ISA Certified Automation Professional (CAP) program

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 n_s is the stator electrical speed and n_r is the rotor's mechanical speed, the slip, S, is defined by:

S = (n_s - n_r) / n_s

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.