ECN Forum Forums Technical Discussion Electrical Theory and Applications Motor Brain-Teaser

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.