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AutoQuiz: How to Calculate the Rated Torque for a Motor

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.

An application includes a VFD that implements a V/Hz method of torque control while maintaining a fixed V/Hz ratio. The motor has a nameplate rating of 460 volts and 60Hz. If the drive is operating at 30Hz, what percent of rated torque will the motor MOST likely develop?

a) 25%
b) 50%
c) 100%
d) 200%
e) none of the above

The volts-per-hertz ratio (V/Hz) controls the torque of a motor. As long as this ratio remains in proportion with the nameplate rating, the motor will develop rated torque. The only answer that represents rated torque is 100% rated torque. The other values do not represent rated torque.

The correct answer is C, 100%.

Reference: Dave Polka, Motors and Drives: A Practical Technology Guide, Second Edition.

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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