Keys to Replacing 2HP (And Under) DC Motors with AC Motors and VFDs

AC Motors and VFDs

When replacing old DC motor variable speed applications with AC motors and VFDs, you must consider the torque required and speed range for the application. Most applications have some sort of gear reduction attached to the motor. Therefore, when replacing DC motors with AC motors and variable frequency drives, you can achieve your desired output speed and torque.

Take a standard industrial one horsepower, 90VDC motor, for example. This motor runs at 0-1750RPM with a 30:1 reducer, giving you a 1-58RPM speed range and output torque of 864 inch pounds based on an 80% eff. reducer. DC motors have relatively flat speed torque curves and are capable of achieving very low speeds, but have fixed top speeds. A one horsepower standard AC TEFC four-pole replacement with the same reducer would yield the same output torque, but only yield a speed range of 6-58RPM if you’re using 10-60Hz of speed on your VFD.

But what if you were using 10-120Hz, instead? Most variable frequency drives can easily output up to 120Hz, which means that you can use the same motor, a 60:1 reducer and achieve a torque output of 1728 inch pounds from 3-29RPM (a minimum of 864 inch pounds at 58RPM). This extra torque at the low end will help you start most of the applications that DC motor was so good at doing. You can also use some combination of gear ratio and VFD output at 0-120Hz to increase your throughput.

Another key to consider is that most variable frequency drives today can achieve 150-300% torque capabilities to help initiate hard-to-start loads.

Over my 28 years in the industry, I have replaced many DC motor applications with AC motors and VFDs. I have found the following simple rules to be helpful:

  • Standard TEFC AC motors need to be spinning in order to cool themselves. Never run motors below 6Hz unless they are specially designed to be used in this manner.
  • Most of the standard AC TEFC four-pole motors made today are capable of running 120Hz without modifications.
  • Try to run your AC motors around design speed as much as possible at 30-90Hz.
  • Remember AC motors have no brushes to maintain, unlike DC motors.
  • In case of a drive failure, AC motors can be run cross-the-line in an emergency.
  • AC motors are more readily available, should a motor fail.

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Scott Frater
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