An inverter is a power supply to control a frequency [Hz] of a three-phase induction motor for variable speed. Since the rotation speed of an induction motor is determined by the number of poles and frequency, a speed control can be done by changing a frequency. A 4-pole induction motor, for instance, constantly rotates 1500min^-1 at 50Hz and 1800min^-1 at 60Hz. By mounting an inverter between a power supply and motor, the speed of motor can be optionally changed regardless of the frequency of power supply.