DC Motor Hum

[scmguru]

We've got a 75HP DC Elevator Hoist motor circa about 1929 that is driven by a motor generator set.

In the last 6 months, this motor has developed a hum similar to a sine wave at approximately 120hz when running.

Initially, the motor would only hum when the elevator was going down (motor is loaded working against counterweights)

We had the motor "serviced" by our elevator company. The tech had no idea what was causing the problem, he put a scope on the current going to the hoist and found nothing that would indicate why the hoist motor was humming.

In a last ditch effort, he cleaned the space between the commutator bars and the motor stopped humming.

About a month later, the hum came back, this time in the OPPOSITE direction (motor is less loaded, counterweights help pull car up).

Is there any kind of electrical phenomenon such as AC bleed into the DC supply to the motor that would cause an approximately 120hz sine wave type hum?

Thanks

 

[maged541946]

Hi, Correct me if I am wrong, I understand that you have ac AC motor, driving a DC generator, The DC generator's DC drives the elevator's DC motor.
If this is the case, there is no way for ac ac bleed through mechanical coupling, even if there was, it would show in all cases, more over, all dc motor speed controllers runing on single phase do not use any filtering depending on the dc motors self inductance to filter out tha ac component, and NO hum was noticed in any case or speed or load, I built several of those speed controllers for some machines that require some extra features.
I am not a motor expert but from the electricity partt I see no problem. My guess would be the load. In elevators a motor turns to generator in cases like you mentioned (counter weight pulls up car) , now we have two generators connected to each other, and one is forced against its will, for that some load (wire resistors or else) are provided to absorbe some power making things smoother..
May be there is the problem. Sorry It is just a guess as I do not have all info at hand but just viewing the problem.
I hope I could help..
Best wishes

 

[Skogsgurra]

It depends. If it is an old elevator (you indicate 1929) and if it is driven by an MG set. Then there is not much that could cause a 120 Hz hum.

The first thing that I can think of is the brake lifter - if it is AC operated. The 60 Hz develops a positive force twice each period (current sqared) which results in 120 Hz.

A "transformer effect" where 60 Hz is induced in the motor cables is not very likely. But, there is a slight possibility that you have modernized the drive by adding a single phase rectifier for the excitation and that the superimposed 120 Hz gets transformed into the armature.

I wouldn't be too worried. A three-phase six-wave rectifier will take care of that problem if you really think that it is necessary.

 

[aolalde]

Shorted conductors in the motor armature could hum at 120 HZ. The frequency will be related to the number of poles and the rotating speed.
The armature winding has a frequency in Hz (F) proportional to the speed (rpm).

F= rpm*P/120

Probably you need at least turn the outside diameter of the commutator and undercut the mica separators. If the commutator has too much wear or the winding has shorted conductors, a total armature replacement could be required.

 

[scmguru]

Ok.. I've done some more homework.. the Hoist is 24HP (it's a model 75) and the input is 208V DC.

maged541946, you are correct, we have a MotorGen Set powering this hoist.

The possible diagnosis of AC Bleed came from one of the motor shops we had out here who stated they had to replace a "rectifier" because he was seeing AC Voltage in the line.

To me this is odd because I thought the MG produced the DC?

I'm curious as to why the hum would have switched from occurring when loaded to unloaded.

I checked the brake and there is no noise coming from the coil picking the brake, so that's not our issue.

The commutators will be turned and undercut as part of a modernization that will replace the MG Set with a Motion Control 12-pulse SCR and updated controller.

Does this have a good chance of resolving our issues?

Lastly, it was suggested on another forum that the pole/interpoles may be a hair loose and that we could tighten the bolts holding these in.

Tigtening of these bolts has not been done in at LEAST 20 years and more than likely never been done because all of the bolts still have unbroken original OTIS green paint on them.

Either way, I think a good course of action would be to have a reputable motor company come out and do a top to bottom assesment of the hoists to see what needs to be done as part of general maint.