Electric motors convert electricity into useful energy that keeps much of the world working. From refrigerators in homes to motors operating large machines in manufacturing plants, electric motors provide necessary power.
In 2019, it was estimated that there were more than 700 million electric motors of various sizes operating in the world. A motor malfunction or failure can cause inconvenience at the least, and often costly disruption of vital systems.
Keeping electrical motors running requires special testers to analyze their operation and diagnose potential problems before they occur.
Eric Winzer, technical support manager at All-Test Pro, Old Saybrook, Conn., at the time of the interview, said basic maintenance tests for electric motors include detecting ground faults; exercising the entire winding system to determine balance and symmetry of each phase of a three-phase electric motor; and detecting any developing turn-to-turn, coil-to-coil and phase-to-phase faults to predict motor failure.
Winzer said All-Test Pro offers energized and de-energized motor-testing technologies to assess the complete health of a motor system.
“Motor-circuit analysis (MCA) is a nondestructive technology utilized for de-energized motor testing,” Winzer said. “MCA can determine electrical faults in the motor system from the motor-control center (MCC) all the way to the motor. MCA can determine ground faults, motor contamination, developing coil-to-coil, turn-to-turn and phase-to-phase winding faults, rotor faults and poor connections in the motor or motor cables, and it allows the user to trend individual measurements to predict motor failure before it happens.”
Electrical signature analysis (ESA) is the technology used for energized motor testing, he said. With ESA, it is possible to evaluate the condition of incoming power, motor efficiency and the mechanical condition of the motor and driven load. Mechanical faults that can be identified with ESA include broken/fractured rotor bars, stator eccentricity, misalignment, bearing condition and many other mechanical problems that can result in motor failure.
Winzer believes multifunction testers are the most suitable for electric motors.
“Utilizing a single instrument that can determine both ground faults and internal winding faults will present a picture of the complete health of the electric motor,” he said. “A motor tester that incorporates MCA is the best technology to troubleshoot motor failure and find the root cause issue. MCA utilizes a low-voltage, nondestructive AC signal that allows the user to detect faults from the motor control center with a motor drive still connected to the cables to the motor itself. By conducting a test from the MCC, it is possible to pinpoint directly where a fault lies instead of blindly changing out components of a motor system.”
A basic electric motor kit, Winzer said, should contain an instrument with MCA technology, a set of test leads to connect up to all three phases of the electric motor and an MCA analysis manual to refer to for proper fault identification and condemning criteria.
Sean Silvey, product specialist at Everett, Wash.-based Fluke Corp., said a visual inspection is the first step in evaluating an underperforming electric motor, because you may be able to see evidence of a problem.
“If possible, remove the motor to see if the bearings are bad. At this time, we would perform an insulation test and validate that the insulation value has not been compromised,” Silvey said. “If all this measures good, replace the motor and verify the voltage and current are balanced. Also confirm that voltage is being supplied at the motor input terminals and check that the current present at the terminals is within the motor rating.”
The testers needed are a digital multimeter (DMM) or clamp meter and an insulation tester, he said. For more in-depth analysis of waveforms and troubleshooting, use an oscilloscope.
“With a 1587FC insulation DMM, we have the capabilities of two meters in one for voltage and insulation testing. With an added current clamp, we can measure the amperage during operation,” Silvey said. “A handheld meter can make visual confirmation of voltage and current waveforms. And with two to four options, it is possible to compare and contrast multiple test points on the motor being tested.”
Silvey said the major change in motor-test equipment is moving from older crank-style insulation testers to digital instruments and new functionality.
“The motor-drive analyzer function on some of Fluke’s oscilloscopes makes it easy for users by providing guided steps to walk them through where to make the measurements and where to connect. This also provides users with enough data to locate/identify the problems and then save data/screens within the tester to compare for future measurements,” Silvey said.
Following safety protocol during testing is essential. “First and foremost, when making any live electrical measurement, confirm the correct personal protective equipment is on,” Silvey said. “When conducting an insulation test, confirm that the test voltage has been discharged.”
