:


> This last note is particularly useful. Most folk with a basic technical background know Ohm's law, but I had never heard of max LRA 'till I saw it in
> this string of posts.



OK here is another one, FLA or Full Load Current which in turn relates to maximum continuous power. LRA can be as high as 6X FLA. Ever wonder why your lights blink when say the air conditioner turns on, or your lamp blinks when you turn on a vacuum sweeper or some other high power motor? That is LRA when you hit the motor with a solid 120 or 240 VAC without current limiting. It is so much current it causes your house voltage to sag and you see it as a Blink while the motor RPM spools up and current tapers to FLA.


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## MattsAwesomeStuff (Aug 10, 2017)

Sunking said:


> Nope you still don't get it, what you know is Academic.


I'd say the reverse, my hands-on and practical grasp of the material is decent, my academic understanding is what's particularly weak.



> Al that means is your dangerous and do not know what you are doing.


It's not me, it's what everyone else is doing too.

From what I understand, motors have many different situations that they would be rated for. Indoor, outdoor, vibration, enclosed, ambient temperature, mission critical, etc etc. The circumstances they're needed for determines what could be expected from them. Almost none of the motors that EVers are repurposing are rated for the circumstances of EV use, so some fudging is necessary.

Generally a vehicle will have high current spikes when accelerating, but not actually be accelerating constantly. Much the same as a Civic isn't expected to be redlined constantly when driving about, and if you did, you'd expect much shorter life out of it.

Most motors aren't rated for being in an airstream of a moving vehicle. They're presumed to have only stationary air cooling. That makes them conservative relative to if they're fancooled, exposed to outside air, water cooled, etc.

You can obviously get massively more power from a motor that is cooled than one that's stuck in a box or around stale air.



> If you ignore it like a DIY as you suggest, you are playing with fire and burn things up. LRA is a maximum thermal limit a DIY would push. As long as you do not exceed thermal limits, it works sort of for a while. But there is huge flaw with that approach. All you are doing is making a lot of heat that is wasted and burning up a motor and wasting energy you do not have to spare


But that's what most EVers do. They're running 36 and 48v motors at 120+ volts (Duncan's running his at 300+, albeit for sprints). Running 200 amp motors upwards of 700 or 1000 amps. Etc.

Now, they're rarely running them that hot for long periods of time, but, that's what they're doing.



> All electromagnets, transformers and like passive devices has a saturation limit where pumping more current does not increase magnetic flux, just makes more heat to burn the insulation off your motor windings. You can certainly push more current and go all the way to thermal limits, but it is all wasted as heat burning up the motor winding and shortening motor life. a properly designed system would not do that. DIY, who knows what you get?


Saturation, flux, etc is something that's eluded me in terms of really understanding what's going on there. The magnetic component of electronics is a particularly weak point for me.

That was one of the things that shocked me most when I first started looking into EVs, was how much more power people were getting out of their motors than they were rated for. I figured for sure they would hit a saturation limit in cores but, *shrugs*, everyone seems fine doing it. 

I can only presume that this is because motors built for industrial applications are more concerned with decades of runtime and the costs of industrial downtime far outvaluing the added cost of overbuilding the motor. Saving $1000 on a forklift motor by undersizing it is a poor choice when a day's downtime for a forklift, let alone the rushed repairs can cost you more than that.


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## DrGee (Aug 22, 2018)

Sunking said:


> DrGee said:
> 
> 
> > This last note is particularly useful. Most folk with a basic technical background know Ohm's law, but I had never heard of max LRA 'till I saw it in
> ...


Great stuff Sunking, 
This is just what I need.


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## DrGee (Aug 22, 2018)

MattsAwesomeStuff said:


> Sunking said:
> 
> 
> > OK Matt I am not trying to beat up on you. What you know is Academic and correct, but you do not know how to apply the knowledge. So perhaps this may help.
> ...


Hi Matt, 
That link to EV Calculations is invaluable!
Thanks so much👍🏾👍🏾


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## DrGee (Aug 22, 2018)

Here's another basic question.
These motors are exposed to wind and rain, when mounted under the bonnet or in the transmission tunnel. Even more so, if you have a fan cooling them- even in the rain- at 100's of cu ft /sec...
How come the water doesn't play havoc with the electrics?


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## MattsAwesomeStuff (Aug 10, 2017)

DrGee said:


> How come the water doesn't play havoc with the electrics?


It does if it gets in there. Especially the control electronics which should very much be sealed (and usually located at the top of the engine comparment) with only their heatsinks exposed if possible. Batteries too need to be kept dry or bad things happen.

Motors have high-silicon iron laminations that will rust just about instantly. Copper will corrode and crumble away with a green version of its own rust. The copper has enamel on it that helps protect most of it. The motor core itself is usually dipped and vacuum-impregnated with lacquer (or if it's really old, beeswax). So that does a lot to help. Sometimes the internals will be dunked as well, sometimes not. There's a tradeoff that, you're insulating the things that are getting hot, making them get hotter, while you're protecting them from corrosion.

Generally people have hoods to keep rain from washing down on the motor, so you're only worried about splash coming up. 

If there are fans, generally it's drier air and, either a splash shield or hopefully not much gets into the motor from below.

Also the spinning parts of the motor are both spinning and hot, which helps dry out the parts that get wet.


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## Sunking (Aug 10, 2009)

DrGee said:


> How come the water doesn't play havoc with the electrics?


Water will play havoc if you do not use a motor designed to work in an electric vehicle. Some motors, particularly AC motors made for EV's use liquid cooling, but they do not use water, they use some sort of oil like ATF, Mineral Oil, or Transformer Oil. Issue is a lot of DIY's do not use motors for EV's, instead repurposed motors not intended to do what you would need them to do. So you end up trying to work around the issues like moisture, water, dust, and contaminants.


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## DrGee (Aug 22, 2018)

Well I suppose forklifts are "ev's" - just a bit heavier and slower than what we build. I'm dreaming up a way to rejig the air filter & duct to the fan that'll be on the front of the 13 inch motor..


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## Sunking (Aug 10, 2009)

MattsAwesomeStuff said:


> I lose you a bit here.
> 
> If in your example the maximum voltage you should use is 100 volts, then that means that you have nothing left at the 5220 RPM you wanted. No ability to climb hills, etc. You have conservatively sized your controller for the momentary starting surge and turned a 13hp motor into a 3hp motor.
> 
> ...



