# AC vs Series DC efficiency



## YohnnyLuksh (Jan 10, 2011)

Hi,

I know, it is discussed before, but how do you think, how much difference is between AC 3 phase induction motor and DC series motor *efficiency*? 

Example DC series motors: Kostov K9'' 144V / Netgain Impulse9 144V
Example AC motors: Siemens 1PV5135-4WS28 / ABB 3GAA 131 316
These motors have different power ratings, but main question is about efficiency (enery loss).

I've heard that generally DC motors have ~60-70% efficiency, while AC motors have ~85-95%. 

But I have doubt about that because:
* DC motor brushes have no such big friction, to give 20% difference?!
* both motors have strator fields, that use electricity.
* assuming that I would use same voltage for AC or DC (~144V).


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## steven4601 (Nov 11, 2010)

Various parameters come to play.

First is resistance. I^2R is very dominant. 
Second is field angle vs rpm. Efficient torque production through a wide RPM range is difficult.

And somewhere the last issue is friction. The air resistance of a vehicle is far great than brush or bearing friction.

Hope this helps.

If you are technically and financially ready for it, go AC.
(Not being negative about DC but its not as 'green' as AC )


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## major (Apr 4, 2008)

YohnnyLuksh said:


> I know, it is discussed before, but how do you think, how much difference is between AC 3 phase induction motor and DC series motor *efficiency*?


Hi Yohn,

Generally speaking for the size and power range of EV motors, I'd give AC about 2 or 3% advantage over DC. And then maybe a percent or so is given back in the AC controller versus the DC counterpart. 

Towards the higher power end of the EV range, AC drives will run higher voltage systems like 300V batteries. There are some AC drives available now in the lower to medium power range rated for about 100V batteries.

That's the way I see it 

major


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## Yabert (Feb 7, 2010)

Seach for motor performance curve. You can see than the K9 motor seem have 84% peak efficiency and Impulse seem have 86% peak efficiency. This peak efficiency is only reachable in a tight range of the motor rpm.

Another really good point for DC motor is the high efficiency of the controller at 98-99% (sometime near 99.5%) compare to 90-95% efficency for AC controller.

Exemple of motor / controller efficiency:

DC: 86% x 0.99 = 85.1%
AC: 90% x 0.95 = 85.5%


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## Tesseract (Sep 27, 2008)

steven4601 said:


> Various parameters come to play.
> 
> First is resistance. I^2R is very dominant.


Yes, and that is true for either AC or DC motors. The problem with a lot of AC vs. DC comparisons is that DC motor efficiency is often given for intermittent duty loads while AC motor efficiency is often given for continuous duty loads. Unless you can get efficiency ratings for both motors at the so-called "S2-60" duty then I wouldn't read too much into them; you aren't comparing apples to apples.



steven4601 said:


> Second is field angle vs rpm. Efficient torque production through a wide RPM range is difficult.


Not really. If you amend that statement to say "Efficient torque production at low RPMs is difficult" then I'll agree with you. This applies to either type of motor, btw.



steven4601 said:


> And somewhere the last issue is friction. The air resistance of a vehicle is far great than brush or bearing friction.


For larger motors, sure, windage and friction are trivial causes of inefficiency. Of course, the rotor of the ACIM can be made much "smoother" than the rotor (armature) in a DC motor, so that gives it an advantage at higher RPMs (e.g. - above 6000).



steven4601 said:


> If you are technically and financially ready for it, go AC.
> (Not being negative about DC but its not as 'green' as AC )


Statements like this, however, infuriate me. How do you figure that AC is "more green" than DC? That's not even a scientifically quantifiable statement! It costs a lot more to make an AC inverter and they use silicon much less efficiently (at best: ~70%) than a DC motor controller (~100%) so they are less "green" right from the start. And even if the AC controller + motor combo is more efficient, how long will it take to pay back the difference in price between the two systems? None of the AC crowd ever thinks about this, they just hide behind the "known fact" that AC motors (only) are more efficient and they can do regen so they must be superior.

Ok, maybe, but please run some numbers first. In fact I'll do it for you. Let's assume that you have two motor + controller systems, DC and AC, that deliver the same average power of 20kW with the same average efficiency (AC motors *can* be more efficient than DC motors. but DC controllers are always more efficient than AC inverters for the same power rating). Let's be extra generous and say that the AC system can recapture an average 25% of the energy with regenerative braking (5-10% is more typical). 

In other words, the AC system uses 15kW while the DC system uses 20kW to do equivalent amounts of work. Wow... 5kW sounds like a lot. Well, at an average price of $0.10/kWh here in the US that means we save $0.50 in electricity _per hour of operation_.

How much does the AC system cost vs. the DC system? How long will it take to pay that back? Here's a representative of each:

Curtis 1238 AC system

There are numerous combinations of DC controller + motor, but ones of equivalent size to the above would cost around $2000 (e.g. - Alltrax 7245 controller + D&D ES-31B motor).

A price difference of $2500 divided by $0.50 means you have to drive for more than 5k hours (an estimated 250k miles) to recover the difference in cost between the two systems.

That regenerative braking is a real benefit when driving in hilly areas is a good reason to go with AC, but claiming it is so much more efficient it will save you money or that it is more "green" is total nonsense.


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## steven4601 (Nov 11, 2010)

Tesseract said:


> Statements like this, however, infuriate me. How do you figure that AC is "more green" than DC?


I knew I was pissing into the wind with posting something like that 
Green is a very vague statement, I wouldn't take that too serious. 


Maybe I was in an unscientific way trying to promote the use of AC. 

Where did you get the figure (at best: ~70%) from? Wild statements like that....


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## etischer (Jun 16, 2008)

Tesseract said:


> In other words, the AC system uses 15kW while the DC system uses 20kW to do equivalent amounts of work. Wow... 5kW sounds like a lot. Well, at an average price of $0.10/kWh here in the US that means we save $0.50 in electricity _per hour of operation_.
> 
> A price difference of $2500 divided by $0.50 means you have to drive for more than 5k hours (an estimated 250k miles) to recover the difference in cost between the two systems.


Adding an additional 25% battery capacity to my car would cost me $3000 bucks and add 175 pounds to my car. Not to mention the space needed to make them fit!


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## YohnnyLuksh (Jan 10, 2011)

> Where did you get the figure (at best: ~70%) from? Wild statements like that....


This told one expirienced EV converter, but he couldn't say exactly where are so big losses are generating, so I wanted to know some thoughts / facts from you guys, in this forum.



> That regenerative braking is a real benefit when driving in hilly areas is a good reason to go with AC, but claiming it is so much more efficient it will save you money or that it is more "green" is total nonsense.


Yeah, regen is one good extra for AC systems, but as you regularry fill little amount in batteries, does'nt this shorten their lives (LiFePO4)? (use more cycles doing so?) Also regen seems to be difficult to set for everyday use, because of need for coasting. So you must adjust regen somehow with brake pedal, etc.?


So maybe we can do conslusion: AC vs DC systems have almoust the same efficiency, if not counting regen, and higher voltage usage possibility? 
Interesting, that DC system in motor actually looks like AC system (because of brushes alternating field)?  So only real difference is where this AC is generated - in controller (inverter) or in commutator! 


Ehh, I want to build controller myself, and it seems that go for DC and OpenRevolt will be best to do. This AC thing is very inviting, but I am afraid that I could stuck there with controller / inverter build.


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## Tesseract (Sep 27, 2008)

steven4601 said:


> Where did you get the figure (at best: ~70%) from? Wild statements like that....


Because the switches in the inverter must be modulated at all times - they can't ever go to 100% on. The highest average modulation depth is 70.7%. There are some exceptions to that (shifting the neutral point around, etc.) but, in general, you have to turn all of the switches off in an inverter for some meaningful percentage of time in order to create sine wave currents in the motor. No way around that, and not so wild after all, eh?




etischer said:


> Adding an additional 25% battery capacity to my car would cost me $3000 bucks and add 175 pounds to my car. Not to mention the space needed to make them fit!


This is assuming that having equivalent range for the two technologies is the main criterion for comparison - a different, and altogether much more difficult argument to make as it depends on terrain, driving style and the ratio of time spent accelerating, braking and at a constant speed.

And, anyway, 25% return from regen is exceptionally generous in the first place and depends on a most favorable confluence of the above factors. Anything less than 10% variation in range is probably unnoticeable by the average driver unless they drive a very precise route every day and are running right at the edge of capacity.

Like I said in my original response, you can make a rational case for AC over DC, but not by invoking economics or "greeness".


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## Qer (May 7, 2008)

YohnnyLuksh said:


> and higher voltage usage possibility?


On which side?

