# Max motor RPM, Volts, and Top Speed?



## john818 (Aug 1, 2008)

I'm trying to decide what motor and pack voltage will give me the performance I want, and I'm unclear on a few points.

If I understand correctly, as a general rule, the more volts you have, the higher your motor will rev, and the higher your top speed can be. Sometimes, an RPM limit is given for a motor, and sometimes, a maximum voltage is given. Does this mean that to get the maximum RPM, you must have a battery pack of the maximum voltage?

To confuse me more, the following info was on Netgain's site.



> 1. All WarP Motors have comms that were tested to over 8,000 RPM, but that does not mean they can be run at that speed indefinitely!
> <snip>
> 3. We like to recommend safe speed ranges from 1500- 2500 RPM, even though we know some of our WarP Motors are peaked around 5,000 RPM for small intervals of time. When working with a customer, please be sure to design gearing so that the customer gets the speed he wants, but the motor will not be at a high RPM for long periods of time.


So at what RPM can they be run safely. If you're going to be driving near your top speed for 15 minutes or so, do you need to gear your top speed at around 2500 RPM? That seems really low! If I get a WarP 9 motor and a 120v or 144v pack, what maximum safe RPM can I expect? I'm considering direct drive w/ gear reduction, so this info is crucial in m application, but even if I used a transmission, knowing the operating RPM of a motor would still be important.

Also, I remember reading somewhere that smaller motors tend to rev better. Would an 8" Advanced DC motor be likely to have a higher max safe RPM or maybe a higher RPM for the same voltage?

Thanks in advance for helping me understand this!


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## judebert (Apr 16, 2008)

You're generally correct about voltage and speed. More voltage leads to higher top speed; more current leads to faster acceleration.

Motor max RPM is not associated with voltage. If you exceed the max RPM, the motor flings itself apart. It happened to me; it's not pretty. Or cheap. Don't exceed the motor max RPM.

If the manufacturer says to cruise at 2500, that's the number I'd use. I always thought ADC and Warp motors were supposed to cruise around 4000, but manufacturer's documentation beats my incoherent ramblings any day.

Yes, smaller motors can usually reach higher RPMs because the centrifugal force (naysayers, derive it in a rotating reference frame and watch it appear like magic!) is much less at a smaller radius. Therefore the commutator bars can move faster before the glue holding them on fails. Of course, you'd still want to stick to the manufacturer's specs.

However, the 8" ADC won't have a significantly higher RPM at any given voltage than a 9" ADC. The 8" has the advantage of higher max RPM before it flies apart; the 9" has the advantage of bigger windings and longer levers, providing better torque and more current (and torque) capacity before it overheats. 

Each will have a similar speed under similar energy inputs and load.


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## Georgia Tech (Dec 5, 2008)

Got to be REALLY carful when I say this, but, kind of got to think of this as you would a normal V6 or V8 engine. Those engines have rev limits well these series motors. If you step on the gas on a sports car in neutral and you go over rev limit, the engine and destroy it self. If you run a series motor without a load or without some sort of rev limit feed back then the motor will burst!

Now yes for a given current the higher the voltage the higher the RPM therefore the more power you’re going to make. Current is directly proportional to the torque while voltage is the RPM. The advantage of going to a large motor is more torque per amp, and the ability to run at a higher voltage, which in turn you get a higher voltage times a larger current which will make more power….this power is normally at the lower RPM rang.


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## booksix (Aug 26, 2008)

Wow! 1500-2500 range?!? That's so low! How does anyone run these in a direct drive setup?


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## john818 (Aug 1, 2008)

judebert said:


> The 8" has the advantage of higher max RPM before it flies apart; the 9" has the advantage of bigger windings and longer levers, providing better torque and more current (and torque) capacity before it overheats.





Georgia Tech said:


> The advantage of going to a large motor is more torque per amp, and the ability to run at a higher voltage, which in turn you get a higher voltage times a larger current which will make more power….this power is normally at the lower RPM rang.]


