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

Why must voltage be reduced along with frequency in a variable frequency speed controller?

A. to let the motor cool off
B. because of capacitive reactance
C. to maintain the volt/hertz ratio
D. to keep the motor from over speeding

CAP Answer

The correct answer is C, "to maintain the volt/hertz ratio. To maintain a constant (rated) flux density, the applied voltage must also be changed in the same proportion as the frequency (per Faraday's law).

Varying the frequency affects both the motor speed and the strength of the magnetic field. When the frequency is lowered (to obtain a slower motor speed), the magnetic field increases, and excessive heat is generated. When the frequency is increased (to obtain a higher motor speed), the magnetic field decreases, and lower torque is produced. In order to keep the magnetic flux constant, the V/Hz ratio must remain constant. This keeps torque production stable, regardless of frequency.

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

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