centrifugal oil filled clutch for electric motors

I will have a punt here that some of the 'oldies' have come across these before.

I have a 22KW motor that has an oil filled centrifugal clutch fitted on the motor shaft before the pully.

The machine was built in the 1970's and I am retro-fitting it.
The customer bought the machine off ebay for £20K unseen and had it shipped to the UK.

The control for this motor is an old fashioned and not very nice method called 'bridge starting'
Whereby the motor is started direct on line by one contactor that shorts out the overloads for a set period. Then a second contactor continues the DOL starting but now through the overload.

The clutch should slip and very slowly bring the motor up to full speed before the contactor changeover.

When the motor is stopped the clutch spins freely but when it starts , although it does slip - it goes too fast too soon and the current starts at about 95Amp and actually slowly rises. (I have not dared take it up to full speed yet for fear of burning the motor out)

I have visited a factory with a similar machine and their clutch really creeps up slowly - but nobody knows anything about it and they have never touched it.

Before anyone suggests it - I have told them that it needs an invertor and he gulped at the price.
But I would like one chance to be able to set the clutch up right but apart from an oil fill cap (and there is plenty oil in there) there doesnt seem to be anything to adjust.

Anyone with any experience with these?

Gordy,
At one time these were pretty common. If you take it apart it will look like a torque converter from a 1950 Buick which is what it is. These are still common on amusement rides,
I would take it apart, run it thru the parts washer, change the bearings and seals and put it back together. 3 hours max, they are very simple. It will be good for another 50 years. I have found that the seals leak so the maintenance guys put in heavier oil, or they do not know what oil to put in. Also over the years heating causes a build up on the vanes.
You may have to experiment with oil viscosity if the label is gone but once you get it it will last another 50/100 years.

Thank you Gas, that's exactly what I will do then
(well not me but the site engineer)
How sad am I, I now can't wait to get back there after the easter break

What time delay is set for the contactor changeover ?

L D[AR2 said:

What time delay is set for the contactor changeover ?

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No idea, the old controls are in a pile on the floor. Replaced by a shiny new PLC.

I was going to watch the characteristics of the current and speed before I set a time or changeover speed function and fit a current monitor relay for starting.

We had a mixer like this - it was dol but I don't recall a shorting contactor. It only took about 30 seconds to run up to full speed but we went to an inverter this year to reduce the risk of explosions (It's a 75 kw drive!)

We had a contractor remove the coupling - took best part of a day!!!

Markie may have something to think about.
If it has been on the shaft for years it may take a long time to remove and depending upon skills something may be damaged.
First just try to change the oil to something lighter and see waht happens.
You may get lucky.

You actually hit on a preferred solution in your post, Goody. The normal and preferred method for adjusting the output torque (and hence acceleration time) of this type of clutch for most brands was adjusting the oil level. There are upper and lower limits, of course, and viscosity etc. plays a part as others have pointed out. There is also quite a large range of adjustment by varying oil level, as well.

Thanks Tom I was wondering about the oil as someone did mention putting transmission oil in and not having it so full.
I shall test and play with it this week.

Goody said:

Thanks Tom I was wondering about the oil as someone did mention putting transmission oil in and not having it so full.
I shall test and play with it this week.

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There was at one time a special oil just for these clutches. As others have said, I suspect that automatic transmission fluid or hydraulic jack oil will work as a substitute. Maybe even these new light weight synthetic motor oils will work.

Tom Jenkins is absolutely correct, oil level does change the accell time.
In the real world once you get it running it will be off everyone's radar for the next 50 years. Try to find a viscosity that works near the full mark. As the oil breaks down and the seals drip the accell time will increases a little. By then we will all be long gone!
There is no mystery with oil, anything with anti foam agents will work. Hydraulic, motor, hytran (ag transmission oil) or turbine compressor oil. Don't get carried away, remember that this was probably designed in the '30s for oil that was available then. Maintenance guys tend to "fix something better", forgetting that it has already run 6000 hours a year for 30 years.
I used to tell my people and now my students " IT WORKED 'TILL I FIXED IT!".

two things come to mind with this.

1. Hyd Oil Type. - the common electrical oil that was both light and used in old factories contained PCB's (i think thats the letters)
this is an extremely light cancer causing oil. also used in oil pot overloads.

I do believe the Trans oil is the right one

2. Contactor bridging the O/Ls - this may have been added after.
If the starting is too fast and the current too high. the only way to prevent the OL trip would be to bridge it.
so this may be a modification due to a faulty coupling.
an electronic O/L my solve cancel this need.

empty the clutch almost and start motor, then fill it with 10 percent of oil etc.
if old oil is not black fill it back in.

I guesss I am just wet behind the ears but I have never heard of bridge starting and I try to learn about the older methods of doing things because you do run into them from time to time and it is just intresting to me.

I found almost nothing on this on the internet. Anyone know where to find a diagram / drawing of it?

What was the reason for bypassing the overloads? I am thinking if they are sized correctly to run then they would nuisance trip on startup? Eutectic alloy type overloads I assume?

If you notice, bridge starting in Goody's original post is in single quotes. I took that to me that was his name for what the thing is doing, and I can see why. So I wouldn't necessarily assume you will find much about this on the net.

I haven't seen this done myself. However, I understand why you would want to. None of the overloads on the market perfectly model the motors they are connected to. The new electronic overloads get the closest but even they aren't perfect. Just think of the multitude of motor frame sizes and build configurations covered by a single overload setting. This is especially true when it comes to short term high current levels. An overload will trip much sooner than required in this case. Do a test with a properly sized overload and the motor shaft locked and see what happens. The motor will be no where near rated temperature after A SINGLE RUN of that test.

If you know your application, starts are VERY infrequent realtive to normal run time and you expect very high starting currents then bypassing the overload may be the way to go. This allows the user to set the overload more closely to the normal run current of the motor without risking nuisance trips. But, as I said, you need to have a good handle on your application and some way to kimit start cycles.

Keith