A. Run as before
B. Overheated and eventually burn
C. Not start
D. Burn
Answer: B. Overheated and eventually burn
Explanation:
Single phasing in a three-phase machine means that one of the three phases of the supply has been cut off due to any reason, one phase fuse blown or removed, or disconnection somewhere in one phase.
The motor, when already running, keeps on running as a single-phase motor making a characteristic noise.
This is a single phasing condition because the current in both the remaining lines now is the same single current.
In star connected stator, the load 1s taken by the remaining two phases so the motor now can take a load of 1/√3 times, (57.7%) it’s rating.
Since now in single phasing operation, for the same current and voltage P = VI cosφ; while in 3-phase operation, P = √3VIcosφ.
So if the motor is operating at a load near full load, the active phases will be overloaded.
If the stator is delta connected which is usually the case, the current distribution in different phases is not the same.
One of the windings gets overloaded even when the current in the active lines is normal.
In a case when any of the three phases fails, in order to compensate the motor, starts drawing more amount of current heating the motor even in some cases could burn the motor.
In 3-φ synchronous motor, if one of the winding is short circuit then the motor will run with excessive vibrations.
More clearly it depends upon the load:
Less than 1/3rd of the rated load, it will continue to operate without any harm.
Higher than 1/3rd of rated load, the motor will continue to operate, but will draw current more than its rated value. The motor temperature will start increasing.
Higher load (near to the rated load), the motor speed will drop gradually to zero.
In this case, the thermal overload relay may stop the motor current or the fuse may blow. If thermal overload relay/fuses are not provided, the motor may get but burnt.