How to Test an Electric Motor: Techniques, Part 2
July 16, 2020
In part 1 of How to Test an Electric Motor, we talked about the importance of testing and different types of tests that you can perform. In part 2, we are going to talk about more of these tests and how you can put test data to good use.
Shaft Current Test
When you suspect that the motor bearings <LINK TO https://www.hecoinc.com/electric-motor-bearings> have suffered electrical damage, then you likely need to perform a shaft current test.
So how can bearings experience electrical damage? When capacitive coupling exists between the motors and the windings, it can create a voltage on the shaft that can be discharged through the motor bearings. When that happens, it leaves behind surface damage like pitting, fluting, and craters. This type of damage shortens the useful life of the motor bearings and can lead to premature motor failure.
You can detect and measure shaft voltage (and bearing currents that result from it) by either using an oscilloscope with special voltage probes or a shaft voltage tester such as the one made by Aegis . If you’re wondering why you can’t use a multimeter, it’s because the voltages occur too quickly for it to capture. The oscilloscope, on the other hand, allows you to observe those changes over time despite this.
Step Voltage Test
In the previous post on testing electric motors, we discussed the hipot test and how you can use it to check for weaknesses in insulation. The step voltage test serves a similar purpose as the hipot test but provides much more information.
In this test, you increase the voltage in steps while measuring and recording the leakage current. You then plot the resulting data, and if it is linear then there are no major issues. A non-linear plot of leakage currents and voltage indicates that there is a problem with the insulation. The problem will lie at the voltage where a jump in leakage current occurs.
A Megger test evaluates the insulation performance of electric motors. The term “megger” comes from the extremely mega-ohm resistance levels that it measures. A Megger test requires the use of a specialized instrument that referred to by its trade name Megger. A Megger instrument can measure extremely high-voltage resistance levels.
A Megger works by applying a high voltage for a certain length of time while a motor is offline. During that time, it measures the current leakage through the insulation in terms of resistance. Note that you will need to power down the motor and disconnect it from anything that is not included in the test. In addition, you need to take care to ensure that you don’t damage the motor or yourself because of the high voltages involved.
Not all electric motor test methods involve electricity: some are visual. One example of this would be infrared thermography. Thermography is a non-contact, online method of obtaining thermal data while a motor running. One of the key benefits of this test approach is that it won’t interfere with the normal operation. This means you can gather data while the motor is working under its normal load.
To perform thermography tests, you will need access to an infrared camera. The infrared camera captures images that provide data about the temperature profile of the motor, including hot spots and heat patterns. Unusual hot spots or areas where you notice significant changes in temperature can lead you to problems such as insulation degradation or failure, insufficient airflow, and unstable voltages.
Motion amplification measures and records motion that cannot be detected by the human eye. The motion is captured using video camera technology that measures displacements with extreme accuracy. Software is then used to analyze and scale up the displacement data for use in visualizations and plots. Included with those visualizations are videos that reproduce the scaled up motion so it can be seen more easily. An example of just such a video below shows the use of RDI Technologies IRIS M system.
Motion amplification <LINK TO: https://www.hecoinc.com/heco-system/motion-amplification> data not only helps you to see where unwanted motion may be occurring, but helps you understand the different components and interrelationships involved with the motion.
What to Do With Electric Motor Test Data
Test data is useful for far more than just troubleshooting. Test data is an integral part of any condition-based maintenance <LINK TO: https://www.hecoinc.com/heco-system/condition-based-maintenance> or predictive maintenance program. As you begin to collect data over time, you can form a baseline for motor properties, behavior, and performance. With that information, you can tell when a motor needs some maintenance or is developing new problems that you need to address.
When you add surplus motors to the testing schedule, then any motor you need to pull for maintenance or repair can be replaced with far less downtime involved.
Over time, the smart use of testing data will lead to reduced downtime, longer mean time between failures, lower M&O costs, and much better performance from the motors in your care.
Electric motor testing is an important part of keeping your motors running. While we haven’t covered every possible test you may need to perform, these two posts should have provided you with a good starting point. And remember: testing is important for far more than just troubleshooting.
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