Thanks to both of you! One of things I was most worried about is running too much current at low RPM. That's one reason I've been leaning toward 144V and the WarP 9 even though it's probably higher voltage and a larger motor than needed for my application. I'd much rather have too much power than too little.

I guess I need to check with NetGain and/or vendors to get a better idea what RPM limit would be safe in the real world. Does anyone have any real world experience they'd like to share?


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## Ed260Z (Dec 27, 2008)

I've been thinking about converting my car, but all the info I've gathered has my head spinning. I have a 2400Lbs car that needs to maintain70-75mph for about 10 miles of my 14 mile commute. 
The big question is what batteries to get? Specifically what should the Voltage and Ah rating be at, to achieve my goal?


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## Guest (Dec 28, 2008)

Your differential is geared too. Match the gearing with your motor. I am thinking of converting a VW Beetle Turbo Diesel. The transmission in that will be better suited to my motors set up. Low motor rpm but high speeds at the wheel. Designed for grunt power not RPM speed. 




booksix said:


> Wow! 1500-2500 range?!? That's so low! How does anyone run these in a direct drive setup?


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## mattW (Sep 14, 2007)

Ok this is just my guess so bear with me but...
The reason I think that many say that higher voltages give higher top speed is because having a higher voltage will mean you can maintain the current in the motor for longer.

At any given RPM the motor is acting like a generator in reverse, opposing the current going into it and giving a voltage drop across the motor. Usually the voltage you are applying is much higher than the drop across it but the higher the RPM of the motor the higher the voltage drop will be (look up back EMF). If at 3000rpm you have a 30V back emf then a 144V system will "see" 114V but a 96V system would only "see" 66V. That low voltage might not be able to draw enough current to keep accelerating up to the motors physical rpm limit.

Basically voltage increases top speed up until the rated rpm limit of the motor (which is dependant on what is holding it all together).

I hope that made sense.


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## Coley (Jul 26, 2007)

You have to remember also that you have full torque at the first turn of an electric motor. 
It doesn't need to wind up like an ICE.
The power is there at low rpm and falls off at too high of rpm....


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## GTWCMT (Jan 22, 2009)

Speed is pretty easy to work out.

find the radius of the wheel and convert to centimeters, there is aprox 1610 meters in a mile.

say a 14" rim is about 65CM, you will travel about 65cm a minute @1 RPM
if youre running at 4000 RPM then thats 65x4000 (2600 meters a minute or 156,000 meters an hour( same as 156KPH))

You then need to take into account any gearbox reduction/addition.
a 2:1 will take down a 4000 RPM to 2000 RPM at the wheel.

The power requirements to move 1KG is 1 watt per meter per second.
(thats only the constant acceleration value)

One more bit of math missing to do with KW of the motor to the total distance and a percentage over the acceleration requirements for the given speed that you want to travel at.


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## john818 (Aug 1, 2008)

GTWCMT said:


> Speed is pretty easy to work out.
> 
> find the radius of the wheel and convert to centimeters, there is aprox 1610 meters in a mile.
> 
> ...


Sorry, I should've indicated I have a pretty good idea how to get from RPM to speed. I use this formula for speed at a given RPM.

(SPEED in MPH) = [(RPM) * (Tire Circumference in Inches) / (Gear Reduction)] * [(60min/1hr) * (1mi/5280ft) * (1ft/12inches)]

Circumference = 3.14 * Diameter
or
Circumference = 3.14 * (2 * Radius)

The terms at the end of the first equation are for unit conversion and can be reduced, but I left them to make it easier to see what they're for. To get MPH from RPM, you need to convert minutes to hours. To get MPH from the tire circumference in inches, you need to convert inches to miles.

GTWCMT, thanks for the response, but when you talk about the radius of the wheel or rim, I think you mean tire. The circumference of the tire is what matters, not the wheel. A 14" wheel with a 185/60-14 size tire will have a different diameter and circumference than a 14" wheel with a 225/60-14 tire.

