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Inductive Ratings
3

Inductive Ratings

Inductive Ratings

(OP)
Gentlemen,
Can somebody explain regarding Control relays why some manufacturers state the inductive ratings for their contacts but some do not, I have problems with a circuit that requires a max of 6 amp @ 125VDC to operate a solenoid, will a standard icecube relay rated 10A @150VDC will suffice, this manufacturer does not provide inductive ratings.
Thanks

RE: Inductive Ratings

If this contact has to break 6 amps at 125 V dc, the ice cube relay contact will probably last right around one operation.  

I suspect the holding current is less than 6 amps, but even so, 125 V dc solenoid switching is tough duty.  You'll probably want to use a good machine tool relay and some type of snubber.   If it is really breaking 6 amps, you'll want to put multiple contacts in series.



    

RE: Inductive Ratings

(OP)
Thanks DPC for the info, the relay in question is being used to open and close a GE 15kv vacuum breaker and it has been in operation for several months and several operations. The owner is concerned about this relay, since the specs of this relay does not state the inductive rating. I would appreciate if you have any comments on this also.
Thanks

RE: Inductive Ratings

An ice cube relay is not appropriate for breaker tripping.  What has saved you to date is that you have only made the tripping circuit, never interrupted it.  If, at any point, your ice cube relay tries to interrupt the trip circuit, that will be the last operation attempted by that relay.  You need a relay with an inductive rating, and then you need to arrange your control circuitry in such as manner that the trip signal will always remain present until the trip coil trips the breaker and the clearing contacts interrupt the circuit. A good snubber will also help.

RE: Inductive Ratings

There are several things that can be done to make a simple little relay's life easier.

One thing is to use a free-wheeling diode parallel to the coil. Then, the relay doesn't have to take care of the inductive part of the load, that current just circulates in the diode until the energy is dissipated and the coil releases. One problem with this is that your breaker will be correspondingly delayed. If you can tolerate the delay, fine. If not, you can use a resistor in series with the diode, a zener or an RC combination. Lots of possibilities. But letting the contact operate the coil all by itself will kill it quickly.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Inductive Ratings

When you said solenoid, I assumed a solenoid valve.  If this is a trip coil, the relay is not breaking the current - there are a couple of breaker auxiliary contacts in the trip circuit that do this - if things are working right.  This is why the relay has not failed yet.  

David and Gunnar have given excellent advice  - I can't add anything to that.  

As to why manufacturers publish different types of ratings, I can't say.  There are NEMA standards for machine tool relay contact ratings that will cover ac, dc and inductive switching.  Those are more useful than a vendor's arbitrary ratings.  

As a general rule of thumb, if a relay contact can break 10 A at 120 V ac, it will have a rating of about 1 amp at 125 V dc, if you're lucky.    

 

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