DO MAG-COUPLINGS SLIP? 1

[Ork1]

I am working at a gold mine in Alaska. We have recently installed several mag-couplings on certain pumps which pump processing chemicals, gland water, potable water. They are installed on pumps with vfd's and without. My question is very general in nature; Does the mag-coupling have a certain amount of slippage on the output to the pump or is this "rock solid" technology" and if you know of a reference I could study on these I would appreciate that as well.
Thank You.

 

[hydromech]

Without question, there is an amount of slip in a magnetic coupling.

The amount of slip will increase with torque through the coupling.

I will search for something for your reference.

Adrian

 

[TenPenny]

Are you referring to the magnetic couplings that are designed to be variable speed? In which case, yes. If you are referring to a magnetic drive style centrifugal pump, it is my understanding that they are synchronous, and there is no slip.

 

[TenPenny]

You might find a little help in here, as well

http://www.eng-tips.com/viewthread.cfm?qid=164990&page=15

 

[Ork1]

Tenpenny,
These couplings actually have an air gap, between the faces on the coupling halves.

 

[hydromech]

Oversized magnets can be used to reduce the slip in synchronous drive conditions, but there is no physical connection so there will be some amount of slip. When load is applied the increased torque will cause increased slip.

In variable applications where the magnets are pulled away from the induction ring, the slip increase is non-linear.

 

[moltenmetal]

TenPenny is correct, and that's an important distinction. Mag-drives for seal-less pumps and agitators are permanent magnet devices which are synchronous and do not slip. Instead, at high torques, they completely de-couple.

 

[ScottyUK]

Agree with moltenmetal - there's no slip in a magnetic coupling. It is a synchronous device, right up until breakaway torque.

An eddy current clutch / eddy current coupling has slip as a fundamental part of its operating principle. The eddy current devices are sometimes used as variable speed controls and can put up with outrageous levels of abuse, but are very lossy and run at blistering temperatures.


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If we learn from our mistakes I'm getting a great education!

 

[Ork1]

I appreciate all responses, The "Mag-Coupling" we have purchased and installed replaces a rigid coupling or a flex coupling. It was intended to help get more life out of our mechanical seals, keep in mind, we are pumping slurry at various speeds, flows, density, pipe stress, cavitation, etc... Despite laser alignments within tolerance, mechanical seals weren't lasting as long as they should. The Mag-coupling does not transfer vibration, thermal growth or pipe strain, which is no longer an issue. the service life of our mechanical seals has at least doubled.

There is no physical connection or mechanical connection between pump and motor. a magnet floats between two circular faces which is slipped on the end of each shaft. There is an air gap between the magnet and the faces. In some cases I have noticed some slight reduced flow. I am wondering if I decrease the air gap would that in fact increase my torque?

 

[BigInch]

Reduced flow, reduced under what? The flowrates you had with rigid coupling.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky

 

[Ork1]

BigInch,
Yes, I noticed a 45 gpm loss in a water circuit that treats underground mine water. It is pumped through a sand filter and discharged to the river. Sand filtering is just one portion of the system. The filters go through several back flush cycles and the sand gets periodically changed. At this point in the process we are dealing with very clean water anyway, however the filters pose a restriction of flow and when I reinstalled the rigid coupling the flow came back up @45 gpm. I am pumping about 400 gpm. I strobed the two shafts and recently found a 181 rpm difference between motor and pump shafts. Without any literature at all on this coupling I was wondering if reducing the air gap would be worth my efforts to decrease the slippage.

 

[BigInch]

I think that if you reduce the air gap, you would get an improvement. It seems logical to me, but I have never played with mag couplings, so I'll let somebody else answer to that.

I had a feeling that you wern't getting the whole truth about mag couplings not slipping. Every liquid, solid and gas is compressible, everything deflects under load and ... everything slips. It might not be much, but you know that it has to. Nothing is perfect. In fact I believe that you have almost proven as much with the 181 rpm difference between the pump and motor rpm that you have observed. If the motor speed was the same with both the mag and rigid couplings, we can find out how much slip there acually was. What were the motor speeds and the pump speeds with each type of coupling?



We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky

 

[hydromech]

I'm going to say it again...

Magnetic couplings have slip!!!

We have a patented pump design for a variable flow pump. There is a magnet array on one side of a membrane and an induction ring on the other. The induction ring is attached to the impeller.

We vary the flow by moving the magnets away from the induction ring. The bigger the air gap, the slower the impeller.

Even when the air gap is reduced to its minimum, there is about 6-8% slip.

The neodymium magnets are sized to allows an amount of torque. If that torque level is exceeded, the slip increases.

So, even in the synchronous position...magnetic couplings do slip.

Adrian

 

[Ork1]

Hydromech,
Yes, it is obvious there is slip. I am truly getting familiar with what applications I can run these couplings and not have it affect certain pump circuits. The information you supplied me will help to research this further.

Thank You,
Scott

 

[ScottyUK]

Adrian,

[red]SOME[/red] magnetic couplings slip. Some do not.

Google [blue]magnetic coupling synchronous[/blue] which will bring up a multitude of synchronous types.


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If we learn from our mistakes I'm getting a great education!

 

[pumpkind]

Even the permanent magnets do have a slip - with every slight change in pressure or conditions of the pumped fluid or with every slight speed change of the motor.
To prove this it will be probably be enough to monitor the temperature of the pump in the magnetic coupling area because the slippage generates heat

 

[pumpking]

There are two types of magnetic coupling, the most common being a synchronous drive coupling which will have absolutely no slip - in fact if it does de-couple, it will completely de couple until both drive and driven magnets completely stop rotation, as I said before these are now the most common couplings on the Market for everyday applications.

The other type, and an older design, is the 'slip ring' type of which were developed as the very first couplings, they are still available, butvtend tonne used for special applications such as hot liquids which become more viscous when cold which start up with slip to the impeller to ease the start up process. These are still quite rare in the current Market and very few manufacturers still make this type due to the very high costs and reducing marketplace.

Hope this clarifies things ?


Ash Fenn

http://www.cdrpumps.co.uk

 

[hydromech]

Hysteresis or Eddy current type...or whatever type, there is no physical link between the couplings. If the resistance is high enough, the coupling will have some amount of slip.

So, with no mention in the original question of load or type of coupling, we can assume that we are all correct..can't we??

 

[BigInch]

Even with a "rigid" physical link, a drive shaft and coupling, torsional displacement of the shaft under load and pin tolerance, or whatever play remaining from manufacturing tolerance in "rigid" coupling will force at least a minute amount of slip.

Perhaps the EEs out there see things through more of a binary mask.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[hydromech]

Possibly..?

Do we need to go back to basics and put a definition on "slip"?

For our magnetic clutch, 5% slip is good considering the amount of neodymium iron boron we use. More would reduce the slip, but its not cheap.

I guess for some 0.0001% may be excessive.