Also, not to be mean, but I'm not sure how you got from a 14" rim to what looks like you're saying is a 65cm circumference. You don't say, and it doesn't make sense. 65cm is almost 26", which could be a reasonable _diameter_ for a tire. If that's what you found, you forgot to calculate the circumference. Just wanted to point this out to avoid confusion.

Matt, thanks. Yeah, that makes sense. Most of what I've heard leads me to believe that 144v on a WarP 9 would be fine, but to what RPM?

Or, in light of Coley's post, a better real world question might be at what RPM will the power drop off be significant? I know "significant" is a bit vague. I guess that's why I'm having trouble with this.

Also, when I worry about running too much current for too long at low RPM with tall gearing, am I worrying about nothing? With "full torque at the first turn of an electric motor", will I accelerate out of any danger zone before significant heat builds up, assuming flat terrain and a light vehicle?

Maybe I need to just take the plunge and see how it works!


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## Coley (Jul 26, 2007)

To be more accurate on the distance traveled per revolution of the tire, chalk mark the tire (on the road/shop floor) and the ground. Roll the car one revloution of the tire and mark the ground.
Measure the distance between marks and divide the feet in mile by that number.
Tire should be at operating pressure.
My 13" tires go 946 revs per mile.


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## Anaerin (Feb 4, 2009)

I've been doing some calculations of my own (using a nice Org spreadsheet with a whole mess of forumlae in), and I'm finding the following:

A car running P235/45 R15 tires at 2,000 RPM has a speed of 226.76KPH or 140.9MPH.

How I calculated that:

"P235/45 R15" works out to:

Passenger-class tires (The P bit)
235mm Wheel Width (The 235 bit)
Tire Depth = 45% of width (The /45 bit)
15" Diameter Rims (381mm) (The R15 bit)

Therefore


Tire Thickness = 45% of 235mm = 110.25mm
Total Wheel Diameter is therefore 381mm + (110.25mm * 2) = 601.5mm
Wheel Circumference = Pi*601.5mm = 1889.67mm
Linear speed = 1889.67mm * RPM (2,000) = 3779335.96mm/min, * 60 = 226760157.74mm/hour / 1,000,000 = 226.76KPH

So, if you had a nice, high amperage, low voltage system, you could hook two motors up to the two back wheels directly (Well, through CV joints) and have a very nice, very fast, and very powerful car.

But instead people are talking of using high voltages to get high RPMs, then dropping them through transmission systems. Is this because low-voltage, high current controller systems are hard to find? Is there a maximum amperage rating for electric motors that makes this unfeasable? Am I missing something (Probably)?


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## Bowser330 (Jun 15, 2008)

Anaerin said:


> I've been doing some calculations of my own (using a nice Org spreadsheet with a whole mess of forumlae in), and I'm finding the following:
> 
> A car running P235/45 R15 tires at 2,000 RPM has a speed of 226.76KPH or 140.9MPH.
> 
> ...


Higher voltage uses less amps to get the same power...so you get greater range from your battery pack as you use less amps to go the same speed...also higher current draw creates greater voltage sag of the battery pack...high current controllers exist but they provide high current and short time intervals, the continuous rating may not be what you are looking for, however some controllers out there are being made to provide higher continuous amp ratings...electric motors can take higher current for short periods of time but not continuously, what exactly is "high current" in your opinion does matter...

I would follow some of the controller discussions, they show what people are working on and developing...higher voltage controllers with higher peak and continuous amp ratings...


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## Anaerin (Feb 4, 2009)

Bowser330 said:


> Higher voltage uses less amps to get the same power...so you get greater range from your battery pack as you use less amps to go the same speed...also higher current draw creates greater voltage sag of the battery pack...high current controllers exist but they provide high current and short time intervals, the continuous rating may not be what you are looking for, however some controllers out there are being made to provide higher continuous amp ratings...electric motors can take higher current for short periods of time but not continuously, what exactly is "high current" in your opinion does matter...


