VFD speed control

Alan Case

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A quick question. What pitfalls are there if any of controlling multiple variable frequency drives (4-20ma) via one analog output. That is, can I series the analog inputs of say 3 drives and control them from the one PLC analog output.
Regards Alan Case
 
vfd speed control.

Hi Alan,

We do something similar in our mill on chain drives,the thing we came up with was to loop the motors into 1 larger drive then you only need 1 analog output and your motors will all run at the same speed providing the motors and sprockets match.
 
Hi Garry. Normally I would do it that way as well but this is a retrofit where they already have 3 drives (pot controlled) so I am trying to save the cost of 2 analog outs.
 
Just as a matter of interest what type of drives do you use most
freqeuntly?
It seems from the forum that you use AB plc's mainly and i just
wondered what type of vfd you use and how you find them with AB.
 
There are several possible pitfalls. As Rick mentioned, the load on the loop is probably too much for one output. Second, depending on the drive I/O card, the "-" side of the 4-20 mA input on the VFD may be tied to common on the I/O card. If that is the case, only the first VFD will get the signal. Finally, if any drive requires service the integrity of the loop is compromised and you may loose control of all of them.

I suggest you run the 4-20 mA output of the PLC into four I/I signal conditioners. These are around $200 each from M-Systems, A-Pi, Action Instrumets, and others. Now you have one PLC output, but a separate current loop for each VFD, plus isolation from the noise (on some VFDs that can be considerable). I'd suggest using the 24 VDC powered isolators, as loop powered ones may have too much impedence.
 
VFD Speed Control

Most modern drives utilize a 500 ohm resistor in their current reference input so that, at 20 ma, they have the same 10V as is usually provided for a speed pot on the voltage reference input. Assuming you have this type of drive, you are looking at 40V max which, almost surely, is too much drive voltage for your current loop power supply.

There are a couple of simple (cheap) ways to beat this rap. Probably the simplest would be to change the loop transmitter to 0 ma minimum instead of 4 ma if possible and rescale the drive input and transmitter output to reach full speed at 8 ma. Now, full speed occurs at 8 ma across 2000 ohms which is a tolerable 16 V. You sacrifice some speed resolution by doing this but that may not be important in this application. You can improve the speed resolution by defining actal motor minimum speed as 0 or 4 ma rather than zero speed at that current loop value.

Another way to accomplish the same thing is to run your current loop to only the first drive and then use the first drive's analog speed output signal to feed the second drive speed input, then use the second drive's analog speed output to feed the third drive speed input and so on. Your drives have to have an analog speed output in order to do this, obviously. Actually, it would be slightly better if each drives' analog output could be reassigned to repeat the speed input but only a few drives have this capability.

Another way to do this cheaply would be to put a 500 ohm resistor in the current loop to translate 4-20ma into 2-10 V. Now use this voltage signal into each drives voltage input. These are high impedance inputs usually so loading should be a big problem. You will have to program the drives with 2 volts offset or change the current transmitter to output 0 ma at minimum speed.

A more expensive but faster and more accurate way would be to install lead-follower software in the drives and set up the drive with the speed input signal as the lead. The others would then follow accordingly.

The suggestion of putting multiple motors on a single drive is a good one if you are starting from scratch and, importantly, if speed regulation and syncronization is NOT important. Motors configured this way are free to wander within their nameplate slip speed range depending on individual loads which can lead to poor speed control if individual motor loads are different.
 
Dick beat me to the punch. I have used the analog output from one drive to the input of the next many times & it works great. Scaling is easy simply using the min./max. speed settings. Of course, mine have all been AB drives.
 

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