Conveyor speed control

Join Date
Feb 2012
Location
UK
Posts
57
Hi All,
I want to run a Conveyor belt at two different speeds depending on the signal from a sensor.
Implementing this logic is easy but my problem is, because the conveyor motor has to suddenly accelerate from slow speed to high speed and vice versa, it will damage the motor.
The time gap between the sensor signals is 30 sec and conveyor motor is controlled through a VFD.
Thanks in advance for the help.
 
The 'jerks' caused by speed going up and down after every 30 sec.
If you set your Variable Speed Drive Acceleration Time and Deacceleration Time to the proper settings, then there will be no jerks at all. That is the simplest method.

You can also create speed ramps, with drive inputs for each ramp, to control the increase and decrease of the speed. I think for only two speeds changing over 30 seconds, this would be over-killing the problem. However, you did not say, once the speed sensor shifts from low to high, then how long is allowed for the motor to accelerate from low to high speed? Is it 10 seconds? 5 seconds? If the acceleration ramp time is less than 5 seconds, then you may have other problems, depending on the difference between your Low and High speed.

What motor RPM is your Low and what is High?
 
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yes there is no issue on ramping up and down a motor but you can't do it faster than the mechanical time response of the system.

accelarating from 50 to 100% or decelering accordingly probably takes about 3-5 sec in a real world with standard stuff and if you want to do it faster, yes you will reduce the life of the system like it would happend if you were using your car for drag strips everyday 24 hours a day.

It would need more current in the motor creating more heat in the motor/drive/wires but it will also create stress on the mecanical couppling/drives and other stuff...The little play in the mecanical linkage will be knocked up and down each time you will change from accelerating to decelerating and so on...
 
Hi Lancie,
I am new to VFD's, can you please tell me more about this Speed ramp, where slould I create this, On PLC or on VFD.
I can set acceleration and deacceleration time on VFD and see how it responds.
To be precise, I dont have any speed sensor, I am getting a digital output ( ON/OFF) from a machine to start and stop feeding it and this machine is fed from this conveyor.
This is a Batch process, what happens is
1.Machine is fed at Certain speed of Conveyor
2. When Machine is full either conveyor should slow down or Stop( I have to determine this after the trial)
3.Machine takes 30 sec to Process and again Machine has to be fed.
This is cyclic process.
 
3. Machine takes 30 sec to Process and again Machine has to be fed. This is cyclic process.
Okay, then I suggest just setting the VFD internal Acceleration time to 10 seconds, and the Deacceleration time to 10 seconds also. Then when you get the On signal, give the VFD a Start signal and it will ramp up to full set speed in 10 seconds, run for whatever time needed, then get an Off signal, ramp down in 10 seconds to 0 speed and stop. Depending on the brand and model of VFD, you may need to set other parameters. Some VFDs have a default setting of "Coast to Stop". You need to make sure it "Ramps to Stop".

If it turns out that you don't want to completely stop the conveyor, then set up a VFD internal Speed Input, so that when the signal goes OFF, your VFD receives a Speed Input that causes it to ramp down to a preset speed. The Speed is set inside the VFD parameter settings, but the signal to Start, Change Speed, or Stop can come from your PLC.

You need to get a copy of the VFD User Manual. It will tell you how to set the ACCEL and DEACCEL times, and also which parameter to set for a Speed Input, and to which terminals to wire your Start, Stop, and Speed inputs.
 
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It is good you are considering question of jerks. I have seen laundry equipment replace reversing gearbox with VFD and 3 phase - reversed every 20 second with no problem.

What are
Motor parameters ie full speed HP type of enclosure
is slow speed motor RPM and high speed motor RPM.
gearbox reduction ratio.

How fast do you want the speed change to occur - this relates to your decel and accel ramps. Critical to know for sizing brake resistors.

At slow speed ie 5 to 10% of full RPM the big thing to watch out for is adequate motor cooling.

Dan Bentler
 
Whilst this application will certainly increase wear and tear on the motor I suspect that it will have a much greater effect on the conveyor. I would recommend that you thouroughly examine the suitability of the belt and mechanical drive components before putting this into operation. If this is a modular belt the lifetime reduction could be severe. As an end user we see this type of thing far too often and end up having to spend our time finding more suitable components.
 
Whilst this application will certainly increase wear and tear on the motor I suspect that it will have a much greater effect on the conveyor. I would recommend that you thouroughly examine the suitability of the belt and mechanical drive components before putting this into operation. If this is a modular belt the lifetime reduction could be severe. As an end user we see this type of thing far too often and end up having to spend our time finding more suitable components.

if the accell/decell change are smooth with enough time for the mechanic to follow, the stress could be less than normal starts with full motor torque...
If there is a lot of play (backlash) on the mechanical drive, it may be necessary to reduce it to avoid ''inertia impact'' when the load move this backlash back and forth with speed change (accel/braking) this is where the most wear will appear and it will broke
 
The drive will do all of this for you. You will need a run signal which will also command one of the speeds. You will need a second signal to use in combination with the first signal to get the second speed.

Internal to the drive will be acceleration ramp times and deceleration ramp times. These are adjustable to whatever your process requires. You will also have to set a parameter telling the motor to ramp to stop rather than coast to stop so you will have a controlled predictable stop.

You can also set a little "S curve" rounding of the accel and decel ramps so, if there is slack in you system, you can pull it out gently before accelerating faster.

It is true that rapid start-stop cycles can cause motor overheating but that is much less of a problem on a VFD since there is little if any inrush current.

If you still encounter motor heating issues, change to a TENV motor if the hp is under 30hp or to an auxiliary cooled motor if above 30hp.

As for your PLC, all it needs to do is generate the two signals and keep track of the timing. The drive will do the rest.

If the above cannot be done due to unfamiliarity with the drive, you should hire someone to do it for you. What I have described above is rather basic and, if its a problem for you, its time to get onsite help.
 
We have at least 3 processes that require this feature. We have a large press with knifes that cut books of veneer we use two steps or ramps comes forward very quickly then hits a prox slows down to a crawl in 1.0 sec.(at to the drive parameter actual never timed it)
We have a conveyor 50 plus feet which 90% of tree bark log ends ECT very heavy. Set of 3 photo eyes depending on the height of the Material 3 different speed settings.
Imagine a reel of peeled wood 1/32 on a reel. This is a constant accell and braking to unreal into a conveyer through a dryer and maintain exact speed so not to break the ribbon. And as the reel gets smaller it increases and decreases speed very fast to maintain a complete ribbon. There is a Little so called slapping on this one due to the very fast accell and braking and speed Fluctuation an occasional chain is all that has been serviced.
If you size the drive and motor up right. I see no problem with premature failure. All 3 of our process have yet (Knock on wood) to have a drive or Motor fail due to the constant speed fluctuation in more than 7 years.
 
Motor Speed Control

InstrumentationUK, you should read what DickDV has posted taking particular note of his last sentence. I have done the impossible with drives and motors, running high speed cyclic functions for years 'feeding' batch systems with no detriment to the 'electrics'. From my experience most failures have occured with in the mechanical delivery system. In any event if you dont know what you are doing read the manual, or get an automation person there and learn from him.
 

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