I'm using a 40A mains contactor to switch a 7kw resistive load (heating
elements in a glass-fusing kiln)
The particular contactor is made by Chint - part no IC40/2.
Can't seem to find a spec online for the expected life (no. of on/off
cycles) of the contactor - and wondered if anybody out there had any
experience of how long these things last, or a copy of the data sheet.
A recent 'play' with a RaspberryPi to monitor the heating / cooling /
switching on the kiln indicates that the contactor is switched some 450
times per 8-hour fusing schedule - which is why I was wondering...
As the kiln doesn't yet have the 'over-temperature' safety circuit (that
some of you helped to design - thanks) installed, a fail-on of this
contactor would be a Bad Thing - so wondering about swapping out the
contactor _before_ it's expected end-of-life...
Thoughts / experiences / actual specs?
I'd thought about using a solid-state relay instead - but I'm not
convinced that they'd be any more reliable, and (apparently) they can
also fail 'on'....
It's a heating coil. I bet it's not pure resistive load. Surely, any
inductance would tend to affect the life of the switch? So, when you
find the spec sheet and it says the contactor will survive 200,000
cycles (or whatever) in ideal circumstances, what fudge factor will you
How bad? Melt the heating coil and cause you annoyance, or burn your
workshop down and shoot molten glass over the survivors - which end of
Probably not _purely_ resistive.... but much more so than (say) a big
motor. Being a cautious sort, I'd probably derate by 50% from the
manufacturer's quoted switching lifetime - simply because I'd rather
replace a £25 contactor every 5 years (or whatever) than sort out the
mess if it decided to stick 'on'.
Either of those scenarios are bad enough..
The most likely failure (I guess) would be the melting of the heating
element at or around 1500c (which would shut off the heating soon
enough). The fibre insulation in the kiln is rated to 1000c - so it
could be damaged & need replacing if the kiln spent any time above that
temperature. If the insulation should break down then I'd have 1000c's
worth of heat let loose in a timber-framed workshop - with thousands of
euro's worth of equipment and raw materials...
So that's why I'm wondering how frequently I should swap out the £20
Given that, I think you absolutely need something in the circuit that
melts before the insulation gives way, but obviously not at normal
Harry says they are commonplace in kilns, and, given what you said
above, we can well understand why.
John Rumm is right to point out that the operating frequency is
irrelevant to the stored energy and back EMF on switching. However, the
sort of inductance we are talking about is not going to store a
significant amount of energy even at the maximum instaneous voltage of
about 350V. If anyone has any figures for practical inductance we could
work it out.
On Fri, 07 Apr 2017 10:48:00 +0100, Roger Hayter wrote:
It's not the maximum instantaneous voltage so much as the maximum
instantaneous current. In this case, a mostly resistive load with a small
element of inductance, the two are almost the same thing.
It's the interruption of ca 29 amps of current that creates an inductive
back EMF to trigger an arc. However, the use of an AC supply allows for a
much smaller contact separation distance than that needed when using a
240v DC supply. The 100 reversals per second of a 50Hz supply ensures
that any arc will self extinguish within 10ms of the contacts separating.
In fact the 'auto extinguish' feature of AC supplies is so effective
that a much cheaper 'toggle effect' can be used in light and socket
switches, one where the end (ab)user can, with some dedication to the
task, defeat the toggling action to admire the SFX and, in some cases,
the 'light show' from behind the plastic switch plate.
The old fashioned "Tumbler" light switches used on DC supplies had a
more foolproof toggle action, along with a contact separation measurable
in centimetres rather than the millimetres of a modern light switch.
On 07/04/2017 06:46, The Natural Philosopher wrote:
What's the frequency content of a switching transient?
Much depends on the heater wire topology. Classic tubular heating
elements with helical wound element wire (like that of a bar style
electric fire) will give you quite a significant back voltage on switching.
Innis & Gunn Oak Aged Original or the Rum Finish ;-)
As much as a millivolt?
Dont be silly.
You simply have no clue do you?
Coils like that are what we used to use to resonate at 10-20Mhz, in
conjunction with less capacitance than you would see between the leadout
wires going to such an element.
"What do you think about Gay Marriage?"
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.