Benefits and Drawbacks of low Inrush Large Induction Squirrel Cage Electric Motors

Abstract

For their reliability and cost effectiveness, large squirrel cage induction motors are used in a wide range of Oil and Gas applications. Most are started direct-on-line and run at fixed speed. When a large motor starts, the inrush current can typically reach 5 to 7 times the full load value. If the power supply grid capacity is limited, starting a large motor can produce a voltage drop that will reduce the motor accelerating torque and potentially disrupt the operation of other equipment connected to the same power grid. This paper will compare alternatives to reduce the inrush of large electric motors with focus on the design and manufacturing aspects of low inrush squirrel cage induction motors. The benefits and restrictions of the proposed solutions will be explored and a relative economic analysis will be outlined. A real case will be analyzed where a 10.8MW, 11kV, 4- pole induction motor was designed to direct-on-line start and run an offshore centrifugal compressor with starting current inrush limited to 300% of full load current. Design and manufacturing challenges will be discussed and final test data will be presented.