Relay with High DC Breaking Capacity

[Farhan Ali]

Hello!

I have to use a relay to operate circuit breaker opening/closing coil i.e. 80W @ 110VDC.
Since the breaker is having some mechanical interlocks, so sometimes when the coil is operating, the breaker may not open. In that case the relay contact has to make & break the opening/closing coil supply.

I initially used normal relay for this operation, but later on I observed that relay is not more functional, when I open the relay (i.e. removed its plastic case), I found the plastic base of the relay poles was melted and when the relay was operating, there was some gap between pole and through.

After studying the issue, I came to the conclusion that the DC Inductive Breaking capacity of the relay I were using was 35W Inductive Load i.e. DC12 L/R = 40ms, while the coil rating was 80W.

I search the relays from different vendors, but only 1 relay found suitable for this application, i.e. Arteche CD-2XR relay with high DC Inductive Breaking Capacity.

Apart from this, we can use some other techniques to achieve High Inductive DC Breaking, as mentioned below.
1. Use Two contacts in series
2. Use Diode across the Relay Contact

Is there anyone who has gone through such problem?
Is there any other brand or relay to use for such High DC Inductive Breaking Capacity?
Is there any other technique to handle this issue?

Regards,

Farhan Ali

 

[jim hardy]

2. Use Diode across the Relay Contact

You'd use the diode across the relay breaker coil (OOPS!)
Or a 'snubber' made for the purpose.

http://www.cde.com/resources/catalogs/Q-QRL.pdf
http://www.cde.com/resources/technical-papers/design.pdf

 

[Tolklein]

For an inductive load you would need a contactor with a sufficient AC-3 rating for your application, the link (http://electrialstandards.blogspot.co.za/2014/04/electrical-contactors-utilization.html) has a short explanation regarding AC ratings.

Industrial suppliers like Siemens, ABB, Schnieder Electric, etc will have massive ranges of such devices for you to choose from, I'm sure if you gave one of them a call they would be able to offer you the correct product.

 

[davenn]

You'd use the diode across the relay COIL .
Or a 'snubber' made for the purpose.

http://www.cde.com/resources/catalogs/Q-QRL.pdf
http://www.cde.com/resources/technical-papers/design.pdf

I know what you are getting at ... but it isn't the OP's problem
that's not going to solve the relay contacts from sticking

Anyway, I don't see what putting a diode across the contacts would do, as he suggested

Dave

 

[CWatters]

You'd use the diode across the relay COIL

I think the OP means the relay is switching an inductive load.

 

[davenn]

I think the OP means the relay is switching an inductive load.

yes

 

[jim hardy]

I have to use a relay to operate circuit breaker opening/closing coil i.e. 80W @ 110VDC.

I think the term for relays in that service is "Pilot Duty Relay"

that's not going to solve the relay contacts from sticking

?? Inductive load so arcing is when contact opens.
http://www.panconcorp.com/PDFs/capacitors/QRL-Quencharc.pdf

Anyway, I don't see what putting a diode across the contacts would do, as he suggested

Agreed 100 % .

 

[CWatters]

Clamps the inductive spike to the DC voltage?

 

[Windadct]

The diode would have to be across the Circuit Breaker's closing/opening coil, not the relays coil.

But ideally you need a relay with contacts rated for the DC voltage and current, AND inductive load. For an air insulated contact it may have an arc chute with blowout coil, there are vacuum and solid state relays for this purpose as well.

 

[jim hardy]

The diode would have to be across the Circuit Breaker's closing/opening coil, not the relays coil.

Well yes of course it would. Sorry i didnt make that clear

old jim

 

[Windadct]

Not sure if someone was being pedantic -

 

[jim hardy]

OOPS now i see my mistake

You'd use the diode across the relay COIL .

I sure 'nuff typed " relay " while thinking " breaker " .

Apology hereby extended to all...

 

[Windadct]

I was not referring to you Jim, but the others... cheers

 

[TonyStewart]

Omron Relays at archive.org have plenty of arc-snubbing advice for relay contact opening. Since ionization can occur at very small gaps as it opens and only takes a microsecond or so, the rate of change is not fast enough to prevent an arc . Also the travel distance is kept small ( few mm) to reduce "Make time" but "Break Current" is gap and inductive current dependent just like holding current in SCR's. But with dry contacts you have very small capacitance reducing as it opens which leads to high voltage (kV) as it opens, V=L dI/dt as dt goes to < 1 us with < 1 pF gap.

The most popular snubbers for low power DC contacts are diodes clamped to the opposite rail. But the low diode resistance also causes slow switching times due to Tau=L/R. So using MOV's or Zener's speed up and quench the switching.

SeriesRC snubbers are also popular for motor relay contacts. This can be in the form of discrete components or very lossy film caps to limit the currents and also prevent arcing from 1kV/mm average air breakdown, but the R value must be damping the resonance as well as limiting contact burnout currents where the highest voltage drop occurs across the highest impedance in the loop..

Vacuum relays are expensive solutions as they raise the breakdown voltage from Paschen's Law.

High power AC breakers for inductive loads must consider high speed and large gaps in these "Contactor" designs. Even though AC has the benefit of zero-crossing current quenching if the plasma is not resonating at some RF value preventing the de-ionization process. After all this is how the process of welding works.

This is an old thread about old technology, but I felt like it needed some overview perspective thrown in.