Wrong and that is where you go off track and your academics fail you. You are correct if you applied 176 volts to the motor, you would pull 176 amps at very low RPMs. That would last for about a second or less when you learned you should have done that when smoke and fire rolls out of the motor. Basically you just designed a motor to self destruct after one use. You exceeded the maximum thermal limits and burned up your motor.


OK can you use a higher voltage like 176 volts on my example? Absolutely and done all the time. That is what the Speed Controller is used for. With a 100 LRA rating you would use a 100 amp or less controller, making it virtually impossible to ever apply 176 volts to the motor at low RPM's because the controller will fold back the voltage to limit current to 100 amps. In practice the maximum voltage you should apply is 2Eb, where Eb = EMF generated by the motor at maximum safe operating RPM's before the motor flies apart. However there are some other thermal risk going beyond the voltage required to produce LRA related to Eb. Once the motor reaches a a specific RPM, Eb goes no higher. Eb is the self limiting factor, so above that point, Eb can no longer push back and limit current. Result is if there is too much load, you exceed the motors maximum thermal limits. The real slap in the face and slam dunk on your batteries is all that power is wasted as excess heat because there is no increase in mechanical power. Put another way your efficiency falls off a cliff and you risk burning up your motor again End result is a lot less range and much shorter motor life is the price you pay for being cheap and cutting corners. It just cost you more fixing it than doing it correctly to start with.


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## Sunking (Aug 10, 2009)

DrGee said:


> Well I suppose forklifts are "ev's" - just a bit heavier and slower than what we build.


Received and acknowledged. Honestly no experience with Fork Lift motors. Could not tell you if they are sealed or not. What I do know is motors made to be exposed to the elements cost more than ones that are not. Considering fork lifts are used indoors, and when outside generally do not typically operate in wet conditions, manufactures may o rmay not used sealed motors. I do not know. 



Just because fork lifts go slow does not mean you can make an EV with them that goes fast. It is just a matter of over all differential ratio.The challenge is fork lift use series motors which is the perfect choice where speed is not a consideration, the primary goal is torque at low operating RPM's like 2000 to 3000 RPM. So to get speed you will have to use a Transmission with multiple gears like an ICE vehicle uses. 



FWIW If you were designing an EV from the ground up, you would never use a series DC motor. Either BLDC or 3-phase Induction motors. Otherwise the vehicle gets to heavy and expensive. Both BLDC and Induction motors have the best of both worlds of torque and speed. They produce very flat torque curves up to several thousand RPM, and operate at much higher RPM' up to 10,000 to 13,000 RPM's requiring no expensive transmission other than a fixed differential direct drive.Yep the motors and controllers are more expensive than a DC motor and Controller, but a heck of a lot less money and weight that a DC motor, Controller, and a transmission to make the DC motors work. Not to mention BLDC and Induction motors are more efficient.


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## brian_ (Feb 7, 2017)

Sunking said:


> Honestly no experience with Fork Lift motors. Could not tell you if they are sealed or not.


Traditional "forklift" brushed series DC motors all appear to be open: you can see the commutator and brushes. Some builders use a commercially available or home-built shroud to close that opening, and provide a connection for a blower, with a filter on the blower intake. An appropriately placed filter will prevent problems with water.

Search this forum for "blower" or check suppliers such as EV West for a motor cooling kit, and you can see examples.


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## brian_ (Feb 7, 2017)

Sunking said:


> FWIW If you were designing an EV from the ground up, you would never use a series DC motor. Either BLDC or 3-phase Induction motors. Otherwise the vehicle gets to heavy and expensive. Both BLDC and Induction motors have the best of both worlds of torque and speed. They produce very flat torque curves up to several thousand RPM, and operate at much higher RPM' up to 10,000 to 13,000 RPM's requiring no expensive transmission other than a fixed differential direct drive.Yep the motors and controllers are more expensive than a DC motor and Controller, but a heck of a lot less money and weight that a DC motor, Controller, and a transmission to make the DC motors work. Not to mention BLDC and Induction motors are more efficient.


If by "BLDC" you mean 3-phase AC synchronous motors with permanent magnet rotors... then yes, this is the nearly universal choice.

Typically torque is flat (at some level limited by current, which in turn is limited by who-knows-what) up to a few thousand RPM (I don't think I've seen higher than 4,000 RPM for a volume production vehicle), then power is flat from that point to near that high speed limit. Whether this is there result of AC operation, or another aspect of motor design, or the relatively high (compared to what an ex-forklift motor can handle) maximum voltage... I don't know.

There is one oddball exception that I've seen: Renault puts a 3-phase AC synchronous motor in the Zoe and Kangoo Z.E., but it has a powered rotor winding (instead of permanent magnets), using brushes and slip rings. Still no commutator.


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## MattsAwesomeStuff (Aug 10, 2017)

brian_ said:


> There is one oddball exception that I've seen: Renault puts a 3-phase AC synchronous motor in the Zoe and Kangoo Z.E., but it has a powered rotor winding (instead of permanent magnets), using brushes and slip rings. Still no commutator.


Does it also have a beefy resistor bank?

LE came out with a new video on slip ring ACIM this week actually. Dunno if it's the same thing you were thinking of.






Gives higher starting torque at startup apparently because of the lower phase angle, the tradeoff being the obvious complexity and cost.

But makes me wonder, is low starting torque an issue for EVs... anywhere? Seems like solving a problem that didn't exist. Or that's not the same thing that you meant.


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## major (Apr 4, 2008)

MattsAwesomeStuff said:


> Does it also have a beefy resistor bank?
> 
> LE came out with a new video on slip ring ACIM this week actually. Dunno if it's the same thing you were thinking of.
> 
> ...


Is this what you reference?

https://m.youtube.com/watch?v=JPn5Ou-N0b0

It is a control method used for mains applications. As such it is completely redundant and undesirable with an inverter driven induction motor in an EV. 

And of course low starting torque would be an issue in EVs.

And beeswax???? Maybe 100 years ago. Why would you even mention it?

Regards,

major


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> Does it also have a beefy resistor bank?


