June 2009 Electrical Apparatus

June 2009 Electrical Apparatus

This is a summary of the Electrical Apparatus June 2009 featured technical article,  by Richard L. Nailen, P.E.  

Despite the simplicity and reliability of the polyphase induction motor, high inrush current limited its usefulness in the early years of weak power system capability. This led to the use of series resistance during starting, to reduce that current. As motor size increased, such resistors required high power dissipation capability, and the autotransformer, or “starting compensator,” became popular.

An alternative was replacement of series resistance by series reactance. Although the solid-state electronic starter is now popular for motors under 375 kW, especially below 600 volts, the motor starting reactor is still used for many large machines and at voltages up to 11,000.

Although some starting reactors are of the air-core type, most contain iron cores because such cores provide higher reactance in a smaller space. One reactor is usually connected in each phase, bypassed by a contactor once the motor reaches full speed. Alternatively, reactors are inserted in the neutral side of a wye-connected stator winding. This permits a lower level of reactor insulation. A less-common scheme includes a single reactor in only one phase, resulting in phase voltage unbalance that reduces total kVA supplied to the accelerating motor.

Reactor starting always reduces motor accelerating torque. Voltage reduction in the reactor is not fixed, as with an autotransformer, but varies with the current. Since this current is determined by total circuit impedance, motor voltage during acceleration depends upon the relative reactor and motor impedances. Complete circuit analysis, and choice of reactor characteristics, involves the vectorial relationships among the circuit voltages and the current.

Reactor opposition to current flow, unlike series resistance, is not necessarily a constant for all values of current. That’s because of magnetic saturation in the reactor core. In some starters the reactors are “saturable”; that is, a separate control winding on the same core allows an external control voltage to vary the saturation and therefore the reactor impedance. Control voltage may be fed back directly or indirectly from the counter EMF of the accelerating motor.

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