(2) PWM inverter and induction motor
In the inverter-type air conditioners 3-phase induction motor is usually adopted for compressor drive while single-phase motor is used together with on-off relay switch in the conventional ones.
In on-off control the motor speed cannot be controlled precisely, but the accurate speed control can be made by PWM inverter with VVVF (variable-voltage variable- frequency) control. Also the input energy is reduced, and the efficiency is improved, and hence the whole control characteristics would be made better