Created Date: May 02,2019

Created by: justamechie

Hello,

I’m trying to determine what type of motor/drive to use for an application.

It’s for a lumber processing line and stacks of lumber must be transferred to different stations via three chain driven conveyors. I need to determine what motors to use for these conveyors. The stacks will move about 10 feet at a time, stop for about five minutes and move again. The line will continue indexing in this manner. I’m guessing I will need somewhere between ½ and 2hp motors.

This must be simple and low cost, so I was originally thinking AC motor with starter and contactor, but I think the stop/starts may be too frequent. Too much power consumption and wear on the components.

Soft starter is the next logical choice, but another issue is speed. For an 1800 rpm motor, I would have to use around a 60:1 gearbox to get the speed down to what I want, which is about a foot per second. That seems doable, but I’m not sure that’s ideal.

A 40:1 gearbox with a VFD set at about 70% speed might be a good option, but this is starting to get expensive and maybe a bit overkill for this application.

Or should I be looking at DC motors?

Any input would be much appreciated.

Created Date: May 03,2019

Created by: Alexandru

You say conveyor speed is a foot per second. Pretty fast.

in assume is a chain conveyor,with a 6” drive pulley. That is 60 rpm. At 1700rpm rated motor speed, you need a 30:1 reduction.

in terms of power is a bit harder to appreciate. I guess a 2kw/3hp motor is needed. At 575v that is 2.7 FLA (full load amps)

Not sure how cheap soft starters are, however I guess half the price of an inverter. Since you don’t need inverter, you may consider soft starters.

you don’t need plc for automation, everything can be done in a control,cabinet.

extra:

24vdc/Ac, for control power

photoeye to control the stop

pedal-start button to start

e-stop and run beacon

control and power field connectors, conductor, all circuits protected, conduit

a controls electrician would be able to do the work. Hardest is not the automation but the safety.

Created Date: May 03,2019

Created by: justamechie

Alexandru,

You are right on the money, that's exactly all the things that are floating through my head right now.

On the speed, I timed some similar conveyors and they looked to be moving about a foot per second but that could change. That's why I thought I better go with a VFD, so I can change the speed if need be.

I was thinking an 8 " pulley, but it's all just rough numbers at the moment, nothing is set in stone. I probably could use 6 ", it would put me in a better range.

The stacks of lumber I'm guessing will be between 200 and 300 lbs and the conveyors will be relatively short. I was thinking 2hp should be plenty, but I have to dig a bit deeper.

The soft starters, as Tinker mentioned, are not that cheap. They turned out to be about the same or some options even more than the GS2.

There are some other things that need to happen, I need to control a few hydraulic cylinders as well, so I did want to use a PLC (BRX).

I'm thinking photoeye as well and definitely need an e-stop circuit.

I want to do everything 480VAC and 24VDC if I can get away with it, to minimize the amount of components needed.

I have a lot of work to do, better get to it.

Thanks for your input, I'll probably be back with more questions soon.

Todd,

That's good to know on the cycle rate, thank you.

Created Date: May 04,2019

Created by: Anvilsoup

Alexandru,

I timed some similar conveyors and they looked to be moving about a foot per second but that could change

Sounds like you already have some conveyors that you could do some experimenting with. I would suggest you do so. Some of the things you need to find out will be will the sample conveyor trip on overload, will the stacks of lumber fall over, during a stop or start,

As Todd Dice suggested when using a VFD the stops and starts in a given time period can be increased greatly. Unfortunately there is not much information available on this subject when using induction motors. There is a lot of information when using servo motors. I'm suspecting your application can be solved with a VFD and a induction motor.

When using an across the line starter, the period of time that the motor is 0 speed to slightly above the breakdown torque speed, the motor will consume somewhere between 6 to 11 times the full load amps. Because this current does not make the load move any faster, it ends up as heat in the motor. When using a VFD, there is the opportunity to use the motor above the above the breakdown torque curve.

Created Date: May 04,2019

Created by: justamechie

Anvilsoup,

Unfortunately, I don’t have anything to test with, the testing will have to happen once the unit is built. But I have to imagine that with a properly sized motor and VFD I can make it work. Manual starters are sounding less and less like a good idea due to inrush current and jerky motion.

If I understand you correctly, you’re saying the inrush current level that would have been reached and that would have been lost to heat, could instead be used to move the load with a VFD. That’s interesting, but I’m not sure I would want to do that in this case. I’d be more inclined to slightly oversize everything, set slow accel/decel ramps to try and keep the current more constant, and keep everything easy and happy. Or am I missing your point?

Created Date: May 05,2019

Created by: Anvilsoup

Or am I missing your point?

My bad, I presumed from this statement that you had some similar conveyors.

I timed some similar conveyors and they looked to be moving about a foot per second but that could change. That's why I thought I better go with a VFD, so I can change the speed if need be.

One point I forgot to make is that you may get the same torque when you slow a motor to less than 60 hz, but the Hp will also drop. Fortunately for you most conveyors require the same torque at different speeds. Also belt conveyors that slide on a flat surface have a condition that I call "stiction ", that is they have a sometimes high breakaway torque to get started.

At this site they have a couple of charts https://electronics.stackexchange.co...for-max-torque

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As you can see the current really drops off somewhere around the breakdown torque point, so the motor is more efficient in converting current into torque when the speed is greater than 75% of synchronous speed, however the motor will not run long when it's giving more than 100% of full load torque.

One strange thing that happens as motors get larger they are limited in the number of starts per hour. Where I work we have 6 700 hp air compressors, the motors are limited to 2 starts per hour.

In round numbers when you use a VFD for many stops and starts a minute the object is to never let the motor get below the breakdown torque "hump ". Some people might call this "oversizing " but in reality you are properly sizing the motor/drive.

Created Date: May 06,2019

Created by: justamechie

Anvilsoup,

Thank you, this is good info. I’ll look into that some more if I end up using VFD’s.

However, as I’ve been looking more into the GS2’s, I’ve run into a few issues. The first is the e-stop, there doesn’t seem to be a good way to do this. Seems the only way to get a true e-stop is to put a contactor in front of the drive, which is potentially damaging to the drive. Not thrilled about that.

Second, the cost of this system is getting out of control. I thought the VFD’s would simplify things a bit, but it’s turning out to be the opposite. They take up more room and produce more heat, so I have to increase the size of the enclosure ($) and possibly add cooling ($). I still need contactors ($), I have to add communication cards to talk to the PLC ($), according to the manual if I use a standard duty motor I have to add an AC line reactor ($), or use an inverter duty motor ($).

I might take a look at soft starters again, maybe they are less in the big scheme of things.