Dc control relay not working

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Luketrician

Senior Member
Location
West Pawtucket
Hello everyone, I have been troubleshooting a lighting control problem today and could use some advice.

Asco 920 RC Lighting contactor feeding our switchyard lights, 120/208. The control circut is 48vdc, with two dc relays (for open and close signal) operated by a photocell.

The 'close' relay is not picking up. It is getting the 48vdc signal to one side of the coil, but I loose the common on the other side. From what I can tell the common for both relays go through the auxillary contacts on the 920. I am thinking that this is where the problem is, maybe someone wired the auxillaries backwards?

This 920 was just installed a month ago, not by me..but I don't think it has worked since it was put in. Now I am just trying to pinpoint the problem.

When put in manual operation the 920 works fine..opens and closes. Photocell circuit is sending the correct signal voltage for both Open and Close relays when in auto...and the close relay recieves the 48v on one side of the coil..but the common isn't there to complete the circuit:confused:

Hope ya'll can make sense of this and steer me in the right direction...
 

micromind

Senior Member
This sounds alot like a mechanically held contactor. Regardless of whether its open or closed, if control power is lost, it'll stay in that position.

DC systems are typically not grounded, so it doesn't do a lick of good to test anything to ground. From your description, you likely already know this. All you can do is test for voltage across each component in the system, eventually either DC will appear where it shouldn't, you'll find a bad relay, or no DC at all.

Alot of DC power supplies have some sort of a dynamic protection scheme built into them, most of them are auto recovery. Make sure you actually have DC at the power supply. (It sounds like you do).

Mechanically held contactors have contacts built into them so that the coil only operates momentarily. These fail frequently. If the contact fails open (I'll bet yours is), it simply doesn't work. If it fails closed, it burns up the coil.

Alot of these are microswitches with a little tab sticking out the end that rides on a cam of some sort, and all it needs is to be tweaked a bit. Sometimes the switch itself fails. Occasionally, the cam doesn't move far enough. Some of these are pressed onto a shaft, not very hard, and can be adjusted. Getting to it can be an exercise in frustration, though.
 

Luketrician

Senior Member
Location
West Pawtucket
micromind said:
This sounds alot like a mechanically held contactor. Regardless of whether its open or closed, if control power is lost, it'll stay in that position.

DC systems are typically not grounded, so it doesn't do a lick of good to test anything to ground. From your description, you likely already know this. All you can do is test for voltage across each component in the system, eventually either DC will appear where it shouldn't, you'll find a bad relay, or no DC at all.

Alot of DC power supplies have some sort of a dynamic protection scheme built into them, most of them are auto recovery. Make sure you actually have DC at the power supply. (It sounds like you do).

Mechanically held contactors have contacts built into them so that the coil only operates momentarily. These fail frequently. If the contact fails open (I'll bet yours is), it simply doesn't work. If it fails closed, it burns up the coil.

Alot of these are microswitches with a little tab sticking out the end that rides on a cam of some sort, and all it needs is to be tweaked a bit. Sometimes the switch itself fails. Occasionally, the cam doesn't move far enough. Some of these are pressed onto a shaft, not very hard, and can be adjusted. Getting to it can be an exercise in frustration, though.[/QUOTE]

That will be another thing I will check on tomorrow. Maybe that is why I am showing an open circuit when I read across the 'closed' dc relay coil...:confused:

I can't wait to get to work tomorrow.;)
 

benaround

Senior Member
Location
Arizona
Glenn,

You say all works well when in 'manual' selection, all 'manual' does is jump

out any 'automatic' device. Q. Are you using a 48vdc photo-cell ? or does the

photo-cell power relays that switch the 48vdc. Q. Does the photo-cell send

48vdc power for 'open' and 'close', if so, like Micromind said, you have MH

contactor.

OK, so on the close coil the common will only be at the coil long enough to

pull in the contactor, when that happens, the aux. contacts open and the

contactor is then MH.
 

Luketrician

Senior Member
Location
West Pawtucket
benaround said:
Glenn,

You say all works well when in 'manual' selection, all 'manual' does is jump

out any 'automatic' device. Q. Are you using a 48vdc photo-cell ? or does the

photo-cell power relays that switch the 48vdc. Q. Does the photo-cell send

48vdc power for 'open' and 'close', if so, like Micromind said, you have MH

contactor.

OK, so on the close coil the common will only be at the coil long enough to

pull in the contactor, when that happens, the aux. contacts open and the

contactor is then MH.

The photocell powers relays that switch the 48vdc, and sends power for 'open' and 'close'.

When the photoeye toggles the 48vdc to the 'close' relay, the voltage reaches the 'close' coil, but it lacks the common to complete the circuit. The common wire lands under 'NO' on the auxillary. Right next to the 'NC' auxillary, which has a wire that runs up to the coil for the 'open' relay.

When this 920 cycles, it should toggle the auxillaries correct? Which should then complete the circuit allowing the 'close' relays coil to pickup and send the 120vac to close the 920 shouldn't it?

I'm about to go back out and look at this some more, thanks for the responses, I'll check back later.


Thanks,:smile:
 

Luketrician

Senior Member
Location
West Pawtucket
Problem solved

Problem solved

Turns out auxillaries were wired backwards. The 'closed' relay was wired to the 'no' side of the auxillary relay. So the dc signal was unable to complete the circuit across the coil to close the relay and energize the 920 contactor.


Swapped the wiring on the auxillaries and tested, now works as desgined.

Thanks for the replies.:smile:
 

benaround

Senior Member
Location
Arizona
Glenn,

Looks like you had it right, right from the start. Thanks for telling us how it

turned out, I wish more Members would do the same.
 
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