The large majority of plant operations are powered by electric motors. Even so, it is rare to achieve full output speed with the most common design type of electric motors, the NEMA B. How do operators select the motor change speeds that are right for equipment processes? Motor speed can be varied through the use of electronic drives or mechanically with belts and sheaves, chain and sprockets or gearing.
In order to make an informed decision, we need to be aware of the differences between these two options. While both electronic and mechanical methods will change motor speed, only one will magnify torque.
Torque is the twisting force that is generated by an electric motor when it is in operation. It is typically expressed in pound-feet or pound-inches and is a component of the horsepower and speed at which the motor operates.
A 1 HP motor generates 3 pound-feet of torque at 1750 RPM. Reducing speed using mechanical means such as a gear box, chain and sprocket, or belts and sheaves will magnify the torque. By applying a 10:1 gear reducer to this application, we multiply the torque to 30 pound-feet, achieving the needed operating speed for the machine and supplying sufficient torque without having to increase the size of the motor.
Mechanical adjustable speed drives function like a belt/sheave and gear reducer by multiplying torque while changing the base speed of the motor.
Current technology has most applications using adjustable frequency AC drives to achieve the reduction in speed, but unlike a mechanical speed change, it will not result in a torque change. AC drives are considered ‘constant torque’ devices: as speed decreases, horsepower also decreases while torque remains constant. This constant torque speed range is closely related to the design and type of the motor being run. For NEMA Premium efficient motors, a 10:1 constant torque speed range is typical. For standard efficient motors, that turn down range shortens to 4:1. Once the controller speed drops below the turn down range, the motor begins to lose torque, becoming constant horsepower.
When you need to make speed changes, always be aware of the torque necessary to operate your application. By choosing the right method, your machinery will continue to run smoothly, more efficiently and will encounter fewer problems during its lifecycle.
For help selecting speed reducing solutions, please contact your Kaman Representative.
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