Motor? Depends on the motor. Yes, most DC motors run below 200 Volt, but so does some AC-motors. The voltage is also pretty irrelevant since what will determine the performance of the car is how much torque and power you get. Torque is proportional to the current, but that is PER MOTOR! If you take two different types of motors and run them at the same current you might very well get totally different torque. Power is power, power in equals power out + losses. So a higher possible motor voltage is just that; higher possible motor voltage.

Battery? All controllers smart enough to have a setting for it can limit motor voltage to a suitable max motor voltage no matter what battery voltage your pack has (ok, it can't increase the voltage, just decrease). You can run for example a Soliton at 300+ Volt pack and only allow up to 170 Volt over the motor. The controller also convert power to power so by having a high pack voltage means your pack doesn't have to handle full motor current since Umotor*Imotor=Ubattery*Ibattery. If Umotor<Ubattery it means that Ibattery<Imotor.

I've said it before and I say it again. You must compare complete systems, you can't just compare AC versus DC and ignore everything else. Before you put your system in a vehicle with a suitable pack it's just a very theoretical discussion without much bearing in real life.


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## aeroscott (Jan 5, 2008)

Many good points brougth up . I remember seening a voltae /eff.chart for the venerable CM 77 ( jet starter/ generator ) used on many early ev's . motor rating 400 A , 24 V , less that 50% at the lower volts and over 

90% in the 100 volt range . I was looking over Remes pmac eff. chart peaks at 95% but can go into the 70's % at some rpms . I think a bigger saving can be had by using a transverse motor / transmission rather then the hypoid differential .


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## aeroscott (Jan 5, 2008)

Just got off the phone With Jim Rowe of Metric Mechanic , he builds racing BMW engines transmissions , differentials . On the super flow dynamometer they get a 27% loss on a stock 318is in the drive train . He thinks that 3-4% in the transmission ,3% in the drive line , 3-4% for the half shafts,the high loses in the shafts is do to the angles he said . He said his nephew built a Ev Geo Metro and he liked the weight and drive train (transverse motor) efficiency of it. sorry for the hijack

WWW.metricmechanic.com


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## 1-ev.com (Nov 4, 2010)

Also, I think DC has a bit more torque right from the start vs. AC needs to build the field... converts into more efficiency when stop and go driving


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## YohnnyLuksh (Jan 10, 2011)

Hello, guys, thank you for answers!

I tried to search for reasonably priced AC motor! 
And I got such question - if AC motors are simpler than DC, why then they are more expensive?  

Maybe someone knows good high voltage (>200V) / >15kW continuous AC 3 phase induction motor for ~1500$? 
I have an idea, maybe if I could find cheap AC motor, I could try to build DIY inverter-controller? (I know it's not close to word "easy", but seems like worth trying).


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## JRoque (Mar 9, 2010)

Hi. 



YohnnyLuksh said:


> Maybe someone knows good high voltage (>200V) / >15kW continuous AC 3 phase induction motor for ~1500


Here's a few 40 HP (~15Kw) motors:
http://desc.shop.ebay.com/i.html?_n...22&_trksid=p3286.c0.m270.l1313&LH_TitleDesc=1

and a 40 HP, 3 ph inverter:
http://cgi.ebay.com/New-40-HP-AC-Variable-Frequency-Drive-Inverter-3-Phase-/290537387072?pt=LH_DefaultDomain_0&hash=item43a560b440


JR


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## YohnnyLuksh (Jan 10, 2011)

Thank you, JRoque!
Wow that's really interesting! OK this will sound very newbie - but can such industrial motors be used for EVs?  they seems to use 460V by default!


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## JRoque (Mar 9, 2010)

Hi. My take on this is that if you can find a motor with the right weight (they're usually heavy) and have sufficient voltage to drive it, you're set. Note that the 300+ volt battery pack to run a 220VAC motor is slightly heavier and more expensive (to connect) than a lower volt setup.

From my searches I've noted that you'd do better with an inverter rated motor due to the high start/stop and current requirements of an EV application. Inverter class motors are also better insulated and can usually spin faster. Look for motors with both C-face and foot mounts. If you have a 460V pack, get a 220V motor and limit the current if necessary.

Don't forget that there will be some tinkering necessary with the inverter to adapt the accelerator, regen and braking. Not terribly difficult but not plug and play either. An inverter with vector drive is essential and one with shaft position input is better.

JR


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## Coulomb (Apr 22, 2009)

JRoque said:


> Here's a few 40 HP (~15Kw) motors:


Um, 40 HP is about 30 kW; 40 HP (continuous) motors are way bigger than you would want. The shipping weight for the first one I saw was 500 pounds... even if a quarter of that is packaging, it's way too heavy and likely way too big (I can't read American style frame sizes... I think it said 324T).

You want 20 to maybe 30 HP motors (and probably compact 30 HP models) for most sedan sized conversions.

Edit: you also don't want to "match" the power level of the inverter and motor. A 20 HP motor should probably have a controller that peaks at about 60 or 80 HP. You'd need to see what the 1 minute rating of the controller is; I'm guessing that the first 40 HP inverter I saw would be called 25/30 kW in Australia, and would have a 50% overload for 1 minute (over the lower figure), in other words about 25 * 1.5 = 37.5 kW peak (about 50 HP). That might be enough for a very small conversion, if high performance was not a goal.


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## JRoque (Mar 9, 2010)

Coulomb said:


> Um, 40 HP is about 30 kW


Whoa! major mistake in my conversion. Here's a link for 20HP motors: http://desc.shop.ebay.com/i.html?_n...22&_trksid=p3286.c0.m270.l1313&LH_TitleDesc=1

Or maybe try these guys closer to you: http://www.baltspektr.lv/eng/gost-standard/

JR


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## 1-ev.com (Nov 4, 2010)

All components are huge !!! will not fit into small EV ...


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## JRoque (Mar 9, 2010)

Hi.



1-ev.com said:


> All components are huge !!! will not fit into small EV ...


http://www.evalbum.com/2441
http://www.evalbum.com/3208
http://www.evalbum.com/1421

JR


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## 1-ev.com (Nov 4, 2010)

I met http://desc.shop.ebay.com/Variable-...p3286.c0.m282&_mPrRngCbx=1&_udlo=&_udhi=1,500


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## sickness1 (Mar 7, 2011)

Tesseract said:


> Because the switches in the inverter must be modulated at all times - they can't ever go to 100% on. The highest average modulation depth is 70.7%. There are some exceptions to that (shifting the neutral point around, etc.) but, in general, you have to turn all of the switches off in an inverter for some meaningful percentage of time in order to create sine wave currents in the motor. No way around that, and not so wild after all, eh?



I cann't agree with you, when you control an AC inverter the mosfet or IGBT act like the thyristor used to do in the past, Open or Close, they don't work in any other state, only on the switch on and off time are not fully closed or open. The efficiency speaks only about the losses in heat that are produced in the circuit. And when a IGBT is on Open state I can ensure you that the looses are practically 0. Taking for example this power IGBT:
http://www.pwrx.com/pwrx/docs/cm600ha24h.pdf
On open state, no current:
P=V*I=300*0.002=0.6W
On close state or current flowing:
P=V*I =2.5V*300A=750W
Suponsing that the PWM is all the time in a ON state, on a power consumption of 300A*300V = 90KW we would have 1500W falling on the 2 IGBTs (this needs to add the on and off switching power loss), if it would be in a 50% on Open state we would have 150A*300V=45KW and 750,3W of looses in the IGBT. And here there are Inverters with a > 90% of efficiency:
http://www.kingsoninverter.com/inverter.html
http://www.metricmind.com/inverter.htm
http://www.brusa.biz/fileadmin/Diverses/Download/Manuals/DMC524_04.pdf

And this is the Wave you would see if looking the signal of an inverter:
http://www.s4wsbox.com/?q=node/25


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## Tesseract (Sep 27, 2008)

sickness1 said:


> I cann't agree with you, when you control an AC inverter the mosfet or IGBT act like the thyristor used to do in the past, Open or Close, they don't work in any other state, only the switch on and off time could have some relevance. The efficiency speaks only about the losses in heat that are produced in the circuit.


Do you agree that all of the switches in an inverter must spend some proportion of each switching cycled turned off?

Do you agree that when a switch is off that it is not delivering additional power to its load (e.g. - the phase winding of an AC motor)?

If you agree with both statements, then you will understand that when I wrote...



Tesseract said:


> ...It costs a lot more to make an AC inverter and they use_ silicon_ much less efficiently (at best: ~70%) than a DC motor controller (~100%) so they are less "green" right from the start.


...that I was only speaking of how efficiently _silicon_ is used in an inverter because the switches must be turned off at least 30% of the time, compared to a buck converter where the switch can be turned on 100% of the time, if necessary.

