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 volts/hertz ratio.
D. to keep the motor from overspeeding.

CAP Answer

The correct answer is C, "to maintain the volts/hertz ratio." The speed of the motor is controlled by changing the frequency applied to the motor:

RPM = (Frequency • 120) / (# of poles in the motor)

Varying the frequency affects both the motor speed and the strength of the magnetic field. When the frequency is lowered (slower motor speed), the magnetic field increases, and excessive heat is generated. When the frequency is increased (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. As frequency changes, we want to maintain a constant flux density to maintain the torque developed by the motor.

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

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