JimG wrote:By the elements being OK "resistance wise," do you mean you have tested the resistance between the element terminals and a grounded portion of the roaster? Or do you mean you have tested the resistance across the element terminals?

If it is the latter, then you should check the former also. The resistance from any of the element terminals to ground should read "OL" on your meter, or many megaohms.

Based on the symptoms you report, though, I suspect they will test OK when cool. But then when it is hot and pieces have expanded, it would show a short, or low resistance.

I don't recall if you have tried running the elements individually, i.e. one at a time? Since they appear to be wired in parallel, I think this is safe? I'd guess that only one of the elements will trip the RCD.

Jim

another_jim wrote:Free tip: Ground fault trip mechanisms are ultra sensitive. When I measured resistance to ground, I got a perfectly good open circuit reading in all ranges except the megaohm one -- the thing was tripping on a milliamp short.

Hi both Jims!

Thanks for the advice. Elements tested across the terminals on the element itself, and also back in the roaster on the contactor. Have tried running it on one loop, same happens, but it does seem to take LONGER for it to occur, but this is a bit subjective since its variable anyway seemingly dependent on how long its had to cool down. Have also tried testing the resistance straight after its tripped down on the contactors and it seems fine.

I thought by trying the element outside the roaster I could then verify the element itself was good, but I see what you mean re earth fault. I will check the resistence between the terminals and a grounded portion of the roaster on the most sensitive scale, good idea, hopefully you might be onto something.

Have guest arriving shortly and the kitchen looks like a mad workshop!

Cheers, Chris

You guys are the best, thanks! Problem identified, but you almost wouldn't believe it...

So, I have three elements, each with two loops. Since they were connected with a bridge across the neutral, I disconnected the bridge, and then was able to independently test the six loops. At rest (ie cold), one loop of one of the new elements showed a short to earth and that was it.

However, on further testing, 5 out of the six loops under HEAT showed a short to earth. The only one that didn't was one loop of the original element. I tested that one running by itself and it worked, the rest all tripped one of the two RCDs (one wall mounted, the other in the fuse box).

Unfortunately, the original element is now a bit bent and I can't fit it properly back into the drum, its kinda hanging there half in at the moment. I was hoping to be able to roast at half power (obviously not ideal) which probably would have worked since I would estimate the heating cycle for the majority of the roast to be less than 50%, especially after a good warm up. Not going to happen though since I can't secure it back in there properly.

Its jerry rigged enough though that I can be back onto the PID/SSR track while I await a couple of new elements from Turkey!

Thanks again for the diagnostic help.

Cheers, Chris

So back onto the PID/SSR.

The current temperature controller gets AC power to run it from contactor 4, same as the Fuji PXR4 or PXG4, so thats good. The problem is, the current temperature controller, takes an AC voltage from contactor 3 (2T1) and then when conditions are met (ie actual temp below set temp) it closes its relay and puts out AC power trigger to contactor 4 (A2).

Therein lies the problem. If I go for a DC trigger SSR to replace contactor 4, and a DC PID, its going to have an AC input coming in from contactor 3. Contactor 3 is closed when the burner switch is turned on. Which is turn is not allowed to be turned on until the drum and exhaust is turned on, which is a signal sent from contactor 1.

If I want to keep the system logic, which I think makes sense, can I buy and use the Fuji PID with the control output one selected as RELAY CONTACT or SSR drive? and if so, can I use it to drive an AC triggered SSR such as this one...

http://hongkong01.rs-online.com/web/sea ... &R=3582971

with its associated heatsink and thermal pad which should just be able to clip onto the 35mm DIN rail.

That way I could start with the SSR which I can get locally, prove that it works, then order the PXG4 from abroad. It would also provide redundancy then as I could revert to my existing temp controller if the PID fell over....

Thoughts or better ideas? Thanks, Chris

If you run a SSR, you power it from the PID controller, not from an external source. You hook up the PID to the mains, then hook up the two SSR control side terminals to the two 12VDC outputs from the PID. These turn on and off to drive the SSR. This is different (and simpler) from operating a mechanical contactor, where the controller just opens and closes a mechanical switch, and it's up to you to supply the right voltage.

OK I understand. Reading through the online manual also indicated a 30 secs duty cycle for an AC relay output vs 1 sec for a DC SSR. I might just wire it so the PID isn't powered until the burner button is selected to ON. Keen to try and keep some of the protection logic intact. That would mean I could only make PID selections while the drum was running, as thats a precursor to the burner being selected on. Do you spend a bunch of time on a regular basis fiddling with the PID controls before roasting, or is it pretty much good to go straight away once all the initial setup/programming/tuning etc is done?

Ie, if its only powered with the drum running, is this likely to be annoyance in the future, or just during setup?

Cheers, Chris

gyro wrote:I might just wire it so the PID isn't powered until the burner button is selected to ON. Keen to try and keep some of the protection logic intact. That would mean I could only make PID selections while the drum was running, as thats a precursor to the burner being selected on.

No no no.

You order the PXG with 2 digital inputs (you only need one, but you have to order two). The digital input is connected to a new "RUN/STANDBY" toggle switch that you put on the front panel. This switch starts and stops your roast profile.

And you also install a new small relay whose normally open contacts are in series with the RUN/STANDBY switch. The coil of this relay is energized by contactor 3.

So the PXG is energized (but merely standing by) whenever your roaster is on. When contactor 3 is pulled in AND the R/S switch is on, the roasting profile begins.

It actually is possible to power up the Fuji PXR and have it execute the program once, then shut the heat down. It will continue to cycle through once whenever it is started This is option 14 on the "nod" variable that appears right after all the ramp/soak parameters in the 2nd block. It's less elegant but requires no inputs at all. This is how I run my roaster PID.

PXG4
Front Panel Size: 1/16 DIN
Control Output 1: SSR drive
Control Output 2: none
Option 1 : RS485 + Digital Input (No.1) (+$55.00)
Digital Output/Alarm Relay: none
Power Source: 100-240 VAC
Option 2: none

How does this look? Got the one digital input and RS485 to connect to computer.

Digital input goes to RUN/STANDBY switch. Do I need another relay, or can I just use a spare pole on contactor 3 to energise the circuit through this switch (when its in RUN). I know the contactor is AC powered, but does it matter if it has DC hooked up to either side of its poles? My understanding is it just brings a bit of metal across to join the poles, but I could be well wrong?

Then when contactor 3 is pulled (by switching the burner on) the circuit to the RUN/STANDBY switch is live, which when made in turns starts the roast profile.

Looks like the Digital input requires a DC current though? which I only have in the roaster when the cooling arms are running, as the transformer is downstream contactor 2 which is made when turning on the stirring arms.

I assume with this setup I can still preheat the drum to a suitable drop temp.

Cheers, Chris

gyro wrote:Looks like the Digital input requires a DC current though? which I only have in the roaster when the cooling arms are running, as the transformer is downstream contactor 2 which is made when turning on the stirring arms.

I'm pretty sure the controller sources its own DC for this input. The states are "high" or "low", corresponding to "open" or "closed" on the external switch/relay/contactor.

I haven't used your particular version of the Fuji, but the above is the way the PXR3 series works.

Jim

Ah right, perfect. Hadn't even considered that for the digital input. Thanks very much.

Do you know if its easily usable as a temperature controller before starting the profile cycle?

The roaster takes around half and hour to get the drum up to drop temp before the profile would be started. That initial drop temp would likely be different depending on the batch size to be roasted.

Cheers, Chris