I was not making a comment about the actual ratio of output power to input power, and worse is that I already clarified this statement in the very post your replied to.


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## JRoque (Mar 9, 2010)

Hi. I too thought you were saying AC inverter switches can only be turned on at a linear 70.7%.

Aren't DC motors more efficient at starting but AC do better at full load running? In terms of cost, AC systems like those from HPEV are a bit more expensive than similar DC systems but not much more.

JR


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## sickness1 (Mar 7, 2011)

Tesseract said:


> Do you agree that all of the switches in an inverter must spend some proportion of each switching cycled turned off?
> 
> Do you agree that when a switch is off that it is not delivering additional power to its load (e.g. - the phase winding of an AC motor)?
> 
> ...


The thing is that in both cases the RMS current will be the same, when a AC motor or a DC motor are working at the same power there won't be any diferences. So it doesn't matter if in one case the switches are closed the 30%of the time, the silicon efficient use will be the same, in one case it will have more peak current in the other more constant disipation, but they will have the same total power disipation (Ac will have a little more due to switching). The difference is that the AC inverter needs 2 more IGBTs and those IGBTs will endure more because they take turns. Suppousing the endure of the silicon elements is the same, at the end they will work the same.

For other side is true that i didn't read correctly that you were talking only about the silicon efficiency.


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## steven4601 (Nov 11, 2010)

This thread is not very good anymore at all......... I feel sorry that because of a hint / joke I made some start putting nonsense claims to facts and the other way arround. 

Hope this helps to put some clarification back to this thread:

1. Silicon utilisation , modulation depth (ratio on/off time of the igbt), switching patterns has effects on efficiency not describable in absolute more or lesser efficient. 

2. Stall torque != to do Efficiency. Stall torque describe steady state (0 rpm) losses.

3. More torque vs less torque at low rpms != to do with efficiency nor the required amount of energy to go to a certain speed .


edit: * != *means* NOT*


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## sickness1 (Mar 7, 2011)

I think that choosing the AC motor is the good choice because of the regenerative capabilities (brake, range extender... ), motor control capabilities, maintained torque below 2000 - 3000 RPM, constant torque reduction on constant RPM increase, high efficiency in all the range between 3000 RPM to max RPM and better heat dissipation.

Graphics of DC motor:
http://www.onosokki.co.jp/English/hp_e/products/keisoku/torque/ts7700.htm
http://homepages.which.net/~paul.hills/Motors/MotorsBody.html
http://www.go-ev.com/motors-warp.html

Graphics of Ac motor:
http://www.metricmind.com/images/mes_200-250_efficiency.jpg
http://www.metricmind.com/line_art/5135ws28.gif
http://www.metricmind.com/motor.htm
http://evcrv.blogspot.com/2011/02/motor-hpgc-ac-50.html

For me the question is Induction motor or permanent magnet motor, here there is a nice discursion:
http://www.doria.fi/bitstream/handle/10024/31238/TMP.objres.448.pdf?sequence=1


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## Coulomb (Apr 22, 2009)

sickness1 said:


> The thing is that in both cases the RMS current will be the same, when a AC motor or a DC motor are working at the same power there won't be any diferences [in efficiency].


Not quite. For AC controllers, there will always be two switches (IGBTs or MOSFETs) conducting in series where there would be one for DC. So an AC controller will lose a little efficiency compared to a similar powered DC controller. Typically however, the motor efficiency will more than make up the difference.

[ Edit: I should add that this is if the AC and DC controllers are running the same voltage system. Most AC systems, with the salient exception of the Curtis 1238, are higher voltage than DC systems. So the doubling in voltage drop cancels out if the AC system is twice the voltage (e.g. 288 vs 144 V), and more than cancels out with higher (e.g. 312, 360, 600+ vs 144 or less). DC systems can go higher in voltage, but you need more sophisticated motors (e.g. with interpoles). ]


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## aeroscott (Jan 5, 2008)

sickness1 said:


> I think that choosing the AC motor is the good choice because of the regenerative capabilities (brake, range extender... ), motor control capabilities, maintained torque below 2000 - 3000 RPM, constant torque reduction on constant RPM increase, high efficiency in all the range between 3000 RPM to max RPM and better heat dissipation.
> 
> Graphics of DC motor:
> http://www.onosokki.co.jp/English/hp_e/products/keisoku/torque/ts7700.htm
> ...


this last link is intense .


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## sickness1 (Mar 7, 2011)

Coulomb said:


> Not quite. For AC controllers, there will always be two switches (IGBTs or MOSFETs) conducting in series where there would be one for DC. So an AC controller will lose a little efficiency compared to a similar powered DC controller. Typically however, the motor efficiency will more than make up the difference.
> 
> [ Edit: I should add that this is if the AC and DC controllers are running the same voltage system. Most AC systems, with the salient exception of the Curtis 1238, are higher voltage than DC systems. So the doubling in voltage drop cancels out if the AC system is twice the voltage (e.g. 288 vs 144 V), and more than cancels out with higher (e.g. 312, 360, 600+ vs 144 or less). DC systems can go higher in voltage, but you need more sophisticated motors (e.g. with interpoles). ]


The DC controllers that I have worked with normally have always 2 IGBTs working in an H bridge (4 IGBTs to make an H bridge), so that you can go forward and backward with the controller without using gears (to inverse the direction of the motor), and when you stop giving current (free throttle) as the motor is a winding it won't stop conducting current so you need the H to make a freewheeling diode with the igbt and the diode of another igbt so that the transistor doesn't suffer when opening.

P.D.: With this configuration is possible to regen with a DC motor:
http://www.reliance.com/prodserv/standriv/appnotes/d7733.pdf


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## Tesseract (Sep 27, 2008)

JRoque said:


> ...
> Aren't DC motors more efficient at starting but AC do better at full load running? In terms of cost, AC systems like those from HPEV are a bit more expensive than similar DC systems but not much more.


I wouldn't make any broad statements about efficiency differences between AC and DC motors. While it is technically possible to make an AC motor cmore efficient than a DC motor for a given frame size, it is not absolutely true.

But this gets hashed out over and over again. I really am agnostic on motor technology. We will eventually make an AC inverter, so why would I trash talk them? However, there is precious little justification for making such a product for the DIY market now - there just aren't enough of you people out there that want to convert a car, much less pay 50% more for an AC system to do so.


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## Qer (May 7, 2008)

sickness1 said:


> The thing is that in both cases the RMS current will be the same, when a AC motor or a DC motor are working at the same power there won't be any diferences. So it doesn't matter if in one case the switches are closed the 30%of the time, the silicon efficient use will be the same,


You're completely missing Tesseracts point. He's not talking about controller efficiency, he's talking about how efficiently you can use the silicon. If you have a 500 Amp IGBT/MOSFET in a DC-controller that's what you get, 500 Amps, but when you have to create a sinus wave to feed an AC motor you can never push the silicon harder than 70% of it's top raiting; ie 350 Ampere. This is what he's talking about:










So let's go back to what Tesseract said originally:



Tesseract said:


> It costs a lot more to make an AC inverter and they use silicon much less efficiently (at best: ~70%) than a DC motor controller (~100%)


"They use silicon much less efficiently", not "they convert power much less efficiently". TOTALLY different things.

If you want 500 Amps out RMS from an AC controller you need transistors that are rated for 700 Amps. If you want 500 Amps out RMS from a DC controller you need transistors that are rated for 500 Amps! So it DOES matter that the silicon never goes above 70% average pulse width, it means you have to buy MORE silicon which makes the controller more expensive.

Add to that that an AC-controller needs 6 transistors (all rated 40% higher than the RMS phase current) where a DC-controller only need one (well, at least for series wound and PM motors) and you end up spending quite some extra money on the silicon alone.

Of course it's more to it than this, but you get the picture (I hope).


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## etischer (Jun 16, 2008)

While it's true AC systems require more silicon transistors, AC systems eliminate the brushes and commutator. I'd rather have microprocessor controlled solid state switches than mechanical commutation and reversing contactors. 

Dollar for dollar, AC systems are cheaper to build despite having more transistors. This is why AC motors are used almost exclusively by electric car companies and in factory automation. I think the DIY electric car market is the only application where AC systems are more expensive than DC systems.


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## Coulomb (Apr 22, 2009)

Qer said:


> If you have a 500 Amp IGBT/MOSFET in a DC-controller that's what you get, 500 Amps, but when you have to create a sinus wave to feed an AC motor you can never push the silicon harder than 70% of it's top raiting; ie 350 Ampere.


