Shaft Current

[puckman]

I have been reading here and else where on the damage current causes to bearings on motors and the different ways to divert the current to ground. Would large 50-100 hp Flight nonsubm. pump motors benifit from one of these types of system?

 

[iwire]

I have been reading here and else where on the damage current causes to bearings on motors and the different ways to divert the current to ground. Would large 50-100 hp Flight nonsubm. pump motors benifit from one of these types of system?

Is it supplied by a variable speed drive?

 

[puckman]

Pumps/motor are powered by vfd's . Forgot to include that in op. Thanks

 

[wireguru]

heres the site for one shaft grounding product, has alot of good info on it http://www.est-aegis.com/index.html

 

[ohmhead]

Well it doesnt matter if its a dc or ac motor or a vfd or a softstarter or just a starter but more so ac power will magnetically induce current in shaft . Pit up you bearings and break them down .
When we install large motors we test the bearings by megging them to ground . Someone has a new type of bearing now ill look it up for you if your interested i read it some where ? Take care be safe

 

[iwire]

Well it doesnt matter if its a dc or ac motor or a vfd or a softstarter or just a starter but more so ac power

It is a much larger problem with VFDs, I do not know why this is so but it is.

Google 'VFD fluted bearings'

http://www.google.com/search?q=vfd+...F-8&oe=UTF-8&sourceid=ie7&rlz=1I7GPEA_enUS292

 

[ohmhead]

Well motor bearings kinda are a capacitor voltage can be induced into shaft current flows thur bearing bearing electrically pits rollers stops them they heat up and burn up !

I think at lower speeds and the type of vfd is what they mean when they say a vfd , most motors 250 hp and larger dont have that problem or high voltage motors there bearings are different .

Its what you pay for they have outer race insulated material now . Thats the ones we megger mostly high voltage motors .

They have oil positive high pressure bearings now .

They have conductive lubricants now .

Some motors have a farad shield in the motor air gap .

But we heard there is a new way to program a PWM VFD to give random pulse modulation to aid in lowering shaft current . some how adjusting the freq or pulse with time to effectively reduce induction into the actual shaft . anyone here of this ? Iam ready for a lesson ? Take care

 

[eric7379]

Because of this same problem and depending on the application, we cut off the ground wire on encoders that are directly mounted to motor shafts. We had some premature failures until we had the encoders analyzed and the manufacturer suggested removing the ground wire (even though it comes supplied with it). Since then, no more problems.

 

[mull982]

I've heard the term "common mode voltage" be used in discussions relating to this problem. I've never fully understood this term however or what it meant.

Has anyone else ever heard of common mode voltage, or know what it means?