Multiple motor startup using ramp instruction

[mordred]

I have a large conveyer system that is currently using an SLC 503
Its motor startup currently uses a 0.5 sec delay on startup on each motor sequentally to prevent inrush current. The problem is that now the loads are starting and stopping more often this effectively
reduces the number of boxes/minute average and backs up production
on our heavier box days I've been thinking of rewriting the program to use the ramp instruction in the SLC 500 set and I am curious if anyone has tried this instruction and what risks I may be inadvertently inducing?

 

[jstolaruk]

I have a large conveyer system that is currently using an SLC 503
Its motor startup currently uses a 0.5 sec delay on startup on each motor sequentally to prevent inrush current. The problem is that now the loads are starting and stopping more often this effectively
reduces the number of boxes/minute average and backs up production
on our heavier box days I've been thinking of rewriting the program to use the ramp instruction in the SLC 500 set and I am curious if anyone has tried this instruction and what risks I may be inadvertently inducing?

I haven't used that instructions but it should work for analog control of devices that are compatible with that type of control. I can't see how it will help you with your production losses though.

 

[ChuckM]

I'm not sure what you are trying to do, but the ramp instruction I don't think is what you want. If you have a lot of conveyors starting and stopping that often, you may want to look into leaving the motors running and using clutches to start and stop the conveyors.

 

[Tom Jenkins]

I guess I don't understand what you are going to do with the ramp instruction. It implies that you are going to be gradually increasing the speed of the motor, which implies that you are using a VFD, which means that inrush shouldn't be a consideration.

It isn't generally a good idea to try to use controls to compensate for a fundamental design flaw in the mechanical system.

What is your motor controller? Why do you need to stop and start frequently? What is the cycle time on the motors? Lots of questions need answering before a good soulution can be found.

 

[mordred]

The system has 15 motors controlling different sections of the conveyer system which is a Bushman design or Pinnacle this conveyer has a brake zone through pneumatics every foott for accumulation control and is a clutch driven system each motor is gear driven at a predesigned speed hence no VFD and each has its own contacts and trip sets the reason for all the start stops is that this in turn feeds a barcode scale system that runs at 24 boxes per minute then these boxes are manually offloaded in our warehouse area for shipping due to our manpower shortages they cannot reliably supply enough manpower to offload them at the 24 box per minute rate. The multiple shutdowns in turn has been causing an increased rate of damage to those motors. My thought is that I may be able to start them up in larger groups using the RMP instruction which in turn will start the boxes moving, clearing the braking zones faster and at the same time help to protect the motors themselves we are also looking into automatic palletizers but haven't been able to find ones that can palletize a line with an average of 10 different products this number varies daily at this rate. Also there is a momentary loss of movement as the rollers tend to initially slip on startup I believe I can reduce that initial slippage by ramping the motors giving them a better chance to grip my average conveyer rate is 60 feet per minute
currently the motors are straight on/off from the PLC outputs to the contacts with the above stated delays

A clutch system may be one option were also looking into switching to hydraulics which I don't particularly like as the pressures are never constant hence throwing off the conveyer timing

 

[Kevin(spark)]

they cannot reliably supply enough manpower to offload them at the 24 box per minute rate

What speed are the boxes entering the conveyor system?

 

[mordred]

What speed are the boxes entering the conveyor system?

that varies according to how many personel are filling the boxes what orders are needed and how far behind they've gotten at the beginning of the system it averages at 10 to 16 boxes per minute they also cannot load faster than 24 due to the accmulation controls, at the end they enter the scaling system at 24 boxes reliably and exit at the same speed. Tom I agree a VFD would be the best overall design however the company does not wish to spend any large amount of money on this conveyer as its life expenctancy is less than 2 years thanks to installing a mild steel conveyer on a high chemical washdown environment

 

[Kevin(spark)]

If the problem is being caused by the consistancy of the speed that the boxes are being unloaded, can you fit a simple gravity conveyor at the discharge point?

 

[mordred]

I know the problem is the end point which is an Intralox accumulation belt
however a gravity feed still does not solve the offloading issue to put it mildly I lose 40% of the capability of the present design due to not enough personel to unload the conveyer at the end

 

[mordred]

What I really need to know is how reliable can the ramp instruction simulate VFD functionality?

 

[OkiePC]

The RMP instruction lets you create linear, acceleration, deceleration, and "S" curve ramp output data wave forms. Use this instruction to ramp analog outputs when using them to control devices such as valves...Rockwell Software 2005

All the RMP instruction does is control the rate of change of a value in a PLC register.

Example: If you want N7:0 to go from 0 to 5000 in ten seconds, this instruction will do it with a linear or S-Curve pattern.

You still need to have a variable speed drive or other analog device to send this value to.

 

[Kevin(spark)]

If you don't have enough personnel at the end to unload this machine even when its running 40% slower than it should, how will they be able to unload it running at full speed? Or am I misunderstanding the process?

My idea of the gravity conveyor was purely as a buffer (if its long enough) so that if the rate of removal is sometimes less than 24 boxes/min there would be a delay before the machine would need to stop, obviously this does mean that at other times more than 24 boxes/min will have to be removed.

But if the max rate of removal is less than 24 boxes/min you will have to accept a reduced operating speed.

 

[mordred]

Beat me to it Okiepc I had just arrived at that conclusion now that I had time to think more about the circuit and the types of motors that are currently on the system Ah well I was hoping looks like It will end up being an expensive fix after all. I believe the best fix is to redesign the warehouse side of things hopefully I can convince them to spend the amount of dollars for the solution. Thanks all for your help.

 

[mordred]

Kevin I'm thinking the most feasible alternative is to look into a recirculation sortation conveyer system with auto palletizers hence less manpower now its just a matter of money and finding a palletizer fast and flexible enough. We now have enough power jacks to move the product with our new warehouse expansion. This in turn may have freed up a considerable amount of room if we remove the two old blast chilling rooms. the cost may reach a million bucks for all of that I know the old sortation conveyer we had at one time was 1/2 a million. However its the only way to gain a consistent running without the constant shutdowns

 

[Tom Jenkins]

Pay close attention to the implications of OkiePC's answer.

The PLC can't directly manipulate the motors. If you have a discrete output, which it appears you do, it can only make the voltage to a field device like the on/off contactors for the motors be on or off. The ramp instruction can't change that.

Even if you go to an analog output, you won't have a solution. The output value can be made to ramp, but the signal must be tied to something that will accept the varying signal and in turn vary the power to something else. If you have a VFD, for example, it tracks the analog signal and in turn varies the output speed (frequency) of the motor in proportion to that input. If you only have contactors for the motors there is no physical mechanism to vary the motor speed and no input for the analog signal.

VFDs for small motors are now fairly inexpensive. AutomationDirect.com, for example, has 10 hp drives for $800. It may be possible for a reasonable amount of money to identify the key motors, fit them with a manually controlled VFD, and adjust the speed so they can run continuously.

If this is a conveyor, note that you may need to de-rate the VFDs for constant torque operation.