That's not my understanding, but I could be wrong. I think if you have a 500 a IGBT/MOSFET, you can get 500 A RMS from it, i.e. although the device sees peaks of 707 A, it heats up as if it is doing 500 A continuous.

You'd want a bit of headroom so the device wasn't teetering on the brink all the time, but you'd want the same headroom for DC as well.

You still need more transistors than DC, of course, because of the three half-bridges. But the factor of 0.7 (sqrt(2)/2) is wrong, as far as I can tell.


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## etischer (Jun 16, 2008)

Coulomb said:


> That's not my understanding, but I could be wrong. I think if you have a 500 a IGBT/MOSFET, you can get 500 A RMS from it, i.e. although the device sees peaks of 707 A, it heats up as if it is doing 500 A continuous.


I'm using 300A IGBTs and I'm pulling 280A battery side, which is roughly 280 amps per phase motor side.


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## Qer (May 7, 2008)

Coulomb said:


> That's not my understanding, but I could be wrong. I think if you have a 500 a IGBT/MOSFET, you can get 500 A RMS from it, i.e. although the device sees peaks of 707 A, it heats up as if it is doing 500 A continuous.


You're absolutely right. Good thing I'm not the hardware engineer, eh? 

It's the Voltage that you only get 70% of, not current. I'll find myself a concrete wall and introduce my head to in now.


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## sickness1 (Mar 7, 2011)

Qer said:


> You're completely missing Tesseracts point. He's not talking about controller efficiency, he's talking about how efficiently you can use the silicon. If you have a 500 Amp IGBT/MOSFET in a DC-controller that's what you get, 500 Amps, but when you have to create a sinus wave to feed an AC motor you can never push the silicon harder than 70% of it's top raiting; ie 350 Ampere. This is what he's talking about:
> 
> So let's go back to what Tesseract said originally:
> 
> ...


I'm going to try to explain it clearly:
RMS:
http://www.kpsec.freeuk.com/acdc.htm
The RMS value is the effective value of a varying voltage or current. It is the equivalent steady DC (constant) value which gives the same effect.

So when we speak about power, current or voltage in AC we will always speak in RMS, when we measure with a voltimeter, amperimeter, polimeter we will see RMS, to see peak Voltage we need a oscilloscope. And what we see in America in AC is 110V, the peak is 2^-1 * 110V, in Europe is 220V 2^-1 *220=311,13V. So a motor that is working in AC at 30KW is a RMS value and will work at the same RMS as a DC motor. 
Now we have a transistor, we will use the same for both cases, it is totally possible because all the power transistor of this world support a big current, the important thing for us is the capacity of disipation that is going to be the same in both cases because their RMS power is the same. For example using the same IGBT of the other day:

http://www.pwrx.com/pwrx/docs/cm600ha24h.pdf

We can see that the nominal current is 600A but it supports 1200A of peak. And I can ensure that if you put this transistor to work on 600A this is whats going to happen:
P = V*I = 2,5V * 600A = 1500W
Max disipation capacity: 0.035ºC/W + 0.035ºC/W
So Temperature = Termal resistance * Power = 0.07 * 1500 = 105ºC 

If we are at 20ºC its going to take a temperature of 125ºC and that counting that is the best performance of disipation (mostly imposible). The only way to improve this is to put liquid nitrogen. So the normal thing is to blow the transistor with a RMS current of 600A, it supports a peak of 1200A, and the peak in AC will be 600*2^-1 = 848,52A when on DC is 600A. But in both cases the disipation is 1500W. So in both cases the transistor will be at 125ºC, will work the same even if it is 80% of the time stopped. But if it is the 80% of the time stopped to make this power (thing that doesn't happend because the current would be enormous) we would need 5 times the current of 600A. Using a maximum PWM of 70% we will need peaks of 857,14A to obtain a RMS of 600A. So this transistor is valid for both cases. Generally in trasistors the important thing is the power disipation once the needed current has been reached, and a transistor that is valid for DC will be valid for AC because peak current is bigger than nominal current always.



Qer said:


> Add to that that an AC-controller needs 6 transistors (all rated 40% higher than the RMS phase current) where a DC-controller only need one (well, at least for series wound and PM motors) and you end up spending quite some extra money on the silicon alone.


I want to know how, with one transistor, you consume the current that is on the winding when you want to download the charge of the motor. I have never seen a DC motor controller with a transistor, like I said before an H bridge is the normal solution, you could use 1 IGBT but also you would need unless 2 power diodes, if you don't do this you will blow the transistor because of the voltage that there is going to create between the source and the drain. Perhaps not in one day or two but it won't last. 
Think that if you have only 1 transistor in DC and an AC has 6 transistor the durability of the AC inverter is 3 times the DC durability with the same components, so it should be 3 times expensiver and it isn't even two times...


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## Tesseract (Sep 27, 2008)

sickness1 said:


> I'm going to try to explain it clearly:


What you should instead be doing is reading more clearly as Qer already said that he was wrong about the RMS current part in the post prior to yours. I had to break the bad news to him this morning.

It is the VOLTAGE that is "wasted" by the switches in the AC inverter, because they cannot be turned on for more than ~70% of the time during any cycle (in order to synthesize a sine wave current in each phase).

That said, the vector sum of the three phase currents is only 1.73x the RMS current in any one phase, whereas the same set of three half-bridges can be used to deliver 3x the current of a single half-bridge in a DC controller. Hence, a better use of silicon once again.



sickness1 said:


> I want to know how, with one transistor, you consume the current that is on the winding when you want to download the charge of the motor.


"..download the charge of the motor"? Clearly a language issue here. Ok, I'll cut you some slack for that.

I guess you mean when the motor current freewheels during the off time of the switch... and, yes, there must be another switch (ie - synchronous rectifier) or a diode to conduct this current. The switch won't survive a _single cycle_ without a freewheeling pathway, much less a day or two.



sickness1 said:


> Think that if you have only 1 transistor in DC and an AC has 6 transistor the durability of the AC inverter is 3 times the DC durability with the same components, so it should be 3 times expensiver and it isn't even two times...


Eh? As far as I know, the currently available AC systems are at least twice as expensive as, e.g., a Soliton1 but with 1/3rd the power (I'm thinking of, e.g., The Tritium WaveSculptor), which is around 6x more expensive by my calculation. The popular 50kW "peak" Curtis 1238 + motor kit costs about the same as a Soliton1 + WarP-9, but the later is capable of a solid 150kW peak (limited by the motor in this case), which makes it 3x less expensive on a $/kW basis.

As for more components improving reliability, generally the exact opposite is considered to be true.


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## sickness1 (Mar 7, 2011)

Tesseract said:


> What you should instead be doing is reading more clearly as Qer already said that he was wrong about the RMS current part in the post prior to yours. I had to break the bad news to him this morning.


I like people that has tips to give but then they won't apply over them, like here:



Tesseract said:


> I guess you mean when the motor current freewheels during the off time of the switch... and, yes, there must be another switch (ie - synchronous rectifier) or a diode to conduct this current.


I said it before... pehaps some of your tips could be applied to you .



Tesseract said:


> The switch won't survive a _single cycle_ without a freewheeling pathway, much less a day or two.


The truth is that is dificult to know how many times it will endure, but the important thing was the concept not the specific thing, you are too picky... But is Ok.



Tesseract said:


> It is the VOLTAGE that is "wasted" by the switches in the AC inverter, because they cannot be turned on for more than ~70% of the time during any cycle (in order to synthesize a sine wave current in each phase).


I want to know what is a VOLTAGE waste, perhaps you mean that the AC inverter needs a higher voltage to create the same RMS voltage? Or maybe putting a lower winding value is enough to create more current and the same power RMS? Resistance = 2 * Pi * F* L? Perhaps thats why AC motor usually work at quite low voltage values(80V,120V...)



Tesseract said:


> That said, the vector sum of the three phase currents is only 1.73x the RMS current in any one phase, whereas the same set of three half-bridges can be used to deliver 3x the current of a single half-bridge in a DC controller. Hence, a better use of silicon once again.


But again the AC inverter would endure 3 times the time it would endure the DC inverter so at last all the IGBTs would work the same (have dissipated the same quantity of W that is the measurement of work) with the diferencce that you will buy 3 DC for the prize of 1 AC inverter, what is a better deal?. And if I put 3 IGBTs in parallel which one would work more? Or all af them would work the same? Putting 3 IGBTs in parallel does it give 3x the power? Why then people don't do this instead of putting 1 IGBT 3 timer bigger? I suppouse you know that the Collecter emiter voltage is different in each IGBT so is imposible to obtain 3x power with 3 parallel IGBTs... You would work with the worse IGBT or in other words with the one that has more voltage fall, having the others to rise their resistance to mantain the physical law, then is a good or a bad system?

