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Old February 11th, 2003, 08:36 AM   #1
Gautham
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VFD , motor , PLC (start /stop/position application)

We are building a Downspout machine for the first time.Actually we thought it would not be much difficult. But we now face a lot of problem.

our main drive(VFD) is rated for 11KW and the main motor is rated for 7.5KW.
Drive is configured for Sensorless vector. (autotune everything was done)

Now we wrote our PLC program and tested the machine without load on very low speed (100RPM). Our motor Max. RPM is 1750.

We have an Encoder which produces 1000pulse for 0.312m of sheet displacement.

Now we have different operations :
1. Straight tube cutting
2. Elbow ( left, right,up & down)

Below is the operation required for straight length tube.
The operator enters the length required and the no. of pieces that he wants.
Say if the operator enters length = 6m and no. of pieces =10

Our PLC has an Encoder card to read the pulse directly and an Analog output card which gives the desired speed command to the drive.

for the straight tube application for 6 m lenght;
from 0 to 5.5 m speed was kept at 100rpm and from 5.5 to 6m speed was reduced to 50rpm and at exactly 6m speed was reduced to 0 rpm.
Now without load everthing worked fine.
when we inserted the sheet 0.7mm sheet, the motor could not even pull the sheet at 100rpm and so we increased the speed to about 400rpm when the motor began to pull the sheet. Now we are actually using sensorless vector mode and i thought that the drive will manage to pull the load at 100rpm.

Below are the finding:
Without sheet the motor took 67% current and torque was 18 to 24% depending upon the speed.
With the sheet the motor took 130% current and torque was 150%.

Does that meant that our motor is underrated for the application.

Now the other thing is that as i said we tested our program without sheet and with sheet we had to increase the speed to minimum 400rpm.
But at 400 rpm i could not exactly stop the sheet at the desired distance.

Now our other option to stop the sheet at the required position is the use of DC injection, Mechanical brake and Dynamic braking.

Our drive manual says that DC injection is not possible for Sensorless vector motor. We donot have any mech brake attached and so our only option is Dynamic braking. Our drive has a brake switch only thing is that we have to try with a brake resistor.

We found one brake resistor which is 36 ohms, 500W. But our manual says a minimum load of 50 ohm or above to be used for a 11KW drive.What will happen if i use a 36 ohm resistor.

The other option that we are thinking about is to use the ramp function in our PLC to ramp down the speed.
Our PLC ramp function has many options and below is what i selected.
1.start value (this will be the current running speed)
2.End value ( we need stop condition so 0 rpm0
3. mode ( 0.01 sec deceleration)
4. Desired time ( if we enterd a time here the motor stops way before it reaches the required position). we actually need this to work as per the position instead of time. any idea on this.

Even if we succeed in the staright position we have lots of problem in the bending side.

As for the elbow.
First the tube of length 0.5m should come out and then the bending cylinder will operate as many times depending on the bending angle requred. Each bend will produce a 11Deg so to have 88deg bend we will have 8 bends and the motor should move the shee 0.025m for each bend. I could not produce that torque for the small lenght at slow speed and when i go to high speed i could not stop it.

So now we have come back to very bad situation that AC drive and motor is not the correct choice for Downspot machine.

Anybody have done the downspout machine with AC drive and motor please tell me the way to start and stop the sheet at exact position.

Many thanks
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Old February 11th, 2003, 09:04 AM   #2
seppoalanen
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Below are the finding:
Without sheet the motor took 67% current and torque was 18 to 24% depending upon the speed.
With the sheet the motor took 130% current and torque was 150%.

Motor current (freely one the floor) is 1/2 of nominal current, it takes Iq-component 'only'.
It seems Your motor motor is underrated for the application.
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Old February 11th, 2003, 09:51 AM   #3
gautham
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Thanks seppoalanen for your posting. Now how do i select the right motor for this application. Now we are using 7.5KW(FLA =15amp).
Do you think that a higher rated AC motor will do the precise start and stop.
What kind of braking will provide precise start stop / positioning.

thanks
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Old February 11th, 2003, 10:54 AM   #4
DickDV
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Gautham, I don't mean to be personally unkind here but what you have described is, unfortunately, a spectacularly misapplied system in several respects.

First, why on earth would anyone specify an 1800rpm motor to operate a load going 100 and 50rpm? What must be done to fix this part of the problem is to get a gearbox or other type of mechanical reduction in the power train so, when the load is going at maximum speed, the motor is turning around 2400 rpm. Your pulses/feed distance will have to be recalculated.

