# Motor theory question

3707 Views 20 Replies 8 Participants Last post by  DIYguy
Since I've joined this forum, I've been doing A LOT of reading around here. One of the things that I've seen in a few threads is that the motor in an EV should be compared to the differential in an ICE car, not the engine. The power is made in the battery pack and the controller, assuming the motor can handle the juice you send to it. This leads me to a question:

Assuming the same battery pack and controller are used, and assuming that you optimize gearing for the different power curves, can different motors produce the same performance? In other words, if all you did was change motors and final drive ratios, would a Warp 9 produce the same performance as a Warp 11 in the same vehicle?
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Since I've joined this forum, I've been doing A LOT of reading around here. One of the things that I've seen in a few threads is that the motor in an EV should be compared to the differential in an ICE car, not the engine. The power is made in the battery pack and the controller, assuming the motor can handle the juice you send to it. This leads me to a question:

Assuming the same battery pack and controller are used, and assuming that you optimize gearing for the different power curves, can different motors produce the same performance? In other words, if all you did was change motors and final drive ratios, would a Warp 9 produce the same performance as a Warp 11 in the same vehicle?
You can get the same performance as long as you don't exceed the motor ratings. Generally a larger motor can handle more power. One could set up a controller to only deliver the current and voltage that say a 6.7 inch motor can handle. Hooking that controller to an 11 inch motor would give you the same wimpy performance. Setting it up in the reverse order briefly gives the same peppy performance, then magic smoke.
Gerhard
Funny I was just wondering that same thing. So lets say your controller and batteries can safely put out 1000A continuous. Would a 8", 9", and 11" motor give the same performance?

My thought would be to pick the smallest motor that can handle the current, since it should weigh less. But isn't there also the factor that the larger diameter means more coil for the field, so for the same current, the larger motors will still give you more torque.

Maybe then you chart Torque/Weight ratios for all motors at the same current and pick the highest. Here making the assumption that if it weighs more but makes up for it with more torque, performance should be better (assuming no tire slip too).

Thoughts?
My thought would be to pick the smallest motor that can handle the current, since it should weigh less. But isn't there also the factor that the larger diameter means more coil for the field, so for the same current, the larger motors will still give you more torque.
I saw that same question in another thread. The answer seemed to be that the larger motors do produce more torque, but it produces it lower in the rpm range. This is why I mentioned powerband and gearing in my question. The posters seemed to imply that optimized gearing would nullify that torque advantage.

I've seen people on here say that you should use the smallest/lightest motor that can handle the juice you want to throw at it.
One of the things that I've seen in a few threads is that the motor in an EV should be compared to the differential in an ICE car, not the engine.
I don't think this makes any sense. Motors are energy conversion devices. Electric power in, mechanical power out. The differential is a multiport mechanical power transfer device.
I don't think this makes any sense. Motors are energy conversion devices. Electric power in, mechanical power out. The differential is a multiport mechanical power transfer device.
Where I read it, I think that analogy was used to make a point. I realize it isn't a very accurate analogy, but such things happen when you are trying to compare ideas from things as different as EVs and ICE cars.
...the motor in an EV should be compared to the differential in an ICE car, not the engine...
I don't think this makes any sense. Motors are energy conversion devices. Electric power in, mechanical power out. The differential is a multiport mechanical power transfer device.
I think the problem is the word differential. The common theme that has been floating around is that the electric motor is more comparable to a transmission, than an engine. The battery is the power source (or engine) and the motor converts that energy into mechanical force.

I find that analogy interesting because an ICE is actually converting the energy stored in fossil fuels into mechanical force as well.
Major or Todd, I know you both know a lot about motor theory and practical application. What do you think about my design metric idea of evaluating motors based on their Torque/Weight ratios for the same current (say that current is limited by something else like the controller, picking that max current)?
I think the problem is the word differential. The common theme that has been floating around is that the electric motor is more comparable to a transmission, than an engine. The battery is the power source (or engine) and the motor converts that energy into mechanical force.

I find that analogy interesting because an ICE is actually converting the energy stored in fossil fuels into mechanical force as well.
Ah, I see what you guys mean now. That analogy probably does work better.
Major or Todd, I know you both know a lot about motor theory and practical application. What do you think about my design metric idea of evaluating motors based on their Torque/Weight ratios for the same current (say that current is limited by something else like the controller, picking that max current)?
Dude, you have me confused with someone who has a clue! I'm learning about EVs - motor theory?!?!? They go round and round...

Major, on the otherhand, is the guru.
My thought would be to pick the smallest motor that can handle the current, since it should weigh less. But isn't there also the factor that the larger diameter means more coil for the field, so for the same current, the larger motors will still give you more torque.
The best answer is, it depends. The efficiency at high power levels tends to increase for the larger motors but they also increase vehicle weight. Somewhere there is the right trade-off. To small will make itself painfully known when gets damaged in use. The White Zombie torched a pair of 8 inch motors on the drag strip so he is running a pair of short Impulse 9's (siamesed) now.

There are some general guidelines for vehicle weight and motor choice floating around. As a general rule for a street vehicle of moderate performance I would suggest a full length 9 inch motor for a vehicle weighing over 3000 lb. If your range is long or your terrain hilly I would suggest the ADC FB-1 or WarP 9 for vehicles over 2500 lb. This assumes you are retaining the transmission. If you are going over 3500-4000 lb. converted weight you may need more motor. If you are racing you most likely will need more motor than a strictly street conversion.
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Funny I was just wondering that same thing. So lets say your controller and batteries can safely put out 1000A continuous. Would a 8", 9", and 11" motor give the same performance?
They have no magic!!!. For same motor design a larger motor will always produce more torque, so more Kw (hp).

But you need to know than a Warp9 and Warp11 haven't the same design!

