Motor Slow Speed: Tortoise and the Hare Scenario

Motor Slow Speed: Tortoise and the Hare Scenario

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

The typical, most efficient running mode of most motors is running at full speed.  Sometimes the motor needs to operate at less than full speed, like at a tortoise pace.

Considerations need to be taken into account for slow speed such as motor branch supply, motor type, controller type, etc.

Both full and slow speed might be needed to accomplish the everyday tasks of an application and both methods should be available to accomplish it. Duration and amount of the slow speed may dictate what method is best.

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

There are several methods you can use to obtain slow speed control in applications, including variable-frequency drives 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

Whitepaper: Slow Speed Applications Using a Soft Starter

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

A soft starter, like the SMCTM-50, does not utilize frequency to control the speed of the motor.  Most soft starters utilize a method called cycle skipping to achieve slow speed.  Cycle skipping has limited torque capabilities.  While this method does provide fixed slow speed operation, usually in a positive and negative direction, the SMC-50 operates differently. 

The SMC-50 offers fully adjustable slow speed capability from zero speed to positive and negative 15%.  Meaning from negative 270 RPM to positive 270 RPM for an 1800 RPM motor.  Instead of cycle skipping, the SMC-50 accomplishes slow speed by firing the SCRs more frequently.

This method allows the development of much higher torque capabilities over the cycle skipping method while providing much more control while in slow speed.  This method provides minutes, not hours, of slow speed operation.  Perfect for lower time durations of slow speed.

Traditional ways of changing direction of a motor, whether it is slow speed or full speed, is to either swap 2 phases of a motor, or utilize a reversing contactor.  The SMC-50 can easily be programmed to perform slow speed in forward or reverse direction without the use of reversing contactors or physically reversing leads.

Examples of use of slow speed with the SMC-50.

  •  Reversing direction of a pump: unclogging the inlet of a pump without the need to send someone to clear the inlet saves time and money.
  • Low-pressure testing: getting the pump to flow just enough to check the system may be all that you need to do without the possibility of causing damage with the pump full on.
  • Mill application: adjusting and checking saw blade alignment by using slow speed.
  • Grain: when an end user would like to verify a grain bucket without having to stop the process.
  • Basic positioning: a door on a tumbler for adding or dumping product. You can use slow speed to get to the right position to stop or to slow enough to complete the process without stopping.
  • Maintenance of equipment: lubrication of gears and mechanical parts can be done at slow speed.
  • Utilizing the Accu-StopTM function to bring a motor from full speed to slow speed and then reapply a brake maneuver for simple positioning.

So the question that should be asked for slow speed needs is the duration. 

If meant to run in slow speed for a very long time, say for a fan, a drive would be a good selection. 

If the application calls for infrequent slow speed for minutes, the SMC-50 maybe the best option.

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 and all, the two working together will get the job done!

Check out our whitepaper Slow Speed Applications using a Soft Starter for more information!

Bill Bernhardt
Posted June 14, 2017 By Bill Bernhardt, Senior Commercial Engineer, Rockwell Automation
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