PWM is not used for TRIACs driving motors. It would require a much lower PWM frequency, than the mains frequency
Phase control is used, which is a totally different thing. A TRIAC is a latching switch and only needs a brief pulse at the gate to turn on, then it will remain on, until the AC cycle passes through zero. Phase control works by delaying the gate trigger pulse, for a certain length of time after the zero crossing point. The greater the delay, the lower the power.
What sort of motor is it? How is it being used?
Phase control is only really good for brushed universal motors. It it not suitable for induction motors, unless they're used to drive a light load, such as a fan. A variable speed drive, is the correct way to control the speed of an induction motor.