No, I'm sure that there are no resistors carrying rotor current. It's from 2018, not 1918. 



MattsAwesomeStuff said:


> LE came out with a new video on slip ring ACIM this week actually. Dunno if it's the same thing you were thinking of...


YouTube links like that no longer work in this forum. Use a URL link instead.

That video describes a real issues for induction motor powered straight across the line without a VFD ("mains applications", as major said), and an interesting solution for it; however, it is not an issue or a solution for a modern EV.

The Renault unit is not an induction motor of any kind, so this doesn't apply. It has a four-pole single-phase powered (so two slip rings) rotor winding, not shorted bars (a "squirrel cage") or a three-phase winding with current induced by the stator field (as shown in the video).

Link to previous Zoe motor thread (with an image of the rotor):
Renault Zoe motor synchronous motor



MattsAwesomeStuff said:


> Gives higher starting torque at startup apparently because of the lower phase angle, the tradeoff being the obvious complexity and cost.
> 
> But makes me wonder, is low starting torque an issue for EVs... anywhere? Seems like solving a problem that didn't exist. Or that's not the same thing that you meant.


I'm not aware of any problem with low starting torque for EVs, even with induction motors, because as major noted in the previous post, EV motors are not powered by a constant-frequency supply like an industrial motor without a VFD.

It is important that an EV has high starting torque, but it's not a problem. For AC motors torque is roughly constant from zero speed through the lower part of their speed range. Strangely, in the Renault specs the peak torque does not go down to zero RPM (just 1500 to 3395 RPM, then peak power from 3395 to 10980 RPM), but I'm sure that torque is still near peak all the way to close to zero speed.


*Stall* (zero speed) is an interesting point to compare the various motor types (assuming that each is driven by a proper controller)... but it's really a synchronous versus asynchronous (induction) comparison, rather than DC versus AC:

In both brushed DC motors (whether the field is provided by windingf or permanent magnets), and in AC synchronous motors the stator and rotor magnetic fields are in synch, so both are stationary at stall (and the stator side of the DC motor is stationary at all times). All currents are continuous (the three phases of the AC motor are at different values from each other, but each stuck on one value). All of the power going into the motor is just being dissipated as heat due to wire resistance.
In an induction motor torque is only produced when the stator field is rotating faster than the rotor field; the stator's magnetic field must sweep through the rotor windings to induce current and a rotor field to react with. The 3 phases of the inverter output are all at a low frequency corresponding to the slip speed. All of the power going into the motor is still being dissipated as heat, but due to a combination of wire resistance and magnetic losses.
In a common AC synchronous EV motor the rotor's magnetic field is provided by permanent magnets; in the Renault wound-rotor motors the rotor's magnetic field is provided by DC current through windings, so it is still in a fixed orientation in the rotor (although presumably variable in strength by varying current to suit operating conditions). The rotor of the Renault motor is like the stator of a common DC motor, powered independently of the rotor as in a separately excited ("SepEx") DC motor.

Of course stall is not a normal operating condition for more than an instant. The vehicle immediately starts moving and the rotor speed is no longer zero. If someone is holding an EV on a hill with power - so stall is sustained - they should move their foot to the middle pedal!


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## DrGee (Aug 22, 2018)

Sunking's information on the higher revs in AC motors reminded me of another question about how a motor behaves under a load. 
Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
We've calculated the max power at this voltage and the current it draws. We are happy that this is sufficient for the chosen vehicle's weight, CD etc. 
It's a direct drive - no gearbox. 
This vehicle used to do 70mph when the ICE was at 2500 rpm in 4th gear (1:1).
What will happen when the DC motor at 144V, tries to reach 5100 rpm, but can't, because it hasn't got enough torque? While it struggles to reach 5100 rpm and fails to, will it overheat?


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## MattsAwesomeStuff (Aug 10, 2017)

major said:


> And beeswax???? Maybe 100 years ago. Why would you even mention it?


I'm sorry for discussing things on a discussion forum that I found interesting. I will endeavor to only reply like a robot from now on.

I've taken apart electronics from the 1980s that were dipped in beeswax.



Brian said:


> YouTube links like that no longer work in this forum. Use a URL link instead.


Odd. They work just fine for me, including in quoted sections it still shows the embedded video player and the video player is functional.


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## major (Apr 4, 2008)

MattsAwesomeStuff said:


> I'm sorry for discussing things on a discussion forum that I found interesting. I will endeavor to only reply like a robot from now on.
> 
> I've taken apart electronics from the 1980s that were dipped in beeswax.
> ...


You were replying to and answering a specific question and stated the motor core was dipped and vacuum impregnated with beeswax if it was really old. I doubt that was ever done. Oh, you once had some electronics that were dipped in beeswax. So you assume beeswax was also used on motor cores? Considering temperature and centrifugal forces? You don't know what you're talking about so please stop pretending you do. 

if you want to share or discuss something which you have found to be interesting, use an appropriate thread or start one. Don't just throw irrelevant or off-topic items into replies to a member's question.

major


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## brian_ (Feb 7, 2017)

*YouTube*



MattsAwesomeStuff said:


> Odd. They work just fine for me, including in quoted sections it still shows the embedded video player and the video player is functional.


Okay, it probably depends on the browser. It's just a big blank space to me. major's comment suggested that he didn't see it, either. 

After a bit of experimentation, I see that the embedded YouTube player uses Flash, which is no longer support by my browser; that's common, so it won't work for lots of people.

A simple URL link is much more compact (good for all of those quoted repeats), and works for everyone.


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## DrGee (Aug 22, 2018)

I'm posting this question again, because it may have been missed in the last flurry of posts-

Sunking's information on the higher revs in AC motors reminded me of another question about how a motor behaves under a load. 
Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
We've calculated the max power at this voltage and the current it draws. We are happy that this is sufficient for the chosen vehicle's weight, CD etc. 
It's a direct drive - no gearbox. 
This vehicle used to do 70mph when the ICE was at 2500 rpm in 4th gear (1:1).

What will happen when the DC motor at 144V, tries to reach 5100 rpm, but can't, because it hasn't got enough torque? While it struggles to reach 5100 rpm and fails to, will it overheat?


