INSULATED SHAFT TO AVOID BEARING CURRENT IN INVERTER DUTY MOTORS 2

[petronila]

Dear All,

I am looking for some good information about procedures and theory of Shaft insulation to avoid current circulation by bearings when the Inverter duty motors are Driven by VFD´s. I know this is practice followed by U.S.E.M

Thanks in advance

Petronila

 

[jraef]

There are some good papers available from a variety of resources.
If you are an IEEE member, this is a good article.
ieeexplore.ieee.org/iel5/6470/17303/00805955.pdf

Most of the best info out there is going to reference Aegis, the leader in shaft grounding technology right now. The majority of inverter duty motor manufacturers are using Aegis kits in their motors. This was a great presentation put on by Aegis at a recent conference. I didn't attend, but people I know who did were impressed.

http://www.e-driveonline.com/images/Presentations/ElectroStatic.pdf

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[petronila]

Thanks Jraef,

The LINKS are excelents, in my case we are repairing a 4160 Volts,600 HP, Inverter Duty motor,manufactured by U.S.E.M,we know to avoid the bearing damage the practices includes install Insulated Bearings or The AEGIS RING,the motor is in our shop and it don´t have any kind of insulation on Shields,Shaft or Bearings, by costumer comments we know this motors was sended years ago to other shop, by a mistake they eliminate the shaft insulation located in O.D.E.For today motors the shaft is insulated in both ends.
The costumer wants we restore the shaft to original conditions.

We was searching in U.S.E.M links and findded the suggested guidelines are apply alumina oxide coating and a phenolic sealer to the surface. They Suggested too this insulation materials: Alumina oxide (Metalceram 25010 or equivalent)- Bonding Material (Metalceram 21021 or equivalent ) both from Eutectic Castolin or Metcoseal AP by Metco Seal, but they don´t suggest any procedure.

We are looking the procedure.

Thanks

petronila

 

[macmckim]

Been involved with many VFD/motor application - including MV machines. We tested number of products including the AEGIS RING. After 500 hours we found the AEGIS RING carbon hairs wore out (completely gone)and were not attenuating any of the shaft voltage. We also tested a product made by Parker Hannifin - ProTech SG seal, which has small carbon brushes that ride on the motor shaft. Right now have had parke SG unit in service for almost 2 years.

http://www.parker.com/packing/cat/english/5278.pdf

 

[aolalde]

Helo Petronila.

I noticed that your posted title starts " INSULATED SHAFT TO AVOID...etc".

The difference of traditional "bearing currents in electrical machines" due to an un-balanced flux distribuition and that due to actual VFD drived motors, is the frequency.

For low frequency and DC, insulated bearings have worked properly for many years. For high frequencies as those imposed for carrier frequencies of VFD's, the current will go though the capacitance between the stator, bearing insulation and bearing-shaft-rotor. Xc = 1/(2*PI*f*C)

Although C is small f is very large.

Then for very high frequency GROUNDED SHAFT is the solution to avoid current through the bearings or other elements like the shaft coupled driven machine.

The problem is how to achieve effective and long lasting shaft connection to ground.

 

[williamoh]

Hello, Petronila,
Please read the IEEE paper published by Dr. Muetze. " A. Muetze, A. Binder, “Calculation of Circulating Bearing Currents in Machines of Inverter-Based Drive Systems,”IAS2004, pp. 720 – 726, 2004" . This paper was republished (with different title and some changes) on IEEE Industry Applications last year, July/August 2006. Insulated shaft or insulated bearing is only partially effective with high frequency VFD signals according to the paper. The only best viable solution is to ground the shaft with insulated/hybrid bearings as far as I know. Or some other combination. Spring-loaded Carbon brush has been used historically and found to be not effective in high frequency signal applications. As you can imagine, it requires frequent maintenance or replacement since the carbon is wears out and creates non-conductive surface called "caking" effect with low current level. One major US motor manufacturer tested AEGIS microfiber brushes over 20,000 hours and still running as far as I know. AEGIS was designed to overcome the disadvantages of carbon block brushes, particularly with high frequency VFD signals.

williamoh