# Differential effect of one controller controlling 2 motors?



## Tesseract (Sep 27, 2008)

Yes, this has been discussed here several times.

Assuming both motors are driven by one controller but each motor independently drives a wheel, the following summaries apply:

1. Two series DC motors wired in parallel will behave similarly to a limited-slip differential. This is because they are receiving the same effective voltage and so will split the available current as necessary. This is the preferred configuration for driving two independent motors from one controller.

2. However, two series DC motors wired in series will behave similarly to a conventional differential, except that if one wheel loses traction then its RPM will shoot up very high, potentially destroying the motor, while the other wheel will stall, also potentially destroying the motor.

3. PM field (brushed) DC motors can be driven in series and this is a preferred configuration, however, they should not be run in parallel, as they have an almost fixed relationship between voltage and RPM. In fact, you can think of them as behaving a lot like batteries, with terminal voltage proportional to RPM. Any difference in RPM between the two motors, then (or differences in internal parameters that cause the ratio of RPM/V to differ) will result in a current circulating between them. This is generally considered a bad thing.

4. AC induction motors can be driven in parallel by a single controller (aka - "inverter") but only in "open loop" V/Hz mode. Open loop control of an ACIM is bad because the maximum available torque suffers. In fact, you can get a higher peak torque out of an ACIM by driving it with an inverter of half the current rating but which employs closed-loop field-oriented control.

5. Multiple "Brushless DC" motors, which are really just AC motors with a PM field, can't be driven by a single inverter/controller regardless. The stator waveform is "locked" to the rotor position.

6. Other motor types are relatively rare in EVs and so I am not going to waste my time addressing them.


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