The Fluke line of testers used for motors includes insulation testers, digital multimeters or clamp meters, infrared thermometers, thermal imagers and oscilloscopes/motor drive analyzers.
Jeff Jowett, senior applications engineer at Megger, Dallas, said that, hands down, multifunction testers are best. Using multiple instruments increases the chances of operator error and confusion over divergent levels of accuracy while adding extra time to the test completion.
He suggests basic electric motor-test “kits” would contain insulation testers, low-resistance ohmmeters, voltmeters/DMMs, winding resistance testers, phase sequence indicators, power factor (tan delta) testers, surge testers and partial-discharge monitors.
Jowett said Megger’s motor-tester catalog ranges from one-test devices up to instruments for complete system analysis, depending on the level of concern.
“Low- and medium-voltage insulation testers are critical for motor maintenance,” Jowett said. “So are low-resistance ohmmeters for connections and contacts. Winding resistance testers are fundamental. Megger offers a handheld motor tester that performs nine fundamental motor tests. Our Baker line offers full-featured diagnostic testers that include surge, insulation resistance, winding resistance, PI, DAR, step voltage and ramp tests, hipot, inductance and capacitance, phase angle and dissipation factor, tan delta and partial discharge, and a unique pulse-to-pulse error area ratio technique. The line also includes live, online systems that monitor power quality for 120 parameters on multiple motors simultaneously and issue alerts.”
Jowett also emphasized the importance of following safety protocols during testing. (Note that all comments regarding safety testing are for information only and should not be considered comprehensive guidelines.) Jowett gave an overview of how to use electric motor testers:
Observe testers’ ratings against IEC 6101-1 for arc flash/arc blast safety.
Testers should be equipped with automatic visual and audible warnings should the device under test (DUT) become live. At the end of a test, motor windings can store lethal capacitive charges. Be certain that testers are equipped with auto discharge and warning features.
The safe maximum limit of a measurement connection is that of the lowest-rated component in the measurement circuit formed by the instrument, test leads and accessories.
Grounding the tester: the devices are grounded through the power cord’s grounding conductor. To avoid electrical shock, plug the power cord into a properly wired/grounded receptacle before connecting the product test leads.
Ensure that hands remain behind finger guards of probes and clips.
Do not touch test lead connections while a test is in progress.
After connecting the test leads to the DUT, route the cables in a safe manner between the DUT and the tester so as not to pose a tripping hazard or interference with the testing process.
Excess test lead cable should be arranged in a manner as to prevent coils from forming near the unit to reduce potential for interference with test data being collected.
The circuit under test must be switched off, de-energized, securely isolated and proven dead before test connections are made. Use a test meter or other appropriate device to confirm that the asset is fully discharged.
When testing from the switchgear, check that all proper lockouts are in place and that proper connection is made to the test points for the asset being tested. Prevent connection to an adjacent live system during testing by confirming with appropriate test instruments that the asset circuit to be tested is dead.
Circuit connections, exposed conductive parts and other metalwork of an installation or equipment under test must not be touched during testing.
Always know what test is being performed and when. For example, do not adjust test leads when operating a test. Leads will have live voltage and severe electrical shock may result.
For capacitor-started motors or systems with surge arrestors/power factor capacitors, be sure to disconnect all capacitors from the test circuit before testing.
After capacitive loads are measured, ensure that the test has terminated and the load is safely discharged before disconnection. Failure to comply with this instruction is an electric shock hazard.
Upon completion of any DC hipot, megohm, polarization index, step voltage or dielectric absorption tests, be sure to short the winding to ground and allow time for discharge before disconnecting the test leads. If you do not do this, voltage may build up on the winding.
Never attempt to test a winding with the host and power pack leads attached to the winding. It will damage the tester.
Dynamic tests. Prior to opening any MCC, ensure that appropriate arc-flash protection clothing is worn. Be sure that appropriate lockout/tagout procedures are properly understood and followed by all personnel.
Depending on the kind of test to be run, be sure there is no physical proximity to the shaft of the motor or any other moving part of the machinery.
Ensure the motor’s phases are not positioned near ground or each other.