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).]
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.
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,
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"?
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