What to Consider When Pairing Your AC Induction Motor with a VFD

When pairing your AC induction motor with a variable frequency drive (VFD), there are a few considerations to make to avoid critical damages. Additional measures may need to be taken when using the VFD to run your motor both below and above the manufacturer rated speed, which will be stated on your motor’s nameplate. Plus, you’ll need to ensure that the entire assembly (motor, VFD, and any other accessories) is fully up to the task at hand.

History of Issues

Before extensive research and innovations, manufacturers and users alike had many lessons to learn through trial and error. Pairing an electric motor with a VFD led to the failure of major components:

• Winding Failures – the VFD caused higher peaks of voltage that fried the windings. Manufacturers realized that the magnet wire insulation needed an upgrade.
• Excessive Rotor Heating – the VFD redirected current in the rotor, causing excess heat. This heat was often enough to fry the stator wire insulation.
• Bearing Failures – the VFD caused high currents to run through the motor's shaft and, in turn, through the bearing. This current caused overheating and vibrations that damaged the bearing. Manufacturers began recommending special grounding measures, including shaft-grounding measures that provided a path past instead of through the bearings. Some manufacturers even recommended using insulated bearings.

Speeds ≤ Nameplate

When using the VFD to run your motor at speeds less than or equal to the manufacturer’s speed rating, there are a few things to take into consideration:

• Did the manufacturer use inverter duty wire for windings and other added insulation components?
• The cooling abilities of your motor decrease as speed decreases, particularly with shaft-mounted fans and rotor fins.
• Did the manufacturer take extra measures to ground shaft current?
• Did the manufacturer use insulated bearings?
• Is the chosen motor capable of continuous operation below the rated speed?

It’s important to note that if the motor was not built to run at below-rated speeds, and doing so leaves the possibility for “unexpected” damages. Additionally, the VFD will limit the amount of available torque at start-up, so you should note the required amount of torque you’ll need for your specific application.

Speeds ≥ Name Plate

Alternatively, when using the VFD to run your motor at speeds greater than or equal to the manufacturer’s speed rating, there are other considerations to note:

• Is the chosen motor capable of continuous operation above the manufacturer rated speed?
• Will the motor have enough torque for the application when run at above-rated speed? Motors generally lose torque the faster they spin.
• Are the motor’s shaft and rotor assembly going to be run above the rated critical speed? (Will the centrifugal force be too high?)

Additional Thoughts

Mechanical and electrical resonance** can be amplified when a VFD runs a motor outside of the manufacturer's speed rating. This can result in overheating and mechanical damage to the torsional system (spinning components). Consider putting a regular inspection/preventive maintenance schedule in place to ensure the quality of the system.

The bearing installed in your electric motor has its own speed rating. When using a VFD to run your motor at speeds above the nameplate rating, ensure that the manufacturer installed bearing can run up to that speed or install an upgraded bearing.

Did you know that the lubrication you use for your bearings has a speed rating too? Using lubrication without a high enough rating can result in overheating, vibrations, and eventually bearing failure. Over-rated lubrication will work just fine but doesn’t result in cost savings.

Want to learn more about VFD basics? Watch the below 5 minute video on what VFDs do and how they work.

Summary

Running a VFD with your AC induction motor is a great way to take better control over the speeds your motor runs at. However, some special considerations need to be thought over before deciding on the best motor and accessories for the job. The main goal is to have your motor run in tip-top shape while avoiding downtime and increasing cost savings.

Read more in this linked EASA article.

This article is posted with permission by EASA.

*Article is based on the use of a standard AC induction motor and a pulse-width modulated VFD.

**Resonance - a vibration of large amplitude in a mechanical or electrical system caused by a relatively small periodic stimulus of the same or nearly the same period as the natural vibration period of the system. ~Mariam-Webster.com, 16 Nov. 2020