Hello all you experts!

Please bear with me..... this may be a long one!

And this will probably sound VERY 'novice'.... because it is!!

I am a software / "machine tool controls engineer" and was recently called to replace a failed Omron PLC controller sitting in a refrigeration compressor control panel.

The customer didn't have schematics of the control panel / system, only hand drawn notes on what appears to be a PLC addition / upgrade to this panel.

Also, there's no record of the program that WAS held in the failed PLC, no software back-up or a printed program.

We have failed to contact anyone who may know the contents of the lost programme. So this is a plea for help / comments / direction / contacts / advice / drawings!? etc.

It was obvious to me on my first visit to the customer, that this was a problem needing specialist refrigeration knowledge, therefore I requested that they contact a refrigeration specialist to assist me with the 'logic' of the process so that I could convert this into PLC 'code' for the replacement controller.

My customer did this, and in the interim period (before procuring a new PLC controller) he advised a 'hardwired' fix to get them back into production (Ice Cream Factory) This involved directly connecting the 'loading stage' solenoid valves to the pressure switches monitoring refrigerant pressure.

I seem to be 'rambling on a bit' so let me first tell you what was originally connected to the PLC controller........

INPUTS....
1 - 'Increase' from a Manometer
2 - 'Decrease' ---------"-----------
3 - 'Relay R2 - Compressor Safety Interlocks all OK'
4 - 'PS1' (Refrigerant Pressures' ?)
5 - 'PS2'
6 - 'PS3'

(The bit in-between is missing! ie. the prog!)

OUTPUTS.....
1 - 'Relay R1 - Start Compressor Motor'
2 - 'SV1 Load Stage 1 Solenoid'
3 - 'SV2 Load Stage 2 Solenoid'
4 - 'SV3 Load Stage 3 Solenoid'
5 - 'SV4 Load Stage 4 Solenoid'

The 'hardwired' temporary mod was to connect the system such that as soon as the motor changed from 'star' to 'delta' SV1 would be energised.

Then wired so that PS1 would directly energise SV2 (Load 2)
PS2 would call SV3 and
PS3 would call SV4.

The system then ran ok for a number of hours, until the temperature got quite low (-0.5bar on manometer) then soon after this the ice that had formed on the compressor suction started to thaw, and the customer informed me that temperature was 'rising' at the 'business end' on the production line!
(sorry for my incomplete specialist jargon!)

The obvious ommission from this 'hardwired' temporary fix is the manometer connections, so while we are trying to find someone to visit site to advise us, can you tell me what effect the manometer inputs should have on the 'loading valves' ? Bearing in mind that the PLC only had control over the Compressor Motor starting and the 4 loading solenoid valves.......

We are having trouble getting the 'specialist' back (too busy elswhere) so in the mean-time, is there anyone out there that may be familiar with this type of PLC modification, a PLC that only has influence over the Loading Valves based on the status of the 3 pressure Switches and the 'increase' and 'decrease' signals from the Manometer?

BTW, please don't SHOUT at me, I realise I know very little about refrigeration theory...... but if you want me to play a tune on a PLC controller, just hum it for me !!

I look forward to your comments.
Regards, Richard.

rlmts@controlit.fsbusiness.co.uk