Faults that can occur in 3 phase induction motors and associated protections are discussed below:
Stator faults:
These faults include phase to phase faults, earth faults, and inter turn faults. Faults in motor windings are generally occur due to failure of insulation which causes due to the excess heating in the windings of the machine. Phase to phase faults are rare because enough insulation is provided between the phases. Earth faults are more likely occur in induction motors. Inter turn fault (phase to phase fault) if occurs lead to earth fault. Hence it is sufficient to provide protection for the earth faults
Rotor faults:
These faults are more likely occur in wound rotor machine. Faults on wound rotors may be either earth faults or inter turn faults which occurs due to severe mechanical and thermal stresses
Other faults which occurs on the rotor of the motor includes failure of bearings and faults in the starters and associated circuits
Abnormal conditions:
Some of the abnormal conditions that are likely occur are:
Prolonged over loading protection:
It is caused by excessive mechanical loading, short time cyclic over loading. This causes excessive rise of temperature of winding and deterioration of insulation resulting in winding fault. Hence over load protection should be provided in electrical machines irrespective of the size, rating of the motor and type of load driving
Stalling protection:
Due to mechanical problem or overload during the period of starting motor may stall and refuse to start. During stalling motor draws huge current. Hence it is not desirable and immediate isolation of the motor from supply mains is essential
Unbalanced supply voltage protection:
The unbalanced three phase supply causes negative sequence currents to flow in the motor that is likely to cause over heating of the motor
Single Phasing protection:
Single phasing in motors cause when one of the supply lines gets disconnected. They may be occurred due to blowing of fuse or open circuit in one of the three phase connection. In such condition motor continuous to operate as single phase induction motor provided that load does not exceed 57.7% of the normal rating.
Single phasing may cause extreme magnetic unbalance, reduction in torque and over heating due to negative phase sequence currents. Operating under this mode lead to damage to the motor and therefore protection is provided for single phasing of the motor
Under Voltage protection:
The under voltage supply condition will lead to drawing more current from the mains for a given load. Hence under voltage protection is also provided for induction motor
Reverse Phase protection:
The direction of poly phase induction motor is reversed if the phase sequence is changed. In some motor applications such protection may be required feature