You can see the answer below. 144v and 1000A
This graphs come from the exellent Maxvtol calculator (a Member).

W9: 209 lbs-pi @ 4250 rpm = 169 hp

W11: 298 lbs-pi @ 2750 rpm = 156 hp

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Right, so it has more low-end torque, but peaks out sooner, so peak power is less. That's why I'm wondering if there might be some 8" motors that have even more peak power, plus lighter weight. Because even 200 ft-lbs of torque is a ton for a Honda CRX

I would love to try this for different motors, but there are only four to choose from in Maxvtol's calculator. What are the minimum specs I need on a motor to add it to the spreadsheet? thanks
They have no magic!!!. For same motor design a larger motor will always produce more torque, so more Kw (hp).

But you need to know than a Warp9 and Warp11 haven't the same design!

You can see the answer below. 144v and 1000A
This graphs come from the exellent Maxvtol calculator (a Member).

W9: 209 lbs-pi @ 4250 rpm = 169 hp
W11: 298 lbs-pi @ 2750 rpm = 156 hp
So at the corner they provide essentially the same power. If you gear down the w9 by about 1.5 you have the same torque.
Gerhard
The common theme that has been floating around is that the electric motor is more comparable to a transmission, than an engine.
Hi Todd,

By common, I think you mean Mr. Bill. I never saw his point and disagree with him, but never saw the value in arguing the point. Like I said, the electric motor is an energy conversion device. And like you allude to, an ICEngine converts chemically stored energy to mechanical power, hence is an energy conversion device. A transmission is a power conditioning mechanism in my mind, converting mechanical power from one ratio of torque and speed to a different ratio of torque and speed. The electrical analogy is the transformer, changing the ratio of volts and amperes.

The battery is the power source (or engine) and the motor converts that energy into mechanical force.
I have never seen a battery do any mechanical work, except when I dropped one on my foot Batteries store charge, or store electrical energy. Perhaps inside the battery cell, it is using a chemical conversion process to do this, but from the macro viewpoint, a battery is a black box with 2 terminals. You put electricity in and store it and get electricity out at a later time.

I find that analogy interesting because an ICE is actually converting the energy stored in fossil fuels into mechanical force as well.
And with the electric system, somewhere along the food chain, there is such or a similar conversion required to get electricity, either from an engine powered generator, or solar cells (which are kind of engines) or some other converter of energy.

But really, it is all just word games They do what they do regardless of what you call them. But the analogy I like is: battery ~ fuel tank, electric motor ~ ICEngine, electric motor controller ~ carburetor.

Later,

major
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Major, would you agree with the statement you should go with the smallest motor that can handle what your controller and battery pack will be sending it? Why or why not?

I've seen this statement, or something like it posted in at least a couple threads and I'm wondering how true it is.
Hi Todd,

By common, I think you mean Mr. Bill. I never saw his point and disagree with him, but never saw the value in arguing the point....
Hey Major. Ditto.

...I have never seen a battery do any mechanical work, except when I dropped one on my foot Batteries store charge, or store electrical energy. Perhaps inside the battery cell, it is using a chemical conversion process to do this, but from the macro viewpoint, a battery is a black box with 2 terminals. You put electricity in and store it and get electricity out at a later time...
100% agreement. The comparison I posted about the battery being the power source or engine was just me attempting to paraphrase that "common" motor-is-transmission analogy for the record, since it was stated incorrectly to start the thread. BTW, I labeled it "common" because it seems like people like to use it more and more lately, not because it is the common belief here.

...And with the electric system, somewhere along the food chain, there is such or a similar conversion required to get electricity, either from an engine powered generator, or solar cells (which are kind of engines) or some other converter of energy.

But really, it is all just word games They do what they do regardless of what you call them. But the analogy I like is: battery ~ fuel tank, electric motor ~ ICEngine, electric motor controller ~ carburetor...
100% agreement again - wait, make that 99%! Can we say EFI system instead of carburetor? Must you reach back into the stone ages Major?!
...the statement you should go with the smallest motor that can handle what your controller and battery pack will be sending it?...
I think that is way too general. There are far too many applications and intended purposes for that to just apply across the board.

Take my application (experimental race car) for example. The most logical path for me was to use the largest motor that "made sense" for the vehicle, and then focus on pushing more and more power through it, until I find its mechanical limits. I like breaking \$#!+ though.

As for the lightest fastest possible combination, in a race scenario, ideology: that is an effective albeit expensive race strategy. Most racers desire some reasonable amount of longevity, meaning they generally over-engineer things a bit. In the case of a motor, that would mean having a bit more than the "smallest", to add reliability. Unless, that is, you like tearing stuff apart and rebuilding it frequently. In other words, if an 8-inch motor is just enough use a 9-inch motor for better reliability.

Road-going vehicles are substantially over-engineered, which is why I can drive a 19 year-old vehicle daily without concern. Then again, I drive a Honda, which is known for its engineering. I work part-time in an auto parts store though, and sell lots of parts daily to people with vehicles that had "just enough" engineering.
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Major, would you agree with the statement you should go with the smallest motor that can handle what your controller and battery pack will be sending it? Why or why not?
Hi roch,

It all depends on what you're building and your expectations. Generally speaking, I'd say use the largest motor you can tolerate. I see a lot of guys sorry they skimped on the motor, and never see any regrets about too big a motor, except the one idiot who put the Warp9 on a go-kart.

Your batteries are somewhat self limiting. Controllers are typically self protected. Motors are at the mercy of the nut behind the wheel and environment.

Regards,

major
Also, generally a forklift style motor (ADC, Warp) 6.7", 7" and 8" are less efficient than bigger 9" and 11".

So for the same power in, smaller motor produce less power than bigger motor. Especially at high amps (1000A).
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