Second, inverters generally are not, by themselves, very good at positioning. This shortcoming can be overcome to some extent by setting up the drive-motor for maximum speed stiffness thru sensor-less vector or flux vector control and setting accel and decel ramps that can be rigidly enforced. If that means a decel ramp so steep that snubber braking is needed, add a resistor to the braking terminals. (No, you can't use a lower ohm value than specified because the chopper resistor will see too much current and fail). Usually, it is better to let the decel ramp be as long as is necessary to avoid freeplay and gear lash in the power train from slamming back and forth. This only beats up the equipment and causes error in the final positioning.

Third, it would be better, in my view, to do the positioning control using the encoder pulses in the PLC and let the inverter slave off the PLC. If that isn't possible, then use a prox switch as close to the final position as possible and enforce a drive decel ramp from the switch location to the final desired location. With the gearbox and careful selection of switch location and ramp rate, you should get good positioning.

Finally, just to provide a little insight into how far off the mark this application is, consider this. You have paid for an 11kw (15hp) AC drive and a 7.5kw (10hp) motor, but are using, at most, only .416kw (0.6hp) of power at 100rpm. And then not getting good performance! Most definitely, the answer is not a bigger motor but using what you've got more effectively. I suspect that, with the gearbox, you will find you have way too much torque and will have to do a torque or current limit in the drive software.

And, without trying to sound to parochial, all sensorless vector systems are not created equal. This is especially true at very low speeds, as this application has demonstrated. Others are capable of managing motor torque and speed quite well down to stall. Given the apparent capabilities of the equipment you have, its just another reason for shifting the motor speed up higher where the drive can manage speed and torque better.

Again, gautham, not trying to be mean here. Hope this helps you get things straightened out.
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Old February 11th, 2003, 11:19 AM   #5
seppoalanen
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DickDV tells many important points for Your application as some suitable gearbox that you can use better speed and torque values of the motor.
Motor Current is decisive factor who limits torque in slow motion,
becourse Current heats motor and it cooler (fan) works imperfectly.

You can calculate speedoutput value in PLC between destination and current point as I think you do.
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Old February 11th, 2003, 11:42 AM   #6
Peter Nachtwey
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?

What type of servo motor and drive would you use Dick?

I think gautham's deceleration time is unrealistic and it is time for a real servo system. I am not familiar enough with different types of servo motors so I can't/won't recommend one, but I do know that many companies use hydraulics to bend pipe and tubes. Hydraulics do very well at low duty cycle applications where force must be controlled.

Controlling this kind of application is a PLC will be a real challenge not matter which way gautham decides to go.
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Old February 11th, 2003, 12:44 PM   #7
Barry Humphries
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We have a lot of this kind of equipment where I work, we mainly use Bosch Servo drives, mind you they cost loads more than motors and inverters, we use ABB Motion controllers.

We have in fact replaced a lot of old hydraulic machinary with servo technology and would NEVER go back, its soo much smoother!

We also have 3 Fanuc Robots that use Fanuc Servo Drives and Motion controllers for 6 axis control.

Servo Drives will do the job you want to do, but the cost.. well may be out of your budget! but have a look anyway!
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Old February 11th, 2003, 01:11 PM   #8
Peter Nachtwey
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Quote:
Originally posted by Barry Humphries
We have in fact replaced a lot of old hydraulic machinary with servo technology and would NEVER go back, its soo much smoother!
If your are comparing servo motor control with bang bang hydraulics, I agree. However hydraulic control is now just as sophisticated as motor servo controllers and in some cases more. Splines or cams, gearing, s curves have come to hydraulic controls a few years back.
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Old February 11th, 2003, 02:54 PM   #9
Gautham
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Thanks all for your comments. I would like to add few more points
1. we are using Gear box ( I did not mention that earlier). Gear ratio 20:1. The 100rpm that I said is the motor shaft RPM and not the Gear output. I just did a scale in my PLC for 0 to 60Hz to reflect 0 to 1750 RPM (motor shaft) and never talked in terms of Gear output. So that 100RPM is 5 RPM at the gear end. So all my test without load was at 100 rpm (5RPM).
2. The reason 100 rpm(5 RPM) to test is because we need to do bending every 2cm so as to have an ELBOW. So only at 100rpm without load I could do that at the time of programming without going too much into breaking. We reduced the decelaration time to 2 sec and we were able to stop exactly by ramping down to 0 rpm without load.
3. Also I noted some strange things. When in operation with load(with sheet) if I suddenly give a stop command to the drive, the drive will not stop but motor stops rotating but I could hear strange noise from the motor(as though the motor is trying to rotate). I realy donot know the reason. But after sometime I could see that my chains will become loose again and at that point the drive also stops. Do you know why this happens.
4. Now my worry is that my start and stop are frequent and at high speed I will never be able to precisely stop at 2 cm interval. For straight tube length we would go for high speed and for bending we need to go at low speed but at low speed we could not move the sheet. So what should I do now. How can I produce enough torque at low speed to have my start stop at 2 CM interval.
5. Also our motor is inverter graded and force cooled.
6. Encoder is not attached at motor shaft instead it is attached at the feedroll so as to measure the sheet distance.
7. I donot think we can use the proxy to do the speed reduction as they need different size of elbow and this proxy needs to be exactly positioned as per different elbow size.
8. I donot have the brake resistor as recommended by the drive manual(50 ohm) and so I will try with my 36 ohm , 500W resistor.
9. Now as I said with load our motor took 130% current and 150% torque with gear box is that normal or my motor is under rated??

