The term “Electric Motor Testing” (EMT) is one we conceived here at Snell to refer to a group of testing methodologies that are called different things by the manufacturers of the various equipment utilized to perform them. We wanted to use a term that wasn’t among those used so we could avoid the appearance of preferring or endorsing one over the other. We differentiate between energized and de-energized testing, but we use EMT to refer to either type of testing. I like the term, but it does leave an element out.
In the various literature out there on this topic, one might see the terms Motor Circuit Evaluation (MCE) or Motor Circuit Analysis (MCA) used to describe de-energized (also sometimes called “static”) EMT. Those terms are a bit more descriptive, in that they include “circuit” in them. When performing de-energized EMT, the typical method is to attach to the motor circuit at the starter, generally on the T leads as they leave the starter itself. De-energized EMT looks at several electrical parameters; phase to phase resistance, phase to phase inductance, capacitance to ground and resistance to ground among them. It also allows for polarization index and dielectric absorption testing, depending on operator selected options.
All the test types described above are looking at the electrical interaction between phases. In the motor itself, the stator windings are compared to one another from one phase to another. The comparison of electrical resistance, capacitance and inductance is also being measured in the cabling. That’s why the inclusion of the word “circuit” exists in the naming convention of these technologies. When attaching to the motor at the starter output, you’re testing both the motor and the motor circuit, hence the names MCA/MCE. When applying de-energized EMT from the starter, the cables and the motor are being tested together.
Some of the failure modes detected with de-energized EMT, that are applicable to motor windings, also apply to cabling. Insulation embrittlement, moisture intrusion, insulation breakdown all can be detected with this test method. However, when testing from the motor starter, the results detected can be caused by either the motor itself or the motor circuit. Isolation is required to determine where the failure mode exists. The next step after discovering one of these failure modes at the starter is to electrically disconnect the motor and test again in the same location. If the failure mode still presents, the motor is the culprit. If not, the cables could then be tested in a stand-alone fashion.
This can be easily accomplished utilizing most motor testers. While these instruments aren’t intended to be utilized just for cable testing, configuring them to appear electrically like a motor can often yield the type of results necessary to identify failure modes. Another option is utilizing a cable tester once the determination has been made by isolating the motor that the cables are to blame for the anomalous data.
No matter what type of tester you have, or what you would call the actual tests run, EMT is a powerful tool for your plant’s reliability program. If you’d like to learn more, consider attending our Electric Motor Testing—Energized & De-Energized course in 2019. Or, order one of our on-demand webinars now and get a jump start on 2019!