Motor system capable of obtaining high efficiency and method for controlling a motor

摘要

In a brushless DC motor system, an RMS value of an input current of a voltage-fed inverter is detected by a current transformer and an RMS value detector circuit. A rotation speed of a brushless DC motor is detected based on a cycle of a position signal outputted by a position sensor circuit. Then, in response to the RMS value of the input current and the rotation speed, the phase of the inverter output voltage relative to the phase of the motor counter-electromotive voltage is set to a phase at which the motor efficiency comes generally to a peak, and a switching command to the voltage-fed inverter is generated. With this constitution, when the applied voltage waveform is controlled only by the voltage-fed inverter in response to the rotor-position sensor signal, the motor attains high-efficiency operation without involving any increase in cost and yet regardless of load conditions.