Faults in Induction and Synchronous Motors
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2.2.1  Damper Winding Fault To produce torque in synchronous machines, the rotor must be turning at synchronous speed, which is the speed of the stator field. At any other speed, the rotating field of stator poles will not be synchronized with rotor poles, but first attracts, and then repels them. This condition produces no average torque and the machine will not start. Using a direct current (DC) motor or a damper winding, the machine can be brought near to the synchronous speed. Damper windings, as shown in Figure 2.5, consist of heavy copper bars, with the two ends shorted together, installed in rotor slots. The currents induced in the bars interact by the rotating air-gap field and produces torque. In other words, the machine is started as an induction motor [33]. The field winding is excited by a direct current when the machine is brought up to the synchronous speed. When the load is suddenly changed, an oscillatory motion will be superimposed on the normal synchronous rotation of the shaft. The damper winding helps damp out these oscillations. Diagnostics of broken damper bars in synchronous machines has not been covered as widely as the other faults like eccentricity and inter-turn faults [34–37]. During transience, the electromagnetic behavior of asynchronous machines with damper winding is similar to that of an induction machine. During transient time, when the machine accelerates from zero speed to synchronous speed, a significant current flows in the damper winding. Excessive
FIGURE 2.5 A salient pole synchronous machine with damper winding and interrupted end-ring. (Courtesy of TECO-Westinghouse.)