Test and Measurement Methods
Dynamic Versus Static Motor Analyzers
What Is the Difference Between Dynamic and Static Motor Analyzers?
In general there are two different methods used to evaluate the health of an electric motor or generator: dynamic and static motor testing. As its name suggests, a dynamic motor analyzer is used while the electric motor or generator is running (online). A static motor analyzer is used after the motor or generator is taken offline then disconnected from its power supply and motor control center (MCC). To optimize motor health, equipment is monitored in predictive maintenance programs (PMPs). To most effectively determine the electrical health of motors and generators in a PMP, both types of test equipment are used.
While dynamic motor testing can be very useful to track a motor’s existing state, static motor testing at over-voltage should be included as a regular part of motor diagnostics. Obtaining complete information about electrical health can improve any predictive maintenance program. This is why winding resistance, insulation resistance, DC hipot, megohm, surge test, and the partial discharge (PD) test, among other tests, have been developed. These tests, with the exception of PD, cannot be performed while the motor is operating online, and are therefore available only through offline (static) testing.
Dynamic Motor Analyzers
Dynamic motor testers use a set of current transformers (CTs) and corresponding voltage probes located at the motor’s MCC or terminals. The CTs and probes collect data about the device under test (DUT) that informs the user about the DUT’s:
- voltage and current levels
- load level
- torque signature
- rotor-bar signature
- operating efficiency
Online motor testing can be very beneficial if data is tracked over time. Starting by logging and analyzing data from the first day, a history of the DUT is collected. Then after it is placed in service, the motor is periodically, or, in some cases, continuously, tested to build a data history. The data accumulated over time can be analyzed to determine whether certain failure modes are statistically likely to happen in the DUT.
Dynamic monitoring or condition monitoring often includes other measurements, such as vibration and temperature. But even with these additional measurements, not everything is known about the motor. None of the methods for online monitoring and analysis alone can provide a complete picture of the motor’s electrical health. Finding the additional key data is where static tests come into play.
Static Motor Analyzers
Static motor testers, or motor analyzers, such as Electrom Instrument’s iTIG, 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 measurements such as the megohm test 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 tests, 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. Although many believe there will be damage from over-voltage tests, this is an incorrect assumption. Numerous tests and studies show that no measurable degradation in the insulation is caused by routine over-voltage testing. See Are Surge Tests Destructive? and Non-destructive DC Hipot Test Methods for more information about this subject regarding both surge tests and DC hipot tests.
The point is that the energy available in over-voltage tests is relatively low. An analogy is the static arc to a metal door handle you are about to touch. When the arc happens, you feel only a pinprick sensation, although the voltage causing the arc can be 20,000V or more. Because your capacitance is low, the result is a low current for a very short duration. The same is the case for a DC hipot and surge test on an electrical motor or generator.