What are the risks associated with static motor testing? Any time you apply voltage to a winding, you face the possibility of causing a failure – either a short between windings or a groundwall issue. However, there are significant differences between how the test process affects the windings and how just starting the motor affects them.
With modern, state-of-the-art equipment, test voltages are computer-regulated, and these testers use low-current DC voltage, which is easier to control and far less stressful to the insulation. Test voltage starts at zero and is monitored for weakness and current leakage to the ground continuously throughout the test procedure. Any deviation or potentially damaging issue is immediately captured and testing stops.
When the tester finds a problem, it locates an area of weakness without causing further damage. On the other hand, when you press that start button, the windings will instantly see full AC voltage that is pushed by eight to 12 times the full nameplate current. Contactor bounce or breaker closures create spiking that can easily exceed five times the line voltage, and these spikes attack the turn insulation.
With regard to medium- and high-voltage motors, insulation breakdown is the underlying cause of more than two-thirds of motor failures. Of those, more than 80% begin as a copper-to-copper weakness. The very thin film of insulation that is baked onto the copper conductors abrades and wears away because of environmental issues the motor endures.
Numerous elements contribute to insulation deterioration; these include starts and stops, heating and cooling, and contamination. During each startup, the windings will experience high voltage spikes because of contact or breaker closure. Magnetic forces imposed on the windings during start-up cause them to expand instantly, allowing the copper to abrade at the weakest point (which is usually within the knuckles or end turns). Copper, insulation materials, and steel laminations all heat and expand at different rates, which allows for some abrasion of the copper’s insulation.
Remember, the insulation system is made up of two components: the copper-to-ground component (or groundwall) and the copper-to-copper component (or turn insulation). The groundwall component is by far the strongest part of the insulation system. The copper is protected from the steel or earth with high dielectric materials and then wedged tightly into the slots with top sticks; it becomes very secure after vacuum pressure impregnation (VPI).
The turn insulation is the only part of the system that can move, and on each start it will “flex” or “breathe.” That movement will allow the copper windings to abrade against each other, creating a weak area and eventually allow arcing between turns. That arcing will get worse with each startup until it creates a carbon path and becomes a hard-welded fault. Once a few turns are shorted together, they will form a loop that will react as the secondary side of a transformer, resulting in extremely high current in that loop, causing extremely high heat that will quickly burn through the slot cell liner and “blow” to ground.
Most winding failures get blamed on grounds, as the consequence of letting the motor run to failure usually results in a hole in the laminations. However, the vast majority of failures will have started as a weakness in the turn insulation. That weakness generally takes months or even years to deteriorate to the point where it fails, so it only makes sense to test periodically and routinely with a complete set of static tests that includes surge testing. Modern surge testers will find the level of weakness in the turn insulation without creating any damage to the insulation. Windings that fail as a result of surge testing would certainly fail at the next startup. It only makes sense to locate potential problems in advance of failure to allow the technician to be in control of when an asset gets the attention it needs.
Preventing unscheduled downtime and expensive repairs should be the primary goal of every predictive maintenance program, and surge testing is a vital part of that program. The benefits of periodic, routine surge testing far outweigh the risks.