Stray Current = Ruined Bearings

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Re: Stray Current = Ruined Bearings

Originally posted by jim dungar: Circulating currents in motor bearings is a known problem with variable frequency drives. It is rare but it does occur. . . . normally, it is not "someone's" fault as much as it is just bad luck.
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I am getting into this one a bit late (didn't check the Forum over the weekend). Also I didn't study the other site (from ccjersey's link). So I don't know if this is new information.

I'm not picking on you, Jim. But your post seemed as good a one to respond to as any other.

I don't think it is bad luck, and I think it is someone's fault. A VFD will, by its very nature, throw a bunch of harmonics into the system. They are going to travel through the bearings and into the shaft, unless you do something to stop them. They are going to cause damage, unless you do what a VFD manufacturer requires you to do. The "unlucky" ones who see the "rare" failures are the ones who did not put on the right bearings, or who did not properly ground the shaft.

The problem is well known. So too is the solution. Whose fault is it then? Well if this was an engineered installation, then it is the fault of the engineer, for not specifying the proper precautionary measures to go along with the VFD equipment. If it was installed by the plant personnel without any engineering, then it was the fault of the lead person in the electrical group.

Everyone should take notice: If there is a VFD, then you must take special actions to protect the bearings. Every time. No exceptions. Check with the manufacturer, and make sure your engineering consultant adds the proper lines to the VFD specification.
 
Re: Stray Current = Ruined Bearings

How far are the VFD's from the motors? The VFD will list a maximum distance for VFD load side wiring. If that is exceeded, harmonics can become more of a problem. Many VFD makers sell line side "reactors" to eliminate the harmonics. That should do the trick.

At any rate, there is one simple solution guaranteed to work. A properly designed filter on the load side of the VFD will eliminate the harmonics and leave the lower frequencies needed to drive the motor. That should fix the problem at the source.

Steve
 
Re: Stray Current = Ruined Bearings

I am not sure on this but I will check for more information. Where I work we had what we call a lunch-n-learn from a local VFD manufacturer, and I recall him saying at one point they actually had a client call about this kind of problem it had something to do with current build up through the mechanical ductwork which led back to the motor and eventually burned up the motor, I am not 100% positive of the source of the current however I thought it had something to do with the VFD itself, the solution I believe was replacing a piece of ductwork with a small piece of flexible connection (rubber) I will try to locate more information on this.
 
Re: Stray Current = Ruined Bearings

Charlie,

Yes, the engineer needed to address the potential problem. A different designed motor, different cabling, or addition of drive output filters were all possible solutions.

The installer needed to follow correct installation procedures for both the drive output wiring and the motor.

The motor manufaturer needed to supply a motor that was designed to mitigate the effects of bearing current, such as incoporating a Faraday shield or additional insulation around the bearings (for reference: http://www.ab.com/drives/techpapers/ieee/IAS96%20bearing.pdf).

The drive manufacturer is probably the least at fault, even though the PWM output is a significant cause.

It is probably impossible for any one party to predict the magnitude of bearing current prior to commissioning, especially if they have no control over what the other parties are providing.
 
Re: Stray Current = Ruined Bearings

At our Paper Mill we have an extensive array of VFD's and struggled with the problem of stray motor currents that cause bearing fluting. This phenomenon is called Electric Discharge Machining or EDM. A buildup of voltage actually causes sparking that penetrates the lubrication film in the bearing. This sparking machines away the metal causing the characteristic uniform pattern in the bearing race.

There are several corrective actions you can take:

The feeder cable from the drive to the motor should be MC cable with a continuous corrugated aluminum armor and three symmetrical ground wires (this construction is critical). In addition the MC cable connectors should be a type like the ones from American Connectors http://www.americanconnectors.com/home.htm that has a 360 degree contact surface. Make sure that the armor is solidly attached to the motor terminal box and the ground bus in the VFD.

Install insulated (ceramic) motor bearings. This often simply moves the problem to the driven device.

Install a grounding brush on the motor shaft. There is a company in Oregon that makes kits.

Install a flat (low impedance at high frequency) braid strap from the motor to any gearbox that may be used. The strap should go back to the motor frame.

Some drive manufacturers such as Siemens makes a toroid choke on each of the feeder cable from the drive to the motor.

Contact the Drive manufacturer Siemens, ABB, and Rockwell can provide a Service Eng. who can use what is called a Rogowski Coil to measure the currents. The coil needs a response of at leas 10 Mhz to be able to detect the entire current. This can also be used to evaluate the effect of corrective actions.
 
Re: Stray Current = Ruined Bearings

UPDATE

OK first I want to say thanks for all the great responses.

I was back there today and for a verity of reasons ($$$) all that is being done is the installation of ceramic bearings and a brush added to ground the motor shaft.

My job is simple I unwire the motor and the motor techs replace the bearings and reinstall it in place, then I rewire the motor.

It remains to be seen if it will fix the problem, and it looks like at best one motor per 10 hour day.

On the up side I have time to catch up on paper work and clean the truck. :cool:
 
Re: Stray Current = Ruined Bearings

Circulating currents in motors due to exitation from a VFD is a well documented event. When you buy a motor that has been designed for use with a VFD you typically get 1. Insulation with a higher impulse rating 2. At least one bearing isolated from motor frame 3. Shaft grounding brush. I will not go into the particulars of why you get circulating currents in the shaft & bearings - there are many papers out there. Just be aware that it exists - so that when the VFD sales guy tells you "Oh, yeah this will work on any AC motor - Just remember this is a definite possibility. We had a 700 hp unit on a pump that was eating bearings within hrs....turned out the insulation material was a new one Reliance had switched to that was expanding due to heat & causing the bearing race to swell & bind the bearing - we had to switch to a bearing with a larger "gap" tolerance.
 
Re: Stray Current = Ruined Bearings

Bob,
Could there be harmonic currents being produced by the VFD's and riding the neutral back to the service and being see on the EGC through the bonding at the service? Go easy on this apprentice I am still learning.......

[ September 27, 2005, 11:33 AM: Message edited by: kevinware ]
 
Re: Stray Current = Ruined Bearings

Probably not. AC drives produce line to line harmonics not line to neutral. Currents in bearings are caused by the output of the drive not the input. Look at some of the sources refered to in other posts.
 
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