There is another thing that intrigates me, the motor you install is capable of giving the peak power all the time?? Or the system is over dimensioned because it can only give it for 1 minute or 2 and then the max power delivered is the nominal power? So where is the so good use of silicon??

I suppouse you know an AC motor is capable of giving peak power for more time than the DC thanks to the termal dissipation



Tesseract said:


> "..download the charge of the motor"? Clearly a language issue here. Ok, I'll cut you some slack for that.


I'm not native English speaker, sorry if something is not correctly explained, I try to be clear but mine English is limitated.





Tesseract said:


> Eh? As far as I know, the currently available AC systems are at least twice as expensive as, e.g., a Soliton1 but with 1/3rd the power (I'm thinking of, e.g., The Tritium WaveSculptor), which is around 6x more expensive by my calculation. The popular 50kW "peak" Curtis 1238 + motor kit costs about the same as a Soliton1 + WarP-9, but the later is capable of a solid 150kW peak (limited by the motor in this case), which makes it 3x less expensive on a $/kW basis.
> 
> As for more components improving reliability, generally the exact opposite is considered to be true.


What I see here is someone trying to sell his product insted of being objective, before i saw you saying this:

much less pay 50% more for an AC system to do so.

I will like to people to post how much they have spent in their controllers and make a comparative table AC vs DC prize. 
The difference is that i try to explain why for me is better an AC system and try to people understand it. If I have sound sarcastic in any moment, please don't take it seriously.

P.D.: This discurssion is much worse (I'm not near to be as good as Tesla) but similar to the one that had Nikola Tesla and Thomas Edison where discussing for which system was better, AC or DC...


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## cts_casemod (Aug 23, 2012)

I have a system designed from scratch using an AC motor. My experience, while AC motos can indeed be more efficient that the DC counterparts this is only true at a given RPM range. 

AC motors have many drwabacks and they relly heavily in a good control algorithm which is expensive

Regenerative braking does not make a big difference if you are not in constant stop/go traffic and even so the difference is not very significant. The constant need for syncronisation from the motor to controller also adds extra losses. It is however a great upgrade to your braking system, speacially if you dont have ABS as the motor will not allow the wheels to lock, if properly set up.
DC motors dont have regeneration but their low torque characteristics also make them efficient to drive in traffic at low speeds. On the contrary for motorway operation an AC version may be more efficient.

I had a DC motor on an e-Bike, after that bought one (that I currently have) with a brushless DC as well and have no reason not to recomend any of both. Same story for Induction motors, both having advantages and drawbacks

There is no better option. You'll need to do some reseach on what you want/have and see which options better suit your needs. Thats the best option I would recomend when thinking about doing any conversion.


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## Hollie Maea (Dec 9, 2009)

You mentioned in the other thread that you were using V/Hz mode. That's not really a good basis for comparison. If you can't get vector control, then AC isn't worth having. The difficulty of getting an inverter with a well implemented vector control system is a legitimate issue, but considering the performance of AC in scalar mode isn't particularly germane.


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## cts_casemod (Aug 23, 2012)

Hollie Maea said:


> You mentioned in the other thread that you were using V/Hz mode. That's not really a good basis for comparison. If you can't get vector control, then AC isn't worth having. The difficulty of getting an inverter with a well implemented vector control system is a legitimate issue, but considering the performance of AC in scalar mode isn't particularly germane.


I was not exactly comparing with my conversion. The advice I posted was generic, not necessarily related to the issues on my build.

In fact, I can work on closed loop using V/Hz with slip control. Vector mode is just a fancy way to be in closed loop using no external encoder, by using two current transducers to check current and rotor speed. This is generally cheaper to implement and requires no extra wiring, but requires a unit with a powerfull processor to make changes quickly enought for the unit to be responsive.

By controlling the slip of the motor I can control the torque and acceleration, hence have a torque mode, the point is how much harder or more expensive the controller ends up being. On my case I will require external circuitry, for those that buy an already made controller it simply means more expensive.



cts_casemod said:


> AC motors have many drawbacks and they relly heavily in a good control algorithm which is expensive


On a DC motor there is not much tweeking to do with the controller as the motor on its own is a torque source and this is probably what you are comparing to when refering to V/Hz. On an AC motor working in V/Hz this is mainly a small energy efficiency and confort as the changes are not so sudden with torque vector. On the other hand I quite like to drive in V/Hz as I can control the car using only one pedal. It gives me a very accurate control, which I will keep when implementing the variable torque mode {-100; 0; +100}.


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## ruckus (Apr 15, 2009)

The OP asked which was more efficient, AC induction or DC brushed.

This is a bit like asking which is more efficient, a Pontiac Bonneville, or a Lincoln Continental. Neither design is very efficient.

The problem with each motor is given in the name. 

Brushed DC. The problem is the brushes. Go into a dark room and spin a regular plug-in brushed drill. You will be amazed at the fireball of sparks in there. Obviously an efficiency, heat, and longevity problem.

Induction AC. The problem is you have to use a bunch of extra energy to create the inductive field. These motors are hugely inefficient at low rpms and low loads. So driving mellow won't help with an induction AC










If you look at the 10 kw line it is 80% efficient at 1500 rpm and only goes up to about 84% at 6000 rpm. That is pretty bad.

If you are looking for the most efficient motor type that is commonly available it is the permanent magnet motor (call it AC or DC if you prefer). The magnet creates the field for "free" so no energy is wasted. They are also good at low loads down to much lower rpm's than induction. Obviously, they have no brush losses.

With the magnet motor below it is 90-95% efficient at 10 kw from 1500-3000 rpm. Exactly the rpm's you typically use while driving.











Dewalt and all the other cordless drill manufacturer's already figured out which motor type provides the highest efficiency and longevity and the lowest weight. 

They all use magnet motors. 

If you want to know the weakness, again, look at the name. Magnets. They are more costly and become heat damaged at a slightly lower temperature. They areusually rated at temperature class F (311 F) instead of H (356 F)). Some make a big deal out of this, but it is only a few degrees. If you are getting any motor over 300 degrees it will not last long.

Cheers.


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## cts_casemod (Aug 23, 2012)

ruckus said:


> The OP asked which was more efficient, AC induction or DC brushed.
> 
> This is a bit like asking which is more efficient, a Pontiac Bonneville, or a Lincoln Continental. Neither design is very efficient.
> 
> ...


That is not true.

An induction motor has the capability to reduce the magnetic field and "overdrive" using very little energy, or work in saturation at low revs with high torque required for direct drive applications. It does, however, require complex control algoritm to operate in an efficient way. They can be driven hard or gentle with similar efficiency curves. Similar because as with any motor as you increase current you'll also increase winding, magnetic or bush losses as heat, everything else being equal.

Permanent magnets... You get nothing for free. The initial cost is higher.
Try to spin a permanent magnet or a stepper motor and you will feel the lag the magnets create just like if you try to push a magnet away from a piece of iron. They are suited for low revs at high torque, an example are e-bikes, at 350RPM typical. There are other types with weaker magnets optimized for higher revs but they will provide little torque (HP = RPM* Torque).

You can not overdrive them. if you want more speed you need more voltage. If you have an high speed PM you wont have torque at low revs, same as a shunt wired DC motor - you have a constant torque at a constant speed, reason for which they are not used as traction motors, but rather applications where a constant speed is required as the load changes.
As with temperatures, yes it is a big deal, because magnets loose their magnetic properties if heated above a certain point and this limits how hard a motor can be pushed, which is much less that what a comparable series DC motor can, so continuous ratings are considerably smaller to avoid this and they are usually more suited for motorbikes


Best application for cars for both efficiency and high operating range is either induction, series brushed DC or syncronous with field powered rotor (Same as a car alternator) where the field can be used to either provide high torque at low revs or high revs at low torque for the same net power output as you would expect on a car (1st gear as much greter torque than 5th gear, but at lower speed and vice-versa).

My opinion: There are no winners, each type has advantages and disavantages. AC is prefered over series DC on the industry because of low maintenance and lower manufacturing costs, where they have to run 24/7 during many years. On a car you would change brushes once or twice unless the motor is too small, however AC is more expensive in terms of control equipment. Foir hybrids is usually cheaper to use a syncronous or even PM because they operate at an optimun RPM range before the ICE takes over and the controll is much simpler than that of an induction motor. Again linking the best of each motor to its own application.


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## major (Apr 4, 2008)

Yet I hear that induction motors win the efficiency contest with PMs at light loads because they have control of the field. And you trade the mechanical commutation for electronic commutation in the AC-DC argument or in other words; replace carbon switches with silicon switches. So it is a tradeoff of efficiency, durability, cost and which type you are offering for sale, isn't it


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## Red Neck (Feb 1, 2013)

A proper AC drive train has proper vector based inverters and efficiency in actual use is equal or greater to that of PM in fully electric vehicles.
And have no issues with magnet placement (glue) or other maintenance issues.


