Every industrial facility has motors. They may range in horsepower, age, OEM, and criticality, but they’re still the most common asset facing maintenance and reliability (M&R) teams. According to the Electric Power Research Institute, improving motor maintenance can improve operational efficiency by up to 15%.
In a webinar he gave for Fluke Reliability in February of 2021, motor testing expert Don Donofrio demonstrated how today’s advanced electric motor testing technologies expose motor failure. Donofrio is the Electric Motor Testing and Power Quality Technology Lead at The Snell Group.
“All mechanisms that lead to motor failure cause increased operating temperature, which leads to insulation failure – and motor testing identifies those failure mechanisms,” he notes.
Some 88% of the more than 350 webinar attendees said in a survey that electric motor testing (EMT) was either “highly applicable” or “somewhat applicable” to the machines at their plant or facility.
Electric motor testing is part of the P-F curve, occurring just to the right of thermography on the curve, as illustrated in Figure 1. In fact, Donofrio says, motor testing and thermography work well together. At that early stage of an asset’s lifespan, the temperature can quickly accelerate failure, and motor testing may be the last step before sending a motor in for rewind.
Figure 1. Nolan and Heap’s P-F curve, showing mechanical asset failure intervals and associated testing methods to identify failure conditions proactively.
What’s involved in motor testing
Also known as motor circuit analysis, EMT measures multiple parameters to help technicians assess a motor’s condition and identify fault-conditions. According to The Snell Group, “EMT analyzes six distinct areas critical to the proper operation and health of electric motors: power quality, winding insulation, power circuit, stator health, rotor health, and eccentricity.”
In the webinar, Donofrio calls out the two principal methods of motor testing, de-energized and energized:
About de-energized motor testing:
- The motor is shut down
- Provides the safest means of testing
- Provides the least opportunity for testing
- Enables assessment of insulation
About energized motor testing:
- It is the most common means of testing
- Provides a power quality snapshot
- Current and electrical signature
- Torque analysis
- Requires specialized equipage for insulation testing (limited)
When asked by the webinar audience which form of EMT is most effective, Donofrio says, “De-energized is the safest but the hardest to organize. Its biggest benefit is the ability to test insulation, and insulation testing picks up all the failures. If you had to pick only one test, it would be insulation.”
Energized testing, however, is a more proactive approach since there’s no need to wait until shut down: the testing is done with the motors running. If you are testing an energized motor, “Start with power quality,” Donofrio says. “You’ve got to have clean and balanced voltage and current.”
What to know about de-energized motor testing
De-energized motor testing is recommended for motor acceptance, such as when adding motors into inventory. It is also appropriate for equipment troubleshooting and commissioning, and condition monitoring. The tests can identify:
- Circuit connection faults
- Cable faults
- Motor winding issues
- Rotor anomalies
- Protective and auxiliary circuitry, including surge protection and power factor correction
Most of this type of testing occurs around the motor control center (MCC), starting at the load side of the contactor. Measurement types include:
- Phase angle
- Fault localization
- Current frequency response I/F
- Rotor influence check
- Capacitance to ground
- Resistance to ground (RTG)
- Surge testing
- High potential (HIPOT) testing
- Step voltage
- Timed resistance testing (polarization index and dielectric absorption)
“We can measure resistance, inductance, and impedance and determine phase angles and then use these values to do fault localization, to determine whether it’s a rotor, circuit, or stator problem,” says Donofrio, as illustrated in Figure 2.
Figure 2. Depending on the values measured for resistance, inductance, and impedance, technicians can localize motor faults to the rotor, stator, or power factor capacitor.
In his webinar, Donofrio reviews each type of testing and the failure modes it detects. At the end, he emphasizes: “The most important test to do is insulation. Go through and meg your motors and make sure you’re getting good resistance to ground readings because that’s what all the failure modes cause, a degradation of the insulation.”
What to know about energized motor testing
Energized motor testing identifies many of the same failure modes as de-energized testing, but by different means. “Energized testing entails connecting three voltage leads, amp probes, and a ground to the motor leads or secondary circuits (CTs, PTs) on medium and high voltage motors,” says Donofrio. This method allows you to assess power quality, current, and electrical signature analysis (MCSA, ESA). Make sure to follow safe practices for energized environments.
The Snell Group recommends energized motor testing for equipment troubleshooting, equipment commissioning, condition monitoring, and power quality fault isolation and localization. Problems identified by energized testing include:
- Power quality problems
- Motor and motor circuit electrical issues
- Motor and drive train mechanical issues
- Most failure mechanisms that lead to motor
The types of power quality testing done for EMT are similar to other power quality diagnostics in many respects, though there is less emphasis on event capture. Figure 3 demonstrates what some of the most common power quality problems look like as waveform distortion. Each distortion pattern indicates a specific type of electrical interference which can, in turn, be traced to the root cause.
Figure 3. Waveform examples of common power quality problems
Donofrio offers Figure 4 as an example of how to correlate power quality data to motor condition. “With the current and impedance values that you get from energized testing,” he says, “if the current unbalance and impedance unbalance are low, everything’s normal.” Moderate unbalance could indicate a connection problem, whereas a high impedance unbalance could indicate a defective power factor capacitor.
Figure 4. Correlating current and impedance unbalance to root cause
For a detailed description of each test and the associated failure modes, please watch the recorded webinar.