Using AC relays with DC power

[Jason Dugas]

I have some G4A relays (ecb.omron.com.sg/pdf/relay/power/G4A.pdf) that I'd
like to use for 72Vdc. Yes, I know they are not rated for DC (they are
rated for 250Vac at 20A). I tried one and smoked it with 72V (at 5A) when I
de-energized the circuit with current flowing through the contacts. I'm
guessing that I'm getting arcing inside? Does this relay design rely on the
fact that AC has a 0-crossover in order to break the current?

Thanks,

Jason

 

[Jason Dugas]

"Jason Dugas" <kb5urq@hotmail.com> wrote in message
news:10jfskk6c4js445@corp.supernews.com...

I have some G4A relays (ecb.omron.com.sg/pdf/relay/power/G4A.pdf) that I'd
like to use for 72Vdc. Yes, I know they are not rated for DC (they are
rated for 250Vac at 20A). I tried one and smoked it with 72V (at 5A) when
I de-energized the circuit with current flowing through the contacts. I'm
guessing that I'm getting arcing inside? Does this relay design rely on
the fact that AC has a 0-crossover in order to break the current?

Thanks,

Jason

The load we were using was resistive. The test setup consisted of applying
the load at 72Vdc and then opening the relay contacts. Upon turning off the
coil voltage, I heard a sustained "sizzling" sound (i'm sure it was the arc)
then puffs of smoke coming from the relay. We tried a snubber circuit-- 1uF
electrolytic cap (rated at 100Vdc) ins eries with a 4 ohm resistor placed
across the contacts and got the same results.

 

[Paul Hovnanian P.E.]

Jason Dugas wrote:

[snip]

The load we were using was resistive.

Any significant inductance in the DC supply?

The test setup consisted of applying
the load at 72Vdc and then opening the relay contacts. Upon turning off the
coil voltage, I heard a sustained "sizzling" sound (i'm sure it was the arc)
then puffs of smoke coming from the relay. We tried a snubber circuit-- 1uF
electrolytic cap (rated at 100Vdc) ins eries with a 4 ohm resistor placed
across the contacts and got the same results.

--
Paul Hovnanian mailtoaul@Hovnanian.com
note to spammers: a Washington State resident
------------------------------------------------------------------
100 buckets of bits on the bus
100 buckets of bits
You take one down,
and short it to ground
FF buckets of bits on the bus

 

[Rich Grise]

Paul Hovnanian P.E. wrote:

Jason Dugas wrote:

[snip]

The load we were using was resistive.

Any significant inductance in the DC supply?

That's kind of irrelevant, since you don't need inductance

to make a contact arc when switching DC. When the gap goes from zero
to not-zero, it starts out small enough that the vacuum between the
atoms breaks down. When the contacts are one air molecule apart,
they're close enough to break down the air, forming plasma, which
continues to conduct until either the gap gets too big to sustain
the arc, or something else interrupts the current.

http://trove.dyndns.org/Blog/News_Item.2004-06-29.3757781614

Cheers!
Rich

 

[CJT]

Rich Grise wrote:

Paul Hovnanian P.E. wrote:


Jason Dugas wrote:

[snip]

The load we were using was resistive.

Any significant inductance in the DC supply?


That's kind of irrelevant, since you don't need inductance
to make a contact arc when switching DC. When the gap goes from zero
to not-zero, it starts out small enough that the vacuum between the
atoms breaks down. When the contacts are one air molecule apart,
they're close enough to break down the air, forming plasma, which
continues to conduct until either the gap gets too big to sustain
the arc, or something else interrupts the current.

http://trove.dyndns.org/Blog/News_Item.2004-06-29.3757781614

Cheers!
Rich


That same one, and some others:

http://205.243.100.155/frames/longarc.htm


--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form che...@prodigy.net.

 

[Winfield Hill]

Rich Grise wrote...

Paul Hovnanian P.E. wrote:

Any significant inductance in the DC supply?

That's kind of irrelevant, since you don't need inductance
to make a contact arc when switching DC. When the gap goes from zero
to not-zero, it starts out small enough that the vacuum between the
atoms breaks down. When the contacts are one air molecule apart,
they're close enough to break down the air, forming plasma, which
continues to conduct until either the gap gets too big to sustain
the arc, or something else interrupts the current.

http://trove.dyndns.org/Blog/News_Item.2004-06-29.3757781614

Is there a way to save that clip?


--
Thanks,
- Win

(email: use hill_at_rowland-dotties-org for now)

 

[nospam]

Winfield Hill <Winfield_member@newsguy.com> wrote:

http://trove.dyndns.org/Blog/News_Item.2004-06-29.3757781614

Is there a way to save that clip?

http://www.ourshanty.com/video/LugoSWR.swf

 

[Tony Williams]

In article <U2p_c.2257$vI2.275@trnddc02>,
Rich Grise <null@example.net> wrote:

That's kind of irrelevant, since you don't need inductance to
make a contact arc when switching DC. When the gap goes from zero
to not-zero, it starts out small enough that the vacuum between
the atoms breaks down. When the contacts are one air molecule
apart, they're close enough to break down the air, forming
plasma, which continues to conduct until either the gap gets too
big to sustain the arc, or something else interrupts the current.

Or until the voltage across the arc falls below
about 22V (the arc sustaining voltage at ntp)?

I have a huge contactor, rated to break 600Vdc at 15A.
There is nothing fancy about it, like magnetic blowout.
It merely consists of about 2 dozen contacts in series,
arranged in a circle, with all contacts simultaneously
switched by a central rotary actuator.

I have always assumed that 600/24 is about 25V per contact
and the simultaneous switching means that the first contact
to open (and arc) only has to withstand the arc for a very
short time, perhaps something less than a mS or so.

--
Tony Williams.

 

[Paul Hovnanian P.E.]

Rich Grise wrote:

Paul Hovnanian P.E. wrote:

Jason Dugas wrote:

[snip]

The load we were using was resistive.

Any significant inductance in the DC supply?

That's kind of irrelevant, since you don't need inductance
to make a contact arc when switching DC. When the gap goes from zero
to not-zero, it starts out small enough that the vacuum between the
atoms breaks down. When the contacts are one air molecule apart,
they're close enough to break down the air, forming plasma, which
continues to conduct until either the gap gets too big to sustain
the arc, or something else interrupts the current.

That effect is the same for either an AC or DC circuit, with or without
series inductance. Its the inductance that provides additional e.m.f. to
sustain the arc's duration beyond the point at which a theoretical
purely resistive circuit arc would be extinguished.

--
Paul Hovnanian mailtoaul@Hovnanian.com
note to spammers: a Washington State resident
------------------------------------------------------------------
professor; n, One who talks in someone else's sleep.

 

[Watson A.Name - \"Watt Su]

"nospam" <nospam@nospam.invalid> wrote in message
news:sjqkj05g9db49fptkvqu4u5qs6fijv4n9c@4ax.com...

Winfield Hill <Winfield_member@newsguy.com> wrote:

http://trove.dyndns.org/Blog/News_Item.2004-06-29.3757781614

Is there a way to save that clip?

http://www.ourshanty.com/video/LugoSWR.swf

You don't _have_ to save it. It is on another URL, called
www.teslamania.com, along with a couple other spectacular arcs!.