Replacing Fluid Coupling
Replacing Fluid Coupling
(OP)
Hi,
I am wondering if you guys have any experience in replacing Fluid Couplings with a soft start. The plant is very old and for safety reasons they want to replace the fluid couplings as they had some explode and cause a lot of damage. Now I know you will say they were filled wrong but they say they were not touched in years except for std maint checks. The safety guys want them gone, so I am looking at the mechanical aspect and I think the fluid couplings give a good smooth start with controlled torque and I think some can give torque in the range of 220 - 250% over motor torque.
Can you let me know what we should look out for when changing to soft start any pit falls or benifits.
Regards
FEG
I am wondering if you guys have any experience in replacing Fluid Couplings with a soft start. The plant is very old and for safety reasons they want to replace the fluid couplings as they had some explode and cause a lot of damage. Now I know you will say they were filled wrong but they say they were not touched in years except for std maint checks. The safety guys want them gone, so I am looking at the mechanical aspect and I think the fluid couplings give a good smooth start with controlled torque and I think some can give torque in the range of 220 - 250% over motor torque.
Can you let me know what we should look out for when changing to soft start any pit falls or benifits.
Regards
FEG





RE: Replacing Fluid Coupling
I have done this many times, but to be sure of a good outcome, some engineering is required.
Firstly, When you use a fluid coupling, you would typically use full voltage starting so that the motor quickly reaches full speed. During start it draws a very high current and is capable of full voltage torque.
When you use a soft starter, you reduce the start voltage which causes a reduction in the start current and the start torque. The start torque reduces with the square of the voltage or current reduction.
If the motor has a very high starting torque curve, then you can reduce the start current and still have sufficient torque to start the load. If them motor has a low start torque, then you may not be able to reduce the current and stil start the load.
You need to study the load torque curve and the motor starting curves to determine if the motor is able to produce sufficient torque to start your machine.
Secondly, during start, there are high slip losses. These losses are dissipated in the fluid coupling during start and are equal to the difference in speed times the torque. Initially, the speed differential is high and the losses are high and reduce as the load accelerates. If the coupling is too small, it will not be able to absorb the slip losses during start.
When you use a soft starter, the losses are absorbed by the rotor of the motor. Some rotors have a very low ability to absorb slip losses aand have a short Locked Rotor Time, or a low maximum load inertia, while others will withstand a lot of slip loss. You need to ensure that the rotor is suitable for the starting losses.
Thirdly, the fluid coupling will dissipate some slip loss during run which is eliminated when a soft starter is used instead of a fluid coupling.
If you get it right, you will have no problems. If you just try it to see, you could come unstuck as there are very large differences between the starting characteristics of apparently similar motors.
Best regards,
Mark.
Mark Empson
Advanced Motor Control Ltd
RE: Replacing Fluid Coupling
A good Transient Motor Starting analysis with something like SKM or ETAP would be a great plan, but you will need full motor and load torque=speed profiles. One thing you can do if that's not possible is to put the soft starter on the motor now and leave the fluid coupling in place, then fully inject it and try to start the load with the soft starter alone. If it starts, you can remove the fluid coupling.
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
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RE: Replacing Fluid Coupling
The only issue I focused on is that the change in thermal severity of the start for the motor as they both discussed.
Fluid coupling start can be less severe than DOL start without fluid coupling.
Soft start can be more severe especially if your torque margins get small and you're not driving a high inertia load.
Combine the above and realize you may be going from thermally easy start to thermally difficult start. So requires some analysis to see if you will remain within motor limits.
Just a question - what does the motor drive?
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(2B)+(2B)' ?
RE: Replacing Fluid Coupling
RE: Replacing Fluid Coupling
RE: Replacing Fluid Coupling
The motor is a very old (70's) 110Kw 986rpm 380V GEC motor and looks like a std sq cage motor. We dont have any info on the cureves and so on. The coupling is a very old one too and it is probably a Fluidrive. This coupling did not blow up, it has happened else where in the group and they want to get rid of them. The elec guys are talking about installing a Siemens or ABB 250Kw soft start.
jraef, I like the idea of filling the coupling if this is what you mean to see if it will start with the soft start but I would be worried if we fill the coupling that it may explode. Can you expand on this.
The application is an inclined belt conveyor in a quarry. The conveyor can start loaded or unloaded depending on what is happening and we dont want to be shoveling off the stone if it stops loaded.
what we are going to do is monitor the starting and running currents and see from there.
Thank you for your replies.
RE: Replacing Fluid Coupling
RE: Replacing Fluid Coupling
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Replacing Fluid Coupling
Have done this many times with units lasting over twenty years and still going fine.
If you use a torque ramp, use an initial torque of 40% and a final torque of 180% and a 15 second ramp time.
You may need to tweak from there.
Best regards,
Mark
Mark Empson
Advanced Motor Control Ltd
RE: Replacing Fluid Coupling
we are looking at the Siemens 3RW44 high feature rated at 356 amps 400V with torque control. We think this should be a good option, what do you think of these or have you ever used them.
I would like to thank everyone so far as there has been good interest n this topic.
FEG
RE: Replacing Fluid Coupling
I have not used the Siemens unit so I can not authoritively comment.
I would ensure that it is a three phase control and that it is rated for 500% current for 15 seconds minimum.
Jraef can probably comment more on this product.
Most of my experience is with the AuCom, Danfoss and Emotron products and these would all do the job fine.
Provided that the Siemens is correctly rated, I do not think it will be a problem.
One recommendation that I would make, is that if you are operating in a weaker supply environment, use an AC3 rated line contactor that is opened when the motor is not being operated.
The starter is most vulnerable to voltage transients when it is connected to the supply and not operating.
Best regards,
Mark.
Mark Empson
Advanced Motor Control Ltd
RE: Replacing Fluid Coupling
The 3RW 4446 looks about right since it has a heavy duty class 30 rating of approx 200 amps. Note that the starts per hour will be limited so check the Siemens info.
Setting options include torque control but suggest you use current limit with voltage ramp as an initial setting.
Regards Chevin.
RE: Replacing Fluid Coupling
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies