Fire and safety issues, wasted energy, unexpected breakdowns, unnecessary wear on equipment, early replacement—these are some of the serious and costly problems that can result from improperly operating electric motors. To keep electric motors running efficiently, they require tests to analyze and troubleshoot operation.
Jeff Jowett, Megger senior applications engineer, said a basic electric motor test kit includes an insulation resistance meter with a thermocouple or infrared thermometer for temperature, voltmeter with supply phase direction of rotation, low-resistance meter, motor direction of rotation meter, inductance and capacitance meter, and a complete set of test leads to perform each test.
“First, distinguish whether the problem is electrical using an insulation tester or mechanical [bearing check with vibration analysis],” Jowett said. “Then proceed step by step in the indicated direction to eliminate potential failures until the correct one is found. If the problem is electrical, testing usually consists of insulation resistance [megohm] tests and a low-resistance test. These tests are essential in determining the health of a motor.”
Jowett said multifunction testers that perform all the tests necessary for basic motor maintenance are available. Individual testers are still important where testing parameters need to be carried to maximum capabilities in troubleshooting operations, and for speed where simplicity of operation is paramount.
Jowett recommended the following tests:
- On the feeder circuit, check that supply voltage remains within the tolerance of nameplate voltage.
- If three-phase, check for correct rotation.
- Test for insulation, measuring resistance phase to ground first, then phase to phase or conductor to conductor.
- Loose or corroded connections can be checked with a multimeter, or a high- current, low-resistance ohmmeter.
- Winding resistance can be tested with a low-resistance ohmmeter or, better, a dedicated winding resistance tester for higher power. If windings are found to be faulty, instrumentation is available to predict insulation degradation, such as partial discharge and surge testing.
- Phase-to-phase inductance measurements can be useful for diagnosing cracked rotor bars, shorted turns, rotor porosity, lamination damage and air gap eccentricity problems.
- Bearings can be checked with a vibration meter or ultrasonic detector. Sources of excess heat can be detected with an infrared detector or thermocouple.
“Modern test equipment of good quality comes with numerous built-in safeguards, including warning indicators and automatic discharge of static voltage,” Jowett said. “The first order of the day is to read the instruction manual. All reliable manufacturers of quality test equipment begin their manuals with a safety section. This isn’t boilerplate. Read it!”
Richard Wexler, director of marketing, Flir Systems, said excessive heat is first and foremost the cause of electric motor problems.
“Heat can be caused by many issues, including overcurrent, current imbalance, harmonic voltage distortion, dust buildup affecting ventilation, inadequate lubrication, motor vibration, bearing problems, rotor bar issues, insulation degradation, internal resistance increase in the motor windings, loose connections inside the motor and connections in the electrical system feeding the motor,” he said.
Another factor contributing to heat is the physical environment, including excessive humidity and high ambient air temperature.
Basic electric motor tests include thermal imaging inspections for predictive maintenance, checking the motor’s electrical connections and components for signs of failure (overheating), voltage measurements, current measurements, phase rotation, motor current signature analysis, vibration testing and ultrasound tests.
“Thermal infrared cameras are used to inspect overheating motors,” Wexler said. “RMS voltage and current testers are multifunctional and very useful for testing motors. Handheld meters that are multifunctional can measure the following: RMS voltage, RMS current, harmonic voltage and current distortion, resistance and VFD output frequencies to the motor.
“Most of the other testers are single function, such as thermal infrared cameras, megohmmeters, digital low-resistance ohmmeters, ultrasonic testers, vibration testers, motor current signature analysis, phase rotation meters, and tachometers,” he said.
Tests typically performed when a motor is energized include thermal infrared inspections of motor, bearings, electrical connections and components; ultrasound; vibration testing; motor current signature analysis; input/output voltage and current testing; and harmonic distortion testing.
For de-energized motors, tests include insulation resistance with a megohmmeter, winding resistance (digital low-resistance ohmmeters), induced current testing for rotor stator and rotor, a test usually performed at a motor shop.
Wexler said the most significant area of change in electric motor test equipment is in software, which has improved analysis speed and broadened the types of tests that can be performed. Also, most field motor test equipment is smaller, lighter and more affordable than ever before.
