Answer: Running the motor.

Electric motors are subjected to extreme conditions when operated. Mechanical stresses, thermal stresses, moisture and chemical contaminants, and high voltage conditions by switching of inductive loads elsewhere on the line.  A motor in operation may encounter voltage spikes over 10 times the normal line voltage, and VFDs can produce voltages even higher and more frequently!  Motors are designed for adverse conditions but they do have their limits.  If they are left to continue operating in damp, hot, and dirty conditions, it will reduce the life of any motor.  But there’s good news – motors are insulated for voltages much higher than the normal operating voltages for just those reasons.

 

From Jacksonville, Florida –

Answer: Yes. The insulation is designed for voltages much higher than the operating voltage.

All motors must have a margin of safety when it comes to being insulated above the normal operating voltage. When tested properly, the insulation is safely stressed to a predetermined level above the operating voltage, yet far below its designed threshold. Insulation testing offers an assurance that the motor winding has the minimum required strength to withstand voltage stresses in actual operation.

 

EXAMPLE: If you’re crossing a 10,000 lb rated bridge with a 3000 lb car, wouldn’t you expect that it could be tested periodically with more than 3,000 lbs?  We want some assurance that the bridge is capable of withstanding something greater than the just the weight exerted during normal use, but not stressing it to the point of damage.  Just as with the bridge example, visual observations and low impact measurements must be performed on a motor before the high voltage testing is performed.  
 

From Detroit, Michigan -

Can over-potential testing further damage weak insulation?

Answer: Not if performed as instructed by the equipment manufacturer with recommended test voltages.

Here’s the fine print. If a motor fails the test and is rendered inoperable afterwards, the fault existed before the motor was tested.  How do we know for sure? Simple, thousands of motors are tested repeatedly without a failure, and other motors fail unexpectedly having had no testing performed.

 

EXAMPLE: When one out of ten motors fails the test, it has to be concluded that this one motor did not have the level of integrity of the other 9 motors that passed the test.  In most cases, the cost to replace a faulty motor is minimal in comparison to the cost of having it fail unexpectedly while in-service. GET IT OUT OF THERE AND KEEP PRODUCTION RUNNING!

 

From Pine Bluff, Arkansas -

Answer: No, not with dc or impulse (surge) testing.

Applying a pressure alone, or a stress, does not degrade the insulation.

 

EXAMPLE: Observe a water pipe with a working pressure of 70 psi household water pressure.  What should we expect this pipe should hold? The pipe itself is rated for 310 psi at 73 degrees F.  If we apply twice its working pressure (140 psi), does that weaken it?  How about 3 times the working pressure?  How about if we test it at that pressure every day for a year?  There is absolutely no change in the integrity of the pipe after repeated tests.  One instrument manufacturer has been using the same coil of Romex NM-B 14/2 wire as a pre-calibration test load for over 10 years with a voltage of 5000 volts DC!

 

From Trenton, New Jersey -

Answer: Not true.

Have you ever walked across carpet and touched a grounded object, like a light switch?  That voltage has been measured at up to 5000 volts or more! How about the ignition system in your car with an output of 20,000 volts or more?  Is it destructive or deadly? 

LAB: To make the point, we can hipot a business card.  As you would expect, it has a very low breakdown threshold and would never be used as motor insulation.  The flashover finally occurs at about 1000 volts DC.  The path that the current took is so small that it’s microscopic. Now with an AC test at 500 volts, the current from the collapsing field of the transformer is so great that it destroys the paper.  This is obviously what’s feared by “hipoting” a motor.  All high voltage tests are not equal.  DC testing is proven safe and reliable.

 

Albany, New York -

Answer: Use dc test equipment.

These motor leads are rated for 600 volts while carrying up to 100 amps of alternating current in high temperature environments and while being subjected to mechanical stress from possible cuts and abrasions. Look closely at the specifications on the leads and you’ll find they have a withstand voltage of over 12,000 volts DC! 

LAB: On the test bench we’ll take a close look at a telephone cable compared to motor leads. The specification on the phone wire shows a 1000 volt dielectric rating. What do you think it will actually withstand? I have 10,000 volts DC applied across this insulation. Don’t you think your motor circuit should be at least as strong as telephone wire? We’re only asking your 480 volt motors to reach 3000 volts as an acceptance test!

Here’s a test that I’ve performed numerous times for skeptics in motor repair shops. I’ll have them bring out the cheapest motor that they have on the shelf (I offer to pay them double for it if it doesn’t pass this test).

The operating voltage is 460, the normal test voltage is 2880 but I don’t stop there, I double that and go to 5760. While they see that this motor is holding just fine at more than 10 times the operating voltage, they obviously know their re-wound motors are much better than the cheapest motor off the shelf. Keep in mind, it’s not the quality of the motor that your testing, it’s the quality of the insulation on the wire. In some cases, you will find a new motor that does not pass the test because of inferior or damaged insulation – that’s why we test!

 

From Atlanta, Gorgia –

Answer: No.  The older motors subjected to the same stresses in service as new motors.

Let’s face it. Some of the older motors are bullet-proof compared to the new motors. But if that old motor (out in the plant) does have an insulation defect, you need to be aware of it before it goes to ground causing a much more expensive repair due to damaged or shorted core laminations. If that older motor has not been damaged significantly, it can be rewound with today’s newer insulation which could make it better than it was when it was brand new!
 

From Appleton, Wisconsin -

Answer:  It's the ability to limit the current.

A step-up transformer is used to generate the high voltage to perform either test. The difference is that through rectification, the dc flash-over current (if a fault is present) can be limited to a few microamps, while the ac flash-over current is not controllable due to the collapsing field. We can watch what this does to actual motor insulation in the LAB.

NOTE: Without a fault in the insulation or contaminants the applied voltage, ac or dc, is just a potential – there is no flow of current.

LAB: After a fault is introduced though, we can get a voltage to flash-over at around 2500 volts. Again, the dc test has detected the flash-over point without making the fault worse – we haven’t lowered the flashover point by performing the test. But again, with the ac test the insulation is severely and permanently damaged.