1. Prolonged overloading:
Generally, all the motors are designed to withstand 15 to 20% overloading for short time.
But prolonged overloading causes an increase in motor temperature and which may further damage the insulation; resulting in winding faults.
2. Stalling:
Due to excessive overloading at the start or due to failure of bearings or due to some mechanical problem motor refuses to start.
This situation is very dangerous since the motor draws heavy current from the supply and may burn.
In fact, this situation is blocked rotor with stater is supplied by rated voltage. i.e. similar to a three-phase short circuit on the secondary side of the transformer when the primary is supplied with rated voltage.
3. Unbalanced supply voltage :
Unbalanced voltage mostly occurs from the supply system but sometimes it may also occur due to blowing of the fuse of any one phase.
The unbalanced voltage causes a negative sequence current to flow in the motor that is likely to cause overheating of the motor.
4. Single phasing:
When one of the supply lines gets disconnected from the three-phase supply connection then this situation is known as single phasing.
Single phasing occurs mainly due to rupturing of a fuse or open circuit in one of the supply lines.
Under these conditions, the motor continues to operate on a single-phase supply.
If the motor is loaded to its rated full load, it will draw excessive current on a single phasing.
Single phasing may cause extreme magnetic unbalance, reduction in torque, and overheating due to negative phase sequence currents.
This condition may cause damage to the motor therefore, protection against single phasing is necessary.
5. Under voltage :
Under voltage supply will cause an increase in motor current for the same load condition.
6. Reverse phase :
The direction of rotation of poly phase Induction motors is mainly dependent on phase sequence. If the phase sequence is changed motor runs in the reverse direction.
In some critical applications, the direction of motor rotation is very important. Therefore, in such cases protection against the reverse phase is important.
7. Stater earth faults and phase to phase faults:
Stater winding faults are mainly caused due to failure of insulation due to temperature rise.
8. Rotor faults:
Rotor faults are very rare and mainly occur if the rotor is wound (slip ring) type. Generally, no separate protection is provided for rotor faults.
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