# Effect of PWM control on Torque



## pyrogaz (Jan 3, 2010)

Could someone tell me what the effect of controlling a PM motor with a PWM controller has on torque. For instance, if the controller is at 50% duty and the motor is stalled should it produce 50% of the torque it would produce if at 100% duty?

Reason I ask is I'm writing a gearing calculator for an electric endurance racer and the real world data we're using to verify the simulator's output tends to indicate much better performance than I would have expected for the PWM controlled races compared to races where a simple relay was used for the motor.


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## major (Apr 4, 2008)

pyrogaz said:


> Could someone tell me what the effect of controlling a PM motor with a PWM controller has on torque. For instance, if the controller is at 50% duty and the motor is stalled should it produce 50% of the torque it would produce if at 100% duty?


Hi pyro,

The torque from the PM motor is proportional to the current, regardless of voltage and RPM (neglecting rotational losses, which are zero at stall). And PWM has little effect on it, except for controlling the voltage, obviously. So if your 50% point applies half the voltage to the motor, it will draw half the current and produce half the torque, at stall.

major


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## pyrogaz (Jan 3, 2010)

Thanks Major. Looks like this is going to be more difficult than I'd hoped, in the simulation current is calculated as a result of vehicle speed derived from the various aero, rolling and drive forces. This works where there's no motor control but presents a bit of a "chicken and egg" situation with a controller! Ah well, always one for a challenge


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