PWM (Pulse Width Modulation) is used to control electric power inside the motor coil.
The output power is controlled by repeatedly turning the output ON and OFF.
Constant voltage operates the motor with the constant period of the pulses.
The resulting voltage in the motor coil depends on the width of the voltage pulses.
It utilizes the semiconductor feature that ON/OFF states have the least loss while the intermediate state has the most loss.
PWM control is widely used in inverter circuits because of its excellent controllability and high efficiency.
It is also used to control brushed DC motors.
In the inverter circuit, AC voltage of a sine-wave that is optimal for driving the motor can be created by periodically changing the ON state (duty) of PWM control.