On 06/11/2019 08:08, Adrian Brentnall wrote:
On 06/11/2019 00:47, Steve Walker wrote:
On 04/11/2019 08:00, Adrian Brentnall wrote:
On 04/11/2019 07:40, Harry Bloomfield wrote:
Adrian Brentnall has brought this to us :
Currently I'm using domestic contactors, which last a few years
(not through being overloaded, simply through the number of on/off
cycles they go through during each firing schedule).
The contactors have screw-down clamps for the connections.

Why not look at converting over from relays, to triac switching?

https://uk.rs-online.com/web/c/semic...E&gclsrc=aw.ds


BTA40-600B is rated at 40amps 600v

HI Harry.
I've looked into the whole 'Solid State Relays' thing, and, to be
honest, I'm not convinced..
When you examine the specs, these things need some serious
heatsinking (as they have a finite 'on' resistance) and, call me
old-fashioned, but I rather like to hear the relay clicking on & off,
as it reassures me that all is running properly. Retro-fitting the
SSR plus the heatsink and probably a small cooling fan would be
difficult in the existing enclosure.

The big worry with glass-fusing kilns is that the main relay fill
fail 'on' (welded contacts) - as when this happens the kiln will
continue to heat uncontrollably until something melts or catches fire.

Use two force-guided relays in series and use the NC contacts for
monitoring? If one relay welds shut, the other should still cut the
power and you'll get a warning that one has failed.

SteveW

Hi Steve
Not sure about 'force-guided' - not a term I know?

Force-guided relays have NO and NC contacts that are physically
connected, so if a NO contact welds together, the NC contact cannot
close, thus allowing allowing failure to be detected.

The idea of the safety relay is that is is in series with the 'main'
relay - but the safety relay is only switched once per firing schedule,
while the 'main' relay can be cycled several thousand times per schedule.

On that basis, the 'safety' relay should wear out less fast, and should
be reliable enough that it can be expected to interrupt the power
when/if asked to do so.

Perfectly reasonable.

If I was designing the controller from scratch I would want to see a
pair of 'signal' contacts on each relay - so I could do the usual
- ask for the relay to close
- wait a bit (in computer terms... to allow the contacts to close and
stop bouncing)
- check the signal contacts to confirm that it's closed
- throw an error / shut things down if it hasn't

and vice-versa when asking the relay to open.

That's where the force-guided relay comes in - allowing the NC contacts
to be running at control voltages rather than power voltages, but
showing when there is a fault.

But, in this case, if I can allow the controller to drop out the
'safety' relay when it feels uneasy about what's occuring, and,
possibly, use the remote monitoring Raspberry Pi as a second sanity
check (by watching the kiln temperature with a second thermocouple and
breaking the 'safety' chain if it goes above expected limits), then I
reckon it'll be a vast improvement over what's there already.

Sounds like a reasonable plan.

SteveW