How to Test an Electric Motor: Techniques, Part 1
July 7, 2020
There are so many different tests that you can perform on an electric motor — and that’s why it’s key that you know what the purpose is of these tests, how they work, and what the data means. This blog post is the first in a two-part series on the subject of electric motor testing.
We’ll start with a quick review of why testing is important followed by a discussion of rotor bar, hipot, surge, and motor winding resistance as well as a discussion of vibration analysis.
Importance of Testing
Bearing problems may be the number one cause of electric motor failure, but electrical faults are a close second. And electrical failures require your maintenance group to go about testing the motors in your care.
The most obvious benefit of testing is troubleshooting. When a motor isn’t working right or fails, you can use testing to track down the most probable cause of the problem. You can also use test results as a measure of a motor’s performance. That data, in turn, can inform decisions about repairs, maintenance, rebuilds, and replacements.
Regular testing is key to your maintenance program and contributes to the performance and reliability of the motors in your care. For example, you can detect minor problems before they become expensive failures. And remember, well maintained motors have much higher reliability, and both the M&O costs and energy costs for them are lower.
Rotor Bar Tests
Cracked rotor bars are a common problem for electric motors. And that’s probably why there are different types of tests for cracked rotor bars. These tests include the growler test, single-phase rotor test, and high current rotor test.
You can use a growler test to look for discontinuities in the flow of current through the motor. You remove the rotor from the stator, then induce a current in it via a laminated core wrapped with wire. Using iron filings, you can visually detect discontinuities from the patterns creating in the filings.
In a single-phase rotor test, you apply single-phase power to the motor so that the rotor slowly rotates. Using an analog meter, monitor the phase while looking for any fluctuations in how many amps are drawn.
To perform a high current rotor test, you remove the rotor from the stator and apply a high current through the shaft of the rotor. Then an infrared camera will allow you to visually inspect the surface of the rotor for localized hot spots. Those localized hot spots reveal problems with the rotor bar.
A hipot (high potential) test, also called a dielectric strength test, checks for weaknesses in cable or wire insulation. To perform this test, you apply current between the electrical circuits and the frame. Note that specific overvoltage levels applied are dependent on the motor and its specified voltage.
During this time, you measure the leakage current and calculate the corresponding meg-ohms. Areas with lower meg-ohm readings have damaged insulation.
Data from a surge test (also known as a Baker test) will help you detect motor burnout and can predict potential motor failure. From the results of a surge test, you can identify failing insulation, dead shorts, loose connections, and unbalances.
You should always perform surge tests in accordance with the IEEE 522 standard. This standard is important: it indicates what voltage level to use depending on winding type and motor condition. When performed according to standard, a surge test will provide you with reliable data obtained without damaging your motor.
During a surge test, you use a special type of testing machine known as a surge generator to apply a voltage pulse (a surge) to each set of motor windings. It is typically performed at twice the line voltage plus an additional 1,000V. This voltage is injected into each phase. The resulting sine waves from every phase must be equal, otherwise a problem is indicated.
Motor Winding Resistance
The goal of a motor winding test is an off-line test used to track down winding failures. You should perform this test whenever you see cracks or burn marks, or you’ve noticed a burning odor coming from the motor.
The drawback of a winding test is that you will need to disassemble the motor; on the plus side, the only items needed to perform the test is an electrical schematic of the motor along with a multimeter.
You will start by cleaning off the windings with shop air and inspecting them. Next, set the multimeter to midrange and configure it to measure resistance in ohms, then touch the leads together to verify that the reading is 0 ohms. Check the motor’s schematic or its motor winding diagram and use the multimeter to measure the resistance of each winding leg. Each leg should have a small resistance reading — if it reads open or short, there is problem
Vibration analysis, an online testing method, can provide you with a wealth of information about the current state of your electric motor when done correctly. The data for analysis comes from a MEMs sensor that generates a varying amount of voltage depending on movement. When you combine this displacement data with time, the result is a time waveform. Using the time waveform data you can perform an FFT (Fast Fourier Transform) that provides even more information.
The results of vibration analysis can point to bearing defects, misalignment, system or component imbalance, resonance conditions, rotor/stator faults, and the presence of broken welds or loose bolts.
Many facilities set up a vibration route so that vibration data is collected according to a schedule, along a predetermined route through the facility. This type of comprehensive data can help you track vibration changes in the motors over time. For example, you can tell from the data when a bearing is just starting to wear out, or bolts are beginning to come loose.
To perform vibration analysis, you will need equipment for measuring and storing the data, as well as software tools to perform the analysis. And you will need someone with a specialized knowledge in vibration to interpret the data (preferably someone that holds certifications for vibration analysis).
Those are just a few examples of useful tests for electric motors. We’ll be covering more in Part 2 (next week), but in the meantime remember that mastering how to perform electric motor tests and interpret the data is key to having high performing, reliable motors.
Posted in Predictive