• Certification Review
ISA Certified Automation Professional (CAP) program

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

CAP question

"Slip" in an AC induction motor is defined as:

A. synchronous speed minus no load speed
B. difference between speed of stator field and rotor speed
C. rated speed plus synchronous speed
D. speed at which motor develops torque

CAP Answer

The correct answer is B, "difference between speed of stator field and rotor speed." Slip is usually expressed as a percentage, and varies by motor, from nominally 0.5 percent for very large motors to about 5 percent for small, specialized motors. If ns is the stator electrical speed and nr is the rotor's mechanical speed, the slip, S, is defined by:

S = (ns - nr) / ns

Motor rotation is developed in an AC induction motor through the effects of a moving magnetic field. As the speed of the rotor drops below the stator speed, or synchronous speed, the rotation rate of the magnetic field in the rotor increases, inducing more current in the rotor's windings and creating more torque.

Slip is required to produce torque. Under load, the rotor speed drops, and the slip increases enough to create sufficient additional torque to turn the load. A very efficient way to control slip is to use a variable frequency drive.

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

Reference: Dixon, P. (2016, September). "The velocity of PID," Control Engineering.

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