20 replies to this topic

[jerryf]

Hi,

This forum is great btw!

I have 5 Conveyors that range from 80 to 110kw that are soft started. The conveyors have had recent gearbox changes to increase speed. The problem we are having is when the conveyors are fully loaded they often trip on motor overload upon start. I have changed the starter settings to around 400-450% breakaway level (supply volatge 415V)a nd decreased the ramp time but to no avail. In once case i have added a 2 second kick start to apply full load voltage. All drives are controlled in voltage mode.

We are looking at upgrading drives to VSD's. Will this help?. Any advice will be appreciated.

Regards

Jerry

[marke]

Hello Jerryf

Welcome to the forum.

If you have changed the gear box ratio so that the conveyors operate at a higher speed, then the effective shaft torque at the output of the gearbox will be lower for the same start current settings. This is because the shaft torque of the motor is increased out the gearbox by the gearbox speed ratio.
Additionally, to accelerate the conveyor to a higher speed, you will need to input a higher total energy. The net result is that the energy required to start the conveyors is higher and the overloads need to be able to cope with this.

What are the overloads that are tripping? are these an external overload, or an integral part of the soft starter. If an external overload, then you will need to change the trip curve. This can be done with a thermal overload by replacing the overload with a higher class of overload i.e. rpelacing a class 10 with a class 20 etc. If the overload is an electronic overload, it may be possible to change the trip class or the start time constant. This is certainly the case with the AuCom and Emotron soft starters.

Either way, you should be able to operate successfully with good soft starters and appropriate protection.

The option of using VSDs will also work provided that you use VSDs that are able to develop a high start torque into a rotor that is not spinning. There are many drives that will give 150% torque about 10Hz, but not so many that can deliver this torque into a stationary rotor. You need the high torque at zero speed to break the conveyor away.

Best regards,

[jerryf]

We are tripping on thermal overload. We have changed the thermal overload units but no luck. It is interesting to watch the conveyor start under full load conditions, 90% of the time the Conveyor does not move an inch.. As you can imagine the motor gets very very hot.

[marke]

Hello jerryf

Ok, if the motor is not developing enough torque to break the conveyor away, you are in trouble.

The next question is whether the motor is actually capable of developing enough torque in the first place.
Do you have speed torque and speed current curves for the motors? Alternatively, do you have the Locked Rotor Current nd Locked Rotor Torque of the motors?
If you have these, then it is possible to predict what level or worst case start current you will need for a loaded conveyor.

There is no point in leaving a motor in start mode if it is not rotating, all that will do is cook the rotor. It sounds like the real problem is insufficient torque to start the conveyor, not thermal overload trips.
Best regards,

[jerryf]

Thanks for the swift reply Mark

Yes for 110kw Locked rotor current is 1282A and Locked Rotor Torque is 261%.

Regards

[marke]

What is the rated current of the motor?

[jerryf]

180A, Conveyor calculations suggest it should be plenty to start the conveyors under full load conditions.

[marke]

OK
So we have a Locked Rotor Torque of 261% and a Locked Rotor current of 712%

I would suggest that we design around a start torque requirement of 160 - 180% for a loaded conveyor at start. (Based on experience)

To develop 160% torque, the start current would need to be 712 times the square root of 160/261 = 560% current.

To develop 180% torque, the start current would need to be 712 times the square root of 180/261 = 590% current.

So you need to lift your start current up to somewhere in the order of 560% - 590%. Trial and error will show you where you need to be.

With your curent limit set to 450%, the start torque is limited to 261 times 450/712 squared. = 104% FLT

If the conveyor is sometimes started unloaded and other times started loaded, then I would recommend a current ramp from around 300% current to 590% current over a 3 - 5 second time period.
This will enable the conveyor to start empty without too much belt slap and associated problems, but also start fully loaded without excessive time in a stalled condition.

Hope this helps,
Best regards,

[jerryf]

Thanks Heaps marke thats great. The only problem is our soft starters have a current limit from 200 to 400% FLC.

[marke]

our soft starters have a current limit from 200 to 400% FLC

That is a problem. I do not know why some manufacturers believe that 400% current is "severe" duty or heavy duty, with modern high efficiency motors, the start torque at 400% is far too low to start difficult machines or high inertia machines.
You can change the rotors in the motors to get a much higher start torque at 400% (lower full load efficiency) or change the soft starters to more useful ones.
Hi torque drives could also be used as above, but should probably be sized for 180% torque at zero shaft speed. A lot more expensive than the right soft starters.

Best regards,

[jerryf]

Brilliant thanks again marke. any recommendations? and will a VSD with high starting torque from standstill provide better starting than a soft starter with the correct current starting settings?

