PLC/Relay help.

DLMUK

Member
Join Date
Jun 2013
Location
Southampton
Posts
311
Hi,

I have to supply a redundant power supply (not a battery powered UPS) for a job which will kick in when a cable is broken/damaged from the OUTPUT of the primary power supply.

What I have is 24VDC output from PSU#1, this drives 45 instrumentation amplifiers. The power is passed through 7 junction boxes by the time it reaches the last amplifier. If the power cable is cut between JB3 and JB4 for example, it would cut the power to about 20 of the amplifiers, meaning that about 0.88A of current draw from the amplifier would be removed. In this instance there is a separate 24VDC from PSU#2 going down a different cable to power the same amplifiers.

What I would like to do is...

Write a PLC program to say that when the current drops below a specified level, activate a relay which switches from PSU#1 to PSU#2. This would be done when the PLC senses the current draw on PSU#1 drops too much, the logic activates a relay on the PLC which drives a high load relay to cut power from PSU#1 and run off PSU#2. There is a UPS attached to the PLC to prevent it cutting out during total power failure/switchover.

This is not something I have ever done before so there may be a better way of doing what I am suggesting?

Does anybody know of a current monitoring device I could interface with a PLC? My total current draw from the PSU is ~7A.

Thanks,
 
AutomationDirect.com has several diffrent types of AC Current Transducers. You can get 0-10Vdc and 4-20Ma outputs to interface with your PLC.

This seems like a pretty simple application. You may be able to get away with just using a Current Switch and a relay. The PLC advantage is that you can "Tune" the system for your operating conditions.
 
I think they are pretty accurate. They work off the same principle as a clamp on Amp Meter that you might use in the field. So should be pretty close.

I have used the units that were set up with a switch. It was just to tell the PLC if a motor was running. So if there was current > 1A close the switch if not open it.

You may be able to do the same thing with a relay and a current switch.
 
Depends on the switch. If you get a switch with form C contracts you will have a NO and a NC side to work with. You can use the contacts you need to get the results you are after.
 
DLMUK,

I have a question for you.
Your system is running and somehow the cable is cut.
You want to sense the current loss and turn off power supply #1.
My question is this.
When you turn on power supply #2 which routes power a different way, how will you power the devices since the main power cable is cut somewhere? You will be turning power on to a cable that has been cut somewhere and is most likely shorted.

I may be missing something, so I thought I would ask.

regards,
james
 
put on every amplifier diodes in both wires and both cables.
this way any cable can break without stopping.
hot swap is no problem.
apart you can detect the voltage in both lines at the ends, so if having 2 rings on two sides.
 
How about one of these at every point along the way - they are quite inexpensive - I use them all the time. Beats re-inventing the wheel. You could then use a voltage monitor at the end of each line to alarm if the DC falls over and alarm but the system would continue to operate provided there is one supply available. http://www.adminstrumentengineering...ing/power-117/din-rail-1/redundancy-module-10

I was just going to suggest something along those lines.

OP, as you are in the UK I would give the guys at PULS a call, they have all sorts of options for this sort of application and have really helped me in the past with DC UPS applications.

http://www.pulspower.com/index.php?reqNav=main&lang=en
 

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