120hz Hum from Otis Gearless

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We've got an otis gearless DC hoist, circa 1929 that is powered by an Otis MG.

Recently, the hoist has developed a 120hz hum that we cannot seem to get rid of.

We were able to reduce the hum by cleaning the gaps in the Hoist Commutator, but the noise came right back.

We did notice the commutator on the hoist looks streaked whereas the other hoist which really doesn't hum looks clean.

Our maintenance company has no idea what is wrong and our tennants are complaining.

Any ideas?

-- Bill Williams (willm@hotmail.com), February 02, 2005


Have you checked that all poles and inter-poles are secured to the frame? Even slightly loose they buzz at an annoying pitch. Have you cleaned the com on the gen and made sure you have a clean output?

-- geoff judge (geoffjudge@bchtgroup.org), February 03, 2005.

genset com is good. armature was redone about a year ago. power to hoist checked w/ scope by tech, it was clean. He also put a filter in the circuit. Nothinh.

please tell me more about the poles..

We're at our wits end. Maint co says they don't know what's wrong.

The only time it went away for a month or so was when the tech cleaned the area between the metal sections on the hoist commutator..

will modernizing to an scr drive eliminate this problem?

Maint co says maybe..

-- bill williams (willm@hotmail.com), February 03, 2005.

Many years ago I looked after a 4 car group and 1 unit developed a hum, even when stationary. After tests it was found to be at 100Hz (the UK runs at 50Hz)and 1 of the interpoles in the hoist motor was found to require tightening up to the casting of the motor. This obviously involves the removal of the armature, and once opened we could not feel the interpole physically move, but when we applied pressure to the fixing Allen bolts they turned, but only about 60 or 70 degrees. This was done whilst the interpole was powered so we actually heard the hum disappear. Just a shame that the fixings wern't external to the motor as we could have done the repair in about 1 minute instead of the couple of hours it took. The giveaway was the fact that the interpoles were carrying current even when the lift was not moving, whilst the main poles and armature powered during travel. Given that your frequency hum is 120Hz you may have the same problem. The easiest way to check is to put the lift onto engineer's control and remove the feed wire to the interpole, if this makes the hum disappear it has come loose. I hope your fixing bolts are external.

-- geoff judge (geoff.judge@ bchtgroup.org), February 04, 2005.

I agree with Geoff. First place, don't mod yet. You'll have two problems - the original problem and a drive that faults out due to existing problem.

This sounds like field pieces in the motor. The motor field is probably line rectified excitation. There's the 120 hz. It's also a big magnet that acts to wiggle anything that is loose. The iron poles of the fields or innerpole could be loose. Tighten all the bolt heads around the motor housing. One of the fields may be sliding up and down on the pole piece. They usually wedge them in with shims. Read voltage drops across each field. A low reading on one field may be a short. This will make its own secondary winding, high current, hot field and possibly vibration. This not as likely because it usually burns up.

-- Jack W. (redmantech@hotmail.com), February 05, 2005.

The maint. co has a "refurbish" scheduled for the hoists including the com and fields, hopefully this will pick it up. The odd thing is, the hum only happens on the up direction, it doesn't do it when the hoise is going down.

A while back, there was a problem with this MG Set and it was rebuit, then there was a problem with AC flowing into the hoist which was "supposedly" fixed by changing a rectifier.

Odd thing is, I seem to recall this hoist originally making noise on the 'up' direction and now it does it on the 'down' and not the up.

I wonder if the rectifier that they replaced had anything to do with this.

In any case, I'll discuss this some more with our vendor and see what can be done.

-- Bill Williams (willm@hotmail.com), February 08, 2005.

Correction.. Humming occurred originally on Down, that has since stopped and now it occurrs when elevator is going up.

I think we're going to have a 3rd party Motor shop investigate as well.

-- Bill Williams (willm@hotmail.com), February 08, 2005.

Whay dont you get a scope out and trace the problem, with the scope you can see the noise with pictures. Jim

-- Jim (elevator555@hotmail.com), February 10, 2005.

sorry that my english is not so good, i work 11j in elevators and i just came on this site, i work now 2 j in otis belgium and i hope i can give you tomorrow a answer...

-- Luc van mensbrugge (lucvanmensbrugge@hotmail.com), March 06, 2005.

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