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## ruckus (Apr 15, 2009)

Guys, I just posted the efficiency map put out by Siemens using their own inverter. How can you possibly claim that with a proper algorithm they are much better. Here is one of the best companies matching their own motor and controller. The efficiency results are not good. I provided data, not wishful thinking.

As I said, the name of the motor gives the weakness. Magnet motors have magnets which are more expensive to produce and have their own limitations such as lower heat resistance and lower rpm potential. 

Magnet motors do not all "cog" unless the leads are connected (regen). This is a huge misconception. My 180 kw magnet motor can smoothly be spun by hand much easier than an 8" brushed dc.

Major, if I thought induction motors were better I would use those. They aren't, so I don't. You were just touting the efficiency of the Remy magnet motor on another thread. Called it a 'real' motor if I remember correctly.

Cheers.


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## Tesseract (Sep 27, 2008)

Shocking - ruckus thinks the BLDC motor is the second coming incarnate!

What non-engineers seem to always fail to grasp is that engineering is a study in compromise. There is no "ultimate" motor. Just like there is no ultimate switch, transformer, resistor, capacitor, etc...

Yes, some portion of the phase current rating of an inverter is squandered creating the field in an induction motor, but that current sloshes back and forth between the input capacitor and winding inductance, with just a few percent lost to the switches. In other words, creating the field in an induction motor does not result in a huge drain on the battery pack.

In contrast, AC motors with PM fields have *terrible* efficiency at light loads because you are stuck with maximum field strength all the time, whether you need it or not.

As for demagnetization from overtemp, I agree that that's an unlikely scenario, but what you seem to be overlooking is that it is quite possible to demagnetize the PMs from too much phase current, or if the phase current is energized at the wrong time (from encoder error or the like).

In the end, BLDC/PMAC/PMSM motors are somewhat delicate and therefore perhaps not the best choice for use in a demanding environment like an on-road vehicle.


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## ruckus (Apr 15, 2009)

Tesseract said:


> Shocking - ruckus thinks the BLDC motor is the second coming incarnate!


har har, that must be why I listed several downsides of each motor type, including magnet motors, right? The question is which is most efficient. I provided the correct answer.



Tesseract said:


> In contrast, AC motors with PM fields have *terrible* efficiency at light loads because you are stuck with maximum field strength all the time, whether you need it or not...


The data I provided proves just the opposite. You can't 'wish' something true. You actually have to show data if you want anybody to believe you. 



Tesseract said:


> ..but what you seem to be overlooking is that it is quite possible to demagnetize the PMs from too much phase current...


Really? What gave you the idea I was overlooking that? Last I checked, every motor can be destroyed by too much current. At least, that's what the non-engineer types tell me (the ones that actually build and use EV's).




Tesseract said:


> In the end, BLDC/PMAC/PMSM motors are somewhat delicate and therefore perhaps not the best choice for use in a demanding environment like an on-road vehicle.


I guess Toyota and GM didn't get the memo..  ..or all those people working in mining, or the folks who build battleship turrets for the Navy..


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## major (Apr 4, 2008)

ruckus said:


> har har, that must be why I listed several downsides of each motor type, including *magnet motors*, right? The question is which is most efficient. I provided the correct answer.


You know magnet motor is the term used by the free energy community for the machine having no windings and your use of it is worrisome.


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## ruckus (Apr 15, 2009)

major said:


> You know magnet motor is the term used by the free energy community for the machine having no windings and your use of it is worrisome.


That's hilarious. You are right. I forgot about those folks. I was trying to avoid the BLDC, PMAC, PMSM letter soup.

So no data yet showing the superior efficiency of Induction motor as you claim?


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## Hollie Maea (Dec 9, 2009)

One thing that people sometimes forget is that the magnitizing current in an induction motor ( or any inductive load for that matter) does not consume energy. I does increase current, but that current is out of phase with the voltage so it does not consume real power. It does decrease the power factor, and the resulting higher current will lead to some real power losses as it passes through resistive elements, but the magnetization itself does not require power.


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## major (Apr 4, 2008)

ruckus said:


> That's hilarious. You are right. I forgot about those folks. I was trying to avoid the BLDC, PMAC, PMSM letter soup.
> 
> So no data yet showing the superior efficiency of Induction motor as you claim?


You got me wrong. I did not say I favored any particular motor type nor am I out to prove anything here. I simply was pointing out you made a statement which is erroneous in the eyes of many experienced people in the field. 

And the correct terminology to encompass all those motors would be simply PM meaning Permanent Magnet and it is understood that it is a PM field with a wound armature.


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## PStechPaul (May 1, 2012)

I'm not sure this is a fair comparison. The AC motor seems to be designed for 8000 RPM while the BLDC is 4000 RPM. It's just a simple change in number of poles or a 2x change in drive ratio to make it more comparable.

The 80% efficiency line of the BLDC applies at 678 Nm at 1000 RPM (71 kW) and 325 Nm at 500 RPM (17 kW). The 85% line applies at 474 Nm at 1000 RPM (50 kW). The 90% line applies at 271 Nm at 1000 RPM (28 kW) and 881 Nm at 3000 RPM (277 kW). But I have difficulty believing that last figure, and the Scott Drive 200 system motor (which I assume this to be) is rated at 200 kW.

Now for the ACIM. The 80% efficiency line applies at 200 Nm at 1500 RPM (31.5 kW) and also at 50 Nm (8 kW), and 10 Nm at 8000 RPM (6.8 kW). The 85% line applies at 190 Nm at 2500 RPM (50 kW) and at 20 Nm at 8000 RPM (17 kW). The 90% line applies at 150 Nm at 4600 RPM (72 kW) and 30 Nm at 8000 RPM (25 kW).

If you use 1/2 the RPM values for the induction machine, you get 80-90% efficiency at "normal driving speeds", which translates to typical ICE values of 1000-4000 RPM. And the induction motor is still 80% efficient at the equivalent of 750 RPM and 400 Nm, where the BLDC is also 80%. 

The BLDC may be doomed for large scale use in EVs, mostly because of the rare earth magnet materials whose cost will skyrocket if the demand goes up by 2x or 3x. BLDCs probably account for only 10% of EVs and a much lower percentage of full size DIY conversions. And the magnets can be ruined by just a short application of very high current, as may happen with a shorted IGBT or MOSFET that blows a fuse at 100 mSec but 20x rated current. An ACIM would easily "shrug it off" with no damage whatsoever. 

You may want to check my calculations and I may not be reading these graphs correctly, but that notwithstanding, motor efficiencies of 80% or better are quite good, and if that occurs at low power levels, the actual losses are not that bad.


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## major (Apr 4, 2008)

ruckus said:


>


Compare to this PM curve: 











> Courtesy of Remy Motors.


I got to wonder where your curve came from ruckus. It reminds me of something someone would just draw off the top of their head. I've tested motors and drawn motor curves and looked at motor curves many times, and that one just ain't right.


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## Coulomb (Apr 22, 2009)

There is also the classic Tesla page, where an engineer explains why Tesla chose induction motors over PM motors:

http://www.teslamotors.com/blog/induction-versus-dc-brushless-motors

It doesn't go into great detail, but it lists eddy current and hysteresis losses as reasons that you'd like to have low field for low power demand situations.

Note that the article was written in early 2007, so some statements like only 3 car models use induction motors are no longer be true.


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## Red Neck (Feb 1, 2013)

1. Siemens does not make all setups for highest efficiency but also for cost and other factors. Inverters for AC motors are a fairly recent thing and the cost is largely related to their functionality. Properly vector supported controls allow MUCH higher efficiencies than your graph. Not to mention that a proper automotive drive train is a very different story in itself.

2. There were so few car makers with AC systems on roads (all of them relied on knowledge of the gentleman making the AEM motors) either fully or partially, because inverters were non existent and problematic until very recently. It means they were not available.

3. As the industry was trying to kill off the full electric car or let it happen on its own, AC motors, which are basically the best fit, took a back seat to PM motors (rare earths did not appear to be an issue at the time) due to going the hybrid route, often meaning constant RPMs, which in principle allows PM motors some mild advantages. For Full electric, the induction motor, when all factors are included, is the best choice. 

4. Lack of their presence was down mainly to inverter issues and the simpler tech (brushed DC, etc) being more readily available and also relatively cheap due to already existing economies of scale. Much like the cost of existing 18650 lithium cells was already low due to economies of scale from consumer electronics field, which lead Tesla and others to use them, even though they are very unpractical. If they had prismatic cells at same cost from get go, they would never touch the cylindricals or would have them in much greater sizes, of 100Ah cells or higher...

