Finding out what went wrong when your electric motor fails can help get you back online sooner rather than later. Unfortunately, determining the cause can be challenging without access to the proper testing tools and standards. So, to help you assemble your electric motor testing tool kit, we’ve asked our highly skilled electric motor technicians to share their go-to testing tools and explain what they do.
Electric Motor Test Standards
For the last 140 years, the Institute of Electrical and Electronic Engineers (IEEE) has worked to establish uniform performance and testing procedures for electric motors of all sizes. Publication IEEE 522 (available in print and digital versions for reference) defines the standards for surge testing, setting appropriate voltage levels for a wide range of winding types and conditions.
Now that you have your surge testing reference guide, let’s examine the tools for troubleshooting electric motors.
Visual Inspection
The first step to diagnosing electric motor issues is a visual inspection. Start by verifying that the power source has been shut off at the breaker panel. Check for visible damage to the motor case, shaft, or exposed wiring.
Next, try turning the shaft by hand. The shaft should turn freely unless you’re working on a massive electric motor. If it doesn’t, there may be an obstruction inside the motor or an issue with the bearings. Lubricate the front and rear bearings and try turning the shaft again.
Digital Multimeter
The digital multimeter is the go-to diagnostic tool because it works with most electric motors. It combines the testing capabilities of many single-task tools, resulting in a comprehensive tool that can measure amps, ohms, and volts. Please note that the multimeter cannot be used to measure power, frequency, or phase angles.
Start by taking a voltage reading at the motor terminals. The voltage displayed should match the voltage of the entrance panel main breaker. Most electric motors require voltage that matches the specific range indicated on the motor tag or nameplate for optimum performance and longevity.
If the multimeter indicates low voltage, the motor will likely not start or will operate at a lower speed. Additionally, frequent motor use at a lower voltage than specified will result in internal damage to the motor. Check that the installed wire matches the circuit’s length and ampacity for low voltage readings.
Next, it’s time to check the resistance of the motor circuits. The reading starts low for components that control circuits but can slowly increase over time due to moisture and dirt accumulation. These contaminants can cause the wiring insulation to break down, resulting in unbalanced resistance. Left unchecked, this imbalance can lead to higher operating temperatures, which leads to thermal degradation of the insulating materials.
The Megger (Megohmmeter)
This tool checks the motor’s insulation condition by applying high voltage for a predetermined time and measuring the amount of current lost (leaked) through the insulation materials. When tested regularly, this information can be used to graph and evaluate the insulation condition over time, as well as wear and damage patterns.
The Megger performs a complex test and needs to be set, connected, and operated very carefully to prevent equipment damage and operator injury caused by the high levels of voltage involved. When testing motors, disconnect the power supply and properly isolate the motor to obtain a full motor winding analysis. A reading of ten megohms indicates the motor insulation is good, while a low reading indicates poor or failing insulation materials.
Surge Test
Surge (or impulse) testing is the most reliable way of detecting motor burnout and predicting potential motor failure. The difference is that surge testing can be conducted in air or oil, while impulse testing must be performed in air. The method uses an impulse generator to apply a safe voltage level to each set of windings to isolate performance. During the test, a voltage pulse is placed across two windings while the third is grounded.
The Ammeter
This tool can test AC (Alternating Current) and DC (Direct Current) devices. A clamp-on ammeter can check the full load amps (FLA), which should match the value listed on the motor nameplate or manual. The ammeter must be in series (on the same circuit path) with the component being tested for it to work correctly.
