Energy saving start-up method by combined use of two position sensorless controls

This paper proposes a start-up method for reducing electrical energy at starting periods. Minimum current startup is achieved by combining two position sensorless control methods that require no harmonic injection. Position sensorless control for low-speed regions utilizes the induced voltage caused by the magnetic saturation of a permanent magnet synchronous motor with conventional block commutation drive. At acceleration, the position sensorless control is changed to the second method based on back-EMF with sinusoidal commutation drive. This paper also addresses when to change the control, as unconsidered switching may result in start-up failure when used with applications that have periodic load properties. A simple yet powerful method to identify the most suitable timing considering load variation is proposed. The proposed methods are validated using a reciprocating compressor with a large periodic load torque.