Your webcast questions answered: Troubleshooting techniques for AC induction motors
Over their operating lives, most AC induction motors exhibit common problems, from transients to voltage imbalances and single phasing.
In April, Plant Engineering presented a webcast on common issues associated with troubleshooting techniques for ac induction motors presented Michael Lyda, PE, Motor and Drive Engineer, Advanced Energy Corp. and Ronnie Alford, Motor Lab Coordinator, Advanced Energy Corp.
Attendees were given an opportunity to ask questions following the presentations. Below are Lyda and Alford’s answers to the questions there was no time to address during the webcast.
Question: What are the concerns of having an unbalanced three-phase voltage supply?
Answer: Unbalanced supply voltage will lead to unbalanced current in the motor windings. This will likely lead to hotspots in the motor, which is not good for long-term motor reliability. Unbalanced supply voltage is also detrimental to motor efficiency.
Question: Which parameters should be checked after rewinding of motor?
Answer: Standard practice following a motor rewind would be to perform an insulation resistance test, polarization index test, surge test, hi-pot test and measure winding resistance. These tests will not only catch faults in the rewind process but can also be used for trending the motor winding over the run life of the machine.
Question: What issues arise from allowing motors to free spin for long periods of time? Example, a motor coupled to a fan that is in an air stream that causes continuous rotation when the motor is not energized.
Answer: As long as the motor isn’t spinning above its rated speed, this shouldn’t be an issue. However, there will be some counter-electromotive force (back EMF) at the motor leads so those should be isolated for safety purposes. Also keep in mind even though the motor is not energized the rotation of the shaft is still putting run hours on the bearings.
Question: What is the ideal color for the motor frame?
Answer: Motor manufacturers have preferred colors for their motors. In general, motor frame color shouldn’t matter unless you are in an environment or application that requires a specific color. Fire pump motors should be red, oil well-pump motors would typically be white, etc. The quality of the paint should certainly be paid attention to, to avoid oxidation.
Question: I have seen numerous bearing failures due to electrolysis related to stray current from variable frequency drive (VFD) use. It is tough to consistently ground the shaft from the stray voltages. Your thoughts?
Answer: Shaft grounding can help mitigate this. There are various products in the industry available.
Question: What will be the allowable current to avoid windings early damage?
Answer: Generally, you should keep the operating current at or below rated current. If the motor has a service factor, it will likely correspond to service factor amperage as well. The motor can operate at the service factor, but it should be treated as a short-term overload factor. If you need to run at a higher load consistently, then it would likely be best to increase the size of the motor, if possible.
Question: What would be too much current imbalance?
Answer: This is relative, of course you don’t want any leg of the current to draw higher than the rated amount.
Question: Why does a cooling fan cover not have openings to safely allow take vibration readings?
Answer: When we measure for vibration, we take the measurements at five points and do not measure an axial point on the non-drive end if there is a fan. If it is a motor without a fan, then you can measure this sixth point. It would not make sense to remove the fan for the vibration measurement since this would impact the readings.
Question: In troubleshooting induction motors, what key components should we take into consideration?
Answer: This was the gist of the presentation, so I won’t go into too much detail, but the best place to start is to use your senses. Listen for abnormal sounds, watch for abnormal looking items, smell for abnormal scent coming from the machine, and feel (safely) for elevated temperature. After that then you will have a better idea of what the next steps may be.
Question: What part does the Industrial Internet of Things (IIoT) play in motor maintenance?
Answer: IIoT is becoming popular in this industry for predictive and preventative maintenance. Once you have sensors installed on machines you can store data either at the machine or to the cloud and use software for predictive and preventative maintenance. You can request sensors come with the motors from the factory through your distributor, or you could install them afterward. IoT has a great future in this industry and end users are just scratching the surface at this time.
Question: How does a motor with a 1.15 service factor differ from a motor with 1.0 service factor?
Answer: The 1.15 SF motor can be operated at 115% of rated load for short periods of time. The 1.0SF motor should not be operated above rated load for any period of time.
Question: For a typical intermittent duty motor, approximately how many cycles should be expected out of the motor before failure occurs? Or I suppose instead of cycles, we could reference run hours.
Answer: No idea. It depends on the quality of the motor build, proper maintenance techniques being followed, and if the motor is the right fit for the application. In some cases, motors can last for decades.
Question: For starting and rated torque curves, could you provide sample applications (i.e., fans A or B, pumps B or C, compressors C or D)?
Answer: NEMA Design A and B “general purpose” motors are popular for centrifugal applications such as fans, pumps, blowers or general industry process use. NEMA design C and D motors are better suited for applications that require high starting torque such as crushers, conveyors, compressors, or elevators.
Question: What does service factor (SF) mean?
Answer: Service factor is a short-term overload factor. By including the service factor on the nameplate, the manufacturer is letting you know that you could operate the motor at that percentage of rated load and the motor will operate continuously at a thermally stabilized temperature below that of the insulation class rating. However, short term overload is a relative term. Additional heat will lead to shorter motor life in the long run so keeping the motor at or below rated load for its lifespan should maximize motor life, assuming you follow proper maintenance techniques discussed.
Question: For vibration, would you explain the difference between measuring in ips versus mils?
