Replacing Fluid Coupling 2

[Feg]

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

 

[Marke]

Hi feg

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

 

[jraef]

Marke is the master of Soft Starters (MOSS), so I can't add much. But one thing I will add is to be ABSOLUTELY clear on the role of the fluid coupling before you begin. Sometimes it is used in a torque-matching application where it is never fully injected. A soft starter cannot do this. Sometimes, along the lines of what Marke said, it is used when the load inertia is very high and the acceleration time is too long for the motor thermal time. Given that your fluid coupling "exploded", I would be very suspicious of this issue. The legitimate replacement application is only for when the fluid coupling was used strictly as a mechanical form of soft starting on applications where the motor thermal damage curve is not exceeded.

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."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[electricpete]

Those guys both know much more than me (MOSS1 and MOSS2).

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?

=====================================
(2B)+(2B)' ?

 

[ScottyUK]

Is it definitely a fluid coupling and not a torque converter? The latter have characteristics which an electronic soft start cannot match, multiplying torque at high slip. They are fairly common on large very high inertia loads.

 

[Chevin]

Replacing a fluid coupling with a soft starter is possible but be careful with the Selection of the soft starter, you will probably have to use higher ratings than for a standard application like a pump for example. When a motor is started using a soft starter the starting current will be about 50% of the DOL value so expect around 300 to 400 % of rated. However the starting torque will only be 25% of the DOL value so if your motor develops 200% when started DOL then the starting torque will typically only be half of its RATED value. With most fluid coupling applications the motor is started off load and then the fluid coupling is used to accelerate the load. With the soft starter the motor is going to be started loaded so the stating current will continue for a longer time, also think about how much torque is required. The advantage of using a soft starter is that there is less maintenance and also no risk of fire due to oil leaks but the downside is prolonged starting currents. Most modern soft starters are sized on the assumption that the motor will be up to speed in less than 10 seconds so if your application requires a longer stating time over rate the soft stater accordingly.

 

[Feg]

Thanks for the replys.

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.

 

[Chevin]

The fact that the motor is a 6 pole helps since the starting torque will be relatively higher. Also an old motor is likely to have good thermal capacity. Good to see you are over rating the soft starter, one with 2 or more starting profiles would allow for starting off or on load.

 

[jraef]

If the motor is sized to start unloaded only, then a soft starter will not likely allow it to restart if loaded. There are soft starters that are capable of providing dual ramps, with one of them being capable of DOL power. But still, if the motor is incapable it is still incapable. Before wasting any money on this, I would commission a detailed study by someone who can see and understand all of the ramifications. If you have to do it yourself, there are on-line inclined conveyor power calculators that can give you a reasonable estimate of the amount of torque you will need to start your conveyor fully loaded, provided you have a good idea of the weight of your material (per unit of length) on the belt. With that, you can calculate the motor kW necessary to start it loaded given your motor base speed and see if you have enough to even think about trying it.

"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

 

[Marke]

Old GEC 6 pole motor, materials conveyor, I would use a current ramp or a torque ramp soft starter with an initial start current set to around 350% and a final current set to 450 - 500% and a ramp time of about 15 seconds. This will allow for a good satr of an empty conveyor and a loaded conveyor. Works very much like a delay fill fluid coupling. You need to select a soft starter that is three phase control, do not use a two phase control unit for this appplication and selct one rated at 550% for 15 seconds should be fine.
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

 

[Feg]

Mark,

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

 

[Marke]

Hi FEG

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

 

[Chevin]

Hi FEG

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.

 

[jraef]

Now that they no longer pay me to care, my professional opinion of the Siemens soft starter is that it is fine as far as functionality goes, but is a little light on overload capability, an important feature for a soft starter. You don't say what the FLC is of that motor, and at 6 poles it will be higher than a standard 4 pole 110kW motor for sure. But most likely a 250kW version is over sized enough to handle this allplication. The 3RW44 has a built-in shunt bypass set of contacts, but it is NOT a true bypass contactor in that you cannot operate the bypass contacts as a DOL starter should the soft starter fail. I think you should have another Bypass Contactor fully rated for DOL starting of that motor just in case the soft starter fails, as most likely this is an impoprtant piece of machinery. In the 3RW44 product however, you are required to isolate the line side of the starter to do that (I cannot explain why, nor could I get anyone at Siemens to explain it to me when I worked there, but it is a requirement nonetheless). By the time you get done with all of that, I think you may find it less expensive to get another brand of soft starter that is more heavy duty rated and get a separate bypass contactor as opposed to an integral one. Not knowing where you are and what kind of support you have near by, it's hard to make a recommendation though.

"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