Overpriced Stuff: Electric Motor Speed Controllers

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.

Oh, I hardly ever bother with formulas. AC-motors work pretty much like brushless motors.

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.

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."...

. So, assuming westfw is right about the MOSFETs*, you're just spreading bad designs around? . If you haven't built it and tested it, how do you know that it "could handle 144v at 690a"? . I just hope the others haven't wasted too much time with your "design." *Not a bad assumption to make - judging by his previous posts, westfw knows what he's talking about when it comes to electronics (and many other things). I've seen no evidence of super-human mental powers, but he's pretty damn smart. You would do well to listen when he speaks.

You have to arrange for V_gs, the voltage between the gate and the source, to have reached the ON threshold. In a low-side switch, the source is tied to ground, and this is relatively easy (although many high power MOSFETs, including the one you reference, have V_gs-on of about 10V, so driving them from a 5V microcontroller can be a small challenge. In a high-side configuration (with the load in between the source and ground), the gate voltage will need to be 10V above the supply voltage to keep the MOSFET on when the load is running.

You didn't mention power dissipation. Max current and max power dissipation are two separate issues in power semiconductors. You can only sustain the max current IF you manage to dissipate enough power that the junction(s) temperature stays "reasonable." With R_ds-on of 1.8 mOhm and, say, 600A flowing through the MOSFET, I calculate that you need to dissipate about 650W, which is no small feat. Does your current cost estimate include a big heatsink and fan?

The circuit you show has no provision for turning the MOSFET off; you'll charge up the gate to V_on (maybe) through your secondary mosfet, but there's no discharge path to a V_off when it's time to stop. A fair amount of attention in high-power mosfet circuits goes to making sure that the turn-on and turn-off events happen FAST, because in-between the MOSFET is going to be operating in a linear region where R_ds is much higher, and the power dissipation will leap-up (linearly with R_ds-on) in neat little spikes, and fry your MOSFET if you aren't careful.

And of course, at 600A, your interconnections and even your wire become things that you have to pay pretty careful attention to.

In short, a speed controller at this sort of power level is one of those things that SOUNDS easy ("just use a huge MOSFET"), but quickly gets more complicated as you have to take into account all the annoying real-world behaviors that exist.

There are some "starting points" in the "DIY electronics" portions of RC Forums, though of course most of those are for far lower currents than you'd encounter in a full-sized car.

Yeah, I could use those too. I have enough stuff to make a pretty neat EV, (except the speed controller).

Thanks! I'll have a good look tonight. I have all the stuff to make an electric motorcycle, and most of the stuff to make an electric Firebird.

DO IT!

The bike is likely, I don't know if I've the time to do the Firebird. My kids may inherit that project.

. Unless you are aiming for all-out performance and aren't worried about reliability, I suggest you add current limiting. Ramping is a good idea, also. Heat is the #1 enemy of motors (and most other electrical components, including the controller). . For proof-of-concept, CL is probably not that important. For everyday use, it is.

. No need for a 1000A sensor. . Ever hear of a CT (current transformer)? . If you know the load resistance, computing the current from the applied voltage is trivial.

. Oops. I don't think a CT will work well with PWM.