Motor backspin detector 1

[LionelHutz]

Hello;

Can you give me some leads for a device to connect to the leads of a 4.16kV motor which would tell me if it is rotating?

 

[ScottyUK]

Do you have VTs available or are you looking for direct connection?

Can you use any of the more conventional methods like a proximitor detecting a flat or keyway on the shaft?


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

 

[LionelHutz]

I'm assuming we'll have to install VTs or voltage dividers.

The motor leads are the only thing available to measure.

 

[MikeHalloran]

Uh, backspin?

As in, rotating in the wrong direction when powered?

OR

Being backdriven, e.g. by a pump, when unpowered?



Mike Halloran
Pembroke Pines, FL, USA

 

[edison123]

My electric hoist will not work if the supply phase sequences are reversed. They have a detection circuit (I don't know the design) that will block the hoist operation until the sequence is corrected.

If you use PT's, guess this type of protection could work.

 

[ScottyUK]

A few of the phase rotation meters have the ability to sense motor rotation when the motor is unpowered. I guess it must rely on some residual magnetism in the rotor. I know BK Precision make one because I got a flyer in the mail, memory says there are a few of these type of instruments. Here's the link the the BK instrument - bear in mind I haven't used one so I can't comment on performance or build quality.

http://www.bkprecision.com/products/categories/sub_categories/models/?model=302

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

 

[LionelHutz]

Mike - this is to detect the motor rotating while it is off.

Scotty - variations of that meter are all I could find.

There doesn't seem to be anything like that meter which can be hardwired to the motor. But then, I don't think there is enough residual magnetism in an induction motor to provide a usable signal once it has been off for some time. This is likely especially true once the measuring device is connected via a VT.

 

[rbulsara]

How about a non-contact tachometer? Will tell you if the motor shaft is rotating. For direction you may have to devise some mechanical set up.

Why connect something to MV leads for that?

 

[jraef]

Lionel,
I have been told that a Mutlin SR469 can do this, something proprietary. I have no direct knowledge of it or how it works, it came up in a competitive situation where what I was proposing did not have it and Mutlin won the order because they got the CE to specify it as a necessity.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[LionelHutz]

Jeff;

It's the 369 that has a backspin connection.

The description says it measures the frequency after the motor shuts down and at the same time it calculates a predicted time to stop. The unit will then use the time calculated when the back emf drops so low there is no signal available anymore.

This doesn't sound very promising for detecting the motor is rotating after the motor has been shut down for hours.

 

[jraef]

Seems to me that's different from what is supposedly in the 469, because the application I came up against it on was in preventing starting into a windmilling fan, and Multilin said they had a way to do that with the 469. I have a friend who is a Multilin rep, I'll see if he can enlighten me on where to find info on that.

In the mean time, a company in Canada called MSE of Canada Ltd. makes a MV motor winding insulation monitor and they have a version with an analog output. With a little experimentation, I'd be willing to bet you could detect movement in the rotor with changes in that output. I did Zero Speed detection for DC Injection Brakes that way, but it involves putting a voltage on the motor. That's what this system does, so I'd think it would not be too difficult to adapt it.

http://www.msegroup.net/

http://www.msegroup.net/

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[roydm]

Connecting anything to the leads of a 4160 motor must be tricky, lots of HV spikes etc. A non-contacting tacho as Rbulsara suggests or perhaps just a proximity detector near a keyway, fan blades or bolt heads would be a simple solution. Any change of state would indicate rotation, two detectors would indicate direction.
Roy

 

[1000V]

How about a PQM with minute current trend function.

 

[MikeHalloran]

... And once you've detected that it's spinning backwards, what are you going to do about it?

The complexity of the proposed detectors is nothing as compared to the complexity and cost of stopping the machine electrically.

Think 'sprag brake'.



Mike Halloran
Pembroke Pines, FL, USA

 

[1000V]

What is it pumping?

 

[LionelHutz]

Jeff - I've looked at the 469, there is nothing in the manual that even suggests it coud monitor the motor terminals in any way.

Mike - The pump would be disabled so it could not be started. Stopping the pump would be up to the customer. The pump rotating would mean there is a valve problem they have to address.

I now think a shaft sensor is the only practical solution after searching for a motor terminal way to do this. I believe to do it via the motor terminals would require injecting DC on 2 leads and measuring frequency on the third.

 

[MikeHalloran]

I've seen a small centrifugal backdriven at scary speed.

It was brought to my attention because it was making funny noises, and the bearings were real hot... in the middle of winter.

I don't think I'd want to be near a big one being backdriven.




Mike Halloran
Pembroke Pines, FL, USA

 

[dgallagher]

http://www.electro-sensors.com

has several solutions for detecting reverse rotation.

Dave G.

http://www.ReliabilityDirectStore.com

 

[dpc]

I'd think a simple proximity sensor at the coupling would tell you if the motor is turning. If it is turning when the motor is not energized (allowing for coast down time), this could provide the reverse rotation indication.

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[burnt2x]

Lionel,
Had same problems with backspinning pumps before too -passing check valves on some pumps in parallel with other pumps! We didn't try monitoring the backspin, we monitored the backflow instead(out-of the-box thinking). I don't know if this works the same way in your case but it's just a thought!