Adjustable Speed Drive with a Brushless DC Motor

The brushless dc motor described consists of a synchronous motor with a rotor position sensor and a three-phase bridge inverter. The commutation in the inverter circuit is performed by the induced voltage of the armature windings. In this situation, since auxiliary commutation devices such as capacitors, inductors, and auxiliary thyristors are not required, the simplest inverter circuit is obtained. However, the commutation ability decreases at low speed and starting difficulty results. A novel starting procedure is described whereby the chopping of the rectifier output voltage is accomplished by control of the delay angle. In order to obtain a rapid interruption of the main circuit current, an auxiliary thyristor antiparalleled to the smoothing reactor is used. Finally, a new method of speed sensing that eliminates the need of a tachogenerator is proposed. Speed stabilization with an accuracy about 5 percent in a speed range of 10:1 is perforned with this new control system.