Some issues are technological and some economic in nature. Economics play a crucial role. They are after all what keeps you from having 500miles worth of lithium in your cars right now...


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## Jan (Oct 5, 2009)

Ruckus,

The Siemens graph is based on real measurements of the Motor and an unspecified Siemens controller combined. The second graph is made up. I'm not saying by you. I think it is a conceptual presentation of tipical efficiency.


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## ruckus (Apr 15, 2009)

major said:


> Compare to this PM curve:
> 
> 
> 
> ...


I realize some of us are lumpers and others are splitters. Both graphs seem to be depicting almost the exact same motor efficiency pattern. The torque and rpm scales are different, causing the apparent change in slope of the 'efficiency vectors' (I just made that up). 

If you would rather use the Remy graph, great. It backs up my point even stronger. Look at 2000-2500 rpm where it is over 90% efficient at extremely low load. Nobody seems able to show an induction graph that is even close.

Again, the topic of this thread is maximum efficiency. The data supports my conclusion that PM motors are more efficient, especially at low loads.

That being said, there are many reasons that induction motors are great.
They are cheaper to build and rebuild (typically).
They have higher temperature resistance (typically).
They have higher rpm limits (typically).
There are many models available from many manufacturers.
The source materials (copper wire, steel) are available from many providers
etc... etc...

As I keep saying, each motor type has pros and cons.

Maximum efficiency = PM motors. 

Unless someone can show data otherwise, this should not be controversial.

Cheers.


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## PStechPaul (May 1, 2012)

I think PMDC motors and ACIMs are fairly similar in efficiency and one or the other may be better or worse under various conditions. Also, you can get motors rated at different efficiencies, especially ACIMs which come in premium efficiency types which may approach 95% in larger sizes as used in EVs. The PM types may be better in terms of power/weight.

But this thread is about AC vs DC series efficiency. Since many DIY conversions use series DC, we should discuss that. I found some performance curves here:
http://www.go-ev.com/motors-warp.html



















Notice that these motors peak at about 85% efficiency (only 72% in series!), and only put out about 30 HP and 140 lb-ft torque maximum at 72 volts. I'm sure they can be overdriven to 144V or higher for short periods of acceleration, but as the losses go by the square of current I would imagine these motors are giving less than 50% efficiency under those conditions. If these graphs are typical, then I wonder why anyone would use a series DC motor, unless they are afraid of high voltage or more complex controls. But for efficiency, maximum range, and lower Wh/mile, they don't seem very good. 

However, I found that the Warp 9 has 90-93% efficiency. I don't know why there is such a large difference. Here is a chart for the newer 2010+ Warp 9 (the older models are about 80-88%):


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## cts_casemod (Aug 23, 2012)

ruckus said:


> I realize some of us are lumpers and others are splitters. Both graphs seem to be depicting almost the exact same motor efficiency pattern. The torque and rpm scales are different, causing the apparent change in slope of the 'efficiency vectors' (I just made that up).
> 
> If you would rather use the Remy graph, great. It backs up my point even stronger. Look at 2000-2500 rpm where it is over 90% efficient at extremely low load. Nobody seems able to show an induction graph that is even close.
> 
> ...


This is an open forum you are free to point your observations. 
We are also free to otherwise say we do not agree with them. 
I have an AC induction setup and do not agree, not obviously telling you PM are not efficient, just telling you they are not good performers at a wide operating range, they are tailored to a particular operating range.

Induction allows you to trade torque for RPM, which you do with any petrol engine (5th gear is an overdrive) and can be used in Direct drive applications.

On a PMDC once you run out of voltage, thats it, you cant go faster and your speed is related to your pack soc, so you will have a lower top speed when your battery has a low soc. Induction looses torque output, but they can be taken to higher RPMs, you may just take a bit more time to get there since there is less torque available/

For a PM to have a decent torque output and high operating speed you need to have some kind of booster like the prius used to boost maximun electric only speed by doubling the voltage. This adds cost and another efficiency loss on the doubler. For them is cheaper because they can keep the same battery but the car is not intented to drive more than a few minutes in electric only mode.

Now what me and other members are trying to say is that you say PMDC is more efficient that AC but only provide some generic graphs. There are inverter rated induction motor capable of 6000RPM which are premium efficiency motors.

I used to think like you before starting my conversion: magnet is there = less losses. True, but magnetic losses do make a huge impact. 

If you ask any e-bike supplier why they dont use regeneration they will tell you that the losses are too high due to magnetic losses when you could be freewheeling, so they simply use a clutch bearing to only allow the motor to turn the wheels (And not the wheels to spin the motor).

But yes, for the operating range of a bike I can never even think of using an induction motor, PMDC is much more efficient. But not in a car.

Try it. Grab an induction and a PMDC and spin both by hand


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## Jan (Oct 5, 2009)

ruckus said:


> Unless someone can show data otherwise, this should not be controversial.


I'm not saying it's not true. You only have to be exact in your comparison.

The SIemens graph you posted comes from this document:

http://forums.evtv.me/file?id=1441130

It's the combined efficiency of the Siemens inverter and motor. In this document the efficiency of the motor itself is also given. You can look it up at Bild 3.4.

That comes very close to the PM graph. It is fair to know if that graph is in- or exclusive the inverter efficiency. If it is inclusive, the efficiency of both motors come very very close.


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## Hollie Maea (Dec 9, 2009)

It's really not that complicated. PM motors have higher peak efficiency because they run at a higher power factor (power factor is NOT the same as efficiency, but a low power factor slightly reduces efficiency, all other things equal). Well controlled induction motors have a higher average efficiency since they can control field strength independently at any speed.
Ruckus prefers the higher peak efficiency motors, and people who drive electric vehicles prefer the higher average efficiency.


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## ruckus (Apr 15, 2009)

Jan said:


> ... It is fair to know if that graph is in- or exclusive the inverter efficiency.


This is an EXCELLENT point. 

We don't really have good detail on what exactly is being measured for any of these graphs.

However, they are the only available data. 

---------------------------------------------------

cts_casemod, I value your opinion and experiences. However, you seem to have a fair bit of assumptions and misinformation.

1. As shown in the graph of the Remy PM motor posted by Major, the PM motor is more efficient at all loads and rpms. If you want to claim otherwise, please show some data. (Holli Maea, that goes for you too)

2. You talk about voltage limiting rpm. Again, please look at the Remy graph which goes out to 8000 rpm. PM motors have a constant torque phase and a constant power phase (with increasing back emf) just like the other motor types. What you say is true. For ALL motors.

3. You are convinced that PM motors do not free-wheel and implore me to "grab one and spin it". I have, and they spin quite freely. I am not sure how you acquired this misconception, but many seem to share it. 

I DO have a 1kw PM motor out of a treadmill which does 'cog' instead of freewheel. I am not sure why, but assume it is hard-wired in a constant state of 'regen' because the treadmill manufacturer's want it to stop very quickly if the power is turned off. The other PM motors I have (of many different sizes and makes) are quite easy and free to spin with no cogging at all.

---------------------------------------------------------------

From the information posted by many above, it sounds as if it might be 'possible' for any of the motor types to approach 95% under 'ideal' conditions.

Yes?


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## Hollie Maea (Dec 9, 2009)

Jan said:


> That comes very close to the PM graph. It is fair to know if that graph is in- or exclusive the inverter efficiency. If it is inclusive, the efficiency of both motors come very very close.


The PM motors are driven by the Scott Drive, which has zero or maybe even negative losses.


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## ruckus (Apr 15, 2009)

Hollie Maea said:


> The PM motors are driven by the Scott Drive, which has zero or maybe even negative losses.


The Scott Drive also powers the Siemens. 

-as for negative losses, I think you might be doing your math wrong.


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## ruckus (Apr 15, 2009)

ruckus said:


> From the information posted by many above, it sounds as if it might be 'possible' for any of the motor types to approach 95% under 'ideal' conditions.
> 
> Yes?



...............


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## Red Neck (Feb 1, 2013)

A proper automotive AC system (inverter and motor) will trump any other system in efficiency in a full electric vehicle. Across the complete operating range, it will defeat a PM, which has a high nominal peak efficiency, which you will not gain from in the actual operating range.

Again, the reason you saw few of them in automotive industry was the late arrival of proper inverters. Think of early 1990ies of actual vehicular inverters coming to market. It is normal for industries to try to use legacy, in terms of economies of scale. Something on market for some time usually already has some economies of scale. Like cylindrical lithium cells.