I was under the impression that Volts = RPM, Amps = Torque. 

I'm suggesting having a motor hooked directly to the drive axle, without any form of transmission or differential. Considering that situation, having high voltage is much less important than having high amperage, as you need as much torque as your motor can give you. Possibly, as other people have suggested, a "Shuffling" controller, that links the batteries in differing configurations to balance voltage and available amperage, would work best in this case.


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## Camaro (Jul 29, 2008)

2500 rpm does seem low . This is what I'm planning on,

Using your fomula:
(SPEED in MPH) = [(RPM) * (Tire Circumference in Inches) / (Gear Reduction)] * [(60min/1hr) * (1mi/5280ft) * (1ft/12inches)]

I should get:
55.9MPH = (5000rpm*pi*25in/6.66)*60*1/5280*1/12
44.7MPH = (4000rpm*pi*25in/6.66)*60*1/5280*1/12
33.5MPH = (3000rpm*pi*25in/6.66)*60*1/5280*1/12
22.3MPH = (2000rpm*pi*25in/6.66)*60*1/5280*1/12
11.1MPH = (1000rpm*pi*25in/6.66)*60*1/5280*1/12

A top speed of 55mph is a ok for my needs. I'd be driving mostly at 30mph - 40mph which is around 2700rpm - 3500rpm. My best guess is that this should be fine for a Warp9, but does anyone have a direct drive system that could give us some feedback?


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## Bowser330 (Jun 15, 2008)

i dont have one, but I have heard a lot of feedback talking people out of direct drive with just a 9"...


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## john818 (Aug 1, 2008)

Coley said:


> To be more accurate on the distance traveled per revolution of the tire, chalk mark the tire (on the road/shop floor) and the ground. Roll the car one revloution of the tire and mark the ground.
> Measure the distance between marks and divide the feet in mile by that number.
> Tire should be at operating pressure.
> My 13" tires go 946 revs per mile.


This is a great idea for real world numbers! Nominal tire sizes are often significantly different from actual measurements. In my case, I don't have the tires yet, so I'm approximating.



Bowser330 said:


> i dont have one, but I have heard a lot of feedback talking people out of direct drive with just a 9"...


Yes, that's why I'm concerned. Of course, the application I'm considering is different from most. I'm thinking about a home-built reverse trike with a total weight of 1200-1300# including driver. With such light weight, I'm hoping a 9" motor, 144v, and a 500a controller will give me the top speed and acceleration I want. A top speed of 70 mph and 0-60 in about 8 seconds would be nice.


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## Zoggman (Feb 17, 2011)

Anaerin said:


> So, if you had a nice, high amperage, low voltage system, you could hook two motors up to the two back wheels directly (Well, through CV joints) and have a very nice, very fast, and very powerful car.
> 
> But instead people are talking of using high voltages to get high RPMs, then dropping them through transmission systems. Is this because low-voltage, high current controller systems are hard to find? Is there a maximum amperage rating for electric motors that makes this unfeasable? Am I missing something (Probably)?



Why not put one motor whit a double ended shaft between the two drive shafts to run both wheels?


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## Anaerin (Feb 4, 2009)

Zoggman said:


> Why not put one motor whit a double ended shaft between the two drive shafts to run both wheels?


Because when you turn a corner the wheels have to turn at different speeds. If they are hard-linked like this, one of two things has to happen, either the wheel slips (so you can do this when offroading), or the transmission (in this case, the drive shaft) gets twisted by the torque and snaps. So you need some kind of differential in the middle so the wheels can turn at different speeds.


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## ceh4702 (Mar 12, 2011)

I am taking a class on conversions. One thing that was suggested is that if you have a donor vehicle that runs, just take it out and run it in the gear you are planning to use at the RPM'S you think you will run at and see what speed you are moving at and just take your magic marker and put a tick mark at that spot.

This brings up the issue of how you plan on knowing what RPM you are going in an EV.


