# AutoQuiz: How to Calculate Rated Torque with Variable Frequency Drives

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

Today’s automation industry quiz question comes from the ISA Certified Automation Professional 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 information about the CAP program. This question is from the CAP study guide, Performance Domain III, System Design. Definition: Design, specify, and procure the hardware/software used in the system.

An application includes a variable frequency drive (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 is in proportion with the nameplate rating, the motor will develop rated torque. The only answer that represents rated torque is 100% rated torque.

The correct answer is C, 100%.

Shares