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nuckolls.bob(at)aeroelect
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 Posted: Fri Apr 20, 2012 12:46 pm Post subject: Non-Aviation Question - Electric Stove Switch |
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At 02:49 PM 4/20/2012, you wrote:
--> AeroElectric-List message posted by: Matt Prather <mapratherid(at)gmail.com>
Feeling nothing to lose, I disassembled the switch and found that a
set of contacts within the switch was stuck (welded?) closed - that
explained the behavior. I managed to pry the contacts open and spent
a bit of time filing on them - not sure that does any good.. I
reassembled everything and it all seems to be working properly now.
If I hold my head close to the panel, I can hear the bimetallic switch
cycling the power to the burner on and off. The technology used in appliances is pretty amazing stuff . . . not so much for the gee-whiz but for the relative robustness. Of all electro-whizzies I've owned, home appliance difficulties have been rare. The biggest thing going for home appliances is the 60Hz AC world where getting things turned on/off is a whole lot less stressful than 28vdc. Now for my questions:
- What caused the contacts to weld closed? It's a fairly new range,
so it doesn't seem like it was just a large number of cycles. I'm in
New Mexico, which has frequent lightning storms. Could the errant
charge from a lightning strike cause the contacts to get welded
closed? Maybe the previous owners of the stove used it for cooking
meth - so it has an abnormal number of cycles on it? Or simply one of those outliers in the statistical study of failure rates. Is it the right-front burner? The service on this burner probably accounts for 90% or more of total use for the stove. My rear burners are almost never used, the left front very seldom.
I would have no other basis for assigning a level of extra-ordinary stress. - Will my efforts at splitting the contacts and cleaning them up
yield a switch that's reasonably long-lived and reliable? Or should I
just order a replacement switch now.. I found an alternate source of
the switch - for $30. Is there something else I can do to the switch
contacts to help them out? Just separating and getting them smooth is all that can or should be done. It may well be that you'll never see this event again. $30 is pretty good. My catalogs show 15-30 for universal replacements 'infinite surface controls' with some brand-specific parts in the $50-70 range.
I'm a bit surprised that a modern appliance has contacts on a bi-metal, duty-cycle control. I would have expected them to go Triac and pot controls like the light dimmer on the wall. I read an article that suggests that contacts get hottest during the
arc associated with the switch opening. And that immediately
re-closing the switch offers the largest risk for welding the
contacts. Welding happens during the contact bounce event that only occurs on closure. Contacts being opened do not have any reason to turn around and re-close.
But contacts that are hammered together (as most are) will bounce. If they're energizing a high inrush load like a motor or incandescent lamp, the little fires that form during each bounce event can a be significant source of welding energy for a deformed contact. This is a low order risk in 60 Hz ac because likelihood of a series of switching events happening right at the peak of the ac wave-form is statistically low. I could imagine an unattended little kid playing with the
dial until it failed closed - it makes a very subtle, but kind of cool
buzz-click every time the contacts close or open... Hmmm . . . 'buzz' suggests less than positive, on-off operation . . . or something loose. Do the other burner controls behave the same way? If this one is different, it may be suffering from a manufacturing defect. If you have a low cost replacement, it wouldn't hurt to have it on hand. But if it's still on hand a year from now, it would be interesting to know.
Bob . . . [quote][b]
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klehman(at)albedo.net
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| Posted: Fri Apr 20, 2012 1:33 pm Post subject: Non-Aviation Question - Electric Stove Switch |
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I'd at least reposition the offending switch to a rarely used rear
burner if they are interchangeable. I've had good luck dressing
bimetallic contacts in small appliances as long as they are not heavily
pitted and have at least some snap action as Bob mentions.
Our new mid range stove still uses bimetallic controls.
In addition to bimetallic controls, the old stove from a large American
manufacturer also used a cheap remote bimetallic relay to control the
oven. I replaced that relay twice. When it failed, the oven top broil
element would turn on and could not be turned off without cutting power
to the stove. Apparently some of this technology is not overly reliable.
Unlike most small appliances, the oven did not have any secondary over
temperature protection. We elected not to use the timer, or leave the
oven unattended, which is perhaps just as well for anything capable of
starting a fire.
Ken
do not archive
On 20/04/2012 4:45 PM, Robert L. Nuckolls, III wrote:
| Quote: | At 02:49 PM 4/20/2012, you wrote:
<mapratherid(at)gmail.com>
Feeling nothing to lose, I disassembled the switch and found that a
set of contacts within the switch was stuck (welded?) closed - that
explained the behavior. I managed to pry the contacts open and spent
a bit of time filing on them - not sure that does any good.. I
reassembled everything and it all seems to be working properly now.
If I hold my head close to the panel, I can hear the bimetallic switch
cycling the power to the burner on and off.
The technology used in appliances is pretty amazing
stuff . . . not so much for the gee-whiz but for
the relative robustness. Of all electro-whizzies I've
owned, home appliance difficulties have been rare.
The biggest thing going for home appliances is the
60Hz AC world where getting things turned on/off is
a whole lot less stressful than 28vdc.
Now for my questions:
- What caused the contacts to weld closed? It's a fairly new range,
so it doesn't seem like it was just a large number of cycles. I'm in
New Mexico, which has frequent lightning storms. Could the errant
charge from a lightning strike cause the contacts to get welded
closed? Maybe the previous owners of the stove used it for cooking
meth - so it has an abnormal number of cycles on it?
Or simply one of those outliers in the statistical
study of failure rates. Is it the right-front burner?
The service on this burner probably accounts for 90%
or more of total use for the stove. My rear burners
are almost never used, the left front very seldom.
I would have no other basis for assigning a level
of extra-ordinary stress.
- Will my efforts at splitting the contacts and cleaning them up
yield a switch that's reasonably long-lived and reliable? Or should I
just order a replacement switch now.. I found an alternate source of
the switch - for $30. Is there something else I can do to the switch
contacts to help them out?
Just separating and getting them smooth is all that can
or should be done. It may well be that you'll never see
this event again. $30 is pretty good. My catalogs show
15-30 for universal replacements 'infinite surface
controls' with some brand-specific parts in the $50-70
range.
I'm a bit surprised that a modern appliance has
contacts on a bi-metal, duty-cycle control. I would
have expected them to go Triac and pot controls like
the light dimmer on the wall.
I read an article that suggests that contacts get hottest during the
arc associated with the switch opening. And that immediately
re-closing the switch offers the largest risk for welding the
contacts.
Welding happens during the contact bounce event that
only occurs on closure. Contacts being opened
do not have any reason to turn around and re-close.
But contacts that are hammered together (as most are)
will bounce. If they're energizing a high inrush load
like a motor or incandescent lamp, the little fires that
form during each bounce event can a be significant source
of welding energy for a deformed contact. This is a low
order risk in 60 Hz ac because likelihood of a series of
switching events happening right at the peak of the ac
wave-form is statistically low.
I could imagine an unattended little kid playing with the
dial until it failed closed - it makes a very subtle, but kind of cool
buzz-click every time the contacts close or open...
Hmmm . . . 'buzz' suggests less than positive, on-off
operation . . . or something loose. Do the other burner
controls behave the same way? If this one is different,
it may be suffering from a manufacturing defect. If you
have a low cost replacement, it wouldn't hurt to have
it on hand. But if it's still on hand a year from now,
it would be interesting to know.
Bob . . .
|
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