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## major (Apr 4, 2008)

He


DrGee said:


> I'm posting this question again, because it may have been missed in the last flurry of posts-
> 
> Sunking's information on the higher revs in AC motors reminded me of another question about how a motor behaves under a load.
> Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
> ...


Hi Dr.G,

Yep, motor will overheat. Controller will limit current and help, but if you're lucky it will self protect and cut back to a current or power level it can tolerate. But it is likely to fail before the motor. That is why you need to use a proper gear ratio.












> The black lines are the motor torque-speed curves for different drive voltages; the red line is an example of a LOAD torque-speed curve. Source.
> The red line I added to the graph is for something like a fan, where the torque increases as some power of the angular speed. The running speed of the combined power supply-motor-fan system is represented by the blue dots marked 1 and 2. Point 1 is a higher-volt drive which causes higher motor current, hence higher torque and the fan runs faster. Makes sense. The engineering edge here is that if we have the actual, measured curves we can really predict the speeds.


From: https://nathotron.wordpress.com/courses/engineering-design-process-530-381/motor-selection/ 

The use of motor speed torque curves and load profile helps understand how to choose the right ratio. The above is just an example. The series motor will have a non-linear (curved) characteristic instead of the straight lines of the PMDC motor. The vehicle load curve will be similar shape as the fan curve.

Regards,

major


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## Duncan (Dec 8, 2008)

DrGee said:


> I'm posting this question again, because it may have been missed in the last flurry of posts-
> 
> Sunking's information on the higher revs in AC motors reminded me of another question about how a motor behaves under a load.
> Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
> ...


Major has answered this - but I would like to answer it a little differently

Your controller will control the Current - so if you have a 600 amp controller then full throttle = 600 amps and half throttle = 300 amps

In the forklift the controller will set to a certain maximum current - with my motor it was 200 amps

Rpm ------Motor Current ----Voltage ----Battery current ----- Battery voltage
0 -----------600 amps-------------10v------------42 amps-------------144v
This is well above the sustainable current - and will melt in a minute or so
1700--------600 amps------------144 v-----------600 amps-------------144v

*NOT* 5100 rpm - because You are using three times the current and will have three times the Back EMF

5100 -------200 amps -----------144 v -----------200 amps ------------144v

In order for the rpm to rise the current must go down - the controller will be on 100% trying it's best to meet your desired current

In practice the dropping torque will intercept the rising wind resistance before them


I did this with my car - changed the controller from a 500 amp and 150v model to a 1000 amp with the capability of 400v
And at the same time reconfigured my batter to get more kwh - but at a reduced voltage - 130v 

It took off like a scalded rat - then the acceleration dropped and at about 3500 rpm and 100 kph the current had dropped to 200 amps - which balanced the increased wind drag - Maximum speed!


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## MattsAwesomeStuff (Aug 10, 2017)

DrGee said:


> Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).


I think you might be confusing max ratings with an expected result. This is a fictional motor I presume, so it probably wouldn't be rated this way.

For almost any other motor type, you can easily predict the unloaded RPM of a motor in wide circumstances. For example, synchronous AC motors will rotate based on however fast the coils get pulsed, asynchronous will always be behind a bit but fairly close. Permanent Magnet DC motors will turn a certain speed based on the voltage you give them, and you can pretty much linearly plot that (double the voltage, double the RPM). You might sometimes hear the term "KV" applied to a motor which means how many RPMs it will spin to, per volt you feed it (so a KV of 100 means if you give it 1 volt it will spin 100 RPM, 12 volts it will spin 1200 RPM, etc. They're telling you the ratio of RPM:Volts). SepEx and Shunt (parallel) wound DC motors will have similar, predictable speeds.

But DC Series motors you can't tell. With any voltage and no load, they accelerate forever, very quickly. The only thing that slows them down is having a load on them, and their speed/current curve isn't linear. So for example, they're not a great choice for any drive system that has a chain, because when the chain inevitably snaps or hops off a sprocket, the motor will just about instantly accelerate until it rips itself apart.

So in your example, if the motor is "rated" for 1700 RPM, that probably means that is its mechanical ceiling before its physical structural integrity can't be relied on anymore (as per whatever conservative degree of its rating and circumstances). Or, perhaps that in that specific application with that load, at 48V it would be expected to spin 1700 RPM. But it would have to be a very specifically known load.

For example, if you give an unloaded DC Series motor 5v, it will reach 12,000 RPM and then maybe explode. If you give it 50v, it will reach 12,000 RPM in much less time, but then maybe explode. It doesn't really matter what voltage you give it, without some drag, it spins up and then explodes. 

By tripling the voltage, you don't physically change anything about the motor. It becomes no more strong at holding itself together. Much like if a chain is rated for 1700 lbs, you can't make it be rated for 5100 lbs. It just is what it is.

Bigger motors will tend to be rated for a lower max RPM than smaller motors because the outside edge of the rotor is moving faster on a larger diameter.

TL;DR - 
1 - Your (fictional?) motor is probably not rated for a max of only 1700 RPM. That's pretty slow. But if it was, max means mechanically the max before it blows up, nothing to do with voltage, and,
2 - Your (fictional) motor is probably not rated for 1700 RPM @ 48volts, because you say it's a DC Series motor and that's nonsensical since there isn't a "no load" speed for that type of motor.



> What will happen when the DC motor at 144V, tries to reach 5100 rpm, but can't, because it hasn't got enough torque? While it struggles to reach 5100 rpm and fails to, will it overheat?


In short yes. Demand too much of anything and it will fail. Almost all electronics have a thermal limit by which they fail (in addition to the RPM limit mentioned earlier which you probably won't reach).

That's probably not the easiest way to think about it, for any motor type.

For example, think about going down a steep downhill. You might barely have to push at all to make the motor spin at 5100 RPM. So it's not how fast you spin the motor, it's how hard it is to spin the motor that fast.

I would kind of abstract the whole "target speed" thing and not think about it that way. 

Think about a normal gas car driving down a highway. You have the accelerator pushed down some amount. Let's say it's flat and not windy so nothing is changing. Because wind resistance increases as a cube of speed (it's increasingly hard to go faster), you will eventually come to a steady speed based on how far you're pushing the pedal.

