I have often heard about problems with low resistance and low inductance motor windings, but I don't understand why they cannot be dealt with. The peak current of each PWM pulse before saturation depends on the winding inductance, applied voltage, and time, so I can see where a low voltage motor driven from a high voltage source could quickly see current rise to the point of saturation, at which point only winding resistance (and the rest of the circuit) limits the current.
The construction and operating conditions of the motor may have some effect on the inductance. If the gap between the rotor and the stator is wide, the inductance will be lower, so for a rewound motor it may be important to choose one that has a tight fit. I would think such close spacing would also provide higher torque, but it may also contribute to friction at higher speeds due to the "windage" or resistance of air. Then again, it may act as an "air bearing".
The effective inductance is also influenced by the coupling of the stator to the rotor, and the inductance and resistance of the rotor squirrel cage will be reflected as a complex impedance at the motor's windings.
In any case, however, there should be a practical lower limit to the possible inductance and resistance that can be presented to the controller by a motor and its connections, and the controller also should be able to withstand a short circuit. For that to be possible, there may need to be internal resistance and inductance that limit the di/dt of the current as well as its maximum value so that internal sensors have time to shut down the system before damage is done.
Adding inductance to the motor lead connections, along with some capacitance, may be able to filter out the PWM carrier frequency to some extent and improve efficiency as well as reduce radiated noise and losses in the cables, motor windings, and bearings. I don't think such inductors would need to be very large or costly. Probably some ferrite, powdered iron, or even tape steel toroids, with a few turns of wire, would be sufficient.
Another idea I had was to make an adjustable voltage bus so that, at low speeds, a higher PWM value could be used, and a much lower dV/dt would be imposed on the motor windings. This could be accomplished with a simple buck converter front end that could reduce the bus voltage from, say, 350 VDC, to 180 VDC or even 90 VDC. The converter need not handle the maximum power requirement, as that could be accomplished by bypassing it entirely with a power relay. Another alternative would be a relay matrix that could connect two or four battery packs in combinations of series and parallel depending on need. Transition may be difficult, but there may be ways to smooth out the doubling or halving of the bus voltage.
This might be a topic for a separate discussion, if anyone thinks it may have merit. I think I have seen articles on something similar, and I may have even proposed it elsewhere some time ago. It may or may not be practical or even feasible, but it may be interesting to talk about.
