DC contactor carrying AC - rerated value?
 

Greetings all,

Haven't had time to stop in much recently, but I'm finally getting my
new shop
close to ready to power up, and in wiring up the phase converter for
the Rivett,
I've hit a parts question for which I can't seem to find a ready
answer:

How much AC current would be reasonable to switch, using
a 40A DC-rated contactor (not a DC coil - magnetically
extinguished DC-rated contacts).

My intuition is to believe that it can safely carry (break) a heck of
a lot more AC current than DC current. I believe that I saw,
somewhere,
once upon a time, a derating rule of thumb, for using AC-rated
contacts
with DC. Even if my memory is playing tricks on me, it stands to
reason
that if it can break and extinguish a 40A DC arc safely, then the
self-extinguishing zero crossings of AC should make it good for a bit
more than that.

The question comes up because the spare contactor from the parts box
that
fits the right hole in the converter box, for the start-cap relay,
happens to be
a Josyln Clark "definite purpose" 40A DC contactor. I'm hoping that
split
between its two NO contacts, I can switch enough start capacitance for
my 15Hp idler.


I suppose that raises another question - should I split the start-cap
bank
into two banks, and run one off of each NO contact, or connect them
all
up as a single bank, and just treat the pair of contacts as parallel
wiring?

If I parallel just the contacts, the trailing contact will be
effectively
breaking the entire starting current by itself - this seems to argue
for
splitting it into two banks, one on each contact?


Here's hoping this beast doesn't let the smoke out when fired up!
Will Ray

DC contactor carrying AC - rerated value?
 

On Sat, 29 Mar 2008 22:30:21 -0700 (PDT), willray
wrote:


Greetings all,

Haven't had time to stop in much recently, but I'm finally getting my
new shop close to ready to power up, and in wiring up the phase
converter for the Rivett, I've hit a parts question for which I
can't seem to find a ready answer:

How much AC current would be reasonable to switch, using
a 40A DC-rated contactor (not a DC coil - magnetically
extinguished DC-rated contacts).

Magic 8-Ball Sez: "Answer Hazy - Call Manufacturer..."

My intuition is to believe that it can safely carry (break) a heck of
a lot more AC current than DC current. I believe that I saw,
somewhere, once upon a time, a derating rule of thumb, for using
AC-rated contacts with DC. Even if my memory is playing tricks
on me, it stands to reason that if it can break and extinguish a
40A DC arc safely, then the self-extinguishing zero crossings of AC
should make it good for a bit more than that.

Ahh, but the zero-crossing is accompanied by the polarity switch of
the waveform every half-cycle, and the fact that it will build up
magnetic forces differently.

AC rated breakers and contactors have specially built arc chutes
inside them, with horseshoe shaped steel stampings mounted in an
insulator (so they are all electrically floating) that magnetically
pull the arc away from the contacts and cool it enough to break the
plasma connection between the contacts. If you don't have arc chutes,
you have to pull the contacts WAY FAR apart to break the plasma.

DC contactors don't have or need arc chutes, they just get the
contacts far enough away from each other to physically break the arc.

You can use an AC contactor for DC, but you often have to wire two
or three sets of contacts in series to get enough raw distance between
the contacts to break the arc. You can't go the other way and run a
DC unit on AC without the arc chutes.

The question comes up because the spare contactor from the parts box
that fits the right hole in the converter box, for the start-cap
relay, happens to be a Josyln Clark "definite purpose" 40A DC
contactor. I'm hoping that split between its two NO contacts, I can
switch enough start capacitance for my 15Hp idler.

Switching a capacitive load will REALLY pull an arc across the
contacts. Don't guess whether a part will work for that application,
find out for sure, or you'll have things burning up and/or blowing up
rather spectacularly...

Unless you enjoy "Here, hold my beer and Watch This!" moments, in
which case feel free to press on with your original plan. And be sure
to roll tape for "America's Funniest Home Videos"... ;-)

-- Bruce --

DC contactor carrying AC - rerated value?
 

On Sun, 30 Mar 2008 10:26:48 -0700, Bruce L. Bergman
wrote:

On Sat, 29 Mar 2008 22:30:21 -0700 (PDT), willray
wrote:


Greetings all,

Haven't had time to stop in much recently, but I'm finally getting my
new shop close to ready to power up, and in wiring up the phase
converter for the Rivett, I've hit a parts question for which I
can't seem to find a ready answer:

How much AC current would be reasonable to switch, using
a 40A DC-rated contactor (not a DC coil - magnetically
extinguished DC-rated contacts).

