# Question about selecting the right kind of motor



## major (Apr 4, 2008)

hyperion said:


> Or, to put it another way, BLDC is a fine choice if your peak/cruise ratio is small. If it's large, the induction motor is best unless range/efficiency is not an issue.
> 
> Do I have this right? Are there other major drivers behind the choice of BLDC vs. induction as the motor for a EV?


Hi there hyper,

I think there is a bit more to it than that. And what you call BLDC is actually a wide range of various motor designs. A lot of it boils down to the pocketbook. How much do you have to spend on R&D and tooling vs using a technology easy to implement? 

I have been really impressed with the few efficiency maps released by Toyota on their hybrid motors. But I'd be hard pressed to call them BLDC. Yes they have permanent magnets in the rotors, but hardly a BLDC architecture. But on the other hand, the 15 to 20 year old EV-1 induction motor kicked butt. And the Tesla ACIM ain't too shabby. So in my mind, it is a wide open call.

But for the DIY guy there are few affordable choices, right? So what are you going to use in your electric car? 

Regards,

major


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## madderscience (Jun 28, 2008)

I dont know of ANY brushless DC (BLDC) motors that are available for use in a full size EV conversion, so I am not sure worrying about the differences between that and a 3-phase AC motor is worthwhile. I do know that in typical driving you are going to spend much less time at full throttle than at someplace in the middle (unless you are building a drag racer) But maybe you know something I don't.

You can get a variety of 3-phase AC motors and inverters for conversions, and there is a nearly infinite variety of series wound DC motors available (and a wide variety of controllers to support them) if you consider new retail motors and the giant surplus motor market from forklifts and other equipment.

I don't know for sure what the tesla achieves in overall efficiency but even with a lowly series wound DC and a basic PWM motor controller you can hit a peak energy conversion efficiency (controller and motor) well above 80%. That's about 90% for the motor at the peak of its curve, and probably more like 97 or 98% for the controller. (if they weren't that efficient, the controller would need way more cooling than just a flat aluminum plate or half square foot finned heat sink that is usually done for low end controllers)

It makes the most sense to try to optimize for peak efficiency at the power output you will use most. It would not be very helpful to have your peak efficiency at the motor and controller's maximum output unless you plan to use a "binary" throttle management policy . In my case, I aimed for peak efficiency someplace at 55-60mph steady cruise. That means about 100 battery amps in my car and puts the motor's efficiency at that power output at about 85%. According to my motors power curves (advanced DC 9"), curves here: 

http://www.amphibike.org/images/750_ADC9inchCurve.jpg

(measurements originally from advanced DC motors, and I found it at evparts.com)

...the motor actually reaches peak efficiency at a bit higher power than what I need at cruising speed, but wind drag would cancel out any range gain due to better motor efficiency at higher power. (all these numbers based on my recollection, but I have more actual calculations buried on my website)

typically 3 phase AC motors can be made slightly more efficient than a similar sized DC motor but if the brushed DC motors peak at around 90% obviously you can't improve on that too much. However the 3-phase inverter is going to be a bit less efficient due to needing 6 times as much silicon. (3 phases * 2 polarities) vs. simple on-off switching for DC PWM controllers. A BLDC controller will also be considerably more complex than a PWM one. Of course you do get regen with AC (and probably BLDC) and that can be significant depending on your driving situation.

To put all this in perspective (at least from my POV). For somebody considering a conversion. You could spend gobs more money or time and improve controller/motor efficiency by 2 or 3 percent over what a typical conversion achieves. Or you could put the money and effort into finding or modifying a chassis to get 20% less CdA than a typical car, or building one from scratch and possibly cutting the number in half over a typical car. What would be the bigger bang for the buck? The chassis. No brainer.


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## hyperion (Dec 15, 2009)

Thanks for the responses! This is such an exciting field; the technology for electric motors has been around a lot longer than the IC engine, and yet it's a very open field in terms of applying electric motors to personal transportation. And there's such a wide market covering that description...

I appreciate the perspective of bang vs. buck. Spending money for the sake of it is not the objective, but I'm looking to optimize across the board for performance, so areas like range, acceleration, efficiency will all be considered.

I'd like to build on the discussion a bit by asking a more general question. My understanding is that regen braking is possible on induction and brushless DC motors. It is possible with brushed motors as well? If so, are their major drawbacks in terms of efficiency, cost, or control?


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## major (Apr 4, 2008)

hyperion said:


> My understanding is that regen braking is possible on induction and brushless DC motors. It is possible with brushed motors as well? If so, are their major drawbacks in terms of efficiency, cost, or control?


Hi hyper,

Let's just call BLDC an AC system, as that is what it is. Then the AC systems essentially get regeneration for free. Well maybe some additional code, but that is usually included. There is no drawback.

With the DC systems, meaning commutator motors, regeneration is possible. With PM, shunt and SepEx motors, it is not difficult but requires an extra switch (transistor) in the controller and appropriate logic (program). These systems are available for low to moderate voltage and power.

The most common DC motor type used by DIYers is the series motor. This presents problems with regeneration as the series motor makes for an unstable generator. It can be done, but presents complexity in the control hardware and software, so none offer DC series motor controllers regen capable.

With the voltage and power levels needed for on-road EVs, the DC motors have another problem. This concerns commutation, resulting in the need for interpoles for regen capable DC motors. This adds to the cost of the motor and such motors are not typically available to the DIYer. That is the major drawback.

Regards,

major


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## hyperion (Dec 15, 2009)

Thanks, Major. It's giving me a lot to think about. It's such an interesting field because you really have to do some engineering to get exactly what you want. Nothing is really "out of the box" when you start pushing the envelope.


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