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White Paper: Evaluating Motor Insulation Systems


The leading cause of electrical failure in motors is deterioration of the insulation system. In an electric motor, the conductors that make up the coils are individually coated with resin or varnish that keeps the current flowing through the winding as intended. There is also insulation which separates the windings from the motor core or frame. Weaknesses or faults in the insulation system will allow current to flow through the insulation, either to adjacent coils/conductors, or to ground. When current flows to adjacent conductors, we have a winding fault. If it flows to the motor frame, we have a ground fault.

Insulation failures can be caused by mechanical (vibrating or moving, overloading), electrical (surges and spikes, over or under voltage), or environmental (heat, dirt, moisture) stresses.

Traditional Measurements

One of the more common tests used to evaluate an insulation system is the polarization index or PI Test. This test is normally performed with a megohm meter, and involves measuring and recording insulation resistance over a period of 10 minutes. By definition, the Polarization index or PI is the measured value at 10 minutes divided by the measured value at 1 minute. The general guideline is that a PI of less than 2 indicates there may be a problem in the insulation system. Challenges involving the PI measurement are:

  1. The PI works by measuring insulation resistance to ground, so gives little insight to insulation within the coils
  2. According to EASA, the test is not applicable to random wound motors – only for taped/sealed coils, generally found in medium and high voltage motors. While often used on 480V machines, it is not a recommended application
  3. It requires a minimum of 10 minutes to perform
  4. Some modern insulations polarize very quickly, showing a low PI, though being perfectly healthy.

An Alternative Measurement Technology

Dissipation Factor, also called loss angle or tan-delta has been used to evaluate the insulation system of power cable, transformers, and generators for years and is now being applied to other systems. Here is how it works in principal: If an insulation system is free of defects, its properties approach those of a perfect capacitor. In a perfect capacitor, the voltage and current are phase-shifted by 90 degrees, and the current flow is all capacitive. In real life, the electrical circuit is a parallel RC circuit, with some capacitive and some resistive current. The ratio of these (Ir /Ic) is the dissipation factor (DF) or tan-δ, often expressed as a percent.

If the insulation system is compromised, the resistance of the insulation decreases, resulting in an increase in resistive current through the insulation, and the DF increases. The higher the DF, the worse the condition of the insulation system.

So what tool would you use to measure DF on a motor?

ALL-TEST Pro has incorporated Dissipation Factor into their latest generation instruments, the ALL-TEST Pro 33 and ALL-TEST Pro 5. The test gives insight into the health of the insulation system (totally separate from insulation to ground), giving the electrician or repair shop new insight into the health of a machine. The measurement has a couple of advantages over the PI test:

  1. It is faster, only requiring a few seconds in many cases; nowhere near the 10 minutes required for a PI test
  2. It does not require a large amount of energy, so the test tool can be compact and lightweight
  3. It gives insight into the entire insulation system, not just insulation to ground.

Of course, if you are going to take the time to test a motor, you want to get as much information as you can. For evaluating total motor electrical health, the ALL-TEST Pro has no equal.

  • We use a combination of AC and DC measurements on all phases to detect virtually every possible electrical fault condition
  • We provide automated diagnostics with good-warn-bad calls for all measurements with a single test sequence
  • With one tool you can test any type, any voltage, and any size motor
  • We can compare and trend to detect small changes indicating a developing fault condition

Give us your challenge! If you have a motor health question or have an interest in improving your motor monitoring program, we can help!

Call 866-225-0182 anytime, or simply email us from our contact page.