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AutoQuiz: Why Must Voltage Be Reduced Along with Frequency in a Variable Frequency Speed Controller?

 

AutoQuiz is edited by Joel Don, ISA's social media community manager.

 

This automation industry quiz question comes from the ISA Certified Automation Professional (CAP) certification program. ISA CAP certification provides a non-biased, third-party, objective assessment and confirmation of an automation professional's skills. The CAP exam is focused on direction, definition, design, development/application, deployment, documentation, and support of systems, software, and equipment used in control systems, manufacturing information systems, systems integration, and operational consulting. Click this link for more information about the CAP program.

 

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
e) none of the above

 

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.

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).

ReferenceNicholas Sands, P.E., CAP and Ian Verhappen, P.Eng., CAP.A Guide to the Automation Body of Knowledge. To read a brief Q&A with the authors, plus download a free 116-page excerpt from the book, click this link.

 

About the Editor
Joel Don is the community manager for ISA and is an independent content marketing, social media and public relations consultant. Prior to his work in marketing and PR, Joel served as an editor for regional newspapers and national magazines throughout the U.S. He earned a master's degree from the Medill School at Northwestern University with a focus on science, engineering and biomedical marketing communications, and a bachelor of science degree from UC San Diego.

 

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Joel Don
Joel Don
Joel Don is an independent content marketing, social media and public relations consultant. Prior to his work in marketing and PR, Joel served as an editor for regional newspapers and national magazines throughout the U.S. He earned a master's degree from the Medill School at Northwestern University with a focus on science, engineering and biomedical marketing communications, and a bachelor of science degree from UC San Diego.

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