I should really post about this on the EVDL or evtech, but I lack the energy at the moment and besides, I’d like to get the idea kicked apart on my journal first.
It seems to me that you could build a very cheap, very powerful drive for a EV that had a lot of the advantages of a DC drive combined with a lot of the advantages of a AC drive by having a standard DC series motor which did not have the commutator mechanically connected to the shaft – instead, the comm would be driven by a stepper motor controlled by a microcontroller which would have a rotary encoder to tell it the current position of the main drive shaft (the output from the ‘power stage’ of the motor), the current position of the commutator, whether any arcing was occuring, etc.
Maybe I misunderstand some of the theory, but I think that such a motor would be able to do regen braking fairly easily, would not fail in a full-throttle on mode (because the only way it produces motive power is if the comm is staying synchronous with the main output shaft, which requires that the microprocesser be doing its job), and that you would be able to get considerably higher power drives in considerably smaller spaces because you would only need a single H-bridge of power silicon to control power to the motor instead of the three H-bridges that a AC drive requires. However, you would still be able to get AC’s flat torque curve because you would in essence have the same amount of control over the magnetic state of the motor that a AC drive has.
Anyone want to kick a hole in it? Yes, it has brushes, which are mechanical and therefore frowned upon. However, I think it could be made simpler and cheaper than a ‘real’ AC drive, and aside from needing to swap the brushes every few hundred thousand miles – which a sensor could tell the micro about – it would have all the advantages of ‘real’ AC.
My next challenge on this vein of thinking is to mentally walk through all the failure modes – i.e. what happens when the comm gets energized in the wrong position -out of sync – with the current shaft position of the rotor..