Overpriced Stuff: Electric Motor Speed Controllers

Commercially, speed controllers for electric motors (electric car size) are very expensive, like on here.

That page has a controller capable of 144v at 550a peak (about 90 horsepower peak) for $1250. I just designed a speed controller with parts off of digikey.com for about $200 that could handle 144v at 690a continuous so it could probably get over 180 horsepower peak.

All things considered, their's costs over 10 times as much per amount of power.

This seems like a BIG markup. I can understand shipping costs, labor costs, etcetera, but something seems a little messed up here.


I am not sure that the design is quite right, and haven't done it myself, so criticism is good.

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omnibot8 years ago
I've been thinking about if a 12v to 230v (DC to AC) inverter could be modified. There should be an oscillator somewhere in there and possibly it could be replaced by a variable one for speedcontroll of a motor. Just an idea.
yourcat (author)  omnibot8 years ago
I really don't know much about AC...
omnibot yourcat8 years ago
Oki .. basically AC is current that alternates (hence Alternating Current) between positive and negative ,actually switching polarity, in a low frequency, usually about 50 hertz.
I'm not sure at all wether the idea would work. I just figured that an inverter already change 12 volt DC to 230 volts AC (or 110 volts) at up to 3 kilowatts. An ACmotor, such as a vacuumcleaner, uses those 50hz to alternate and rotate, instead of with a mechanical commutator. If I'm right this is done with an oscillator somewhere at a low voltage, exchange/modify that for a squarewave oscillator that can be controlled and you would have a PWM curcit that drives an AC-motor at a controlled RPM from 12v batteries with 90% efficiency in the transfer without making changes to the motor.
As I've said it's just an idea, I could be wrong for any number of reasons and I'm probably never going to try it out.
yourcat (author)  omnibot8 years ago
I know what AC is, I'm just clueless as to a lot of the formulas, etc. I think an AC motor generally would only need special circuitry to start itself, and thats the part I really don't understand.
omnibot yourcat8 years ago
Oh, I hardly ever bother with formulas. AC-motors work pretty much like brushless motors.
yourcat (author)  omnibot8 years ago
If the actual running performance range is the same as BLDC, then what I don't get is the messed up ohm's law. I know for AC, volts times amps doesn't quite equal watts, and if I don't know the wattage then I can't figure the HP.
omnibot yourcat8 years ago
Yeah, tell me about it. *sigh* Ohms law, though useful, has a lot of shortcomings especially with high voltage AC. But it's the best we've got atm so if your wattage is about right just leave it at that. I think it's because the effect is not linear and at higher voltage/amps/resistance while Ohm's only gives a reliable value in a linear curcit at constant and low voltage/amps/resistance.
Whenever I need to use formulas I reluctantly look them up, put'em into Calc (excel on MS) or some other mathprogram. I then take the answer I get, tweak the formulas and values until I get an answer that aprox equals my measured values.
Frankly I've been frustrated with Ohm's since childhood and it's mostly because it's only an approximation and the materials we use are only precise down to 1-5%, after that it's all quantum.
LinuxH4x0r8 years ago
Can I get your design? I'm currently converting a 1980 Fiat Spyder Thanks
yourcat (author)  LinuxH4x0r8 years ago
Here they are.

I think I got most of the stuff on there, and a lot of the parts aren't critical as to which one you use, but the really big MOSFET is here. There are some others on there that are almost as powerful and that you don't have to order two of, and those should work fine too. If you have any questions, just ask. I'll get back to you with programming for the microcontroller (for a picaxe 08M).
westfw yourcat8 years ago
N-channel MOSFETs are usually used for low-side switching. No offense, but it looks like you have a long way to go before you'll have a working speed controller. I suggest you start with some cheaper MOSFETs to do "proof of concept."...
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