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I am trying to understand the impact of a triac based speed control on the torque/speed curve of a motor. For as easy example, let's use a corded drill as an example. If I pull the trigger all the way the motor gets the full waveform and the torque/speed curve will look like a typical universal motor:

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(image source)

Now let's say I only squeeze the trigger down 50% so that the waveform now is chopped up by the triac. What happens to that curve?

Another way to ask the question is does a triac change the speed more, the torque more, or both in some proportion?

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With reduced voltage, the curves for all the motors except the AC induction motor will move downward with approximately an equal speed reduction for every speed. For the universal motor, that approximation may be a bit crude.

For the induction motor, reduced voltage will reduce the torque at every point approximately proportional to the per-unit voltage squared.

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