You don't have to know the math or quantities behind how much gas is pouring into the engine every second, exploding and propelling the car. You know that if you back off the gas, the car will slow down and reach a slower steady speed. If you step on the gas, the car will speed up and reach a faster steady speed. You don't have a pedal position that dictates "speed" and half way down is half speed, all the way down is full speed, etc. It depends on wind, whether you're climbing, whether you're towing, whether you're accelerating, etc. More is more and less is less.

Same thing for electric motors. Tell the throttle to give you "more", it will demand more. Tell the throttle for "less", it will give less. 

If you redline a gas engine, suppose you have a small engine and you're trying to tow a heavy trailer up a hill, or, suppose you're trying to go 200mph on a crappy car... you will be demanding More and More and More. At some point you will only be able to put so much gas into the engine and it can't give you move (pedal to the metal), but more troublesome is that with you redlining the engine it's going to be getting hotter than you can keep cool, temperatures will rise, and something will blow up.

Ditto for electric motors. If you have too small of a motor and you're trying to do too much to it... accelerate too fast, reach too high a speed, drag too much weight up a hill... it's going to overheat and something will blow up. Unlike with gas engines though, motors have a bigger swing in what you can demand from them. You can only shove so much fuel and oxygen into a piston, and chemistry can only happen so quick. But electric motors you could abuse to a wider rate, several whole multiples of their rated steady power for very short periods, hitting only magnetic limits in the core I suppose. 

I would think more in terms of power.

How much power is required from your motor, and can it sustain that?

You can know about the power it takes to travel a certain speed, or accelerate a weight at a certain rate, or travel up a hill a certain speed from that calculator I linked earlier. Those are physical facts. It will take X amount of power to get a certain car of a certain weight and shape, to travel a certain speed up a certain incline in certain weather. That amount of power has to come from the motor.

When you press the accelerator, you're in the end asking for more power. The actual sequence will be something like:

1 - Pedal position tells a circuit what duty cycle to pulse the motor on/off at since changing the actual voltage is impractical.
2 - The power transistors do as instructed, and pulse the voltage to the motor with those proportions of onff time.
3 - This averages a net voltage on the motor (i.e. 50% on/off time for 100 volts, appears like 50 volts and the motor would act the same if you gave it 50v steady).
4 - Voltage causes current (amps) to flow.
5 - Current makes the motor spin.
6 - The harder it is to spin, (not speed directly, but all the things that make a car difficult to move like wind, hills, and accelerating) the more current flows for a given voltage.
7 - The more current the more heat.
8 - Too much heat and something melts, either power transistors or motor brushes or motor wires or, something.

So you're not really telling it "Go this fast", you're telling it "Push this much harder". And pushing hard is what makes heat. 

Think of perhaps, a toy electric car that you put your foot on. What happens? The motor can't pull you, it tries as hard as it can, and it overheats and melts. Ditto for an undersized motor in a car. It tries, it overheats, and melts.

By whatever method you're asking for "more", if it's more than the motor can provide, it dies.


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## DrGee (Aug 22, 2018)

Duncan said:


> DrGee said:
> 
> 
> > I'm posting this question again, because it may have been missed in the last flurry of posts-
> ...


😂😂😂 Would have loved to have seen that take off!..


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## DrGee (Aug 22, 2018)

Duncan said:


> DrGee said:
> 
> 
> > I'm posting this question again, because it may have been missed in the last flurry of posts-
> ...


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## DrGee (Aug 22, 2018)

MattsAwesomeStuff said:


> DrGee said:
> 
> 
> > Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
> ...


Thanks Matt, 
There's a lot of information here. The complexities of using AC or Sepex motors & controllers is my challenge. So far, I'm still likely to use a repurposed forklift motor.


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## MattsAwesomeStuff (Aug 10, 2017)

DrGee said:


> Thanks Matt,


Just FYI,

On forums it's generally good etiquette to exercise selective quoting, especially for larger posts. Selective quoting is where you only quote the part of the previous reply that is relevant to what you're going to refer to.

If it's not specific, and just refers to the whole post, then you can skip quoting entirely and just say what you want to say if it's obvious.

That way the content is a bit more streamlined and easier to follow.


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## DrGee (Aug 22, 2018)

MattsAwesomeStuff said:


> DrGee said:
> 
> 
> > Thanks Matt, /QUOTE]
> ...


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## DrGee (Aug 22, 2018)

This is a 13" forklift motor. It's similar to the imaginary motor in my question earlier. It says it's rated at 1380 rpm. This looks like a good one to repurpose doesn't it?


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## Duncan (Dec 8, 2008)

Hi - looks good

You will need to find something that matches that spline - probably not difficult

How heavy is it?

What top speed/rpm do you need? - as a bigger motor it will have a lower "burst speed" - I have been told my Hitachi 11 inch will be OK at 6500 rpm - a 13 inch - maybe 5000 rpm??- we need Major to tell us about this one


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## DrGee (Aug 22, 2018)

Duncan said:


> Hi - looks good
> 
> You will need to find something that matches that spline - probably not difficult
> 
> ...


It's listed as weighing 250 lbs. I'm aiming to cruise at 70 mph. This requires an rpm of 2600 or thereabouts in my jeep (it'll be a direct drive).
Thanks Duncan!


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## boekel (Nov 10, 2010)

For people worrying about water ingress in DC-motors (forklift motors in particular)

I wouldn't recommend it...but the (dual) DC motors in our little (7meter) boat have gone trough some abuse in the last 7 years...have been under water at least 10 times, sometimes run under water...

We did change to sealed bearings when converting the boat, that's higly recommended.

There is a thread on the forum...but due to the 'new' forum the pictures don't work anymore 
https://www.diyelectriccar.com/forums/showthread.php?t=55318

https://www.youtube.com/watch?v=BeDhIDnUrnM


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## Sunking (Aug 10, 2009)

DrGee said:


> Sunking's information on the higher revs in AC motors reminded me of another question about how a motor behaves under a load.
> Let's take a simple brushed DC series wound motor for this example. It's rated as 1700 max rmp @ 48V. We overvolt it to 144V. It's max revs are now 5,100 rpm ( 3x 1700 - in theory).
> We've calculated the max power at this voltage and the current it draws. We are happy that this is sufficient for the chosen vehicle's weight, CD etc.
> It's a direct drive - no gearbox.
> ...


