Crane Slip Ring Motor

Someone may have a "magic idea", but otherwise, I think you have to dig out (and then provide) some more information. Some things to do off the top of my head:

• Have you checked (and maybe even monitored--i.e., put a chart recorder on) the motor current--that is, both the incoming current from the power supply, and the current in the wound rotor?

• Does the crane duty vary, either in cycle time or load--try to measure those current under the heavy loads.

• Are the crane operators operating the crane properly--are they trying to use too high a speed for the given load?

• Do the same group of operators operate both cranes (or is even one operator using both cranes)--when collecting the load currents, correlate them with the operator.
  

That should give you (and maybe us) some clues.

The cranes operate at high speed as magnetic loading cranes. The line current isn't abnormally high, there's no change from a new motor to one that's on its way out. The motors are Brook Crompton (can't remember the frame) 60HP. The brushgear is inside the main motor body. 2 brushes to a ring, bush wear is OK until the rings start to pit.

The operators are a bit heavy handed (they don't know what slow is) but this has been going on for years with several changes of driver crews in that time. The cycle time can't be slower or they fall behind.

Will try to get more details when I go to work.

Sounds to me like it's down to a money thing.

Which will cost you more?

Running motors hard and having a reduced life. Or slowing down and extending motor life.

If you're thinking long term, it may pay to invest in another crane and operator.

Any piece of equipment that's pushed to it's limits on a regular basis is not going to last as long.

Yes eccentric rings will soon cause a problem.

To check this on a new motor, test the run out with a dial indicator it will soon pick this up.

With existing motors do some inspections to see if there is a burnt patch on one part of the slip rings, if there is pull the motor and have it machined as it will only get worse.

Excessive vibration of the motor mountings can also cause this.

I've seen the same effect on slip ring motors when the M-lead resistors aren't balanced or have loose connections. Check your resistor bank resistances at the motor (all three) while bringing in the speed contactors (without directional T's). Since it's 60 HP, I assume this is the hoist motor. If the speed contactors & resistors are not trolley mounted, give your festoon wiring a thorough ringing. I had a very similar anomaly while working as a crane service tech and it finally turned out to be an intermittent open in the trolley festoon. Good service technicians hard to find where you are? I have a friend...

Steve

I've had the same fault with crane resistor banks and festoons. It usually causes a patchy patina on the rings not burning. The last one, one ring was fine, the other two looked like a bit of modern art. A b**tard to find though!

For sure everything has to be perfectly true. You say magnetic loading cranes, there could be a lot of aggressive, maybe conductive dust around. I would examine the ring area for contaminants, this sounds like a good place for WD40, too.

Yard Cranes are subjected to heavy duty work. What is rating of the motors are they rated for Class IV contineous duty? If motors are not rated for the application required then they will performance problem. Crane mfr can suggest some solution as Crompton motors are good motors.

I've checked the rings on the spare motor, they run out by .5mm. I'm sending it back to the repair shop to get it put right.

All I've got to do now is persuade the powers that be to systematically change the motors so they can all be checked / trued up. At least if its planned there won't be the mad panic changing them.

Tony, how big a job is it to turn the rings in the lathe?

Unlike others I don't see the operation or operator as the fault. I appears to be something with the rebuild or wiring to each unit. I would assume that the same folks are working on each of the two cranes having the issue with motor. If so you need to take them by the hand and properly trouble shoot the cause not repair the after effects. These is one of the issues I have had to deal with with Maintenance folks over the years, proper trouble shooting.

I, for one, did not put the blame on the operators. I'm not sure anyone did. But, I did suggest that this is one of the things to be investigated.

The burning occurs when not proper contact between brush and ring! This motor need to be taken care of quite often. Thus it is not popular any more. The answer is frequent maintenance, especially elimination of dirt and metal shavings and lubrication with silicon lubricant should eliminate that. Also frequent start-ups add to the problems. How often have you polished the ring-slip surface? It seems you prefer more work than maintenance! Do you have proper spring on the brushes? Perhaps you need to replace brushes rather than rings.

Dirt.

The primary cause is persistent unequal load distribution between brushes

This slip ring file may help.