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EK-General : 2021/DEC/10
Question
Q10. with reference to a.c. induction motors:
(a) state, with reasons, the type of fuses used for protection; (2)
(b) explain the effects of single phasing; (3)
(c) describe how thermistors can be used to protect the motor; (3)
(d) state TWO types of relay that provide electrical overload protection. (2)
Asked in Year/Month/Ques No.
Synopsis
>> Image for ‘a’ is not mandatory, if you have time you may draw a free hand sketch.
>> ‘b’ you may find it large but I have tried to explain you so you can reproduce answer in your own words.
Marker's Feedback
>> About 50% of candidates did well with this question and the rest did not.
(a) The answer to this is HRC fuse, which not that many got.
(b) It is the effects of single phasing on the motor, not what happens if you ignore it. There were all sorts of answers about the motor bursting into flames or people getting electrocuted. I just want to know that if you walked past a motor that was single phasing what would it tell you? Noise, vibration, slowing down and if you press the stop button it will not restart. If you ignore single phasing, yes it can go on to bigger problems but that was not what was asked.
(c) A number of candidates were confusing thermistors with bi-metallic strips, saying they expanded under the influence of heat and seemed unaware of their location within the motor.