Bearing failure, motor or pump? 1

[Mwater]

If you have a centrifugal pump, inverter duty motor and VFD, which bearings has the highest risk of failure(motor or pump), what kind of failures is expected and what are the steps needed to prevent these type of failures.

Are there any features that can be added when ordering the motor and the pump in order to reduce the risk of failure?

I talked to dealers about insulted bearing in inverted duty motor; one said is not needed and another said is only a myth.

I found some articles about the of PWM drives on bearing and general bearing failure but I still looking for article about the effect of centrifugal pump on Motor bearings and vice versa. if you have any reading materials about subject feel free to add them.

Thanks in advance for you suggestions and advices.

 

[ischgl99]

Electrical flutting from VFDs is a real problem, not a myth. It comes from shaft currents that are generated from the VFD during operation, but does not effect all motors. If the VFD is installed properly and the motor is grounded correctly, you probably will not have a problem that would need insulated bearings. If you do find the flutting, it is better to solve the problem causing the flutting instead of covering it up with an insulated bearing.

Most bearings fail from lubrication problems, either not the right type, too much, too little, water or dirt contamination. Only a small fraction actually reach their design life. What to do depends on the size of the pump and the type of lubrication. For grease, using an automatic lubricator helps prevent all the problems mentioned, for oil, a good circulation system with filter will go a long way to prolonging life. Then of course make sure the pump is installed correctly. A good firm base, proper alignment, no pipe strain, etc.

 

[Mwater]

thanks mbensema it is very helpful.
I was not very clear before; the myth is not about the electrical fluting but about insulating the bearing to solve the problem. I think the dealer agree with you that is better to prevent the problem with good grounding and electrical installation than putting an insulation.

do you have an idea if the bearing failure are more frequent in the pump or in the motor that drive it (vertical motor)?

Thanks,

 

[ischgl99]

It is impossible to say which will fail more often, it will depend on the installation. Electrical problems such as mentioned from VFDs will cause motor failures, but could also cause pump bearing failures if the shaft current grounds through the pump instead. There could be a number of problems related to the pump itself that causes it to fail, ie misalignment, resonance, improper greasing, excessive speed or load, etc. How reliable or unreliable the motor of pump is will depend on how well it is designed, installed and maintained.

Sorry for the vague answer, but there are so many factors involved to give an answer one way or another.

As for the insulated bearings, they stop the arcing through that bearing, but may cause another problem elsewhere. You can use either ceramic balls or insulated coated bearings to isolate from the housing, but if you do not then take into account the voltage will want to find an alternate path, you might actually make your problem worse. By isolating the motor bearings, the current may find a path through the pump bearings and destroy them instead. My advice would be to run the motor with the normal bearings and install a shaft grounding ring to drain the voltage to ground or install a filter between the VFD and motor to minimize the high frequency pulses that are the causes of the flutting. Which way you decide to go will depend on how big the motor is and the costs of each strategy.

This article might be helpful in what you decide to do.

http://www.hpac.com/GlobalSearch/Article/47488/

as well as this one

http://www.eng-tips.com/viewthread.cfm?qid=102205

 

[Skogsgurra]

No, bearing problems with VFDs are not a myth. But few people understand this complex field.

"Correct installation" is a myth - I would say. And so is "Correct grounding". It all depends.

There are three main failure mechanisms with motors and VFDs: Induced shaft voltage, Capacitively coupled shaft voltage and Frame voltage due to bad grounding/long PE.

The induced shaft voltage is most common in large motors - like 300+ HP while capacitively coupled shaft voltage is more common in small motors. Frame voltage can be a problem regardless of motor size.

If your motor is a fairly large one, like 300 HP and up, the motor bearings are at risk. But not only the motor bearings, the pump bearings can also have EDM. Especially if the coupling is conductive.

Insulated bearings have a limited sucess in preventing EDM. That is because the HF components in the PWM waveform pass through the rather high capacitance of the Al2O3 layer that constitutes the insulation. Typical capacitance values are between 2 and 20 nanofarads for motor bearings.

My advice is that you shall do some reading yourself and try and understand the problem. There are too many "knowing" persons telling you whatever she/he has misunderstood. And that is particularly true in this field.

Google EDM (Electric Discharge Machining, which is the term for bearing damage through electric erosion). That will give you a good start.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

Addendum: Googling "EDM" produces millions of answers. Use +bearing +EDM instead. That sorts out the useful hits for you.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Fugeguy]

While an expensive option using ceramic rolling element bearings will solve the problem.

A very good option for critical applications.

 

[Fugeguy]

Addendum:

A flyer on bearings with cermamic elements:

http://www.koyousa.com/brochures/pdfs/cat185ex - Insulated bearings.pdf

This one has pictures of bearings with fluting damage:

http://www.koyousa.com/brochures/pdfs/cat2000ex - Failure Chart.pdf

Disclaimer- I do work for Koyo but the information above is pretty general.

 

[mayt4u]

Let me see, vertical motor on a centrifugal pump, with a VFD? Bearing failures?

1. Why would you install a VFD on a centrifugal pump? The pump is designed for the requested head and flow caracteristics. You will miss the BEP with a VFD.

2. There are no bearings in a vertical pump! They have guides (bushings) and wear rings.

3. Flow on a vertical centrifugal pump is controled by a bypass line or pinching in the discharge valve.

4. The most problems will come from the seal. Mechanical or stuffing box. Be sure to get the flush lines right, if any.

Hope this helps.

 

[TenPenny]

The above post has so many inaccuracies and misleading statements in it that I'll completely ignore it.

Getting back to the original post, there are a host of possibilities, but in order to discuss the correct ones, you'll need to give more details.

Here are a couple of thoughts:

If the VFD allows the pump to run under no flow/deadheading conditions, you can expect the pump bearings to give you trouble.

If the VFD allows the pump to operate at a critical speed, you can expect the pump bearings to give you trouble.

 

[ztlbearings]

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