Exam Question
Briefly explain the Principle of Operation of Induction Motors. (4)
(a) What is Slip for an Induction motor? (4)
(b) Draw a simple ladder logic diagram of star delta starting of an Induction motor. (8)
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📅 Oct 2025
Reference Answer
(a) answer will be here soon :)
(b) Slip can be defined as the difference between the flux speed (Ns) & the rotor speed (Nr). Speed of the rotor of an induction motor is always less than its synchronous speed. It is generally expressed as a percentage of the synchronous speed (Ns) & represented by the symbol ‘S’.
% Slip= Ns – Nr/Ns X 100
(c) In operation, the main contactor and the star contactor are closed initially and then after a period of time, the star contactor is opened, and then the delta contactor is closed. The control of the contractors is by the timer built into the starter. The star and delta are electrically interlocked and preferably mechanically interlocked as well. The control circuit is provided with a manual stop button and a thermal overload contact to shut down the motor.
(b) Slip can be defined as the difference between the flux speed (Ns) & the rotor speed (Nr). Speed of the rotor of an induction motor is always less than its synchronous speed. It is generally expressed as a percentage of the synchronous speed (Ns) & represented by the symbol ‘S’.
% Slip= Ns – Nr/Ns X 100
(c) In operation, the main contactor and the star contactor are closed initially and then after a period of time, the star contactor is opened, and then the delta contactor is closed. The control of the contractors is by the timer built into the starter. The star and delta are electrically interlocked and preferably mechanically interlocked as well. The control circuit is provided with a manual stop button and a thermal overload contact to shut down the motor.
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