Your questions answered: Troubleshooting techniques for ac induction motors

On March 3, 2020, CFE Media & Technology hosted a webinar on troubleshooting and maintaining ac induction motors. The webinar invoked numerous questions from the audience. Here are some of the most interesting, which were answered by Michael Lyda, a motor and drive engineer at Advanced Energy Corporation. Ronnie Alford, motor lab coordinator at Advanced Energy Corp. also presented as part of the webinar. Much thanks to them both.

1. You state, “Every 10 C cuts insulation life by half.” Does that mean for every 10 degrees above ambient temperature rating?

No. Think of it as every 10 C rise above normal winding temperature at full rated load.

2. These days, technology can provide electronic protections for over/under voltage and heat rise. Why it is not implemented?”

These things are available, just with added cost to the initial purchase. Temperature relays are standard these days for certain types of motors.

3. For the voltage unbalance, does the ground help in any way?

The ground helps for safety. The relationship between the ground and voltage unbalance is this: When voltage unbalance is present in a 3-phase system, you will likely see current running through the ground. This is due to the sum of all currents not being zero in an unbalanced system, and the ground is the only place for the excess current to go. This also is referred to as the zero-sequence current.

4. Some utilities’ standards allow a 3% voltage unbalance and state that the user should just de-rate their motors if utility-supplied voltage is unbalanced at their location. Is derating enough or does it cause other problems?

This really depends on the application. I would look at each on a case-by-case basis. For example, if you already have an oversized motor on the application, then de-rating it due to the unbalance will still likely keep you running below or at the rated full load operating temperature on even the line with the highest current. But if your motor is already designed near the rated load with balanced voltage, the increased current due to the voltage unbalance could lead to early failure (excess heat). Looking at the impact on motor efficiency, voltage unbalance is detrimental to efficiency in almost every case. Looking at voltage unbalance at the input to a variable frequency drive (VFD), the VFD may trip off if the unbalance is too high.

5. Do you feel 500 V is a proper level of voltage when testing motors used on VFDs?

The 500 volts level is specified for infrared (IR) testing for motors rated under 1,000 V.

6. How would you remedy connecting a 230 V motor to a 208 V service?

Provide a motor rated for 208 V.

7. Is there a way to measure negative sequence current which is normally produced during current balance?

You can always check the current flowing through to ground (if there is any), which gives you a good idea. There are fancy meters that you can buy to do this.

8. I often see extremely distorted sine waves (voltage and current) because of harmonic distortion on the ac line. How much distortion of the sine wave is allowed?

Tough question. IEEE 519 is a resource you could take a look at. Power utilities don’t like harmonics because they can’t charge you for harmonic current or reactive power. They can only charge you for real power. They also don’t like the negative impacts that increased harmonics have on the grid and other customers. I don’t know of any regulation around this topic, but the IEEE 519 standard will get you headed in the right direction.

9. How does harmonic mitigation on the input of a VFD affect its output?

Adding a line reactor to the input of the VFD adds inductance to the circuit thus lowering the harmonic impact of the capacitance on the output. This is good news for the motor. Motor performance and reliability are improved when it sees less current harmonics.

10. I have not seen many VFD-driven motors with bearing ground protection. Noting that bearing is the lead cause of failure, how much can this protection impact those statistics?

The bearing ground protection on VFD-driven motors is to mitigate shaft bearing currents originating from the pulse width modulation (PWM) signal from the VFD. Voltages will be hitting the motor winding at much higher voltage than nameplate rating and the PWM switching frequency will be many orders above the driven frequency of the motor. The high frequency switching can lead to current if there are any temporary shorts (like shaft to bearing through the ball bearings). One reason you may not see many of the grounding rings is cost. They aren’t cheap.

11. If the service factor (SF) is 1.25, what percentage of increase in overload current should be applied if the SF is to be used in design? I’m thinking that air compressors seem to use all of the available SF.

Most motors will have an SFA column with the service factor amperage listed. If your motor does not, then just multiply the full-load amps (FLA) by the SF to get the SFA. For the air compressors generalization, I’m not completely sure, but maybe the compressor manufacturer is using a smaller motor to cut the overall cost of the unit. This may give them a competitive advantage while staying capable of delivering the required output (some of the time).

12. What temperature of a motor surface with insulation class C is considered permissible?

The insulation class temperatures refer to winding temperature only. I am not aware of any ratings around surface temperatures. But if I could point you to a resource it would be UL 1004. I may be wrong, but I believe surface temperature would be more of a safety concern.

13. You can run a 50 Hz motor at 60hz. It’s called field weakening area. Output power should be kept constant, which means torque should be reduced.

Good point, yes you can. You can also drive 60 mph in a 50-mph zone, but one day it may backfire. If this is a critical application, I would be weary of mismatching the frequency of the supply. An application where you may be typically running at 50% to 75% load of the motor anyway, you will likely not see much of a difference. On the flip side, if your motor is more closely matched to the rated load (at 50 Hz), then you will see overheating when running at 60 Hz — and we all know what that leads to.

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