Class 4 (4th Engineer) MET 📅 Dec 2022

Exam Question

(a) With reference to single phasing applied to a.c. motors: (10)

(i) Explain the meaning of single phasing;

(ii) Describe its effect

(iii) State the most common cause of single phasing.

(b) Sketch a simple diagram of a direct on-line starter, showing in detail the overload and single-phase protection trip. (6)

Reference Answer

### (a) Single Phasing in A.C. Motors
#### (i) Explanation of Single Phasing
Single phasing is a critical fault condition that occurs when one of the three phases of the supply to a three-phase AC induction motor is lost or disconnected. The motor, designed to operate on a balanced three-phase supply that creates a rotating magnetic field, is now forced to run on only two phases. This effectively results in a single-phase supply being applied across two stator windings, while the third winding remains un-energized.
This can be caused by an interruption anywhere between the power source and the motor terminals, such as:

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