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Class 4 (4th Engineer) MET 📅 Jul 2023

Exam Question

(a) Explain the term single phasing as applied to poly phase induction motors. (4)

(b) State the likely causes of single phasing and the consequences if motors are not adequately protected (6)

(c) Describe with the aid of sketches THREE methods for motor protection should single phasing occur. (6)

Reference Answer

### (a) Explanation of Single Phasing
Single phasing is a fault condition in a three-phase electrical system where one of the three supply lines to a polyphase induction motor is lost or disconnected.
A three-phase induction motor's operation relies on the production of a Rotating Magnetic Field (RMF) in the stator. This RMF is generated by three alternating currents, phase-displaced by 120° from each other, flowing through three sets of stator windings. The RMF induces currents in the rotor bars, creating a torque that causes the rotor to turn.
When single phasing occurs, the effect on the motor depends on its state:

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