Can Any Motor Be Used with a VFD?

While the AC induction motor was a marvellous invention that forever changed the way we do work, the single-speed ability quickly became a problem to be solved. Over a century, hundreds of innovations were introduced to solve this problem. But none of them could do so quite as effectively or efficiently as the variable frequency drive (VFD).

Use the below links to jump to different sections of this article.

• What do VFDs do?
• What motors can be used with a VFD?
• How to tell if a motor is inverter rated?
• What does it mean if a motor is inverter rated?

VFDs have been used in various industries since the 1980s, effectively solving the single-speed issue of the three-phase AC electric motor.

What Do VFDs Do?

An AC motor's speed and torque are determined by the frequency and voltage of the circuit. Variable frequency drives vary the frequency and voltage to manipulate the speed and torque of your motor so it can complete its operation as efficiently as possible. This ultimiately helps save energy and optimize energy consumption.

You'll find VFDs in many applications, such as elevators, conveyor systems, irrigation/pumping systems, and any applications that require variable speed and torque to complete operation.

What Type of Motor Can Be Used With a VFD?

• AC Synchronous Permanent Magnet Motors: Specifically designed for use with VFDs.
• AC Synchronous Brushless Motors
• AC Asynchronous Wound Rotor Motor: Most common in applications where a high starting torque is needed, but the power from source is inadequate.
• AC Asynchronous Squirrel Cage Motors: Most common type of motor to be used with a VFD in industrial applications.

How to Tell If a Motor is Inverter Duty Rated?

Because most motors made today are now inverter rated (VFD compatible), most manufacturers won't indicate 'inverter rated' on the nameplate. But, if your motor is more than 5-10 years old, it may be indicated on the nameplate. If you see the rating 'CT/VT' on your nameplate, it typically means your motor is inverter rated. This stands for 'constant torque' and 'variable torque'. You may also see 'PWM' indicated on the nameplate, meaning the motor is rated for a Pulse Width Modulation drive. Inverter rated motors are designed to handle voltage spikes without the insulation failing, and can handle much lower speeds without overheating.

Here are some examples of motors that are not inverter rated:

• Pre-EPAct Motors (pre-1992): Can only be run with a VFD if they are VFD rated, have Class F insulation or higher, and the VFD has a constant torque ratio of no higher than 2:1.
• EPAct Motors: With a 2:1 constant torque rating and a 4:1 to 10:1 variable torque rating. Additionally, fractional and 56-frame motors are not VFD rated.
• NEMA Premium Motors: With a 4:1 to 20:1 constant torque rating and a 10:1 to 20:1 variable torque rating.

There may be cases where the nameplate of the motor you intend to pair with a VFD is not available for checking ratings.

What Does it Mean if a Motor is Inverter Rated?

It's important to note that while most motors are typically compatible with VFDs, there are still a few important factors to review before adding a VFD to your electric motor system.

Motor Winding Insulation

Due to their internal functions, VFDs have been known to cause high-frequency voltage spikes in the motor windings. Ensure that your motor's winding insulation is Class F or higher to help protect your motor's internals. Alternatively, depending on your motor manufacturer, the nameplate may indicate "Inverter Duty" for motors compatible with VFDs.

Insulated Bearings For Electric Motors

The high-frequency voltage spikes mentioned above can cause damage to more than just your windings. This extra voltage builds up on the motor's shaft, causing bearing lubrication to break down, eventually damaging the bearing. You can prevent this damage from occurring by installing insulated bearings and shaft grounding rings on your motor.

Shop our selection of insulated bearings.

Speed Ratings

Since VFDs vary the frequency to manipulate the AC motor's speed and torque, they can run a motor outside of its rated speed range. But that doesn't mean you should.

When running your motor at speeds lower than the manufacturer's rating, the cooling system's capability is decreased. If you intend to run your motor lower than base speed, an auxiliary cooling system may need to be installed.

When running your motor at speeds higher than the manufacturer's rating, the motor attempts to draw additional power from the VFD. This power draw can lead to overload situations and other critical damages.

Lead Length

In your typical VFD-motor circuit, the cable length doesn't exceed 50 feet. However, sometimes there is no available mounting space for the VFD in the vicinity of the motor. If you have a lead length of over 50 feet, you will need to install additional filters (load reactors or DV/DT) to mitigate voltage spikes.

Summary

While we've solved the single-speed problem of AC induction motors by introducing the variable frequency drive, not all motors are VFD compatible. Some motors are specifically designed to run on VFD power, while others need a few components added to the system to be compatible. Ensure that you understand your motor's capabilities and limitations before installing a VFD.

Not sure if your motor is up to the task? Get in contact with one of our technical experts today.