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?.
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.



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

Could it have a run capacitor that is making up intermittently?

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

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.

Is the motor being back driven by the machine? Perhaps the machine was turned off mid-cycle.

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 )
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.

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

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?

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

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,
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"?

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>

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

Alan, you gave 60Hz #s instead of the 50Hz #s that Mike and his friend are dealing with, but that's all I can think of. I don't know if you can lose half the poles and not all of them internally. I would imagine that the torque would go down the tube but if the process is already running with alot of gear reduction, maybe it's possible.
Joe

Mike


I have no idea what your trying to say here.

In my parts 'Maintenance Mechanics' are usually very skilled at the jobs they do, most are not simply untrained handymen.

All I was trying to say is when given somewhat unusual information you should take it with a grain of salt.

Many times 'eye wittinesses' are mistaken.

IMO if your not bringing an instrument that can determine the actual RPM of the motor shaft you are wasting your time.

I have a hard time believing this motor is somehow over speeding regardless of the high tech theories offered here.

It seems more like likely that it is coming up to full rated speed than slowing down to a lesser speed.


Bob



[This message has been edited by iwire (edited 08-20-2006).]

Here is what makes the "harmonics" theory implausible. Over design frequency of an AC motor, torque decreases in inverse proportion to the square of the frequency, while the corresponding slip remains nearly constant. Even if there were enough harmonic current to drive a 5HP motor, as the frequency increased on that motor to twice the fundamental, the motor's torque would be cut too much to keep spinning the load. It is, after all, still a 5HP 50Hz motor. So if you had 18 ft-lb of torque at 50Hz, you would have only 4.5 ft-lb at 100Hz (1/2 squared) and 2 ft-lb at 150Hz (1/3 squared).

Voltage would need to be 460V if 100Hz were to have any appreciable effect on making the machine run faster, 690V if the "harmonic" frequency were to be 150Hz.

The machine is mechanically running away on you and that 5HP motor cannot exert enough counter-torque to prevent it. It needs a brake or you need to replace that motor with a 3 phase + VFD + dynamic braking resistors. If it worked before, it must have had a mechanical brake system of some sort, and the brake system has now failed.

Mike:

I think Alan Belson hit the nail on the head:


There have been cases in the past, with cinema projectors' motors (usually 1/4hp, 1725 rpm capacitor start) where a short in the stator's run winding would cause "runaways".

Note that the problem could be either a shorted winding (turn-to-turn) or an intermittant ground fault. (The ground fault was the most common cause with the projector motors. It was usually accompanied by a loud pop in the motor and also a massive hum spike in the sound system.)

In your case, would it be easier to just replace the motor first and have the bad one checked out by a motor shop?

edited for dropped letters..

[This message has been edited by mxslick (edited 08-20-2006).]

Bob,
Sorry mate,I was going off in an entirely different direction there!.

Sorry it's taken me a while to reply to this thread but, I've since started there as a Maintenance Supervisor and the hours are horrendous coming up to speed on the gear they use and the operation of it.

I took my junk down there and had a look at what they had.
This problem is only a recent one.
About 8 weeks prior to the problem being noticed, the company had installed a brand spanking new collating machine.
3 phase, 230/400V and it is PLC controlled and uses a VSD as the drive for the main motor in the unit.
That's OK.
We had a couple of guys there to help, using these rolling machines with the power off to the collating machine, no worries.
(By this time I had a 'scope on the mains)
The collating machine was set up and started on low speed and run, again no problems.
The collating machine was then run up to full speed with rather heavy papers running through it and then Bang!!.
It showed up on the scope (harmonics all over the place!), the rolling machines started acting as was described.
We are currently working with the supplier of the machine (in Australia, no less) to fit some sort of harmonic filters to the thing.
Thanks to one and all that replied to this thread, it has been an education for me and I managed to score a new job after my last one fell threw.
No worries though, I have a great bunch of tech's working with me and free newspapers too!.

Isolation XFMR.