Single Phasing of Three Phase Induction Motor

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Electrical Motor is an important component of an industry. Squirrel cage induction motor is most widely used in power stations and industries. To protect the motor from different faults conditions various protections are provided. Single Phasing is the most common problem found in three phase induction motor.

Single Phasing Condition:

when one of the supply fuse of a three-phase motor blows off or a terminal connection comes out, single phasing at the motor may occur. In such case, motor may continue to rotate, but the two healthy phases may draw high current leading to thermal stresses on the insulation.

Causes of Single Phasing:

There are many reasons for single phasing to occur.Single phasing means opening of any phase of a three phase supply system. This can occur on either the primary side or secondary side of a distribution transformer.

Single-Phasing on Transformer Primary – Causes

1. Opening of Primary phase conductor of the transformer due to any reason.
2. Open winding in any phase of the transformer.
3. Primary fuse open.
4. Failure of circuit breaker automatic recloser to close all the three phases at one time.
5. Defective Contact in any of the primary side breaker or switches.

Single-Phasing on Transformer Secondary – Causes

1. Open winding in motor.
2. Open winding in one phase of transformer secondary.
3. Any open circuit in any phase anywhere between the secondary of the transformer and the motor.
4. Open fuse or open pole in circuit breaker on main, feeder, or motor branch circuit.
5. Open cable anywhere between transformer secondary and motor terminals.
6. Any loose connection present inside the motor terminal block.
7. Damage motor starter contact or Breaker.

Effects of Single Phasing of Three Phase Induction Motor:

Single phasing represents the worst case of an unbalance voltage condition the effects are similar to the unbalanced voltage condition. Due to single phasing the induction motor performance severely effected. The following are the effects of single phasing.

• The motor draws more current from remaining two supply lines. Approximately twice the earlier current.
• The torque and speed characteristic is seriously affected when a 3-phase motor operates on a single phase. Torque developed is reduced and the motor will continue to run if the load is less.
• There will be severe vibrations.
• Phase windings will get heated.
• The breakdown torque decrease to about 40% of its original value, and the motor develops no starting torque at all.
• Besides, the Negative Phase sequence (I₂) component of the unbalanced current, produces a reverse reaction field which cuts the Rotor iron and winding at approximately double the speed, thereby inducing double frequency eddy currents, causing over heating of the rotor.
• Overheating of the motor damages the windings hence decrease the efficiency and life of the motor.
• The excess heating of rotor and stator will give noise, heat and smoke.

Action Needed for Single Phasing of Induction Motor :

• Single phasing is a hazardous condition and steps should be taken to de-energize the motor.
• After single phasing also the three phase induction motor can continue to run, and the motor will get heated quickly. It is essential the motor should be disconnected from the service by opening of its power supply circuit anyway.
• The thermal relays, if provided, will protect the motor from overheating but they are unable to detect rotor heating due to unbalanced currents. To detect this phase failure relay (single phasing relay) should be connected in the supply circuit.
• The protection device should able to detect the resulting increase in current and respond quickly to save the motor from burning.

Single Phasing Protection Using Relays:

• For H.T Motors:

(I₂) based single phasing protection having Inverse or definite time delay is used to protect the motor against this eventuality. This is called Negative Phase Sequence motor protection provided with Numerical Relays. It is also called as Phase Loss protection.

• For L.T Motors:

For small L.T Motors, single phasing preventor (unbalanced voltage V₂ or current I₂ based) is used to detect single phasing and isolate the defaulting Motor. Phase failure relay connected in the supply circuit detects the single phasing condition and trips the circuit breaker or contactor in the motor control circuit.