OK a few points you need to understand about motors, especially Series Wound DC Motors. Remember that wirty dord Back EMF and Motor Resistance? 

Lets start with Motor Resistance and how LRA works with controllers. Lets say you have that 48 volt motor and it has a Resistance of say 0.167 Ohms and peak power of 15,000 watts or 15 HP. What size controller are you looking for? If you are going to use say 48 volts, just call it 50 volts. The absolute maximum current a 50 volt battery can push is 50 volts / .167 Ohms = 300 amps. Here is the point I am trying to make I see a lot of golf cart guys making an expensive mistake. They go out and buy a 500 amp controller they have no use for nor would they want if they new how much money they wasted and the damage it could cause. All a 500 Amp Controller means is the Controller can supply up to 500 amps before it limits current and folds back voltage to limit the current. . It does not mean it can push and force 500 amps into a motor if there is not enough voltage to push 500 amps. You could use a 50 amp controller and all that means is you limited torque.

Back EMF is your friend and protects your motor from DIY's and beginners ignorance, but only up to a point. Take that same 48 volt golf cart motor where if running at 5100 RPM produces no Torque or draws current. It quit producing Torque and drawing current because at 5100 RPM back EMF = Battery Voltage, thus leaving you ZERO VOLTS across the motor. Back EMF prevented your motor from burning up and over speeding causing it to fly apart. That's a good thing and the way the manufacture intended it to work. Now you apply 144 volts. Issue is your motor can only produce up to say 60 volts Back EMF and you have a 1700 amp controller. What happens? Smoke On The Water and Fire In The Sky turning your balls Deep Purple. So now you have 144 volt battery - 60 volts Back EMF = 84 volts across the motor. That would mean your motor is drawing 84 volts / .167 ohms = *500 amps *on a motor than can only handle say 75 amps continuous, or 300 amps peak LRA for a few seconds. That is a real problem that would show up first time you used the motor, and be a very expensive lesson. Loosing a lot of money is a great educator. Education is not cheap learning that way.


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## major (Apr 4, 2008)

Sunking said:


> OK a few points you need to understand about motors, especially Series Wound DC Motors. Remember that wirty dord Back EMF and Motor Resistance?
> 
> Lets start with Motor Resistance and how LRA works with controllers. Lets say you have that 48 volt motor and it has a Resistance of say 0.167 Ohms and ...


Hello Sunking,

Realize that the real resistance of a series wound golf cart motor is more on the order of 0.0167 ohms and that of a lift truck motor, half that. It requires a Wheatstone bridge to measure. 

Also LRA is a term rarely used with these types of DC motors. LRA is common with induction motors.

Regards,

major


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## MattsAwesomeStuff (Aug 10, 2017)

I'm confused about the motor plate on DrGee's 250 pounder. Is that actually a Series wound motor?

Why does it list an RPM at all? Doesn't say max RPM, and 1300 is quite conservative if it was. Doesn't appear to necessarily be for a very specific known purpose.

Is it perhaps a SepEx or some other type?

Clearly has 4 terminals for the 2 sets of coils so it's not permmag.


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## Sunking (Aug 10, 2009)

major said:


> Hello Sunking,
> 
> Realize that the real resistance of a series wound golf cart motor is more on the order of 0.0167 ohms and that of a lift truck motor, half that. It requires a Wheatstone bridge to measure.



THX and yes I know. It was a KISS exercise in theory. As for requiring a Whetstone Bridge, not so much IMO, at least not the actual Test Set. A battery, lock the rotor down, volt meter, and amp meter will get you just as close to a real working measurement. Essentially the same thing as a Bridge or DRLO Meter. 

While I am thinking about it, if you have a DC Motor and no info, you have to do some experiments to find working values. You would need to know Locked Rotor Current. Motor KV RPM, No Load Current on shaft, and Eb if I recall correctly. LRA gives you gives you Max Peak Power, No Load Current gives you max Continuous Power, and Eb gives you max safe battery voltage right? 

I ask because DC motors are not my area of expertise. My thing is Batteries, Battery Plants, Power Generation/Distribution, and Electronic Communications related to telecom. Had to go through all that when I designed my first golf cart when I used DC Series Motor and that has been a while. Today I use a HPEV AC15 Induction Motor operating at 96 volts with a 650 amp controller.


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## major (Apr 4, 2008)

Sunking said:


> ...
> While I am thinking about it, if you have a DC Motor and no info, you have to do some experiments to find working values. You would need to know Locked Rotor Current. Motor KV RPM, No Load Current on shaft, and Eb if I recall correctly. LRA gives you gives you Max Peak Power, No Load Current gives you max Continuous Power, and Eb gives you max safe battery voltage right?
> ...


Hi Sunking,

No, that is not right. Here is a tutorial which turned up quickly on a google. It appears pretty good. 

https://www.electrical4u.com/types-of-dc-motor-separately-excited-shunt-series-compound-dc-motor/ 

Also some other parts.

https://www.electrical4u.com/working-or-operating-principle-of-dc-motor/ 

Regards,

major


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## Sunking (Aug 10, 2009)

major said:


> Hi Sunking,
> 
> No, that is not right. Here is a tutorial which turned up quickly on a google. It appears pretty good.
> 
> https://www.electrical4u.com/types-of-dc-motor-separately-excited-shunt-series-compound-dc-motor/


Thx Major. Honestly I do not see anything on the link under Series Motors that invalidates what I have said or expressed. From what I can see mathematically, theoretically, and application are correct. Not arguing or debating, just my observation.


The formulas in the link are pure Ohm's Law which I believe I have stated accurately. I see a lot of info missing, and the application the link addresses is NOT applicable to Continuous use and Variable Speed required for EV use. They specifically say, direct battery, no current or voltage controls. They further add if operated other than a Starter Motor would burn up which I agree with when used in this manner. The link is a very specific application of short burst of low RPM very high torque for starting operation of like an engine or to get a Load moving like a craine making the initial pull to get RPM's up and then use a SEPEX or Induction motor take over. 