According to John Olobri, director of marketing, AEMC Instruments, it seems electric motors always fail at the most inopportune time, and although the original cost or replacement cost may not be significant, it is the damage or loss of production and product that can be extremely expensive.
“For 80 percent of testing and for ‘annuals,’ megohmmeters are well suited,” Olobri said. “For pre- and post-rewinding applications, multifunction testers are suitable for additional tests, including shaft and bearing erosion caused by harmonic currents, damage from vibration and/or misalignment, and contamination from dust, vapors, moisture and fluids.”
For troubleshooting underperforming motors, input power is the first order of business, followed by winding continuity, insulation testing with temperature compensation, and shaft alignment.
Tester software packages ideally have setup parameters, run tests automatically, provide test results, and generate reports automatically.
“A significant advance in equipment is megohmmeters with the addition of ramp/soak mode, step change mode and oscilloscopes for harmonics,” Olobri said.
The development of motor drive analyzers offers technicians a new way of gathering information on the motors they service and a new way of interpreting and sharing the results, said Frank Healy, product specialist for analysis and diagnostic tools, Fluke Corp.
He said basic motor troubleshooting tools continue to be digital multimeters, clamp meters and infrared thermometers. Insulation testers are used in preventive maintenance programs; vibration testers and laser alignment tools quickly perform vibration analysis and evaluate shaft alignment.
Power quality analyzers locate, predict, prevent and troubleshoot power quality problems in three-phase and single-phase power distribution systems.
Motor drive analyzers are a newer and less common product group.
Key motor-drive measurements include voltage, current, DC bus voltage level, AC ripple, voltage and current unbalance as well as harmonics. More in-depth maintenance tests include voltage modulation and motor shaft voltage discharges. Extended harmonic measurements will help identify the effects of low and high order harmonics on the electrical power system.
“Motor drive analyzers can take the form of a handheld oscilloscope and, in many ways, present measurement data in a form that will be familiar to oscilloscope users,” Healy said. “Motor drive analyzers save time and eliminate the hassle of setting up complex measurements while simplifying the troubleshooting process for variable frequency drives. The user simply selects a test and then follows a step-by-step guided measurement process. These tools can show the user where to make voltage and current connections, while preset measurement profiles ensure the capture of all the data needed for each critical motor-drive section—from the input to the output, the DC bus, and the motor itself. These tools can also include built-in report generators to quickly and easily generate as-found and as-left reports.”
Healy said guided testing with graphical step-by-step voltage and current connection diagrams make setup and connection to variable frequency drives a simple process. Preset measurement profiles collect data based on the chosen test procedure and reduce the need for complex configuration. In addition, built-in report writing capabilities assist in producing professional as-found and as-left motor-drive troubleshooting reports. In short, users get more in-depth information while the motor drive analyzer performs most of the complicated steps.
A basic motor test kit should start with a go/no-go voltage detector. Close behind, Healy said, would be a multifunction true-rms AC/DC clamp meter. An insulation tester can prove highly valuable as part of a preventive maintenance program. An in-depth motor test tool would be a motor drive analyzer already discussed.
Multi- and specific function testers are available. Which is best depends on the user’s purpose and needs.
“For general maintenance or first-line, opportunistic troubleshooting, a dependable AC/DC clamp meter is an excellent choice,” Healy said. “Such a clamp meter can tell much about the immediate condition of a motor and often point to something other than the motor—a supply line, for instance—as the actual cause of the problem.
“Multifunction motor testers excel in two different areas. First, they can perform specific tests that are beyond the capability of a clamp meter. These include such measurements as pulse-width modulated voltage measurements, voltage/hertz ratio, voltage unbalance drive input, voltage unbalance drive output and motor input, current unbalance drive input, rise and fall time (voltage difference, time difference, voltage versus time difference, overshoot), harmonics and spectrum and shaft voltage,” he said.
These measurements can be recorded over time and used as a component of preventive maintenance programs. Significant changes in one measurement compared to past readings can provide an indication of equipment that is beginning to degrade, allowing scheduled service as opposed to a sudden breakdown.