Verify Motor And VFD Compatibility

Fig. 1. This poorly wound stator is full of cross-overs and loose conductors.

Can you rely solely on testing for the answer?

By Howard W. Penrose, Ph.D., CMRP, MotorDoc LLC

How reliable are your electric motors in a variable-frequency-drive environment? In the past several years, there seems to have been an increase in the number of inverter-applied electric motors that require investigation from electrical and mechanical standpoints. The mechanical faults are normally failed bearings, with evidence of fluting, while the electrical failures typically manifest as unexpected or undetected winding faults—which can be difficult to verify. Those types of faults, most of which go virtually undetected in winding tests, are often identified when a variable-frequency drive (VFD) trips
a motor.

WHAT TO DO

How do you determine the reliability of an electric motor for application on a VFD? Are there tests that can be performed to verify the condition and provide a high level of confidence the machine will start and run far beyond the manufacturer’s warranty? 

Common insulation-resistance and ohm-meter testing will only detect the instant condition of a motor and whether it is open or grounded. To detect potential problems with the bearings, a shaft-voltage or current test would have to be performed with an oscilloscope. It’s important that this test be conducted with the motor in place, as the conditions surrounding shaft currents are dependent on the location, cables, connections, and application. For shaft currents, in-place solutions can be provided.

Winding faults are a different, and more challenging, animal. That’s because the reliability of motor windings starts with material selection and the winding process, whether performed by the manufacturer or repair center. Traditional test methods—resistance, insulation-resistance, high-potential, and surge-comparison testing—will rarely uncover defects such as out-of-order turns and cross-overs. These types of issues, such as those shown in Fig. 1, are latent issues in which greater potential will exist between the conductors. Small gaps and loose conductors are other winding weak spots that can generate partial discharges, conductor vibration, or areas where contamination can be trapped, resulting in damaged insulation.

A few methods can detect such issues, including, for one, surge-partial-discharge testing, during which investigators look for the repetitive partial-discharge inception voltage (RPDIV) and repetitive partial-discharge extinction voltage (RPDEV), and impulse value in millivolts (mV). These tests are based upon IEC/TS 61934-2011, “Electrical Insulating Materials and Systems–Electrical Measurement of Partial Discharges (PD) under Short Rise Time and Repetitive Voltage Impulses,” which specifies when those values are detected. (Note: RPDIV is the point where 10 subsequent discharges are detected. RPDEV is the point where the subsequent discharges fall below 5.)

Fig. 2. This screen shows detection of partial discharge (PD) in a 50-hp, 460-V motor.

As shown in Fig. 2, a 50-hp, 460-V motor has an RPDIV of 1,652 V, RPDEV of 1,514 V, and energy of 2,402 mV. Based upon this information, so long as the drive pulses are less than 1,652 V, there should be very little partial discharge. It is also important to note where the partial discharge is located, all the way to the left under the first wave of the ‘sine wave,’ which is actually a resonance that normally indicates weak insulation, or an arc versus higher-frequency PD, which normally occurs later in the dampening sine wave, such as in Fig. 3.

Above. Fig. 3. This screen shows detection of partial discharge (PD) in a 1-hp, 460-V motor.

Figure 3 shows a 1-hp, 460-V motor with an RPDIV of 2,165 V, an RPDEV of 2,096 V, and a high potential of 6,220 mV average impulse. As the DC bus of the VFD is about 650 V, with a theoretical maximum peak of twice the DC-bus voltage for reflective waves, if the RPDIV remains above 1,300 V, the motor should have a reasonable winding life. In most electric motors without winding defects, the RPDIV will be well above the calculated test voltage of the off-line partial-discharge instrument.

The challenge, though, is cross-overs and loose connections (as shown in Fig. 1 above). These conditions require a visual inspection, as they can’t be seen with test instrumentation. Sadly, in light of many manufacturers moving to areas with lower labor rates, these issues are becoming more prevalent, as is also the case when low-quality repair facilities and inexperienced winding staffs are thrown into the mix.

Is your operation experiencing a high rate of winding failures? If so, in addition to inspections and tests surrounding filtered VFD output, or lack of, the windings should be inspected to determine whether their quality relates to part of the fault. Keep in mind that winding failures frequently occur when conductors cross each other or loose and moveable conductors exist. Thus, it’s important to ensure that all work is neat and tight, with conductors next to each other, phase separators in place, and winding ties at every slot on the connection and opposite connection sides. The windings should be trickle impregnated or dipped and baked in such a fashion to create a solid mass and no loose conductors.

NOW THE SHORT ANSWER

Mechanical faults due to shaft currents in VFD applications are relatively common, and can be readily detected and corrected. The apparent increase in winding faults in these applications seems to be due to a combination of the application and new motor-manufacturer and motor-repair quality-control practices. Errors such as loose conductors and turn cross-overs, and those associated with insulation systems, however, are difficult to detect.

In short, they require motor owners to either check quality in-process or specifically inspect windings should a high rate of failure be determined. Unfortunately, such conditions are extremely difficult to detect with winding-test instruments alone. EP

Howard Penrose, Ph.D., CMRP, is founder and president of MotorDoc LLC, Lombard, IL (motordoc.com), and most recently, past chair of the Society for Reliability and Maintenance Professionals, Atlanta (smrp.org). Contact him directly at howard@motordoc.com.