How to determine whether a VFD is appropriate for a Motor?

What Functions do VFDs perform?

The circuit’s voltage and frequency determine the speed and torque of an AC motor. With the use of variable frequency drives, you may adjust the voltage and frequency to control your motor’s speed and torque and maximize its efficiency. In the end, this optimizes energy use and assists in preserving energy.

VFDs are used in a wide range of applications, including irrigation/pumping systems, conveyor systems, elevators, and any other application where variable speed and torque are necessary for the system to function.

Which type of Motor is compatible with a VFD?

  • AC Synchronous Permanent Magnet Motor are made especially to work with VFDs.
  • AC Asynchronous Wound Rotor Motor are most frequently used in cases when a strong starting torque is required but the available power is insufficient.
  • AC Synchronous Brushless Motors
  • AC Asynchronous Squirrel Cage Motors are the most popular motors used in industrial settings with VFDs.

How can I determine the Inverter Duty Rating of a Motor?

The majority of manufacturers won’t list “inverter rated” on the nameplate because the majority of motors manufactured recently are inverter rated (VFD compatible). If the rating ‘CT/VT’ appears on the nameplate, the motor is probably inverter rated. This represents both “variable torque” and “constant torque.” ‘PWM’ may also be written on the nameplate, which indicates that the motor is suitable for a Pulse Width Modulation drive. Motors with inverter ratings can run at considerably lower speeds without overheating and are made to withstand voltage spikes without the insulation failing.

What does an Inverter-Rated Motor mean?

Even though most motors can work with VFDs, there are a few important considerations you should make before integrating a VFD into the electric motor system.

  • Insulation for Motor Windings
  • Insulated Electric Motor Bearings
  • Speed Ratings
  • Lead Span

Insulation for Motor Windings

VFDs have been stated to create high-frequency voltage spikes in motor windings as a result of internal functionality. To assist preserve your motor’s internals, make sure the winding insulation is Class F (or) above. As an alternative, “Inverter Duty” may be mentioned on the nameplate for motors that are VFD-compatible, depending on the manufacturer of the motor.

Insulated Electric Motor Bearings

It’s not simply the windings that can sustain harm from the high-frequency voltage spikes indicated above. The motor shaft experiences an accumulation of excess voltage, which leads to the breakdown of bearing lubricant and subsequent damage to the bearing. By equipping your motor with

  • Insulated bearings and
  • Shaft grounding rings,

you may stop this harm from happening.

Speed Ratings

VFDs can operate an AC motor outside of its rated speed range since they change the frequency to control the motor’s torque and speed.

The cooling system’s capacity is diminished when the motor is operated at speeds below what the manufacturer has recommended. It could be necessary to add an auxiliary cooling system if you want to operate your motor below base speed.

When your motor runs faster than the speed recommended by the manufacturer, it tries to take more power from VFD. This power drain may result in overload conditions and other serious harm.

Lead Span

The average VFD-motor circuit has a cable that is no longer than fifty feet. However, there are conditions when the VFD cannot be mounted in the area near the motor. To reduce voltage spikes, you will need to install extra filters (load reactors or DV/DT) if your lead is longer than fifty feet.


Although the variable frequency drive (VFD) has allowed us to solve the single-speed difficulty with AC induction motors, not all motors are compatible with VFDs. While certain motors can only be powered by VFDs, others can only be used with a few more system components. Before installing a VFD, make sure you are aware of the capabilities and limitations of your motor.