So is there any way out without using Servo drives and motor.

I agree that I realy donot know much about the selection of motor and drives but I always thought that if this does not work then use the bigger one and that is how I did things so much year. Even if I asked my senior, they would say replace that with the bigger one and thatís it and I never got a chance to learn though I was interested to know.

I would like to start it from scratch and If you can provide me with a website were I could learn from basics of torque calculation , brake resistor selection for different load and so. I would be realy thankfull to all of you.

Thanks again.
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Old February 11th, 2003, 03:12 PM   #10
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Gautham, seriously, this is not the right kind of application to "learn" about drives. Please, get a drive application specialist to help you.

That will give you time to learn the basics first and then work up to more difficult applications like the one you have here.

And, as I said earlier, using that 36 ohm resistor will likely blow the brake chopper transistor. If that is what you want to do, go ahead. Otherwise, do the right thing and buy a resistor with at least 50 ohms.
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Old May 23rd, 2006, 03:00 PM   #11
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Wink

This may or may not help your application.
However i have an application where I use a PLC High Speed Counter to count encoder pulses and output a 0-10VDC VFD speed control or a multi step speed command. In the PLC logic i can aproach the target and slow down the motor to "creep" up on the final target value. (This setup is known as a poor mans servo).
This works very well in my application. However I have a 1000PPR encoder and a 78:1 gearbox. So my resolution is very high. Also with your app. I hope you have a seperatly driven cooling fan for the motor.
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Old May 23rd, 2006, 04:09 PM   #12
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Peter,


Don't you have experiance in veneer lathes? I've seen cariages powered by both DC motors, and hydraulics (Bosch servo.) The hydraulics had much better control, but the DC worked faster. I guess Gauthum is not yet worried about production numbers. Still, the ratio on the DC motor was much greater than 20:1