To use a Series Motor in an EV requires regulating both current and voltage to prevent a Series Motor from self destruction and some control of speed which is going to be poor to start with using a Series Motor. Had to go back and look at my notes because like I said DC motors are not my daily thing. But when I ran through this fire drill 5 or 6 years ago, I had good specs on Golf Cart Motor that included a full set of peak/continuous power power, voltage, current, torque, and temperatures. Interesting to note they give two sets of specs, one for 36, and once for 48 volts. 



I designed just off the curves, but those curves match the specs and you can use the math to recreate the curves. Example they give you Rm of the resistance of both Ra and Rs totaled, max peak power, max continuous power, Eb and so fourth. Basically from what I can tell is thermal and mechanical RPM limits. 



Where am I off at?


That link is fine is you want to use a 12 volt starter motor for a diesel engine to put on a wheel chair and use a 24 volt battery. Speed control with an off/on switch operating a 1000 amp Contactor to connect the battery directly to the motor. Would work at least once and knock you unconscious when you turn the switch on, flips the wheel chair backwards upside down and landing on your head while the motor flies apart.


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## boekel (Nov 10, 2010)

MattsAwesomeStuff said:


> I'm confused about the motor plate on DrGee's 250 pounder. Is that actually a Series wound motor?
> 
> Why does it list an RPM at all? Doesn't say max RPM, and 1300 is quite conservative if it was. Doesn't appear to necessarily be for a very specific known purpose.
> 
> ...


It's more information than on most motors!

9,x kW at 1380 RPM
254 A @ 36V = 9 kW 

1380 RPM will probably be the nominal speed at 36 or 48 volt

then there is the part number:
5BT1366B137B
http://www.jwarfieldelectric.com/shop/5bt1366b137b-g-e-drive-motor/
http://bullseyeindustrialsales.com/ge-5bt1366b137b-36-48v-98kw-forklift-motor-138937

All four connections are the same size, so probably a series motor.


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## major (Apr 4, 2008)

Sunking said:


> LRA gives you gives you Max Peak Power, No Load Current gives you max Continuous Power, and Eb gives you max safe battery voltage right? ...


I'm sorry that I can't tell you why, but these three statements are incorrect. I can't speculate on your thought process or logic used to arrive at those conclusions. But please read up on the basic theory and don't rely on nameplate data. Older text books are great sources. Internet offers the good, bad and ugly. Sometimes you can find lecture notes which are outstanding, like from the Navy.

Regards,

major


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## MattsAwesomeStuff (Aug 10, 2017)

> 1380 RPM will probably be the nominal speed at 36 or 48 volt


How are you defining nominal?

Speed at any voltage on a series motor is nonsensical without known load.


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## brian_ (Feb 7, 2017)

MattsAwesomeStuff said:


> How are you defining nominal?
> 
> Speed at any voltage on a series motor is nonsensical without known load.


Yes, but that placard gives a speed in the context of power output and both current and voltage. The load would be 9 kW worth; the nominal condition is the condition described by all of these values, and could be either the speed and load for maximum continuous power, or the design conditions (intended application).


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## DrGee (Aug 22, 2018)

Sunking said:


> major said:
> 
> 
> > Hi Sunking,
> ...


You're just hilarious Sunking! 😂😂


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## Sunking (Aug 10, 2009)

major said:


> I'm sorry that I can't tell you why, but these three statements are incorrect. I can't speculate on your thought process or logic used to arrive at those conclusions.


No you can't, but I can shine some light on it. Reference this material from Mizzou Eng, and note Motor Data Table Specifications, motor 118748 a 48 volt nominal motor.


I have a different opinion from a user/engineer that name plate ratings do not tell you everything except once extremely important number from an engineer's POV supplying power. Nameplate VOLTAGE tells you the voltage for maximum Efficiency. I agree you can go Higher or Lower but there are trade-offs doing so and two of them are at the expense of Efficiency and more waste heat generated as a result. So if a manufacture tells me the most efficient voltage is say 48 volts nominal, I would consider important data when used in an EV trying to squeeze every mile of range possible with limited battery capacity. 

OK let's start with *LRA*. I stand corrected in terminology. You are correct LRA is typically specified in Induction motors. The correct Term in a DC Series Motor spec is Stall Current or Starting Current. Term is different, but is the same characteristic as LRA in AC motors. Guilty as charged using wrong Term. Reference Starting current = 1.56 amps in the Tables. Input Power = 48 volts x 1.56 amps = 75 watts. My understanding this is the maximum peak thermal limit of the motor windings. Heat is watts or power and directly related to Ohm's law. Your controller for this motor would need to be at most is 1.5 amps or les to protect the motor windings. Voltage based on nameplate is 48 volts. So maybe that shines some light on that logic. Now find Terminal Resistance (motor winding resistance = 30.9 Ohms) and see if it squares up with my logic using Ohms Law. We have a Stall Current = 1.56 amps squared x Terminal Resistance of 30.9 Ohms = *75 watts*. Being a Thermal Limit from what I can see is just short of Fussing Current of a given wire AWG. Example Fusing Current on say 12 AWG Solid Copper = 235 amps. Just I^2R heat protection on the motor windings from my prospective. Does not mean thou shall push the Thermal Limits. One just as easily could use say a 1 amp controller and eliminate most heat and stress issues. 



I stated maximum continuous power is Motor No Load Current x Battery Voltage.I got that wrong and erred on faulty memory. For some reason I confused it with AC term FLA. The correct DC Series motor term is Nominal Continuous Current .356 Amps from Table. If you used No Load Current as I sated would be 6.95 mas @ 48 volts is 1/3 a watt. Again like Peak Power is a Thermal Limit based on what the motor winding can handle. So Max Continuous Power is .325 amps x 48 volts *15.6 watts*, let's just say 15 watts. I was close but no cigar on that one, thx for making me look and taking a refresher. FWIW what No Load Shaft Current can give you is rpm/volt, however the Table gives you that in the specs.


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## DrGee (Aug 22, 2018)

boekel said:


> For people worrying about water ingress in DC-motors (forklift motors in particular)
> 
> I wouldn't recommend it...but the (dual) DC motors in our little (7meter) boat have gone trough some abuse in the last 7 years...have been under water at least 10 times, sometimes run under water...
> 
> ...


