Considerations for Protection of Motors

Considerations for Protection of Motors


Video length: 2:34 3 minute read


Hi, this is Ted Wilke with SPOC Automation, I want to talk today about protecting motors from the output of variable speed drives. Kind of a different topic for us. We get a lot of questions about it. People ask me: I’m having variant problems with this. Could it be because of harmonics on the utility? The answer to that is no, that isn’t it. Harmonics on the utilities don’t cause motor problems on the output of a drive.

So, there are two issues that we need to be concerned with on the output of a drive that we use to protect the motor and cabling system. The first issue is what’s called reflected wave phenomenon. It simply means that when we have longer cable lengths, we can actually get a doubling on the voltage; we can get voltage spikes at the motor that exceed 1000-volts. They go up to 1400-, 1500-, 1600-volts depending on the cable length.

A NEMA motor has a specification: NEMA MG-1, part 31 for inverted-duty rated motors. And those motors today would have the insulation system required to be protected by their installation voltage rating. And the motor will not be damaged when being used by a frequency inverter variable speed drive. However, you do need to be worried about your cabling system. So, any time you have more than about 300-feet, we recommend a DVD key filter, which will keep those voltages clamped down and keep them from happening.

The second issue is called bearing fluting, and it’s caused by what’s called common-mode voltage and common-mode currents. Shaft energization is another way people refer to them. And we recommend when you’re buying a new motor to use with a variable speed drive, to use the insulated bearing. The insulated bearing will protect you from that and keep your bearings from being fluted, damaged and living a short life.

If you’re doing a retrofit, then using a shaft grounding ring is a very effective way to do it. You can put a shaft grounding around some smaller motors as well. It’s very cost-effective, particularly on the smaller motors.

The third way to protect against common-mode voltage is what is called a common-mode choke or a common-mode filter, and that mounts inside the drive cabinet and keeps those common-mode voltages from happening. Now, of course, if you ever change a drive in the future and you don’t have insulated bearings, you would then need to make sure that your new drive also has the common-mode choke.

So, a couple of different ways to skin the cat. And I hope this has been helpful for you. If you have any more questions, give us a call.

Thank you. Have a great day and be safe.

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A variable frequency drive should only be worked on by QEP certified professionals and only following all corporate, local, state and national regulations.