Q&A

Question: I run a motor repair shop, and we use the SKF discharge detector (TKED 1) to detect bearing current. It seems to give very variable results. At what number of discharges per second is there a major problem? On one motor, I recently measured 6000 in 10 seconds.

Martin Deiss: That instrument counts the number of discharges per second. But it doesn't tell you the voltage at the instant of discharge, and that's going to determine how much energy is released, and how much damage is done. So the detector will tell you when there's a discharge, whether it's at 5 or 50V. The damage increases by the voltage squared, so if you double the discharge voltage, you quadruple the damage. So the voltage level is important, as well as the number of discharges. That's why we recommend monitoring shaft voltage and looking for the voltage that discharge occurs at.

With that said, in your example, you were seeing 6000 discharges in 10 seconds: that's 600 per second. In motors without bearing protection, the average that we see is 1000-2000 per second, so that's relatively mild.

Question: The root cause of electrical bearing damage from VFDs is an imperfect sinewave in the drive's output. In that case, a drive manufacturer/model with the most sinusoidal waveform will have less risk of damage. Is that right?

Martin Deiss: In theory, yes. In practice, it doesn't make much difference. When you hook up a drive's output to a motor, you have current running through the windings. Windings have high inductance, which actually makes them act like a filter for the current. So all drives' current waveforms are pretty close to sinewaves. They still have a lot of high-frequency spikes, but there's obviously a sinewave in the waveform.

But the motor doesn't filter the voltage. The voltage waveform still looks like a mess of pulses. It is those voltage pulses that cause capacitive coupling between the motor's components and produce the shaft voltage that discharges through the bearings.

This will be a problem for virtually all drives. The possible exception we know of is matrix drives, which don't include a capacitor bus. Instead of putting out pulses of constant voltage from a huge capacitor, they directly patch together the three input phases into variable frequency output. Matrix drives don't give perfectly clean output waveforms either, but they are orders of magnitude cleaner than what you get from a regular drive.

Question: In connection with the last question: Can't you make the drive's output more sinusoidal with load reactors, dv/dt filters, and sinewave filters?

Martin Deiss: Sort of. Load reactors, dv/dt filters, and common mode filters will smooth out the current waveform to some degree. But they don't affect the voltage waveform much, so you will still have a problem with shaft voltage and bearing discharge.

There are actual sinewave filters out there that will smooth the voltage waveform into a sinewave. These are mostly used in petroleum and mining applications where the drive is very far – sometimes kilometres – from the motor. Because they filter the voltage, they're great at cutting down reflected waves, which can kill bearings and motor windings.

But they are expensive, bulky, and hard to find. They're out there, but there are also a lot of so-called sinewave filters that actually only filter the current. So you've got to know what you're getting. Read the specs, look at the before and after pictures carefully, and ensure the product does what you think it does.

Question: I have heard that shaft grounding is unnecessary if the drive is closer than 20' (6m) to the motor. Is that true?

Martin Deiss: VFD manufacturers say you should keep the drive within 100-150 feet (30-46m). At higher distances, you get reflected voltage waves. Voltage reflection can double the voltage level at the motor terminals. This idea is shown in the figure: The blue wave is voltage coming from the VFD, moving right, and the green wave is reflected voltage, moving left. The red wave is their sum: the total voltage at each point in the cable. (The voltage coming from a drive is a square wave, not a sine wave as shown here, but a square wave can be treated as a sum of many sine waves of different frequencies. We show only one frequency here for clarity.)

Voltage reflection has two consequences. First, it can burn up the motor windings, and second, it makes for higher shaft voltage. Higher shaft voltage means you have more energy available to discharge through the bearings. So more damage per arc, and damage builds up faster.