ABNORMAL OPERATING CONDITIONS (Induction Motor)

3.5
Abnormal operation of an induction motor drive may be caused by internal or external problems. The most common electrical and mechanical faults in the motor are:
Pole changing: (a) four-pole stator winding,(b) eight-pole stator winding.
FIGURE 3.6 Pole changing: (a) four-pole stator winding, (b) eight-pole stator winding.
1. Short circuit in the stator winding, which can occur between turns of the same phase (interturn fault), between different phases (interphase fault), or between a phase winding and ground (ground fault). Serious stator faults cause the overcurrent protection circuits to react immediately, but minor faults take time to spread.
2. Cracked rotor bars, resulting from frequent thermal and mechanical stresses, for instance in often-started motors. The cracking usually occurs at the junction with the end ring. The damage to the rotor reduces the torque of the motor and introduces low-frequency harmonic torques. Healthy bars must then carry an increased load, so they are likely eventually to crack too.
3. Bearing failures, caused by wear and accelerated by such mechanical imperfections as rotor unbalance and eccentricity or misalignment of the motor and load shafts.
Interestingly, the incidence of stator and bearing faults in induction motors in adjustable-speed drive systems has been found to be significantly higher than that in uncontrolled drives. It turns out that the switching operation of power electronic inverters supplying the motors in variable-frequency drives causes increased voltage stresses on stator insulation and, in certain cases, microsparking in the bearings.
External factors that may cause abnormal operation of the induction motor are:
1. Poor voltage quality, such as sags or unbalance. As already explained in Section 2.4, voltage sags result in quadratic reduction of the developed torque, so that the motor may stall. Voltage unbalance produces harmonic torques, and it increases losses in the motor. High losses and poor power factor also occur when the stator voltage is too high. Generally, the voltage is considered to be of good quality when it does not strain from the rated value by more than ± 10%, from the rated frequency by more than ±5%, and from ideal balance by more than ±2%.
2. Phase loss, typically resulting from the action of protection relays in the power system or fuses in the supply line. Basically, the induction motor can run on two phases, albeit with a significantly reduced torque and increased stator current. Eventually, thermal overload relays will trip the circuit breaker and disconnect the motor from the line.
3. Mechanical overload, which may cause overheating or even stalling the motor. Although induction motors have a significant torque margin (see Figure 2.18), prolonged operation with
overload is hazardous and prevented by the already mentioned thermal overload relays.


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