ISA Certified Control Systems Technician (CCST) program

Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

What must be done to reverse the direction of a three-phase motor?

A. Turn off one phase
B. Put DC to the stator
C. Reverse any two input leads
D. Switch all three phases

CCST Answer

The correct answer is C,“ Reverse any two input leads." In a three-phase induction motor, reversing any two of the three wires will change the rotation direction.

The windings in a three-phase motor, when activated by a three-phase supply, produce a rotating magnetic field in the rotor of the motor. Swapping phase A with phase B reorders the fluxes so that the flux rotates in the opposite direction. Swapping B with C does exactly the same thing, so does swapping A with C.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.

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