An inverter sets the speed by changing the frequency supplied to the motor. In an induction motor, synchronous speed = 120 × frequency ÷ number of poles; if the frequency is halved, the speed of the rotating magnetic field, and with it the motor speed, falls to roughly half.

The inverter (frequency converter) first rectifies the mains voltage to DC, then uses power transistors to produce a three-phase output at the required frequency. The output voltage is changed together with the frequency (V/f ratio). Below rated frequency this ratio is kept constant, so the magnetic flux stays roughly constant and the motor can deliver its torque.

Example

The 4-pole 7.5 kW 132M-4 motor has a synchronous speed of 1,500 rpm at 50 Hz and a rated speed of 1,460 rpm on the nameplate. Fed at 25 Hz, the same motor has a synchronous speed of 750 rpm; the actual speed is lower by the slip caused by the load. This is an approximate calculation; the exact value depends on the load and the inverter settings.

Vector-controlled inverters can compensate for slip so that speed is less affected by load. Which control mode is needed depends on how precise the application must be.