Start up problem in Conveyor Systems 1

[gokulkrish2]

Hello all,

I am working in a mining company and working on a conveyor project. The conveyor actually wont start if tried to start up with load on it. I analyzed the data of the motor current and it shows that the motor withdraws 180 amp for a maximum load on the conveyor and it's rating is 270 amp. The motor rating is 250 HP, 270 amp FLA and 1.15 service factor.

So with this result i figured out that the motor's rating is okay, but there is a problem with the starting torque or the starting current. My conveyor system is driven by the motor through a delay fill fluid coupling and started by a soft start.

I think that taking out the soft start and connecting the motor across the line would solve the problem. Any comments on this. Any repercussions??

Can you people suggest me what i should really do to overcome this problem

thanks
gokul

 

[dpc]

Starting across the line will give you more starting torque, but that may create a bigger problem. The soft starter is probably there to protect the conveyor from excessive torque more than to protect the motor.

The soft start can probably be configured to provide more starting torque, but you need to determine the maximum torque that the conveyor should be subjected to.

This could be a basic limitation of the conveyor, or you may just need to change the configuration of the soft starter.

 

[jraef]

Actually, the soft starter is not a contributing factor to the starting torque in this application because of the delayed fill fluid coupling. The soft starter is just serving to reduce the starting current of the motor. Starting it across-the-line will not change the situation with the fluid coupling. That is the more likely component that you need to get rid of.

 

[edison123]

Don't they have something called scoop control on these fluid couplings ? If I remember right, you could control or set the starting torque with this scoop control.

 

[dpc]

Either way, the reduced torque is probably intended to protect the conveyor. Some conveyors cannot be started when full.

 

[Skogsgurra]

dpc is very right. Many (most)mining conveyors refuse starting after being stopped fully loaded. Always empty the conveyor before stopping. In case of an outage, shovels are the solution.

Some installations have adequately dimensioned conveyors, but they are few and far between. Most designers seem to assume that a conveyor never starts fully loaded.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[roydm]

250 HP, that's quite a lot of shoveling!
I have never seen a soft start on a conveyor or hydraulic drive for that matter.
You don't say if it's the motor that fails to turn or the fluid coupling slipping.
What is the normal starting sequence, soft start then slowly engage the fluid coupling after the motor is up to full speed?
Is this a new conveyor you are commissioning or a new problem on an existing conveyor?
Is it just the load on conveyor pulling up an incline or is the conveyor pulling from a plugged feed chute?
When it stops is the load it starts with normal or does it get additional tonnage from something that takes a while to stop e.g. crusher or vibrating screen?
What starting current do you observe?
Please provide a better description of the conveyor and what you see when it tries to start.

I have often seen conveyors often struggle to get away with DOL starter, the motors spin for a few revs then stall stretching the belt to jerk the load slowly away. I don't see how a soft start can help the situation.

You say "I think that taking out the soft start and connecting the motor across the line would solve the problem. Any comments on this. Any repercussions" I doubt you would damage the drive train but you also say "My conveyor system is driven by the motor through a delay fill fluid coupling and started by a soft start". We need a bit more information about what is happening.

Regards
Roy

 

[gokulkrish2]

Hi Everyone,

Really amazed to see the response of this forum. I thank everyone who took their time to answer to my question.

Here is some more information regarding the problem.

It is an existing conveyor system

It is an horizontal conveyor, so no elevation at all. And it is a conveyor pulling from a plugged feed chute

It runs perfectly ok with the full load on it when running but when stopped, it wont start again until the load is shaved out from the conveyors

It is a soft start which starts it and a Fluid coupling is also there in between. Im sure about it. and as i already said it is a delay fill coupling.

No additional weight is added from any crusher or other equip but some weight may add from chute.

I dont know if its the motor which fails to start or the fluid coupling. But the conveyor starts once the load is shoved out. so i believed it was the motor

Starting sequence is as u mentioned. soft start then slowly engage the fluid coupling after the motor is up to full speed

The specifications of the Soft start and the fluid coupling i dont have right now. but will try to post them soon.

Can u guys figure out some thing with this data?

waiting for your response.

regards

gokul

 

[LionelHutz]

You need to determine what is keeping it from starting.

I've seen an installation like this before. The soft-starter is installed to limit the inrush on the power system and the fluid coupling is installed to produce the necessary torque to start the conveyor. So, you need to determine which is failing to get the job done.

It could be a simple matter of adjusting the soft-start to get up-to-speed a little quicker or changing the delay in the fluid coupling to not begin coupling the load up so quickly. The soft-start could still be in current limit mode when the load is applied which limits the torque the motor is producing and stalls the motor.

It could also be that the motor or fluid coupling just can not produce the torque needed to start the conveyor loaded.

 

[macmil]

You mention 180 amps on the motor ~ is that in normal operation ? or during the start ? If you are only getting 180 amps on a 270 amp motor for starting there's a problem. Have you measured the motor speed before it ( presumably ) trips ? In other words, is the fluid coupling slipping ? How long is the motor enegised before it trips ? Is there any movement on the belt at all ? Is there any slippage at the drive pulley ?

