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#130608 - 08/19/06 01:57 AM Motor Brain-Teaser  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,223
SI,New Zealand
Hi Guys,
Here's one for you.
A mate of mine that works as a Maintainence tech in a factory that prints newspapers, related this story to me recently and I must admit, it has me stumped.
OK,
There are a set of machines that they use to roll plastic film over the papers (to prevent them from getting wet after they sit on your lawn after being delivered).
The machine uses a set of rollers and various other mechanisms, to roll the paper and wrap the film over it.
The machines are driven by a 230V 50Hz 1 HP single phase , cap start Cage induction motor and this in turn feeds a set of chains and sprockets to effect the rolling, etc.
Now, here is the crux of the problem, the motor, when started up for each roll, drives the chains at the full speed of the motor and is geared down to suit the rolling machine.
However, they have noticed that every now and then, the motor will accelerate to speeds beyond what the motor is ought to (or even designed to) run at.
How on earth is this even remotely possible?. [Linked Image]
I was under the impression, that the speed of an induction motor was controlled by the supply frequency.
BTW, this motor has no speed controller or governer on it to reduce the operating speed under normal operation.
It's the first time I've ever heard of such a thing.
Can anyone help at all?.
Thanks,
Mike.
[Linked Image]


[This message has been edited by Trumpy (edited 08-19-2006).]


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#130609 - 08/19/06 08:39 AM Re: Motor Brain-Teaser  
WFO  Offline
Member
Joined: Sep 2005
Posts: 202
Cat Spring, TX
Could it have a run capacitor that is making up intermittently?

[This message has been edited by WFO (edited 08-19-2006).]


#130610 - 08/19/06 08:49 AM Re: Motor Brain-Teaser  
SteveFehr  Offline
Member
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
Is the speed exactly 2x or 3x or 4x the design speed? Is it lightly loaded when this happens? If so, I'm thinking it might be harmonics at 100Hz or 150Hz allowing a faster drive speed.


#130611 - 08/19/06 08:56 AM Re: Motor Brain-Teaser  
LarryC  Offline
Member
Joined: Jul 2004
Posts: 783
Winchester, NH, US
Is the motor being back driven by the machine? Perhaps the machine was turned off mid-cycle.


#130612 - 08/19/06 09:08 AM Re: Motor Brain-Teaser  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,223
SI,New Zealand
WFO,
No,
It only has a start capacitor, that is switched out of circuit, once the centrifugal switch opens the start winding.
Steve,
That was what I was leaning towards, as far as harmonics are concerned.
But if that was the case, would it not run at this odd speed all of the time?.
I'm only going by what I've been told and all that was, was that the machine goes wildly out of control, once in a while.
(If I was a cartoonist, I'd love to be able to draw that scenario [Linked Image])
It's definitely more than twice the normal speed of the motor and it has the same loading on it (Rollers and chains etc) most of the time, apart from when a paper is inserted and rolled, the load on the motor would increase.
Sorry I can't be of more help.


#130613 - 08/19/06 09:22 AM Re: Motor Brain-Teaser  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
As I was a maintenance mechanic I have learned to take peoples impression only so far.

No offense to your friend at all but I would want to measure this with some type tool and actually verify an over speed condition was happening or if something else is going on.

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#130614 - 08/19/06 10:15 AM Re: Motor Brain-Teaser  
SteveFehr  Offline
Member
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
Trumpy, the 3x harmonic is produced by 6-step switched power supplies, and often as a pretty sizeable % of the main current. Do you have any large UPSs or DC loads on that circuit that you might be able to correlate with the over-runs? Harmonics are wierd and hard to model- the slightest variation might trigger a motor to sync with a strong harmonic vice the mains. Might not be the cause, it's it's worth checking out. I don't suppose you or your mate have an o-scope, do you? [Linked Image]

[This message has been edited by SteveFehr (edited 08-19-2006).]


#130615 - 08/19/06 10:42 AM Re: Motor Brain-Teaser  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,223
SI,New Zealand
Guys,
I'm going to head down there later on today (the place doesnt work on Sunday's) and have a look for myself.
Conventional thinking would tell you that this situation could not happen.
Bob,
Quote
No offense to your friend at all but I would want to measure this with some type tool and actually verify an over speed condition was happening or if something else is going on.

This guy is a very clued up young fella, I trained him at the local PoCo as an Electrical Apprentice.
I will be taking a scope with me, because the harmonic situation sounds very plausible, considering that this factory is based in an area of town called an Industrial Park.
They aren't known for their clean AC supplies, with most of the parks inhabitants using VSD's (Variable Speed Drives) on quite large machinery.
BTW Bob,
A maintenance person can be as smart or as dumb as they come, unfortunately, it's the dumb ones that tar the rest with the same brush.
I'm not looking to insult you at all, but why is the term "Maintenance" often associated with the lesser respected term "Handyman"?


#130616 - 08/19/06 11:12 AM Re: Motor Brain-Teaser  
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 791
Chicago, Il.
Mike, If it happens only infrequently during weekday hours, you probably won't get to see it on a lazy Sunday.
Joe

Or maybe it's a 4 pole motor that's becoming bi-polar mechanical depressive.<G>


#130617 - 08/19/06 12:35 PM Re: Motor Brain-Teaser  
Alan Belson  Offline
Member
Joined: Mar 2005
Posts: 1,803
Mayenne N. France
Joe, could that be it? Is it possible to 'lose' a winding by an intermitant fault, [ that is, say, one winding off a 4-pole wound configuration = 2 pole ], and thus double the speed? Probably a horribly unbalanced rotor but....

4 pole 1800 r/m less 4% slip, 1728
2 pole 3600 r/m less 4% slip, 3456

Alan


Wood work but can't!

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