The single phasing of three phase induction motors forms the most common hazard to motor operation, because of the possibility of overheating and burning-out of the motor under this condition.
Causes of Single Phasing of Induction Motor
The single phasing of induction motor can occur because of causes external to the motor which are
- a blown fuse on the transformer primary
- a blown fuse on the motor branch circuit
- a loose or poor connection in the switch or starter
- a loose or broken lead.
It can also occur on account of causes within the motor itself like badly soldered winding connections or an ‘open’ within the winding.
Effects of Single Phasing in Induction Motor
If single-phasing occurs when the motor is not working, the motor will fail to start. It will merely give a humming sound. Current drawn by it under this condition will be 87% of the starting current under the normal condition.
If single-phasing occurs when the motor is working, it will continue to run with a peculiarly squeaking noise until it is isolated by the over-load device or the single-phase preventing device. Current distribution during single-phase operation, if the motor continues to deliver the same power, will be as below:
If the motor is star connected, line and phase current will be √3I, where ‘I’ is the current under 3-phase balanced operation. Current will flow through two phases only.
If the motor is delta connected, current distribution will be as shown in Figure. The line current is √3I with the heavily loaded phase carrying I(2/√3), which is twice the phase current under healthy operating conditions as normal phase current is equal to I/√3.
In the case of a ‘delta/delta’ or ‘star/star’ transformer, the effect of blowing of a fuse on the primary side is the same as the opening of the secondary circuit.
In the case of a ‘delta/star’ transformer, the heavily loaded line carries, twice the normal current. In actual practice, the current distribution may be somewhat different in some cases.
Protection from Single Phasing of Induction Motor
The protection against overloads is normally provided by thermal type overload relays. These relays provide adequate protection to motors if the setting of relays is related to the load conditions because in that case, the relays can sense the overload and isolate the motor before any damage to the insulation can occur as a result of overheating.
The most critical condition occurs where a motor is loaded between 40 to 60% because below 40% load there is no overloading and the motor can deliver the load without damage till single-phasing is detected.
For loads above 60% of the rated load, the rise in the line current is such that the overload relay can take notice of the over-current even if the relay is set for 100% current. In the range of 40 – 60% load, protection by overload relays will be provided if their setting corresponds to the load on the motor.
Where motors above 15 KW may have to work for long periods on part loads of 40 – 60%, additional protection by single-phase preventers, which trip the starter on any serious imbalance in the currents of three-phases will be necessary.
In other cases, protection by thermal relays is more than adequate. In motors below 15 KW, the designs are such that no serious over-heating of the winding takes place even in the range of 40 – 60% load.
The occurrence of single phasing of induction motor can be minimized or eliminated by proper maintenance. The use of re-wirable type fuses is not recommended for motor circuits because of their unreliable operation.
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