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## ceh4702 (Mar 12, 2011)

You were discussing just mounting the motor to turn the rear wheels. If you look at some of the kits that Riley designed in the 70's, it looked like he used the rear end differential turned upside down and then mounted the motor directly on a mounting plate and was directly turning the differential using a pulley. Of course I wonder if that would work with a 4,000 pound EV as the axle is bouncing up and down over the potholes. You might look at his Trike Design.

Differentials use gear ratios. Vehicles like a fork lift use a different gear ratio, to speed it up.

We were taking about the concept of force as it is applied to drive the vehicle. The more rotational weight that is turning, the more force there is required to turn the drive train. i.e. transmission, adapter plate, flywheel, clutch, drive shaft, differential, final axle, bearings, hubs, wheels, accessories from the front spindle, etc. 

So basically less weight to spin = less force, or less elecrical amps being used to go that same mile.

When doing conversions it is generally accepted that we use the existing transmission as it is set up. So if you had a vehicle that could handle a heavy load and also had a transaxle like volkswagon engine in the rear like a sand rail, that might be an advantage. I guess you would get the same advantage from a front wheel drive car like an escort except for the battery placement and weight distribution. I remember they also made an Escort station wagon model.

So maybe an Eagle Talon might be nice.


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## fixallthings (Sep 4, 2009)

I believe he said a reverse trike. This is also called a tadpole design with two steering wheels in the front and one powered wheel at the rear. RQ Riley has two of these designs on his website rqriley.com. One is the Tri Magnum with a motorcycle providing the power and the XR3 with a diesel engine running the front wheels through a VW transaxle and a direct drive electric motor running the rear wheel. The big advantage to this design is a very simple rear suspension without the need for a differential.


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## LithiumaniacsEVRacing (Oct 9, 2010)

judebert said:


> You're generally correct about voltage and speed. More voltage leads to higher top speed; more current leads to faster acceleration.


You can also increase your top speed with current by dropping your gear ratio while racing, as soon as the rpm's drop, the amps will start pulling you faster, this process can repeat itself.


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## John Metric (Feb 26, 2009)

booksix said:


> Wow! 1500-2500 range?!? That's so low! How does anyone run these in a direct drive setup?


I run a 3.08 rear end direct drive and a 26.6" tall tire. Works pretty well. Burn rubber at 60mph.
I am dropping to a 2.73 right now.

I also found a 2.26 which I will keep in the garage until I need to LSR.

Metric


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## DIYguy (Sep 18, 2008)

john818 said:


> "3. We like to recommend safe speed ranges from 1500- 2500 RPM, even though we know some of our WarP Motors are peaked around 5,000 RPM for small intervals of time. When working with a customer, please be sure to design gearing so that the customer gets the speed he wants, but the motor will not be at a high RPM for long periods of time."
> 
> So at what RPM can they be run safely. If you're going to be driving near your top speed for 15 minutes or so, do you need to gear your top speed at around 2500 RPM? That seems really low! If I get a WarP 9 motor and a 120v or 144v pack, what maximum safe RPM can I expect? I'm considering direct drive w/ gear reduction, so this info is crucial in m application, but even if I used a transmission, knowing the operating RPM of a motor would still be important.


This quote isn't really correct. I think it was George Hamstra who admitted this to be a typo. Anyways, it was publicly refuted... somewhere.


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## dladd (Jun 1, 2011)

DIYguy said:


> This quote isn't really correct. I think it was George Hamstra who admitted this to be a typo. Anyways, it was publicly refuted... somewhere.


I know this is an old thread, but yes I specifically asked Netgain about this a little while ago, and the response in an email from George was that it's a typo on the online document. He said the motors need 2500-3500rpm to cool themselves properly (not 1500-2500rpm), and to keep extended use below about 4000rpm.


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## John Metric (Feb 26, 2009)

Just for another data point on this old thread. We just ran 155mph at 180 motor volts on stock netgains, with plenty of power all along the way. 2.73 rear gear. one speed overdrive. 1000motor amps and still accelerating at 155mph.


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