Answer: When you are measuring in mils you are measuring displacement. This can be a preferred unit when speaking of unbalance. Velocity of vibration is measured in peak units such as inches per second (ips). Another way of looking at velocity is distance per time or how much the machine is moving every second, in three important directions at all main bearing points (axial, vertical, horizontal). Velocity measurements and monitoring of vibration is the most common unit to identify various problems or acceptability such as: unbalance, misalignment, looseness (machinery structure, foundations, or bearings), harmonics, and many other issues in the machinery frequency range.
Question: Short circuits between winding turns. How can I diagnose the damage without measuring instruments?
Answer: It is hard to diagnose any winding failure without some type of measuring instrument. If the motor winding has failed, you could use a preferred repair shop to help you diagnose and understand why the motor winding failed.
Question: Should vibration sensors be installed on motors for continuous monitoring of motor vibration?
Answer: Continuous monitoring of vibration is a great practice on critical machines. It can allow you to monitor real-time vibration at all times and not risk a critical fault to occur during a normal quarterly rotation of data collection. This method can become expensive but can prove well worth the money on critical applications.
Question: What type of meter should be used for measuring voltage at the motor leads for a VFD controlled motor?
Answer: You will need a meter that can filter out the pulse-width modulated signal of the VFD. I’m sure there are some smaller hand-held products available for this. If your VFD has a monitor screen, you can likely navigate to and check the output voltage there, as well.
Question: How do we measure power quality at the motor for VFD controlled motors? Such as for measuring for reflective wave?
Answer: We use an oscilloscope with a potential probe for measuring peak voltage and rise time. We use a precision power analyzer to measure the voltage and current total harmonic distortion levels.
Question: Can you please explain shaft run out?
Answer: Shaft run out would cause more of an issue with vibration and is fairly easy to diagnose. High vibration due to run out of a shaft can cause excess stress and heat on the motor bearings. In our laboratory, we use a dial indicator with a magnetic base to check shaft runout.
Question: Should a 480 V motor ever be tested at 1000 V? I was told to do that by a supplier and it seemed wrong. It was not their motor.
Answer: There are tests that apply high voltage for a short period of time to verify the integrity of the winding insulation. These are referred to as dielectric tests and can include: high-potential tests, insulation resistance tests, polarization index tests, etc. These tests can be run with ac or dc voltage so be sure you are using the correct voltage and meter before running any of these tests.
Question: How far can a motor be away from its VFD without a problem?
Answer: Ideally, the motor should be as close to the VFD as possible, but in many cases this just isn’t realistic. Use VFD-specific cables as prescribed by the manufacturer for best results, especially for long distances.
Question: What is motor saturation, and how is it caused?
Answer: Induction motors may reach a point of magnetic saturation where small increases in torque can lead to disproportionate increases in current. This is detrimental to motor reliability and longevity. Saturation mostly occurs when the V/Hz ratio of the motor is interfered with. For example, if you adjust the input frequency without adjusting the input voltage then you need to derate the motor for that application. This is a common issue if you try to use a 50Hz rated motor on a 60Hz power source.
Question: Is there a generally acceptable HP rating at which a motor should be rewound vs. replaced?
Answer: This depends on your motor population. This also depends on the age, efficiency rating, run hours, etc. of the motor. It is great to have motor repair and motor purchase specifications so that you don’t have to make these decisions on the fly. For example, a repair/purchase specification could state to replace all motors under 50HP, and for motors above 50HP replace if the repair is to be 60% or higher of the replacement cost.
Question: In a blower application, should the motor be run at or near the full load amps (FLA) for best efficiency?
Answer: This is true for basically any application. Most general-purpose ac induction motors will reach peak efficiency at approximately 75% of rated load. Efficiency will still be very high at full load. Some motors will even list 75% load efficiency on the nameplate.
Question: Should I use an AEGIS grounding ring on a motor that is rated for “inverter duty”?
Answer: The grounding ring would only be recommended if the motor is being operated with a VFD.
Question: When using a VFD, what type of meter should be used for measuring motor voltage and current?
Answer: At the input VFD terminals you can still use your standard multi-meter. You can use a standard ammeter to measure the current, but a standard multi-meter will not measure the voltage since it is pulse-width modulated. You will need a more advanced meter that can filter out the high-frequency switching.
Question: Can you provide any recommendation to follow when doing motor re-wind?
Answer: EASA AR100 provides a set of guidelines for motor shops who are doing motor rewinds. Using shops with third-party motor repair accreditations is recommended.
Question: Should you regularly grease bearings in sealed low HP (“can”) motors?
Answer: It is best to make sure that the bearings can accept grease. In a lot of applications where the motor is sealed, they will also install sealed bearings. In this case the bearings will not be able to accept adding more grease than that was applied when manufactured. You can retrieve this information from your motor supplier.
Question: How critical is it to replace the motor bearings with bearings rated with C3 clearance for motor duty?
Answer: Bearing clearances are important when it comes to the run life of the bearing. There are times when a motor bearing would need to change for instance from a C3 to a C4 when seeing expansion of the inner ring due to interference fit on shaft. Just keep in mind that this is something that should be looked at very closely before making changes. The higher the fit such as a C4 or C5 means there is more clearance in the bearing itself and can lead to higher vibration. Unless you have a reason for the possible change always go back with the same clearance fit that the motor came with.