motor snynchroniczation

[leitmotif]

I think you have a choice of using one VFD as a master to drive the other two at the same frequency OR you can use the PLC to send 3 speed signals one each to each of the VFDs.

If you go with the VFD master then they will all be at same frequency the only variation between them will be the slip. So if you can live with each one speed varying by say 5& you are OK.

The PLC option lets you set a specific speed for each VFD and motor ie the input multiplied by some factor to give different speed signals to each VFD.

IN either case
You will not get fine speed control ie cannot control for slip under varying load conditions and will have to live with any slip thus each motor gearbox varying a little unless you go with encoders to measure actual speed and feed that back to result in real fine speed control.

Dan Bentler

 

[mordred]

Ok well couple of questions which PLC are you using and analog input card and analog output card? Do you want the conversions to RPM or is Frequency sufficient? I'll also need a good gear ratio for the RPM for at least one of the motors and I'll see if I can work out an example.

 

[mordred]

You will not get fine speed control ie cannot control for slip under varying load conditions and will have to live with any slip thus each motor gearbox varying a little unless you go with encoders to measure actual speed and feed that back to result in real fine speed control

there is a slip compensation boost available on the PF 4 however its not 100%

 

[Adam303]

Slc 5/05
1746-ni4
1746-n04I
gear ratio i belive is to be 40:1 on one of the gear boxes

 

[Adam303]

The slip should not make that much of differance as the load will be at it's minimum .
RPM WOULD BEST

 

[mordred]

Ok I'll just work out the 4-20 output here is what you need to do

first work in frequency to start with.

at 4 ma=4 hertz
20 hertz will be max frequency so for argument sake lets set that at 80 hertz.

1)we know the following the SCP uses y=mx+b
2)At 60 hertz RPM = 1750 RPM. and we have a 40 to 1 reduction on one gearbox.
3)the Analag card has a range of values of the following now I don't have the output card you have for the manual so you will have to confirm this range 4-20 = 0 to 16384 this is contained in the manual for your input card and output card and can have different units depending on make and model.

here is a very useful article for you to read

http://www.plctalk.net/qanda/showthread.php?t=15069

lets just work with the data above.

essentially here is the steps you need to do

1) determine the range of units your analog card uses for the type of signal example 4-20ma is 0-16384
2) determine max frequency
3) determine your hertz to RPM scale.
4) determine the scale changes for the Gearbox ratios.

In the article Ron discusses and shows a 4-20 scale example

so what you need to to is determine how many hertz = 1 RPM at the output shaft.
well as stated we know that 60 hetrz= 1750 RPM input shaft and a 40 to 1 gear reduction.
so one hertz= 1750\60= 29.166667 RPM
devide this by your gear ratio 40 1 hertz at output shaft = 0.729 RPM

so reverse it what RPM will the output shaft be at at 80 hertz.

we can direct ratio this

60\1750= 80\x
60x=80*1750= 2333.333.... RPM
now devide by gear ratio \RPM =58.33 RPM output shaft @ 80 hertz.

follow Rons example on how to set up the scale for this However I've shown how to switch from Frequency to RPM input shaft and output shaft.
with all the info provided you should be able to handle this now.

 

[Adam303]

Thank you mordred i will update one i will get this going

 

[mordred]

no prob Ron's article is highly recommended on the scaling and how it works and as I've shown the calc's to RPM are simple in nature if your not worried about slip etc.

 

[The Plc Kid]

How critical is the speed? How much error can you tolerate between each motor.

One way is to have encoders measuring the output shaft of the gearbox not the motor and use this as your speed reference and use the plc to give each drive the same speed reference in rpm's. So the single scaled pot would increase the rpm spped ref to each drive and they would be corect as the drive would make the necessary changes to keep the shaft at the target rpm which is much more reliable.

Are the motors the same or different? If they are different they most likely will have a different amount of slip which could be compensated for by using the method above.

 

[Adam303]

mordred I'm a little confused about your calculations as far as what RPM will the output shaft be at 80 Hz?? Can you please explain

 

[Adam303]

Ok so i got some more info about the motors and the gearing.
1st motor
1750 rpm with a 40:1 gear box
as morder calculated this before
1750/60=29.166667 rpm at 1Hz
29.166667/40 gear ratio = 0.729 rpm @1hz
1750/40=43.75rpm @ 60hz

2ND MOTOR

1750 rpm 30:1 gear ratio
1750/60 =29.166667 RPM @1 HZ
29.166667/ 30 GEAR RATIO = 0.972 RPM AT 1HZ
1750/30=58.33 RPM @60 HZ
So to sum every thing up

1ST MOTOR
0.729 RPM @ 1 HZ
43.75 RPM @ 60 HZ
2ND MOTOR
0.972 RPM AT 1HZ
58.33 RPM @ 60 HZ

Now that i have some better understanding as to what the speed is of the output shaft, i need to come up how to use the SCP.come up with a linear relationship, as the two motors must rotate at the same RPM.
Any input as to how this could be done is very welcomed.

 

[Adam303]

Hey guys. Its been about three harsh days trying to synchronize thease motors. I still cant figure out what number to place in the scp instruction to figure this out. Any help? please!

 

[Paul B]

I've done this on lines that are not paper. When we say "not paper" it is understood that some difference in speed between the motors is acceptable. Paper will not stretch and will bunch up.

The way I've done it is to pick one motor to be the master. The Line speed setpoint from HMI is scaled 0-100% using a ratio setpoint set via the HMI and manually checked with a tachometer.

Once the first motor is set, this motor's speed 0-100% inside the PLC program is multiplied by a ratio for the second motor. The ratio setpoint for the second motor is set from the HMI. Repeat for however many motors you have.

With modern sensorless vector drives this achieves a very good level of following. It just will not do paper.

With a Siemens application I am simply taking the 0-100% speed inside the program and using the Unscale instruction to convert to raw counts for the speed reference to the drive.

I realize your question is specifically about AB's SCP instruction, hopefully someone will contribute further.

 

[Adam303]

would i have to pick one motor of the slowest rpm as the master ???

 

[Paul B]

With my method it does not matter. I always pick the most significant motor to be the master. This is usually the motor that feeds the rest, hence the term "following".

The 0-100% speed sent to the master is used for the followers, times a ratio. Each follower has its own ratio setpoint.