Derating AC rated contactors for DC use

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Hi as above but with some further details.

Say I have a contactor which is rated at 32A AC-1 at < 690V is there an appropriate derating factor for DC use when manufactures specifications cannot be located?

I'm wanting know if the above contactor is capable of breaking 24V 20A dc loads.

I've emailed the manufacturer but am interested to know from other people's experience whether there is a rule of thumb.

Matt
 
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As I understand it, the DC problem is due to the arc produced as the breaker opens. With only 24 volts it wouldn't be a concern. Car/commercial switches for 12/24V have internals which are similar in construction to 240V AC switches. Poor contacts in any switch would lead to some burning/pitting. One question - 24V and 20A; why not buy a switch or relay from Maplin's for a couple of pounds?
 
One question - 24V and 20A; why not buy a switch or relay from Maplin's for a couple of pounds?

The contactors are forcibly guided safety contactors that are in an existing installation, I would like to use one pole from each contactor in series so that my DC supply Is e-stopped with the rest of the devices that are supplied via them.

My feeling is that they wil be ok at 24V as it is related to arching as you say.

I will add that this is for a supply to an inductive load, but the load itself is switched by local relays and also suppressed by 1N4007's

Matt
 
Say I have a contactor which is rated at 32A AC-1 at < 690V is there an appropriate derating factor for DC use when manufactures specifications cannot be located? ... I've emailed the manufacturer but am interested to know from other people's experience whether there is a rule of thumb.
I don't think there could be a 'rule of thumb' because it depends so much on the design and construction of the particular item. One can produce a contactor whose DC breaking capability is very much lower than for AC (probably the common situation) or, alternatively, one whose DC-breaking capacity is appreciably closer to the AC rating than that. Only the manufacturer of a particular product could tell you where the product was on that spectrum.

Kind Regards, John
 
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Thanks John,

You are probably correct, and as I say. My gut feeling is that I'll be ok, due to the nature of the load (solenoid valves, which are unlikely to all be energised concurrently)

I was just curious to see if anyone had any bad experiences with a similar situation .

Ill wait for rockwells response, wine beckons.
 
Having worked as an auto electrician I have seen what DC can do but I would think the twin contacts of a contactor must be 100% better than the single contacts of a relay so I would not really be worried.

However JohnW2 is spot on. And only people who can answer are the manufacturers. However I tried to find a washing machine that would work on a simulated sine wave from an inverter and 99% played safe and said I could not use there product. Only LG said their product would likely work OK.

It is a hard question to answer.

A) Ask the manufacturer knowing they will likely not say yes in case there is a problem.

B) Don't ask so you can say hand on heart I did not anticipate a problem.

Unless you are using a solid state contactor I personally can't see a problem but at the end of the day the but rests with you.
 
One alternative is to switch the AC supply to the 24 volt power supply and leave the 24 volt circuit as it is.

The advantage is that you are not putting the ELV 24 volt onto contacts adjacent to contacts that are carrying 230 volts. True the contact sets will be isolated from each other but in a fault condition where the contact arcs badly that isolation may fail.

You will need to ensure that total loss of 24 volt is a safe condition.
 
However JohnW2 is spot on. And only people who can answer are the manufacturers. However I tried to find a washing machine that would work on a simulated sine wave from an inverter and 99% played safe and said I could not use there product. Only LG said their product would likely work OK. ... It is a hard question to answer. A) Ask the manufacturer knowing they will likely not say yes in case there is a problem.
I don't think it's necessary that bad in this case. The specification/datsheet/whatever for the contactor will very likley have had a DC rating on it - the OP merely needs them to read that information to him, since he does not have any documentation himself. I cetainly agree that if youy ask a manufacture a question relating to something which is not in their documentation/spec, they will err very much on the side of caution (to the extent that the answer will probably be predictable and useless!) - but I don't think that's the situation we have here.

Kind Regards, John
 
One alternative is to switch the AC supply to the 24 volt power supply and leave the 24 volt circuit as it is.

The advantage is that you are not putting the ELV 24 volt onto contacts adjacent to contacts that are carrying 230 volts. True the contact sets will be isolated from each other but in a fault condition where the contact arcs badly that isolation may fail.

You will need to ensure that total loss of 24 volt is a safe condition.

Unfortunately the 24V cannot be switched off completely as its used for many other functions in the panel, including feedback from the very contactors I'm talking about.
 
I would like to use one pole from each contactor in series so that my DC supply Is e-stopped with the rest of the devices that are supplied via them.
Should you be cobbling together your own e-stop controls?
 
Should you be cobbling together your own e-stop controls?

I won't be, I'll simply be having my device e-stopped by the machines existing e-stop circuit.

Even If I created my own e-stop controls, I'd still have to interface with the machines e-stop circuit as its all part of a single automatic line.

All done in accordance with BS EN 60204 and BS EN ISO 13849
 
Oh an also, Rockwell came back to me;

32A AC-1 contacts can be used as 25A DC-1 contacts, so happy days
 

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