ISA Certified Automation Professional (CAP) program

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Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.

CAP question

The function of the rotor in an AC induction motor is to: 

A. reduce speed
B. reverse the direction of the motor
C. produce torque
D. create a magnetic field

CAP Answer

The correct answer is C, "produce torque." An alternating current is passed through the stationary windings (stator) of an AC motor. The stator windings are wrapped around an iron core, thereby creating a rotating magnetic flux. The flux generates a magnetic field in the air gap between the stator and the rotor and induces a voltage, which produces current through the rotor bars. This creates a magnetic flux in the rotor, opposite to that of the stator. Because opposite magnetic fields repel one another, the rotor begin to rotate, imparting a force (torque) on the motor shaft.

Reference: Trevathan, Vernon L., A Guide to the Automation Body of Knowledge, Second Edition, ISA, 2006.


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