centrifugal oil filled clutch for electric motors

[Tom Jenkins]

I haven't seen this done myself. However, I understand why you would want to. ......
Keith

I've got a different speculation about the reason, and this is one I've done myself - the clutch part, not the "bridge starting, which doesn't look like good practice to me.

Back in the day starting a high inertia load like some fans or pumps, or a load with a high starting torque like a bucket conveyor, caused a lot of trips on the overloads. We didn't have good reduced voltage starting, much less VFDs. Sometimes you just couldn't get the motor up to speed in the ten seconds or so available before the overlaods tripped - the acceleration torque wasn't sufficient. That's often why you would use a torque limiting clutch like the fluid clutch Goody described. My SWAG (Scientific Wild A$$ Guess) is that the contactors bypassing the overloads were there first, and the clutch added later to mininmize the shock load when starting the motor across the line.

 

[iant]

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?

As Tom has stated the only reason is to prevent false tripping.

It is simply a hard wired contactor in parallel with the overload set. and a timer in the start circuit.
These days electronic Overloads have both Thermal and Magnetic trip settings that can look after this.

I still suggest this may be an after market change, because the other reason to do this bridging, is to allow multiple starts with out a trip. simply by increasing the timer set point.
the old overloads would heat up and not be easily resettable.

 

[Gil47]

Where I work we have 12 of 22kw Fluid drives and we use tellus 32 hydralic oil with the level set at 10 o'clock -- 2 o'clock and just use a thermal over load

Most times our loads start no problem, but sometimes they are at a glueey stage and can be difficult to start hence the fitting of the fluid drives.

From memory ours do not stay more than 50 % overload during run up unless they are bogged

 

[Goody]

I got the term 'bridge start' from a CAD program I use.
It was already there ready assembled and called bridge start

And this drawing below matches the configuration I have on my 40 year old panel. Incuding the different rated fuses for starting and run

 

[JesperMP]

We actually used these [edit: I mean oil-filled centrifugal couplings] a lot in the past. Nowadays we use softstarters in stead, mostly because they are a lot less expensive.

Nothing much to add what has already been written.

Bypassing the thermorelay I have seen for extreme hard starts.
As a better solution, I suggest to remove the thermorelay completely, and use monitoring of the winding temperature in stead (if the motor has sensors in the windings).
A more expensive solution is to exchange the simple thermorelay for a more advanced programmable computerized relay intended for extreme hard starts.

I am thinking that maybe the bypass contactor is redundant today as already suggested. What is the motor nominal current, what is the starting current, and for how long does the starting current stay on ?

 

[Goody]

After a lot of trial and error - oh sorry - research and development, I got the clutch working to an acceptable level.

We tried transmission oil first and even when full to the top it still slipped for far too long virtually boiling the oil.

Then hydraulic oil. This also ended up being filled right to the top before it ran ok. The clutch still gets hot but I suppose it would and the reason it has cooling fins on it.

On the bridge starting control, I was under the assumption that the bridge contactor came in first then swapped to the run contactor.
No!, I realised that both bridge and run contactors have to come in together then the bridge contactor drops out.

The motor is over current only for a short while and I have placed a current relay to detect over-current for too long on start up.

The whole machine has been running fine for two weeks now but is still being watched.
They know an invertor will be fitted if problems occur.

 

[Tom Jenkins]

Thanks for the update! Glad it is running.

 

[GlennA]

Can anyone suggest why a wye-delta starter wasn't used instead of the bridge starter ?

 

[JesperMP]

A wye-delta would have resulted in less torque, and thus even longer start-up time, and thus even more likely to trip the fuses. Maybe it would not even be able to get up to speed, even if it would not trip the fuses.

 

[mostafa bhaa]

first there is nothing called (bridge start) it is only abbreviation used by goody to explain the situation but it may be old way to control the staring of the motor without damage to the main contactor or tripping for the overload by means of installing large contactor without overload for aperiod of time around 7 sec. and after thet change over to the main contactor installed with it the overload relay to protect against over current or locked rotor.
to solve the problem you have to remove the clutch and depending on the application of the motor you can install either softstarter or inverter if the application for this motor is constant speed use softsarter and the price for 22 KW is around 900 USD.
but i am afraid that may be this motor not induction machine may be this one is DC motor and this clutch is bruches for carbon slip ring!!!

 

[blueyedpeter]

I have only ever seen this bridge style of starting used once on a fan circuit , that wrongly used a three wire dol type of starter .
Where it could or should have used a 6 wire star delta motor starter , the high pressure fan took far too long to start up and reach final speed and for the proper size of the overload of the motor circuit , it was something like 0 to 3000 rpm in just under 45 seconds .
It was a long time for the overload to handle the starting current of the motor.

 

[Goody]

first there is nothing called (bridge start) it is only abbreviation used by goody to explain the situation

The picture I posted was from a CAD program and was labeled BRIDGE START

I didn't name it, it is an old fashioned way of soft starting a big motor that needs lots of time to build up to speed.
It has been running perfectly for a year now (it's a big spin drying machine that spin dries 10 ton of soaking wet yarn)
Like the spin cycle on your own washing machine, it builds up slowly to allow the water to be gradually spun out.

The clutch takes all the strain off the motor under the initial heavy load gradually building as the water is pumped out.

It's not perfect in today's modern control ways but I suspect it will still be running beautifully long after I have retired.

 

[blueyedpeter]

As for your oiled filled drive unit yes i haven't come across a unit like that in the last 15 years or so , but in an old gantry over head crane , it was a brilliant way of allowing the drive to slow down the long travel of the crane with out creating harm to either the motor or motor circuit .
And as it has already been pointed out oil type and level controls the torque output the amount of slip is crucial , another magnetic unit that allowed for drive adjustment was a magnetic coupling and the coupling device was filed with an iron powder , adjusting the voltage on the coupling allowed the drive motor to break free from the load and allow adjustments to a strip accumulator on the fly while not stopping production . They were great old units very reliable .

 

[Goody]

It's not perfect in today's modern control ways but I suspect it will still be running beautifully long after I have retired.

It is my sad duty to report the demise of this fluid clutch.
I couldn't get the full story but I suspect it suffered severe mechanical damage, either way it was leaking oil faster than they could put it in.

On the plus side, I got to fit a new Ac Drive that I haven't worked on before - a Vacon 100. What a delightful drive to fit, wire and configure.

The control panel is like a mini HMI with wizards to set everything up - all in good plain English (with a help screen for every parameter) and when it is running, 1 screen tells you everything you need to know.

I shall recommend Vacon drives in the future.

 

[HJTRBO]

I shall recommend Vacon drives in the future.

Don't if you want to put them on an Omron ethernet IP network! LOL> An issue I had last week with one... Solved it in the end.

But aside from that I agree, they are very easy to use and they last. Case in point; We have hundreds of them.