Can someone assist me as to how many maximum loops can I connect to SLC analog output 4-20 ma channel. I realize the output load is 0-500 ohms. We are driving 4 reliance AC frequency drives in a loop. The drives are for belts on our high speed machine. We do see non linearity of speeds at full 20 ma output. I am wondering if it is loading of the output module?. We are using GV3000 drives.

Thanks in advance for your help.

 

If they are in a loop, all the drives would see the same signal.

It could be that with four of them in series the output cannot drive it all the way to 20mA, but all four drives would still see the same signal.

It all depends on the drive input.

easiest thing might be to put a meter in the loop and see what the current signal actually is when you are trying for 20 mA.

 

Quite possible. Assuming 250 ohms burden for each input, four would be 1000 ohms. This will drop 20 volts at 20ma in a 24VDC system, leaving only 4 volts for the the current source and wiring drops. Their spec says they would like about 14 V for compliance and wiring drops. I take your question to be how many devices in a loop, which then defines how many loops. The 250 ohm burden is almost a given with inputs that can accept either analog voltage or current loops because it gives 1-4V with 4-20ma. 500 would be easier for them at 2to 8 volts but would limit you to half as many devices in a loop, so 250 ohms is the normal compromise. You would be in spec at 2, and 3 would probably work, but 4 is too many.

Regards,
cww

 

Here's a quick answer.

- Each of the drives should be connected in series (I assume the obvious here).

- Measure the resistance of the loop through all the drives. R= V/i = 24 / .020 = 1,200 ohms max resistance.

- If you want all drives to run same speed it is a question of tolerance. The control current will be dropped across a resistor in each drive so the control voltage will be slightly different for each drive. Therefore, the drives will not run a 'exactly' the same speed. How much deviation you can live with is a call by you. Also, as the load varies on each drive, that will change the speed of the motor as well.

- Don't know if you have a vector drive or a volts/hz drive. Vector is better but can still yield speed differences.

- You may need to use digital encoders for feedback to get exact speed sync of each drive. This will still be somewhat difficult if you are trying to control them with an analog signal.

- If you still can't get sync speed control you require then you may have to digitally tell the drive what speed to run by using DeviceNet to each drive. This will eliminate the error in the analog values.

Rodney Wirtz
Rodney at BoxxEngineering dot com

 

Hi Rodney Wirtz, Curt Wuollet and Bob Peterson,

Thanks for your replies. They are very useful information. We do have issues with maintaining speeds at 20 ma. Hence the loop loading is causing it. Even though we have encoder feed back, the drives are in saturation. I may look into splitting into different loops using other output channels or use transmitter with high impedance. I really appreciate your replies.

Thanks again.
Dinesh

 

First of all you need to determine, what is the impedance of the input, as others have stated in series you add all the impedances. You may be able to connect them in parallel as voltage in.

You also have the option of using signal isolators or a seperate output for each drive.

"We do see non linearity of speeds at full 20 ma output. I am wondering if it is loading of the output module?." This sounds like your load impedance is too high.

Regards,
Roy

 

Depending on the features available in the drives that you have, you may want to consider the "speed follower" option. In this configuration your analog output signal would control the first drive. The second drive would be controlled by an analog signal from the first, which would be proportional to the "actual" speed of the first motor. In this way, you can cascade as many motors as you wish, without having to worry about 4-20ma drive issues. This feature also allows you to do individual scaling if for some some reason you need to run with differences in speeds on the individual motors. If they are all scaled the same, they all run at the same speed. As was mentioned earlier, if the loads are different, you may have variations.

Cheers,
Gerald Beaudoin