Variable-frequency drives (VFD) allow sophisticated control by AC motors by allowing their speed and torque to be precisely adjusted. Control is achieved by varying the frequency to the motor. It does not, however, send a pure sine-wave frequency to the motor. Rather it sends a series of pulses, whose width varies, to the motor. This technique—pulse-width modulation—supplies the drive with the same power profile as a sine-wave frequency. PWM also allows the effective voltage to the motor to be varied.