Tortoise and the Hare Scenario Plays Out in Motor Speed

Motor Speed: Tortoise or the Hare?

Not every motor application needs to run at full speed, like a rabbit racing a turtle. Moving slowly might be what’s needed to get the job done.

By Bill Bernhardt, senior commercial engineer, Rockwell Automation

Sometimes a motor needs to run at full speed like a rabbit.

The typical, most efficient running mode of most motors is full speed. Sometimes the motor needs to operate at less than full speed, like at a tortoise pace. When a motor needs to operate at slow speed, considerations such as motor branch supply, motor type and controller type need to be taken into account.

Both full and slow speeds might be needed to accomplish an application’s everyday tasks, and both methods should be available to accomplish them. The slow speed’s duration and amount may dictate what method is best.

Many applications benefit from the ability to adjust a process’ speed while operating. Reasons for this include positioning; checking or adjusting alignment; inspecting equipment, such as a band saw; or reducing fan speed.

You can use several methods to obtain slow speed control in applications, including variable-frequency drives (VFDs) and soft starters, in addition to traditional gearing.

VFD control uses frequency to control speed. Motor synchronous speed is a function of applied frequency and the number of poles on the motor. This example helps you to understand the point of frequency adjustment:

  • N = 120F/P
  • N = Revolutions per Minute (RPM)
  • F = Applied frequency
  • P = Number of motor poles

Because a motor’s number of poles doesn’t change, changing the frequency changes the motor speed. The VFD can operate at slow speeds while simultaneously providing up to full torque, depending on the control. It can run at slow speed almost indefinitely, depending on the motor insulation and type. All this while performing positioning function accurately.

While some soft starters use frequency to control motor speed, not all do.  Most use a method called cycle skipping to achieve slow speed. Cycle skipping has limited torque capabilities. This method provides fixed slow speed operation, usually in a positive and negative direction.

Both Options Have Their Place

Not every application needs to run at full speed, like a rabbit. Sometimes moving at a turtle’s pace will assist with the application. All in all, the two working together will get the job done.

Learn about Allen-Bradley® soft starters from Rockwell Automation.

The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.


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The JOURNAL from Rockwell Automation and Our PartnerNetwork™ is a bimonthly magazine, published by Putman Media, Inc., designed to educate engineers about leading-edge industrial automation methods, trends and technologies.