Are Surge Tests Destructive?
Since surge testers find weak windings by detecting an arc from one turn in the winding to another turn, people sometimes ask what such an arc does to the insulation. And, even if there is no arc, do the surge test pulses weaken the insulation?
Another reason for the question is the misconception and related statements made by some that surge testers are destructive. There are probably other reasons. These questions will be addressed below.
- Most winding insulation failures start as a turn to turn weakness which can only be found with an over-voltage surge test before it is too late. It cannot be found with low voltage measurements because the insulation still has some dielectric strength.
- Over-voltage surge tests are not destructive when done correctly. The voltages used are far below what a motor is designed for both when new and old. See results of 8000V surge tests on an old 460V motor below.
- Motors typically see voltage spikes routinely during normal operation that are significantly higher than the peak operating voltage, and often higher than the surge test voltage.
- With Electrom iTIG II motor testers the surge test process is automatic. The voltage and number of surge test pulses are controlled by the tester rather than the operator.
The root cause list for weak insulation is long and includes age, over-heating, vibration, large frequent voltage spikes, voltage spikes from variable speed drives, humidity, dirt and grease or oil where it should not be, and manufacturing defects to mention some.
It is estimated that about 80% of winding insulation failures in motors are the result of turn-to-turn weaknesses or shorts. A turn-to-turn failure typically starts with a turn-to-turn insulation weakness that progresses to a short. The turn-to-turn short then progresses to multiple shorted turns, and eventually to a ground fault that is listed as the reason for the failure. This progression happens because a shorted turn creates a loop in which current is circulating, and the energy is transferred to heat. The heat weakens the insulation further and eventually excessive heat will cause a ground fault.
Finding insulation weaknesses
Windings and coils have to be tested to a voltage higher than the operating voltage of the equipment to find weaknesses before it is too late and the motor has a catastrophic failure. This is what a surge tester does. A weakness is a condition where the motor runs for the time being, but has a “defect” that will lead to electrical failure.
A turn to turn arc caused by the higher test voltage is the result of an insulation weakness that will eventually lead to a catastrophic failure.
Normal Voltage Spikes during Operation
Motors see steep voltage spikes during normal operation all day long. They come from breakers and contactors being opened and closed, starters, power circuits being switched, VSDs or inverter drives etc. They can also come from lightning strikes, faulty breakers or malfunction of switching gear. Many of these spikes can be much higher in voltage than what is used during a surge test.
Use of Surge Testers versus Low Voltage Testers
Surge testers have been used in some form or another since the 1950s and became common in the 1990s. They are used worldwide in increasing numbers by the vast majority of motor shops and motor and coil manufacturers. They are considered critical to their operations. Industrial users of motors and generators use surge testers for predictive and preventive maintenance purposes and reliability programs. Not finding turn-to-turn, coil-to-coil or phase-to-phase weaknesses or faults result in critical motors failing while in operation with major ramifications, both financial and other.
Low voltage measurements and tests are used by industrial end users to analyze and monitor the condition of rotating equipment. Some argue that they can be a substitution for surge tests. This has been proven wrong by people using both low voltage and high voltage test equipment. Low voltage measurements are very useful in various applications, but they can be difficult to interpret, and they can produce false results when used to find winding insulation weaknesses. There is no substitution for tests performed at “high” voltages. Weaknesses are found that simply cannot be detected or measured at low voltages.
Can a Surge Tests Be Destructive?
When an inter-turn weakness is found there will be arcing or discharges from one turn to another. If the surge test is run for too long while arcing (too many pulses applied), carbon tracking could result and the arc-over will occur at a lower voltage next time.
- This is not the way surge tests should be done. The surge voltage should be taken up to the test voltage fairly fast and the test should be terminated quickly. The IEEE 522 standard requires a minimum of 5 pulses at voltage.
- All Electrom Instruments iTIG II Surge Testers automatically control and limit the surge voltage and the number of pulses using our proprietary Quick Surge™ and Surge Guard™ technology.
- If a weakness is detected, the insulation is not good in the first place. The test has served its purpose and found a fault or weakness during testing. The motor should be scheduled for repair or replacement or considered for a rewind.
Results from an Arc Test Experiment
In one typical example Electrom repeatedly tested a 35 year old used 460V 4-pole motor (made in 1980, picture below). Normal test voltage for surge and DC hipot for this motor is 1920V.
- The surge test voltage was raised until the windings arced. Arc voltage: 8000V
- 5 tests were done before the arc voltage dropped to 7500V.
- This was followed by 4 tests that did not arc at 6500V.
- DC hipot tests were done successfully to 7000V after the surge tests.
Do not hipot or surge test working motors unless the megohm insulation resistance meets EASA or IEEE recommendations, or use your own higher requirements.
A surge test does not shorten the life of a winding because the pulses are but a few among all the spikes a motor absorbs during its life, and because the surge test voltages are far below what new and used windings are designed for.
If a weakness is found, the tester served its purpose. The motor has a serious defect that will lead to a catastrophic failure later unless the situation is dealt with.
If high voltage surge testing was a problem, or shortened a motors life as a result of the “stress” from a surge test, this practice would have gone away a long time ago.