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Electrical Apparatus, Jun 2008

You are invited to send questions to this department. Your identity will be kept confidential, but we ask that you include your name, affiliation, postal (not e-mail) address, and telephone number. If you 'd like a pre-puhlication reply, enclose a stamped envelope and we'll try to respond promptly. We cannot provide personal consultation or supply winding data.

Does a motor winding have a 'BIL' rating?

After we had a motor fail after only a few months of service using a low-voltage vacuum starter, the starter supplier said their experience was that voltage spikes from the vacuum contactor wouldn't exceed the "BIL rating" of a standard 460 volt motor. What is such a rating?

"BIL" means Basic Insulation Impulse Level, defining a transient overvoltage rating assigned to apparatus that's tested by withstanding a standardized transient voltage impulse.

Such a rating exists for stationary apparatus such as transformers and switchgear, but not for any rotating machine. The motor manufacturer may be able to tell you the impulse withstand capability of their product, but it won't be a "rating " as such, nor will it be defined as BlL.

Insulation resistance tests may not suffice

Insulation resistance measurements are most often recommended to check motor winding insulation.

But we've had several motors that showed excellent values for a couple of years and then suddenly failed. Why wouldn't a deteriorating condition show up in the test?

A megohmmeter reading is a quick, reliable diagnosis for moisture or other contamination of insulation between a motor winding and core.

Aging can also contribute to lowering of insulation resistance. However, most winding breakdowns start between turns in a coil, where pinholes or taping defects can be present without affecting any insulation resistance reading.

Authorities agree that, unfortunately, no single test can reliably diagnose insulation condition or predict future life.

Take care with kVA code letter

We're having trouble applying a soft starter on a 60 hp 460 volt motor that is nameplated for 71 amps full load. It's marked with Code Letter G, so we assumed 600% locked-rotor current, which would be 426 amps.

But we're measuring as much as 460 full voltage. Should there be this much variation?

Nameplate Code Letters aren't based on percentages, but only on the locked rotor current plus rated voltage and horsepower. For any motor, Code G means that the maximum locked-rotor WA per horsepower is 6.3. The current calculated from that is 474 amps; your figure of 460 is not out of line.

Is megohmmeter testing destructive?

One of our electricians doesn't like periodic insulation resistance testing because he says the megohmmeter applies an overvoltage that eventually breaks down insulation. Yet all the recommendations we can find say that the testing is safe. Can you give us some clarification?

Insulation resistance tests using a megohmmeter don't apply voltages that exceed normal a-c motor ratings.

For example, a 460 volt motor insulation system normally offers normal service life at any a-c voltage up to 600. Using a 500 volt d-c megohmmeter is equivalent to applying only about 315 volts a-c.

For a 4,000 volt machine, a megohmmeter voltage of 2,500 is equivalent to less than 1,600 volts a-c (an in-service d-c hipot test, that is potentially destructive, would be 11,000 volts). So repeated insulation resistance tests should not of themselves shorten insulation life.

Several factors influence breaker operating time

In setting up a transfer scheme that could cause a quick reclosure on a spinning motor, we have worked with the motor manufacturer to arrive at a safe reclosure interval.

We now have to choose breaker or switch operating time to suit that. We're looking at a five-cycle maximum breaker operating time. How accurate is that likely to be?

We don't know what tolerance may exist on the breaker operating time itself; the supplier should be able to tell you. However, the real problem is that a breaker can't operate until it receives the appropriate control signal.

That usually comes from a relay in a CT circuit. So you have to add in the relay operating time plus any slack in the CT response. That could double the operating time of the breaker itself.

Repaired vs. replaced: Which is more reliable?

Please help settle this question between some of our operating people. One group says it's always better to replace a failed piece of equipment because a new one will always be more reliable and last longer-that once anything breaks down and you repair it, the life will be shorter. Another group claims that normal life should be restored by proper repairs. Who's right?

They're both right-and both can also be wrong. Anything new is subject to what's called "infant mortality" or the "bathtub curve." Unavoidable weaknesses result in an initially high failure rate (that's why short-term warranties exist).

Once that's over with, in-service failures drop to a low level for a long time until the onset of "wear out." A new item will usually come with a broader warranty than a repair, but that won't cover the full cost of downtime or changeout resulting from premature failure.