Last night I ran into three drives on the same speed control 4-20. I had never seen this before and thought multiple devices would cause inconsistent speeds? Was hoping for some info on this if anyone cares to chime in. Thanks in advance for this forum.
Daisy Chaining a 4-20mA loop
- Thread starter dcam
- Start date
Tom Jenkins
Lifetime Supporting Member
A 4-20 mA signal sends the same current to all devices in the loop. If the drive analog inputs are properly calibrated and the total load does not exceed the allowable load on the transmitting device all three should run at the same speed.
If these are sequential conveyors it is common practice to have each section a little faster than the previous.
If the 3 need to be monitored and constantly adjusted, then a single output control might not be good if the loads can vary on them.
If the 3 need to be monitored and constantly adjusted, then a single output control might not be good if the loads can vary on them.
Peter Nachtwey
Member
The problem is that the 20ma of current is going through 250 ohms at each device. The total voltage across all ofthe devices in series must be less than the current sources maximum voltage.
Saffa
Member
Some devices also don't have isolated analog inputs (one leg tied to 0V or ground) which will also cause a problem if inserted incorrectly into a loop. There are 4-20mA loop isolators and retransmitters designed for dealing with this.
cardosocea
Member
This is very popular in process industry where you want to have a readout of an instrument locally. So they stick a 4-20mA display in the loop. In fact, a lot of instruments that have a display on top do it by placing two different bits on the current loop, one to display another to measure.
There should be no problem providing the loop resistance is not too low, after all the current is constant through all devices so if the card is outputting 12ma this is same thru out the loop (it has to be ohms law) but it may not be 12ma because of the load, this will not be a major problem as any error could be calibrated out. Assuming all series loops are of the same type or loop powered and not grounded 2-4 units should not be a problem.
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