Variable impedance for starting cage induction motors - uses thyristor control and introduces progressive short circuiting to adjust impedance level

摘要

The object is to build a starting circuit for very large cage type induction motors without using a large number of controlling semiconductors. In the starting mode the motor (9) is connected to an intermediate busbar (2) which is in turn connected to the mains (1) through a reactor with an air gap core (3) and two mutually coupled windings (4, 5). One winding (4) is in series between supply and motor whilst the other can be short circuited by two anti-parallel connected phase controlled thyristors (6). The series reactor winding (4) is designed to absorb about 25% of the vailable voltage when the other winding is open circuit. As the motor runs up the second winding is progressively short circuited for longer periods until the short circuit reactance is reached. At this point, the motor can be switched (10) directly onto the supply (1) and the starting circuits isolated (7, 8).