Hi Boekel, 
Didn't realise I hadn't replied to your post.. 
That YouTube clip is amazing! 
I'll still protect my motor from moisture, but I feel alot better about it now.


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## boekel (Nov 10, 2010)

DrGee said:


> Hi Boekel,
> Didn't realise I hadn't replied to your post..
> That YouTube clip is amazing!
> I'll still protect my motor from moisture, but I feel alot better about it now.


here's a bit more:
http://boekel.nu/foto/10/2010-11sloep/index10.htm

http://boekel.nu/foto/11/2011-03sloep/index2.htm


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## PStechPaul (May 1, 2012)

Here is Boekel's boat project in PDF form so images are visible. 



https://www.diyelectriccar.com/forums/attachment.php?attachmentid=109653&stc=1&d=1538004372


Let me know if you have any issue with that, and I can remove it.


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## boekel (Nov 10, 2010)

PStechPaul said:


> Here is Boekel's boat project in PDF form so images are visible.
> 
> Let me know if you have any issue with that, and I can remove it.


No problem with it. it does stop after 20 pages unfortunately.

so what browser / browser plug in can you use to have the images work?
Such a shame they don't fix it 

attaching images sucks...


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## PStechPaul (May 1, 2012)

I use Mozilla Firefox, which has a "Toggle Reader View - F9" button in the address bar. I think it is supposed to be for portable devices like phones and tablets. When I switch back to standard view the images are still visible. Then I just used Nitro PDF Pro to print the page. I did notice that there were three pages although page 3 did not have much content. 

I like your boat project!


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## boekel (Nov 10, 2010)

PStechPaul said:


> I use Mozilla Firefox, which has a "Toggle Reader View - F9" button in the address bar. I think it is supposed to be for portable devices like phones and tablets. When I switch back to standard view the images are still visible. Then I just used Nitro PDF Pro to print the page. I did notice that there were three pages although page 3 did not have much content.
> 
> I like your boat project!


Thanks!, did another one beginning of last year:
http://boekel.nu/foto/17/2017-03-zwaansloep/

But we might be getting off-topic here


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## DrGee (Aug 22, 2018)

boekel said:


> DrGee said:
> 
> 
> > Hi Boekel,
> ...



Great project Boekel! The idea of an electric boat is as exciting as an electric car...
I had no problem viewing the pictures ( using Google chrome on android). 
Tell us about your two motors. 
Are they AC or DC? 
What voltage & current do you run them on? 
What power do they put out? 
rpm's?


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## boekel (Nov 10, 2010)

DrGee said:


> Tell us about your two motors.
> Are they AC or DC?
> What voltage & current do you run them on?
> What power do they put out?
> rpm's?


Please use the boat-Thread for these questions, (they're Series wound DC as you can read in the thead) (Second boat, with 4x tesla battery module: Sepex motor)


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## brian_ (Feb 7, 2017)

brian_ said:


> ... Renault puts a 3-phase AC synchronous motor in the Zoe and Kangoo Z.E., but it has a powered rotor winding (instead of permanent magnets), using brushes and slip rings. Still no commutator.


Apparently, according to a comment buried in an article about Volkswagen's coming MEB platform, the Audi e-tron package uses a synchronous AC motor with an externally excited rotor winding as well, and so will vehicles built on the MEB platform (in AWD variants).
Volkswagen details the foundation for 10 million electric vehicles (see the _Two different kinds of motors_ section)


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## johnsiddle (Jun 22, 2011)

I am sure this has been answered somewhere, but I cannot find it so here goes in hope.
For an average mid size compact 4 door car, is a surplus industrial 3 phase motor suitable for an EV?
If so what sort of size and power rating would be considered as good.

So far I have seen 3HP, 5HP and 10HP advertised would any of these be considered suitable?

regards John


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## DrGee (Aug 22, 2018)

johnsiddle said:


> I am sure this has been answered somewhere, but I cannot find it so here goes in hope.
> For an average mid size compact 4 door car, is a surplus industrial 3 phase motor suitable for an EV?
> If so what sort of size and power rating would be considered as good.
> 
> ...


Hi John, 
I'm relatively new to the world of EVs, but here's what I've gleaned. To keep a car moving at a constant velocity, you need to overcome two main opposing forces - 
Rolling resistance and air resistance (or drag) . Drag is particularly important at higher velocities, since it increases exponentially with speed.
Still, for a small relatively aerodynamic car like a Toyota Corolla, the power required to overcome those two forces at 65mph is only about 20hp. 
Acceleration however, is a completely different kettle of fish. You may need 3x or even 4x the 20hp to get decent acceleration. 
I assume the motors you are speaking of are 3 phase AC. I don't think you can safely increase their power output by simply increasing the voltage you run them on. 
However, you can safely "over volt" a series wound DC motor. Hence the popularity of DC series wound motors with EV enthusiasts. Used forklift motors 
are ideal candidates because they are usually DC series wound, they are cheap and they are easy to find.
Hope this helps, 
Greg.


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## johnsiddle (Jun 22, 2011)

DrGee said:


> Hi John,
> I'm relatively new to the world of EVs, but here's what I've gleaned. To keep a car moving at a constant velocity, you need to overcome two main opposing forces -
> Rolling resistance and air resistance (or drag) . Drag is particularly important at higher velocities, since it increases exponentially with speed.
> Still, for a small relatively aerodynamic car like a Toyota Corolla, the power required to overcome those two forces at 65mph is only about 20hp.
> ...


Thanks Greg, 
that settles that idea. I will go back DC and look for a Forklift one, do you happen to know if a Hydraulic pump/Motor from a forklift would be any good.
I believe my milk float motor is failing under high current and blowing the electrics.
John


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## MattsAwesomeStuff (Aug 10, 2017)

> do you happen to know if a Hydraulic pump/Motor from a forklift would be any good.


Probably not. It'd be okay for motorbike size and weight.

Two issues:

1 - Pump motors are usually only designed for lower duty cycles, so they lack cooling fans.

2 - They're undersized by at least 50%, if not 66% compared to what you need. You need the traction motor from the lift.

There's a whole gigantic stickied thread on repurposing treadmill motors.


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## johnsiddle (Jun 22, 2011)

Thanks Greg.
the one I am looking at is rated as cont and has plenty of cooling.

thanks again.
John


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