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

Which of the following statements correctly defines the relationship between the shaft speed of an AC squirrel cage motor, the frequency of the power input, and the number of poles for which the motor is wound, assuming negligible slip?

A. The shaft speed is directly proportional to the number of poles and does not depend on the motor frequency.

B. The shaft speed is directly proportional to the frequency and does not depend on the number of poles.

C. The shaft speed is directly proportional to the frequency and is inversely proportional to the number of poles.

D. The shaft speed is inversely proportional to the frequency and is directly proportional to the number of poles.

CAP answer

The correct answer is C, the shaft speed is directly proportional to the frequency and is inversely proportional to the number of poles. The shaft speed is related to frequency and the number of poles by the following equation: N = [(120 x F) / P] - Slip

where,
N = shaft speed (RPM)
F = frequency of the power supply (Hz)
P = number of stator poles (pole pairs)

Assuming no "slip," if the power supply frequency increases (assuming the number of poles remains the same), the shaft speed will increase (directly proportional).  For the same motor, as the number of stator poles is increased (assuming a constant power supply frequency), the shaft speed will decrease (inversely proportional). 

Reference: Dave Polka; Motors and Drives: A Practical Technology Guide, ISA Press, 2003.