Regards

[marke]

Hi Jerryf

I have successfully used both Emotron and AuCom soft starters for this application, but there are many others such as Motortronics that you can also use. It does depend where you are and what the local support is like.

One caution, the AuCom would need to be rated for the high start current and slightly over sized as the maximum start current setting is 550% and the standard rating is 300%. The Emotron would also need to be oversized as the maximum start current setting is 500%. This would apply to any soft starter that you were to choose.

It may be possible to get the conveyors away at less than 550%, only trial will confirm that. We used the figures that I normally apply for starting loaded conveyors and in your instance, they may be higher than required.

Perhaps you should try one only to get a feel for what you need. If you were in New Zealand, I could arrange a loan unit to try out and talk ou through the set up. You may be able to find someone local who has the experience to do similar.

The advantage of the VSD is that you will have a low start current, but you have a much higher capital cost, about 5% additional losses and a limited life expectancy, commonly 7 years for VSDs.

Best regards

[marke]

Another way of starting loaded conveyors is to use delay fill fluid couplings. With the coupling chambers empty, no torue is transferred from the motor to the load and so the motor starts very easily. The chambers are then filled and the coupling acts as a torqu limiter. In this case you would need to set the torque up for around 180% torque.

You could use your existing starters and motors for this, just insert the delay fill couplings between the motor and gearbox.

The disavantage of the fluid coupling is the power dissipated in the coupling during start and run. Like the VSD, the efficiency is reduced by about 5%.

Best regards,

[mariomaggi]

Dear jerrif,
I'm convinced that there are different kind of situations, where you could take some risks. This is not the case, I think, this is not a cheap and small motor where such modifications - like a gearbox - can be made (with a risk) withouth the necessary design.
To give you an honest suggestion, we MUST know more details on your application, otherwise we can only give you partial suggestions.

Please collect all info in only one document (you will lose some tens minutes). Motor degree of protection, motor label data (star/delta? frequency? etc.), type of mechanical coupling on the shaft (pulley, plexible coupling, etc.) ambient temperatures, duty cycles, overload sequences, motor insulation, protections, type of application (horizontal, tilting) and type of factory, main cable lenght between soft starter and trafo, etc. ), location (cold mountains, desert), country (for power quality considerations). Historical info will help (how many year ago was installed? etc.)
Such document will be useful to discuss possible solutions without mistakes.
-----------------
If you need to have only 180% of first-detachment friction, an inverter need to output something more, i.e. 200 % during starting period. This means that probably you have to select a bigger inverter.
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Is possible that the sudden start will happens because there is some "glue" or "gum" effect between mobile and fixed structure in the conveyor? In this case you have to operate chemically (lubrication, powders, etc.)

Regards
Mario

[GGOSS]

Hello Marke/Jerrif,

I belive there is something very important here that has been overlooked.

In relation to the soft starters, you will probably find that the Motor protection and Start Current Settings are directly related.

So, if you were to consider a product like the AuCom IMS2 for example, and wanted to retain the motor protection functionality of that product, you cannot upsize the starter to achieve a higher level of start current than the starter settings allow. Why, becuase to achieve a higher level of start current, you would have to artificially increase your motor FLC setting and this of-course would compromise your motor protection.

Hope that helps.

Regards,
GGOSS

[marke]

Hello GGOSS

You are of course correct, but in this case there are thermal overloads fitted (that is my understanding anyway), so if that is the case, there is no problem in oversizing if necessary. In doing so, you may loose the close protection of the motor within soft starter, but you will have the same level of pretection as already exists.

Best regards

[GGOSS]

Your right, there was something being overlooked, and it was me who was overlooking it.

[kens]

It all depends how 'rough' you are willing to be. With the IMS2 I believe there are two motor data sets that can be used. If some one was willing to risk it they could set a higher FLC in one set and then the correct FLC in the second set and use the run output to switch between data sets. As soon as the motor is running the starter would provide full protection of the motor. Not much help during start unfortunatly. In this example it could end up being a 'belts and braces' type approach where the thermal overload will protect during start with an input into the auxiliary trip input and then both the thermal and IMS2 motor protection during run.

Kens

[GGOSS]

Hello Kens,

The IMS2 has two motor parameter sets however the input must be closed before a start is initiated if the secondary parameters are to take affect. It is therefore not possible to start with one overload setting and move to another without firstly stopping the motor.

Regards,
GGOSS

[kens]

Thanks GGOSS, I couldnt remember the details of the operation of the parameter sets. Back to the simple option, using the thermals.

Kens