 

[dpc]

I worked on a lot of long belt conveyors for coal handling at power plants many years ago. The conveyor supplier required wound rotor motors to limit the starting torque. We designed in purge cycles via timers to empty the belts prior to stopping the motor. If the belt was stopped full (belt misalignment, pull cord sw. etc), the belt generally had to be at least partially shoveled off before it could be restarted. And when it was -10 deg F, the conveyors did not like being started, even when empty.

 

[Marke]

Hello Gokul

I would suggest that the best solution is to lift the start current of the soft starter so that it develops enough torque to quickly spin the motor up to full speed before the fluid coupling increases the torque transfer. This will give you the advantage of a reduced starting current.
Then, the motor, operating at full speed can deliver up to it's maximum torque which may be in the order of 200% torque for a relatively low current of 200 - 250% current. You will not achieve this torque with the motor operating at less than full speed except by operating with a slip ring motor.
Provided that the maximum torque transfer of the delay fill fluid coupling is higher than that required to break away the conveyor, it will start. If the maximum torque transfer of the fluid coupling is less than the required breakaway torque of the loaded conveyor, you will not be able to start it.

If the start current of the soft starter is too low, the motor will not get to full speed and so you will not get maximum torque transfer trough the coupling.

step 1. Increase the start current so that the motor can easily spin up to full speed even though the conveyor may not be moving. This may require a start current of around 400%. If the soft starter is a voltage ramp type, I would suggest that you lift the initial start voltage to around 60%.
Step 2. check to see if the maximum torque transfer of the coupling is sufficient to start the loaded conveyor. If you monitor the current drawn by the motor once it is at full speed, you should see the current rise as the torque transfer increases. You should also be able to determine when the coupling is "full" as the current will not rise any further. If the current reaches a maximum and it is less than say 200%, then the coupling is limiting the torque and an adjustment to the coupling may result in a higher torque transfer, or an alternative coupling may be required. From the current, you can get an indication of the torque being transferred and make decisions from there.

Replacing the soft starter with a DOL will not achieve any additional torque provided that the soft starter is set up to accelerate the motor to full speed before the load is transferred by the coupling.
Removing the coupling is going to reduce the maximum torque transfer and is OK if the starting torque is less than 100%. If the breakaway torque is in the order of 180%, then you will have difficulty achieving this with a soft starter and no fluid coupling.

Best regards,

Mark Empson
L M Photonics Ltd

 

[DickDV]

I think that the later responders in this thread have overlooked the single most important point about this conveyor system and that is: WHAT IS THE MECHANICAL TORQUE LIMIT OF THE CONVEYOR AND DRIVE SYSTEM?

Without knowing that, all this speculation on how to increase the starting torque may simply result in broken mechanical pieces.

Better get first things first and identify the mechanical limits of the machinery.

 

[Marke]

Hi DickDV

That is a good point, but in this case, the maximum torque that can be applied is limited by the fluid coupling, so changes in the settings of the soft starter should not be an issue. Changes to the fluid coupling may become an issue, but as it is a delay fill, the torque will be applied gradually and this is much kinder to the equipment than the "shock" treatment of DOL and no fluid coupling.

Best regards,

Mark Empson
L M Photonics Ltd

 

[roydm]

All,
This is a mining operation, they need to get the conveyor back up ASAP. In my experience they will not be worried about such niceties as "mechanical torque limits" etc. the operators push till it breaks then back off a tad. I worked at a 100,000 tpd plant where they would keep resetting the O/L relays on a bogged crusher until the rotor bars melted. I still wonder about the wisdom of having any sort of soft start on a conveyor unless it can develop the same sort of torque as a DOL e.g. wound rotor which will take up all the slack and jerk it away. I think easing into it will just cause the feed chute to pack down making it harder
Several of the posters suggested running the load off before shutting down, of course that's the normal shutdown but Gokulkrish2, is not worried about that, it's the unexpected trips caused by downstream equipment interlocks. With a belt that size (I'm guessing > 1,000 tph) you could easily end up with an additional 20 tons of excess material to deal with.
My suggestion, use the soft start to get the motor running at full speed, once the soft start contactor energizes then engage the fluid drive as quickly as possible. With the additional inertia provided by the spinning motor it should pull away.

Could we have a bit more info.
normal tonnage, speed, length, ore size
BTW, I assume there is a contactor to short out the soft start.
Regards
Roy

 

[Skogsgurra]

Conclusion: If you cannot empty the conveyor before stopping it - you need shovels.

And that would not be so bad. If that kind of stops are exceptions.

But if they are standard - then a redesign is needed.

No Band-Aids will work in the long run.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[roydm]

Skogsgurra, what are you doing up so late, me I can't sleep>
Regards
Roy

 

[itsmoked]

Skogs lives on the other side of the planet!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

I am glad you didn't say I live on another planet!

FYI: It ia a quarter past noon over here now.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[HCBFlash]

Shovels also encourage the grunts to go a little easier on the equipment. I've actually, even recently, seen "management" disable E-stops so the conveyor wouldn't get shut down loaded.



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Me wrong? I'm just fine-tuning my sarcasm!