Minimum Current Start-Up Method by Combined Use of Two Position-Sensorless Controls

A start-up method for reducing the start-up current of a permanent-magnet synchronous motor (PMSM) is proposed. Minimum current start-up is achieved by combining two position-sensorless control methods that require no harmonic injection. Position-sensorless control for low-speed regions utilizes induced voltage caused by the magnetic saturation of a PMSM with a conventional block commutation drive. During acceleration of the rotor, the position-sensorless control is switched to the second method, which is based on back electromotive force with a sinusoidal commutation drive. Since unconsidered switching may result in start-up failure when the proposed start-up method is used in applications with periodic load properties, a method to determine the correct timing to change the position-sensorless control in consideration of load variation is also proposed. The proposed methods are validated in start-up tests using a reciprocating compressor with a large periodic load torque.