Motors & Drives Information

Typical Question

We have had a relatively high failure rate in one geographic location for Adjustable Frequency Drives of a particular frame (25 to 40hp). In most cases, the users have tried many other brands, all with worse MTBF than ours. Most begin having problems with tripping on overvoltage, even though the supply is well within nominal ranges. Eventually they “blow up.”

Our units lasted longer, probably due to a higher bus voltage tolerance than most, but now we are also succumbing to the problem. This happens in more than one plant, but all within a 100 mile radius and on the same utility.

Here is the interesting part. One of the users hired a consultant on their own since this seems to cross over manufacturers’ lines. This consultant has put forth the theory that the utility has, somewhere in their grid, poorly separated power lines that are occasionally touching each other in the wind, sending very short duration, but repeated, spikes down the lines.

Without much detail, I find this a bit difficult to swallow, but not being a utility guy, I thought I would throw it out here for those who may have heard of this.

Discussion Group Answers

• “Line slap, huh? That does not sound at all like a likely cause to me, especially with the fast acting protective relays around today. I think the best bet is to get a high speed recorder on one of the feeders to get an idea of what kind of voltages you're dealing with. Many utilities will provide this type of device for you to use temporarily. Once the magnitude and duration of the voltage are known, it may help get you in the right direction or at least help identify a short term fix. Discussions with the utility may also shed some light on it.”
  
  
• “I can sympathize with you. Not too long ago, we too experienced repeated failures at three sites within in a common geographical area. At each installation, a number of soft starters and motors had failed over a period of about 12 months, further inspection strongly suggesting transient over-voltage damage in each case.
  
  “The ‘events’ however were not captured by the recording equipment installed by the supply authorities, and for some strange reason, we were discouraged from installing our own. This eliminated any possibility for us to confirm one way or the other.
  
  “On further investigation we found that the HV supply comprised an automatic tap changer that we believed (but could not confirm due to a lack of available information) was open transition by design. Although I cannot confirm the automatic tap changer was the cause of those failures, you might be interested to note (as we were) that the sites have been trouble-free since we outlined our suspicions (10 months ago) to the 3 clients involved.”
  
  
• “I have seen a similar situation happen on a supply system where the supply transformer 22KV/440V was delta primary and wye secondary. The earth neutral link was loose, resulting in a high impedance between the earth and neutral.
  
  “When the power factor correction was switched in and out of circuit, it caused transients on the line. This was capacitively coupled through the transformer, causing the whole secondary to leap above earth momentarily. The 440V three-phase appeared normal phase-to-phase, but there were very high fast transients relative to ground on all phases at the same time.
  
  “This lifted the DC bus away from ground and effectively injected a very fast high voltage transient into the output of the drive (due to the capacitance of the cable and motor to earth). This caused overvoltage trips, instantaneous over current trips, and very occasionally, caused output device failure.”
  
  
• “We had a small Adjustable Frequency Drive tripping frequently on bus overvoltage during run, deceleration, and acceleration. I researched potential causes for the nuisance tripping and went back and re-read the manual word by word. It placed particular emphasis on grounding.
  
  “I had the ground checked at the drive ground terminal to a known good ground. There was 15-19 ohms resistance. We re-grounded the Adjustable Frequency Drive to a known good ground and started the drive...phase-ground fault.
  
  “Meggar checks from the Adjustable Frequency Drive to the field did not pick up anything, even at 1,000VDC on the 480V system. We should have megged from the motor back, but that was a difficult location to get into. The bad ground at the Adjustable Frequency Drive was not allowing the fault to blow through insulation, but it was enough for the drive protection circuitry to pick up and trip on high DC bus voltage.”

Clarifications

Just a little comment on transient recorders. It is equally important to monitor line-to-ground transients as line-to- line. This is particularly important if the substation is using an ungrounded delta secondary.

Theoretically, the “float” voltage can go anywhere until some insulation or protective device conducts, and then if there is power behind the transient, bang! It is even worse to operate continuously with one leg deliberately grounded on a delta secondary substation. Most electronic equipment is not really designed to see full voltage-to-ground on the phase legs and the result is more insulation stress than necessary.

With Adjustable Frequency Drives, manufacturers protect the input by including line-to-ground suppression. Typically, MOVs are used from line-to-line and line-to-ground to limit voltage transients.

The potential problem with this type of protection exists when the supply system impedance is low and the voltage transients exceed the capabilities of the MOVs. Often Adjustable Frequency Drives will first exhibit overvoltage trips indicating the presence of voltage transients, line-to-ground. Then, after many “hits”, the MOVs fail. Since many drives have the MOVs and power rectifiers packaged as modules, a complete rework of the front end of the drive is needed.