Gauthum, I'd first guess that you need more ratio than 20:1. A higher ratio will give you better ability to control position. As far as ristors go, its a lot cheaper to get the right resistor than to burn up your drive.
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Old May 23rd, 2006, 04:21 PM   #13
Thomas Sullens
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Quote:
when we inserted the sheet 0.7mm sheet, the motor could not even pull the sheet at 100rpm and so we increased the speed to about 400rpm
i nothing about motion control, but can help a little on the power end. If your power is 60 hz when you were your motor was running 100 rpm`s you were about 3.4 hz when you went to 400 rpm you were about 13.7 hz. Several years ago we did a project more blood and guts than motion control and where told that the drives at that time developed full torque and hp the last 75% of there bais frequency. So it looks like since the drive and motor would turn the load at 400 rpms this could be your problem and the faster you run the motor the less control you are going to have with your setup. i maybe wrong, but i don`t think you will develop full hp until you reach 1750 rpm. Also without a breaking resistor you have practically no stopping torque which is probably not good when trying to do motion control. To do what you want your going to have plenty of torque both ways. Until you have this i believe your P*ssing into the wind. At the low hz`s i guess you could always boost you`re v/hz ratio, but your motor is probably not going to like this for extended periods of time. You do realize you don`t have 7.5 hp at below base hz of 1750 and i could be wrong about this. This is going to be a good learning curve keep us informed as to your progress i`m sure there are plenty that will help. Some that have responded to your post are truely motion control people.
Good luck!
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Old May 23rd, 2006, 06:30 PM   #14
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Quote:
Originally Posted by Thomas Sullens
i nothing about motion control, but can help a little on the power end. If your power is 60 hz when you were your motor was running 100 rpm`s you were about 3.4 hz when you went to 400 rpm you were about 13.7 hz. Several years ago we did a project more blood and guts than motion control and where told that the drives at that time developed full torque and hp the last 75% of there bais frequency. So it looks like since the drive and motor would turn the load at 400 rpms this could be your problem and the faster you run the motor the less control you are going to have with your setup. i maybe wrong, but i don`t think you will develop full hp until you reach 1750 rpm. Also without a breaking resistor you have practically no stopping torque which is probably not good when trying to do motion control. To do what you want your going to have plenty of torque both ways. Until you have this i believe your P*ssing into the wind. At the low hz`s i guess you could always boost you`re v/hz ratio, but your motor is probably not going to like this for extended periods of time. You do realize you don`t have 7.5 hp at below base hz of 1750 and i could be wrong about this. This is going to be a good learning curve keep us informed as to your progress i`m sure there are plenty that will help. Some that have responded to your post are truely motion control people.
Good luck!
Thomas you are correct about the HP dropping as the speed drops, however remember the HP is the measure of work done over a unit of time.

With modern VFD's running with either sensoreless vector or flux vector control you can have the motor produce full torque all the way down to 0 HZ. At 0 HZ the HP will be 0 because no work is being done. The things to keep in mind are, you either need a TENV (totally enclosed non ventilated) motor or a motor with an external cooling fan. I have never seen a motor this small with an external blower so I am hoping the motor he has chosen is designed to run at these speeds for an extended time.

In hindsight I'm sure the original poster now realizes he should have gone with a servo and it might still come to that. I am guessing he is in the position where he doesn't want to return the hardware that he has, or can't. And maybe doesn't have time to wait for a new drive system.

I have never had any experience with sensoreless vector on a Siemens drive. I have however withnessed that the performance of this technology VASTLY differes between manufacturers. I have been very dissapointed with a couple of the sensoreless vector drives that I have set up (basically anemic performance in one case). I have had much better luck with flux vector control. I would suggest that the original poster check and see if there is a flux vector option on his drive. This would require hooking up an encoder directly to the motor, I don't know if this is possible with the drive/motor combination that he is using. The reason I suggest this is that if his motor will pull the load at 400RPM and not 100RPM then his drive and motor combination is either not set up right or the sensorless vector portion of the control is not working right, with a proper system the motor should develop the same amount of torque at 100 RPM as at 400 RPM

And no don't hook up the 33ohm braking resistor unless you want to find out what electricity is made of.

As far as the positioning goes, trying to stop to a position at varying speeds will never work. I would suggest as others have of slowing down to a fixed speed everytime you get close to your stop point and then issuing a stop command. You will have a better shot at stopping in the right place. Whether or not it will be accurate enough is impossible to say.

At the end of all of this you may end up needing a servo anyway, I am just trying to give you a fighting chance with what you have.

Good luck.

Last edited by allscott; May 23rd, 2006 at 06:33 PM.
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Old May 23rd, 2006, 09:05 PM   #15
Peter Nachtwey
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This is an old thread

Quote:
Originally Posted by plchacker
Peter,

Don't you have experiance in veneer lathes? I've seen cariages powered by both DC motors, and hydraulics (Bosch servo.) The hydraulics had much better control, but the DC worked faster.
Yes, lots. I used to work for a company that installed the mainly the scanning veneer lathe optimizer. Now we sell to these companies. I was quite literally thrown to the wolves when it came to motion control but the systems were well designed so scaling and tuning the motion controllers was easy. I didn't know it could be hard. It all seemed intuitive so here I am 25+ years later.

I have movies .wmv and .mp4 file on our ftp site.
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Quote:
As far as the positioning goes, trying to stop to a position at varying speeds will never work.
Our customers do it all the time. One must only do a simple calculation each scan ramp distance =(Velocity^2)/(2*decelertion rate). When the distance to go is less than or equal to the deceleration rate then the axis must start decelerating. Sometimes it is desirable to start ramping down one time period earlier so the equation becomes:
ramp distance =(Velocity^2)/(2*decelertion rate) + velocity*scan time.
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