So even it has less or no sense, it is viable if it has a starting advantage, due to already being available and relatively affordable.

Even Chinese, which govern rare earths, do not wand them to be the centerfold of electric drive trains.


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## Qer (May 7, 2008)

Now, if we could only replace the inefficient driver too and we'd be getting somewhere...


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## cts_casemod (Aug 23, 2012)

ruckus said:


> cts_casemod, I value your opinion and experiences. However, you seem to have a fair bit of assumptions and misinformation.
> 
> 1. As shown in the graph of the Remy PM motor posted by Major, the PM motor is more efficient at all loads and rpms. If you want to claim otherwise, please show some data. (Holli Maea, that goes for you too)
> 
> ...


I could not care less on what you use, remember my conversion is finished 

Make it clear i am not saying PM are not efficient, what I say is that each motor has advantages and disavantages and you are selling the PM as the holly grail as if they were superior to others. I gave you a list of reasons. much before i had a car I always had an ebike and made mods to it, so I know what i am talking, however as stated earlier there are thousands of PM motors out there and you need to make sure you get the right one, the same way you wouldnt use a shunt DC motor on a car, for example.

As someone pointed out controlling a PM is much easier than an induction, reason why manufacturers use them in earlier years on hybrid vehicles. The prius oil cooled motor could be self contained. A DC can not and in 1995 thats pretty much what they were able to use. Induction control was still quite expensive. Today is a bit different, but as I said for hybrids PMDC is still the best choice for power versus weight. In an all electric vehicle I dont agree with this, but this is my opinion and you are free to think otherwise.

Another point is finding a suitable PM to use in a car. I tried and all I could find were quite expensive and small. Car size controllers are not something very easy to find, altought is pretty straightfoward to do your own if you wish. It would be nice to interface the prius drivetrain but I know no one that has gone that route.

So my point was correcting someone about the low efficiency of the induction motors when, as you say, not enought data is provided for any particular model. I did not at any point said anything about efficiency, just metioned some drawbacks of each type as I can give you a list of drawbacks of an induction motor, if you wish.


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## ruckus (Apr 15, 2009)

cts_casemod said:


> ... you are selling the PM as the holly grail as if they were superior to others.


How did you come up with that? Are you responding from the hip without actually reading my posts? I said the available data points to PM being more efficient but also listed a bunch of limitations of the PM design as well as a bunch of pros for the induction design. So a pretty balanced view. Where are you getting this holy grail garbage?


Qer makes an EXCELLENT point!  The driver (and vehicle) can make a larger difference than the motor/controller selection.

You can take a motor that is 10% more efficient and put it into a car that is 15% heavier and 20% bigger and you have definitely LOST efficiency.


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## rochesterricer (Jan 5, 2011)

Didn't Nissan go to an induction motor in the 2013 LEAF, from the PM motor in the 2012, to improve efficiency and increase range?


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## Jan (Oct 5, 2009)

rochesterricer said:


> Didn't Nissan go to an induction motor in the 2013 LEAF, from the PM motor in the 2012, to improve efficiency and increase range?


They use a synchronous motor, yes. If it was PM, I don't know.


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## PStechPaul (May 1, 2012)

It seems that the synchronous motor was in the 2010 model Nissan Leaf:
http://livingleaf.info/2010/11/nissan-leaf-electric-motor-and-transmission/
http://www.nissanusa.com/ev/media/pdf/specs/FeaturesAndSpecs.pdf

But I think it is a permanent magnet AC motor:
http://www.johnwmorehead.com/nissan-leaf-ev-and-what-is-a-permanent-magnet-ac-motor/

The 2013 Leaf will use inductive charging:
http://www.engadget.com/2011/12/05/nissan-leaf-to-get-inductive-charging-lose-its-stem-in-2013-vi/

Toyota may be shifting from PM to ACIM:
http://www.greencarreports.com/news...-ditch-rare-earth-metals-from-electric-motors

Synchronous motors usually use permanent magnets or a wound rotor supplied with DC through slip rings. There is a reluctance type synchronous motor which is suitable for EV traction use, but it is not self-starting and loses torque if it goes out of sync. Hysteresis type synchronous motors are self-starting but are only for small sub-fractional HP motors such as electric clocks and turntable motors which need to run in sync with the power line frequency. ACIMs have higher torque and other characteristics which make it much better for EV traction use:
http://en.wikipedia.org/wiki/Synchronous_motor
http://www.engineersedge.com/motors/synchronous_motor.htm
http://ecee.colorado.edu/~ecen4517/materials/SynchronousMach.pdf
http://www.allaboutcircuits.com/vol_2/chpt_13/2.html

The only real advantage I can see for a synchronous motor in an EV is that it can easily be used as a generator. 
http://www.pdhonline.org/courses/e171/DOE-HDBK-1011-v3.pdf
http://www.hss.doe.gov/nuclearsafety/techstds/docs/handbook/h1011v3.pdf
http://www.youtube.com/watch?v=pIbSMpHQ9a8


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## ruckus (Apr 15, 2009)

rochesterricer said:


> Didn't Nissan go to an induction motor in the 2013 LEAF, from the PM motor in the 2012, to improve efficiency and increase range?


The press release I read said they were reducing their 'rare earth' materials by '40%'. So they are still using PM motors with 40% less magnets.

OEM's will often choose price over quality, so their use of a particular system is not necessarily indicative that it is the 'best' for the application.

The fact that all OEM's use either PM or Induction motors indicate that they provide the combination of the highest long-term reliability and return on the investment.

The biggest issue with induction motors is gearing. Most car transmissions are not designed to run at 10,000 rpm in 2nd gear while cruising on the highway. The E-gear drive (8.28:1) or using very short gears help to make induction viable. 

Like I keep saying, each motor type has it's challenges. 

cheers.


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## Hollie Maea (Dec 9, 2009)

Don't think anyone uses wound rotor synchronous motors in EVs. Yes, you can do field weakening, but an induction machine is a better option, and doesn't need brushes. The vast majority of synchronous machines are utility scale generators. They work great for that, since real power output can be manipulated independent of reactive power output.


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## rochesterricer (Jan 5, 2011)

ruckus said:


> The press release I read said they were reducing their 'rare earth' materials by '40%'. So they are still using PM motors with 40% less magnets.


Actually, after doing some searching, the amount of magnets did not change. They used a new process that requires 40% less of the rare earth metal dysprosium for the protective coating on the magnets.


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## Red Neck (Feb 1, 2013)

AC motors win out for full electric vehicles. You will not see a leaf or Prius winning a consumption competition until on AC.

The Monaco ECO Rally this year has only AC driven cars in top spots.

http://www.acm.mc/documents/6/EN-24-Classement_Consommation.pdf

In actual real world use, across the range, AC is more efficient. And there are
ways to make them even more efficient but the cost of each percentage point of hugher efficiency is simply not worth it and is better spent on battery tech..


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## major (Apr 4, 2008)

Red Neck said:


> AC motors win out for full electric vehicles. You will not see a leaf or Prius winning a consumption competition until on AC.


The Leaf and Prius both use AC drives. Certainly not DC series motors


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## Red Neck (Feb 1, 2013)

major said:


> The Leaf and Prius both use AC drives. Certainly not DC series motors


My bad. I meant Induction vs anything with magnets 

Prius and Leaf (Leaf is supposedly now going induction also) are
permanent magnet based drive trains.


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## 1-ev.com (Nov 4, 2010)

Well...

I think we should point out what are the "AC vs Series DC efficiency" (as Title states) differences for OFF-SHELF components and not what OEM are using, due to the fact that OEM have $$$ to spend on R&D and we are buying that available to us as a off-shelf components.

Please correct me if I am wrong... Txs.


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## gunnarhs (Apr 24, 2012)

OK, now I want to add some stuff

1) Don't look at the numbers, look at efficiency through the driving circle
(AC induction motors for example have high efficiency only around specific RPM)
2) Don't forget to count in forced cooling. Most Permanent magnet motors only have 90% + efficiency if they are cooled with liquid. Meaning you are loosing up to 10% of your battery energy in the cooling system for the motor.
3) Controllers and motors do not drive continuously with 1000A +. Meaning that when you are reaching 100 kW performance, you have to use higher voltage (most DC equipment does not support high voltages).
4) It is important that you can adapt (with software usually) the control-box /motor combo to your existing drivetrain (torque/speed) limits.
5) Power requirement What driving experience are you looking for?
(drag (brute force), driving in city (high regen), highway driving (constant speed), racing at high speed. )
6) Cost, maybe the most important for DIY.
7) Complexity of the system

I am myself prefer AC-induction but based on experience for DIY the
Series DC is in my opinion the best compromise for DIY under 50 kW (30 kW continuous)


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