Help needed regarding a synchronus motor allowing a grab unit to drop.

[bluespark1978]

Good morning ladies and gentlemen,

I have been having an issue with one of the machines in our manufacturing plant. The piece of machinery in question is a grab unit. The issues we are having are that it is suddenly dropping during running, sometimes it only drops an inch but it has been seen to have dropped up to 36 inches. It doesn't happen all of the time and is very intermittent.

The grab weighs approximately 1000Kg maybe slightly more and it is lifted with a chain drive arrangement which is run by a Bosch-Rexroth synchronous motor and gearbox with the motor brake rated at 32NM.

I was doubtful about the motor being faulty as it is only 4 months old but the grab has been reworked and made slightly heavier than before. I am curious whether the added weight means the brake may not be rated high enough anymore of whether it is simply a motor fault. Any ideas or suggestions on either option would be appreciated please, I have not really done any motor torque work since college which was more years ago than I would like to remember.

Thanks all.

 

[7anoter4]

See for instance:

https://www.researchgate.net/public...r_Motor_Sizing_with_Calculations_and_Examples

 

[waross]

99/100 Your chain is loose and jumping on the sprockets. Possibly one section of the chain has become damaged, possibly some of the sprocket teeth have become damaged.
999/1000 Not the motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

I don’t know that it COULDN’T be the motor... what about pole slippage? He said it is a synchronous motor and I for one have never seen a synch motor used on any hoisting/lifting applications. That’s not to say it isn’t done, but I question it. Nonetheless, assuming someone knew what they were doing on this originally, adding weight to the machine without then going back and reevaluating the motor data would be a serious error. Same for the brake unit.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[lukin1977]

What is the type of the brake? Maybe you need to adjust the breaking force due to normal wear of the break shoes

 

[bluespark1978]

Thanks for the replies. Chain was checked and is good and tight. Following conversations with someone at Bosch-Rexroth and then looking into the drive state it seems to be happening when the drive hands over to the brake. Their opinion is a motor fault with the possibility that the brake is rated high enough.

 

[RRaghunath]

The synchronous motor can slip poles if the load is excessive for its rating.
Jraef could be right.
Why don't you check if you have a power factor trace or some kind of record from the motor feeder!
Power factor and the impedance undergoes for a rapid change during pole slip.
How about motor protection - does it include load jam protection, pole slip protection etc.!

Rompicherla Raghunath