Exam Question
(a) What is single phasing of a 3 ph induction motor? (6)
(b) What are the causes of single phasing? (6)
(c) Explain any method of protection against single phasing (4)
Class 4 (4th Engineer)
MET
📅 May 2023
Reference Answer
### (a) What is Single Phasing of a 3-Phase Induction Motor?
Single phasing is a fault condition in a three-phase electrical system where one of the three phases supplying power to a motor is lost or disconnected. While the other two phases remain energized, the motor is subjected to an unbalanced power supply, leading to severe operational abnormalities and potential damage.
**Consequences and Electrical Principles:**
1. **Collapse of Rotating Magnetic Field (RMF):** A balanced three-phase supply creates a constant-magnitude magnetic field rotating at synchronous speed within the stator. This RMF induces currents in the rotor, generating torque. When one phase is lost, the RMF collapses and is replaced by a pulsating or oscillating magnetic field. This field is incapable of producing a net starting torque.
Single phasing is a fault condition in a three-phase electrical system where one of the three phases supplying power to a motor is lost or disconnected. While the other two phases remain energized, the motor is subjected to an unbalanced power supply, leading to severe operational abnormalities and potential damage.
**Consequences and Electrical Principles:**
1. **Collapse of Rotating Magnetic Field (RMF):** A balanced three-phase supply creates a constant-magnitude magnetic field rotating at synchronous speed within the stator. This RMF induces currents in the rotor, generating torque. When one phase is lost, the RMF collapses and is replaced by a pulsating or oscillating magnetic field. This field is incapable of producing a net starting torque.
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