×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Motor backspin detector

Motor backspin detector

Motor backspin detector

(OP)
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?

 

RE: Motor backspin detector

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?
  

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

RE: Motor backspin detector

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

The motor leads are the only thing available to measure.
 

RE: Motor backspin detector

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

RE: Motor backspin detector

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.

RE: Motor backspin detector

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
  

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

RE: Motor backspin detector

(OP)
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.

 

RE: Motor backspin detector

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?

RE: Motor backspin detector

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  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Motor backspin detector

(OP)
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.
 

RE: Motor backspin detector

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: Eng-Tips.com Forum Policies  

RE: Motor backspin detector

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

RE: Motor backspin detector

How about a PQM with minute current trend function.  

RE: Motor backspin detector

... 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

RE: Motor backspin detector

What is it pumping?

RE: Motor backspin detector

(OP)
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.
 

RE: Motor backspin detector

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

RE: Motor backspin detector

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

RE: Motor backspin detector

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!

RE: Motor backspin detector

If it's backspin prevention you are after there are devices to attach to the motor that will prevent that, some sort of ratchet device that flings the pawls out once the motor is running. I have seen them on vertical pumps.
It's common in mining also for uphill conveyors but there they use something on the gearbox.

RE: Motor backspin detector

(OP)
burnt2x

We've already suggested a flow sensor as a possible solution. The motor isn't accessible so they won't be installing a proximity sensor.

 

RE: Motor backspin detector

A very tough problem I guess!
We do have another unit employing DC braking system, but we used feedback from a tacho-gen mounted on the shaft of the motor to prevent starting of motor when backspinning. The gadget kicks-on when the motor is backspinning, bring the rotor to a stop before the softstarter engages to speed-up the motor.
But, as you said, they won't be installing a sensor due inaccessability!
If your pump is a vertical well pump, roydm's suggestion could be a solution. The equipment is just attached to the non-driving end of the shaft; uses steel balls that locks-in on backspin, but stays out on the correct rotation!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources