Abstract
Induction motors have widely been used in industrial factories due to their simplicity of design and operation, low cost of maintenance. The most common approach for starting them is a direct-on-line (DOL) method. However, it produces a large transient torque, causing a shortened lifetime of mechanical rotating parts of the motor, driven load, and gearbox. Therefore, this paper presents a generalized zero-crossing switching approach to reduce an unnecessary peak torque amplitude during the DOL start. Unlike the conventional DOL technique, the proposed method requires zero-crossing detection. When a random closing command is initiated, one of the three-phase voltages that go to zero first is regarded as a referenced voltage for connecting a pair of the remaining two voltages simultaneously at the exact instant when it crosses the zero point. The motor is initially started under two-phase operating mode. After a given ½ cycle of delay time is expired, the referenced voltage is subsequently supplied for a continued operation in full three-phase mode. The experimental and simulation results confirm that the proposed technique can help to alleviate the starting transient torque peak arising from the serious drawback of the DOL starting.
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