Question re: power relays & RC snubbers

[Ross Herbert]

On Sat, 31 Jan 2009 01:37:13 GMT, Ross Herbert <rherber1@bigpond.net.au> wrote:

:On Sat, 31 Jan 2009 00:23:24 GMT, Ross Herbert <rherber1@bigpond.net.au> wrote:
:
:It occurred to me that you could also try connecting both relay contacts in
:series to increase the effective contact gap at opening which is where the
:majority of arcing will occur.
:
:A search of literature on arc suppression capacitors for a load such as yours
:indicates that a value of about 13uF would be required. Since it would need to
:have a rating of 300Vac this would make it bigger than the relay itself so it
is
:not really a goer in my opinion. The cap could remain charged to line potential
:so that when the contacts closed again it would cause a high discharge current
:which would result in an arc of its own. A series resistor of say 5 - 10
hms/10W would need to be included in series with the cap to limit this
:discharge current.
:
:A search of patents revealed a fairly novel approach using a thyristor in
:conjunction with a standard relay to suppress arcing. Note, you may have to
:create a free account to view.
:http://www.freepatentsonline.com/3555353.html


Looking at the circuit, the thyristor idea might be able to suppress arcing
during closing of the relay contact but it would not do so when opening.

 

[Phil Allison]

"Ross Herbert"

It occurred to me that you could also try connecting both relay contacts
in
series to increase the effective contact gap at opening which is where the
majority of arcing will occur.

** False reasoning.

An arc is created at the exact moment of opening, when the gap is
microscopic.

Series contact connection is useful for breaking larger voltage-current
combinations than one contact can reliably do - but at the expense of a lot
of arcing. Hence the number of operations is quite limited.

A search of literature on arc suppression capacitors for a load such as
yours
indicates that a value of about 13uF would be required. Since it would
need to
have a rating of 300Vac this would make it bigger than the relay itself so
it is
not really a goer in my opinion. The cap could remain charged to line
potential
so that when the contacts closed again it would cause a high discharge
current
which would result in an arc of its own. A series resistor of say 5 - 10
ohms/10W would need to be included in series with the cap to limit this
discharge current.

** Such a series resistor would ruin the ability of the cap to suppress
arcing at opening - since the resistor will allow the voltage across the
contacts to rise instantaneously to 30 - 50% of the supply voltage value at
that moment = up to 170 volts.


....... Phil

 

[Phil Allison]

"whit3rd"
Adam Tenenbaum

[about a relay showing contact damage]

It's a Finder relay, part# 66.82.8.110.0307 -- rated for 30A, DPST NO,
switched on 110VAC with AgCdO contacts.

A bad relay contact can result from foreign material (dust, etc.)
so first, clean it out. Then, burnish the points (clean down
to bare metal, and smooth it). That might fix the problem.


** Not bothered to look at the link given by the OP ??

The relay in question is a * sealed unit *.


Even if the load is intended to be a heater, it might be
an inductor, too;


** Ridiculous shite.


Sulfide corrosion can cause black stuff that looks like
char, but it's not a sign of overheating.


** The relay in question is a * sealed unit *.

Piss off - moron.



....... Phil

 

[whit3rd]

On Jan 29, 4:50 pm, Adam Tenenbaum <ac...@torfree.net> wrote:

[about a relay showing contact damage]

It's a Finder relay, part# 66.82.8.110.0307 -- rated for 30A, DPST NO,
switched on 110VAC with AgCdO contacts.

A bad relay contact can result from foreign material (dust, etc.)
so first, clean it out. Then, burnish the points (clean down
to bare metal, and smooth it). That might fix the problem.

Even if the load is intended to be a heater, it might be
an inductor, too; consider installing a surge suppressor
(MOV disk thing, looks like a capacitor) across the contacts
if you can do so safely.

Sulfide corrosion can cause black stuff that looks like
char, but it's not a sign of overheating.

 

[Ross Herbert]

On Sat, 31 Jan 2009 12:56:56 +1100, "Phil Allison" <philallison@tpg.com.au>
wrote:

:
:> Ross Herbert wrote;
:
:> It occurred to me that you could also try connecting both relay contacts
:> in
:> series to increase the effective contact gap at opening which is where the
:> majority of arcing will occur.
:
:
: ** False reasoning.
:
:An arc is created at the exact moment of opening, when the gap is
:microscopic.
:
:Series contact connection is useful for breaking larger voltage-current
:combinations than one contact can reliably do - but at the expense of a lot
f arcing. Hence the number of operations is quite limited.
:
:
:> A search of literature on arc suppression capacitors for a load such as
:> yours
:> indicates that a value of about 13uF would be required. Since it would
:> need to
:> have a rating of 300Vac this would make it bigger than the relay itself so
:> it is
:> not really a goer in my opinion. The cap could remain charged to line
:> potential
:> so that when the contacts closed again it would cause a high discharge
:> current
:> which would result in an arc of its own. A series resistor of say 5 - 10
:> ohms/10W would need to be included in series with the cap to limit this
:> discharge current.
:
:
:** Such a series resistor would ruin the ability of the cap to suppress
:arcing at opening - since the resistor will allow the voltage across the
:contacts to rise instantaneously to 30 - 50% of the supply voltage value at
:that moment = up to 170 volts.
:
:
:...... Phil
:
:

I agree with you Phil. Arc suppression at high current is a huge compromise in
values anyway. There is no really effective way to do it cheaply (using
capacitor/resistor or even no resistor) without the protection being bigger than
the relay itself.

 

[Adam Tenenbaum]

On Jan 31, 7:26 pm, Ross Herbert <rherb...@bigpond.net.au> wrote:

On Sat, 31 Jan 2009 12:56:56 +1100, "Phil Allison" <philalli...@tpg.com.au
wrote:

:
:> Ross Herbert wrote;
:
:> It occurred to me that you could also try connecting both relay contacts
:> in
:> series to increase the effective contact gap at opening which is where the
:> majority of arcing will occur.
:
:
: ** False reasoning.
:
:An arc is created at the exact moment of opening, when the gap is
:microscopic.
:
:Series contact connection is useful for breaking larger voltage-current
:combinations than one contact can reliably do  - but at the expense of a lot
f arcing.  Hence the number of operations is quite limited.
:
:
:> A search of literature on arc suppression capacitors for a load such as
:> yours
:> indicates that a value of about 13uF would be required. Since it would
:> need to
:> have a rating of 300Vac this would make it bigger than the relay itself so
:> it is
:> not really a goer in my opinion. The cap could remain charged to line
:> potential
:> so that when the contacts closed again it would cause a high discharge
:> current
:> which would result in an arc of its own. A series resistor of say 5 - 10
:> ohms/10W would need to be included in series with the cap to limit this
:> discharge current.
:
:
:** Such a series resistor would ruin the ability of the cap to suppress
:arcing at opening  -   since the resistor will allow the voltage across the
:contacts to rise instantaneously to 30 - 50% of the supply voltage value at
:that moment  =  up to 170 volts.
:
:
:......    Phil
:
:

I agree with you Phil. Arc suppression at high current is a huge compromise in
values anyway. There is no really effective way to do it cheaply (using
capacitor/resistor or even no resistor) without the protection being bigger than
the relay itself.

Thanks to everyone for the advice -- I agree that arc suppression
isn't really feasible in this case AND that the original relay is way
underspecified. I'll officially declare the topic closed + suggest
that an SSR be used in place of the relay.

Adam