For proper working of any 3 phase induction motor it must be connected to a 3 phase alternating current (ac) power supply of rated voltage and load. Once these three phase motors are started they will continue to run even if one of the three phase supply lines gets disconnected. The loss of current through one of these phase supply is described as single phasing.
Effect of Single Phasing
The following are the effects of single phasing:
1) Due to single phasing the current in the remaining two phases increases and it is approximately 2.4 times the normal current value.
2) Single Phasing reduces the speed of the motor.
3) The motor becomes noisy and starts vibrating due to uneven torque produced in the motor.
4) If the motor is arranged for standby and automatic starting then the motor will not start, and if the overload relay provided fails to function then the motor may burn.
5) The windings will melt due to overheating and can give a fatal shock to the personnel.
6) It may cause overloading of the generator.
Causes of Single Phasing
Single Phasing is usually caused when:-
1) One of the three back up fuses blows (or fuse wire melts).
2) One of the contactor for motor is open circuited.
3) Single phasing might also be caused due to wrong setting of the protection devices provided on the motor.
4) Contactors are coated due to oxidation hence not conducting.
5) Relay contacts may be damaged or broken.
How to Protect Motor from Damage Due to Single Phasing?
All motors above 500 KW are to be provided with protection devices or equipment to prevent any damage due to single phasing.
The rule stated above does not apply to motors of the steering gear system installed on the ship. Only on the detection of the single phasing, an alarm will be sounded; however, the motor will not stop as it is essential for safety or propulsion of the ship.
The most commonly used protection devices for single phasing are:-
1) Electromagnetic Overload Device
In this device, all the three phases of the motor are fitted with an overload relay. If there is any increase in the value of the current then this relay activates automatically and the motor trips.
This device works on the principle of the electromagnetic effect produced due to the current.
As the current value increases, the electromagnet in the coil also increases which pulls the relay and activates the trip relay and the motor is stopped.
The time delay is provided in this system because during starting the motor draws a lot of current which can trip the motor.
Thermistors are small thermal devices which are used together with an electromagnetic overload relay. The thermistors are inserted in the three windings of the motor. Any increase in the current will cause heating in the windings, which is detected by the thermistors that send signals to the amplifier.
The amplifier is connected to the electromagnetic relay. As soon as a signal is received from thermistor about overheating, this amplifier increases the current value in the coil of an electromagnetic relay which activates the trip and the motor stops or trips.
3) Bi-metal strip
In this method, the bimetallic strip is placed in such a way that it detects the overheating in the circuit. As soon as overheating is detected, this bimetallic strip tries to expand due to two different metal used and because they have a different coefficient of expansion. The strip tries to bend towards the metal having a high coefficient of expansion and finally completes the trip circuit and the motor trips.
If you liked this article, you may also like to read How to Minimize Electrical Shock?