3.6
Most induction motors in industrial and household use operate in an uncontrolled manner, being supplied directly from the power system. The motor speed is roughly constant, but the developed torque is sensitive to changes in the stator voltage. In many applications, significant energy savings could be realized by replacing the uncontrolled drive with an adjustable-speed one.
Motors with difficult starting conditions require means for assisted starting so that the motor does not overheat. Autotransformers, series impedances, wye-delta switches, or electronic soft-starters are commonly employed.
For assisted braking,
when the load torque alone is insufficient to quickly stop the motor, plugging or dynamic braking can be applied. Plugging, consisting in the reversal of magnetic field in the motor by changing the phase sequence, is a harsh operation because the accompanying energy losses in the rotor amount to three times the initial kinetic energy of the drive. Moreover, the braking torque in most motors is relatively low. Dynamic braking, utilizing a dc stator current to produce a stationary field, offers better operating conditions, but it requires a step-down transformer and a rectifier. Prolonged stopping can be realized using soft-starters. Reversing an uncontrolled drive involves stopping and restarting. Plugging can be used for motors running light. The gear train itself can be of a reversible type.
Pole changing allows two or more different synchronous speeds in specially constructed motors. The stator windings are so arranged that they can be switched into configurations producing various patterns of the magnetic field.
Abnormal operating conditions of induction motor drives can be of internal or external origin. Motor faults can be electrical or mechanical. Most common faults are short circuits in the stator, cracked rotor bars, and bearing failures. Poor quality of the supply voltage, phase loss, and mechanical overloads are the most common external causes of aggravated operation of induction motors.
