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Originally Posted by P-hack
You would probably have to sacrifice FOC with one controller and multiple motors that aren't bolted together  , |
With FOC you track frequency, but it does not appear to be part of the control strategy, per se, after you start rolling (maybe 3 or 4 Hz). As far as I can see, you control Id and Iq. The frequency is adjusted to generate the torque. You keep track of the frequency and use that to limit rpm.
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which doesn't look like a deal breaker, since you can adjust your driving style (FOC seems most helpful in lighter loads AFAIKT and seems to lose any advantage at heavier loads, you can regain some starting torque with slip tuning).
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Hmm. As I understand it, running without FOC (I call it Volts per Hertz, or V/Hz), is not as efficient. In FOC, the voltage used for a given frequency is a function of the torque required at that frequency. So the current required determines the voltage applied. Lightly loaded uses lower voltage. Heavily loaded boosts the voltage as required.
V/Hz is very simple but a bit wasteful. In the case of a constant torque load, like a car, you have a straight line from 0 speed to rated speed. An example helps. 1800 rpm rated at 60 Hz, 230V. 0 speed = 0V. 900 rpm = 115V. 1800 rpm = 230V. So if you are driving at 1800 rpm but are running at anything below maximum load (max load you should be accelerating) you are using too high a voltage and the motor has to dump this energy as heat.
If you are cruising at 60 mph, using 20 HP, and you have a motor that can do 30 HP at rated voltage .. you are wasting 50% of your battery capacity in the motor and you have to use even more energy to cool your motor and your controller.
FOC is quite important, IMHO. I know that our industrial controllers (VFDs or ASDs) - used at the day job - do not allow multiple motors for FOC. But they are not using encoder feedback. They are measuring back-EMF from the motor and determining rotor position with some space vector math.
If a workable theory - not the right name - workable set of rules? can be hammered out, I'd like to test multiple motors in parallel ( likely on a bench) and see if something can be empirically worked out to give all the motors some torque. Perhaps the slip of each motor would need to be characterized separately? We need to sum the required currents so that the controller knows what current, at what phase, to supply.
I have a couple of 5 HP, 230V, industrial motors that I can bench test with, should the opportunity arise.
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Haven't completely thought through which sensor to use for slip, maybe average with individual limits? I don't know of a good way to make the slower wheel not have more torque than the faster, except to stop feeding the motors (or don't feed it much, just keep your speed up).
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Keep thinking - maybe we can all work out something!