Magic 8-Ball Sez: "Answer Hazy - Call Manufacturer..."

My intuition is to believe that it can safely carry (break) a heck of
a lot more AC current than DC current. I believe that I saw,
somewhere, once upon a time, a derating rule of thumb, for using
AC-rated contacts with DC. Even if my memory is playing tricks
on me, it stands to reason that if it can break and extinguish a
40A DC arc safely, then the self-extinguishing zero crossings of AC
should make it good for a bit more than that.

Ahh, but the zero-crossing is accompanied by the polarity switch of
the waveform every half-cycle, and the fact that it will build up
magnetic forces differently.

AC rated breakers and contactors have specially built arc chutes
inside them, with horseshoe shaped steel stampings mounted in an
insulator (so they are all electrically floating) that magnetically
pull the arc away from the contacts and cool it enough to break the
plasma connection between the contacts. If you don't have arc chutes,
you have to pull the contacts WAY FAR apart to break the plasma.

DC contactors don't have or need arc chutes, they just get the
contacts far enough away from each other to physically break the arc.

Sorry buddy, but you have it backwards. DC rated contactors
DEFININITELY require either arc-chutes or magnetic arc deflectors. DC
contacts arc WAY MORE than AC

You can use an AC contactor for DC, but you often have to wire two
or three sets of contacts in series to get enough raw distance between
the contacts to break the arc. You can't go the other way and run a
DC unit on AC without the arc chutes.

My experience is opposite to that.

The question comes up because the spare contactor from the parts box
that fits the right hole in the converter box, for the start-cap
relay, happens to be a Josyln Clark "definite purpose" 40A DC
contactor. I'm hoping that split between its two NO contacts, I can
switch enough start capacitance for my 15Hp idler.

Switching a capacitive load will REALLY pull an arc across the
contacts. Don't guess whether a part will work for that application,
find out for sure, or you'll have things burning up and/or blowing up
rather spectacularly...

An AC capacitive load is different than a DC capacitive load. I'd
switch an AC capacitive load without too much trepidation, while
switching a DCcapacitive load would give me pause.

Unless you enjoy "Here, hold my beer and Watch This!" moments, in
which case feel free to press on with your original plan. And be sure
to roll tape for "America's Funniest Home Videos"... ;-)

-- Bruce --


--
Posted via a free Usenet account from http://www.teranews.com

DC contactor carrying AC - rerated value?
 

"willray" wrote in message
...

Greetings all,

Haven't had time to stop in much recently, but I'm finally getting my
new shop
close to ready to power up, and in wiring up the phase converter for
the Rivett,
I've hit a parts question for which I can't seem to find a ready
answer:

How much AC current would be reasonable to switch, using
a 40A DC-rated contactor (not a DC coil - magnetically
extinguished DC-rated contacts).

My intuition is to believe that it can safely carry (break) a heck of
a lot more AC current than DC current. I believe that I saw,
somewhere,
once upon a time, a derating rule of thumb, for using AC-rated
contacts
with DC. Even if my memory is playing tricks on me, it stands to
reason
that if it can break and extinguish a 40A DC arc safely, then the
self-extinguishing zero crossings of AC should make it good for a bit
more than that.

The question comes up because the spare contactor from the parts box
that
fits the right hole in the converter box, for the start-cap relay,
happens to be
a Josyln Clark "definite purpose" 40A DC contactor. I'm hoping that
split
between its two NO contacts, I can switch enough start capacitance for
my 15Hp idler.


I suppose that raises another question - should I split the start-cap
bank
into two banks, and run one off of each NO contact, or connect them
all
up as a single bank, and just treat the pair of contacts as parallel
wiring?

If I parallel just the contacts, the trailing contact will be
effectively
breaking the entire starting current by itself - this seems to argue
for
splitting it into two banks, one on each contact?


Here's hoping this beast doesn't let the smoke out when fired up!
Will Ray



Splitting the capacitors into two banks will force the contactor contacts to
properly share the load. Paralleling the contacts works but does not prevent
load-hogging by one set of contacts. My sense is that the contactor will be
adequate for the purpose.

Don Young