Motor Failure

[Rachit]

So this is a little pre-mature in information, however I would like to begin my research in determining the failure of the motor, and would like to see if some of you can point me in the right direction for research topics to determine why a motor would burn out. Here is the information I have thus far:

-The AHU motor has pitting in the fan shaft where the bearings meet the rotor.
-They have lost 9 motors in the last 2 years.
-A grounding kit/strap has been added to the VFD feeding the AHU.

I am still trying to get information like the current on each phase, if there is any current on the ground wire, impedance per phase, voltage levels, and so on and so forth to determine if there is any mistakes in the power quality, but I want to know if there is anything else I should be measuring or if there is any other reasons for motor failure that I am not thinking about. I did read an article once about reflection waves that stated if the VFD is located 500 feet or more from the motor there is a chance of motor failure due to reflection waves. This is an example of what I mean. I would have not looked at the distance between the two without reading that article. So if anyone can think of some other motor failure causes please give me topics to look into.

Thanks.

 

[rrr_usty]

If you are getting pitting in the shaft, I would look at the conductors, check all of your grounds at all points from the VFD to the motor, Experience has taught me to would not rule out the conductor condition P-P & P-Gnd in addition to Vd from the VFD out to the motor.

Good Luck

 

[GoldDigger]

There are several good threads here about bearing damage when a motor is fed from a VFD with high dV/dt.
It is not caused by leakage but by high frequency currents induced from one end of the motor shaft to the other or from the motor and load shaft to ground.
Beyond filtering the VFD output, you can use a motor specifically qualified for VFD use or install insulated bearings.
If you go that route you will have to also put insulated bearings on the load shaft or use an insulated coupling between motor and load.
In some cases adding a brush to ground the shaft may help, but that will become an ongoing maintenance issue.
Search the for for VFD, and "bearing pitting" for more info.

Sent from my XT1080 using Tapatalk

 

[weressl]

So this is a little pre-mature in information, however I would like to begin my research in determining the failure of the motor, and would like to see if some of you can point me in the right direction for research topics to determine why a motor would burn out. Here is the information I have thus far:

-The AHU motor has pitting in the fan shaft where the bearings meet the rotor.
-They have lost 9 motors in the last 2 years.
-A grounding kit/strap has been added to the VFD feeding the AHU.

I am still trying to get information like the current on each phase, if there is any current on the ground wire, impedance per phase, voltage levels, and so on and so forth to determine if there is any mistakes in the power quality, but I want to know if there is anything else I should be measuring or if there is any other reasons for motor failure that I am not thinking about. I did read an article once about reflection waves that stated if the VFD is located 500 feet or more from the motor there is a chance of motor failure due to reflection waves. This is an example of what I mean. I would have not looked at the distance between the two without reading that article. So if anyone can think of some other motor failure causes please give me topics to look into.

Thanks.

Motor size?
Cable length between motor-ASD?
Carrier frequency?

There is an inherent magnetic imbalance on both the rotor and stator side. This causes a voltage generated in the rotor and discharge to the ground. (Rotating the motor leads can minimize it.) With ASD's this imbalance is exacerbated to the degree where it causes arcing along the bearing surface and eventual mechanical failure of the bearing. Since the grease film on the bearing surface acts as an insulator the voltage builds up and eventually discharges. Experiments were conducted with conductive lubricants and so far no success. Larger motor specifications call for one bearing to be installed insulated to avoid a closed loop opportunity. Shaft grounding has also been succesfuly applied and protected from bearing failure and they do last just as long as long life bearings.

http://www.shaftgroundingsystems.com/

http://www.est-aegis.com/

Beware that these may present an unacceptable arcing risk in Classified locations.