What is the difference between dynamic and static motor analyzers?

Introduction

There are, in general, two different methods used to evaluate the health of an electric motor or generator. The two are dynamic and static motor testing. As the name suggest, a dynamic motor analyzer is used while the electric motor or generator is running, or what is often referred to as “online”. The second type is a static motor analyzer, which is used after the motor or generator is taken offline and is disconnected from its power supply and motor control center (MCC). In order to optimize and most effectively determine the “electrical health” of motors and generators in predictive maintenance programs (PMPs), these two types of test equipment can be used together.

Dynamic Motor Analyzers

Dynamic motor testers use a set of current transformers (CTs) and corresponding voltage probes located at the motor’s MCC or its terminals. These collect data which informs the user about the device under test’s (DUT’s) voltage and current levels, load level, torque signature, rotor-bar signature, and operating efficiency. The online motor test can be very beneficial if data is logged and analyzed from day one of the motor’s life and periodically, or continuously in some cases, tested after being placed in service. This data can then be used to statistically determine certain failure modes in the DUT. Dynamic monitoring or condition monitoring often includes other measurements as well such as vibration and temperature. However, none of the methods for online monitoring and analysis provide a complete picture of the motor’s electrical health, and this is where the static tests come into play.

Static Motor Analyzers

Static motor testers, or motor analyzers, such as Electrom Instrument’s iTIG II, are used to test the DUT by emulating the worst-case-scenario conditions it will see while in operation, and by doing so teasing out small and large defects without causing damage or degradation to the motor. Low voltage testing such as capacitance, inductance, and impedance have been proven to be helpful, but not sufficiently effective in detecting insulation weaknesses alone. Winding resistance measurements provide added information such as resistive connections, hard shorts in windings, and partial blow outs. Insulation resistance test such as the Megohm and Polarization Index (PI) are used to detect current leakage from the windings to the ground plane due to weak insulation and/or contamination. However, the tests that provide the most information about the condition of the insulation system in a motor are the over-voltage tests. These tests include Hipot and Surge testes, and are done at voltages higher than the peak operating voltage (RMS x 1.41).

By testing the insulation at voltages higher than peak operating voltage, weaknesses in the insulation can be found before the motor has serious problems detected by analysis done at operating voltage. This helps operators plan replacements and determine which motors should be taken out for reconditioning without causing unexpected or unnecessary downtime.

In theory, the higher the voltage one tests to, the more information is gathered. If a serious weakness, such as high leakage during Hipot or a large line to line percent difference during surge is found at 1.5xRMS voltage or less, the motor has a serious insulation issue. Not much time is left before it needs to be taken out of service to be replaced or reconditioned. On the other hand, if the fault is found at 2xRMS there is significantly more life left, and time to schedule proper actions.

The fact that more insulation weaknesses are found when the test voltage increases is also true for Partial Discharge (PD), especially in low voltage motors that should have no PD. When found above peak operating voltage, the life of the motor can in most cases be extended through several different means.

Over-voltage tests done properly on a new, reconditioned, or used motor will not damage the insulation or reduce the life of the motor, something that is incorrectly assumed by many. Numerous tests and studies have been done on this subject showing that there is no measurable degradation in the insulation caused by routine over-voltage testing. For more information on this subject for both Surge tests and DC Hipot tests, take a look at the following links:

The bottom line is that the energy available in these tests is relatively low. One good analogy is that of a static arc to a door handle you are about to touch. When this happens, you do not die, you finger is not even burned, you just feel a pin prick even though the voltage causing the arc can be 20,000V or more. Your capacitance is low, thus resulting in a low current for a very short duration. The same is true for a DC Hipot and Surge test on an electrical motor or generator.

Conclusion

Dynamic motor testing can be very useful to track a motor’s status quo, however, static motor testing at over-voltage should be included in order to obtain information about the electrical health that will help improve any predictive maintenance program. This is why winding resistance, insulation resistance, DC Hipot, Megohm, surge test, and the partial discharge (PD) test, to mention some, have been developed in order to provide a more complete diagnosis. These tests, with exception of PD, cannot be performed while the motor is in operation, and are therefore only available through offline testing. To learn more about the above-mentioned tests and what failure modes they can and cannot find, use the following links: