Sanhua sec-612

[davidk]

Hello all,

I've built two custom air conditioners for a special vehicle. The units are equipped with single phase 230V compressors and a Sanhua SEC-612 controller for the EEV. Everything works just fine except one thing: After compressors stop, the pressure tends to equalize between condenser and evaporator. After one minute or so the controller shows "PST" meaning it sees the pressure sensor as shorted due to high pressure feed from condenser. In this situation the controller fully closes the EEV and the equalisation can't continue. As a result, the compressor won't start unless I wait a very long time. Do anyone knows if there's a setting I missed that will leave the valve open?
If I power cycle the SEC controller one or two times, during initialisation it will open the valve and will allow pressure equalize and I can start the compessors.
I use R410a refrigerant and the refrigerant setting is correct in the controller (as I said, the SH modulation works as intended) . This situation happens on both units but on one unit the error show with a delay and I'm able to start the compressors after 10 minutes.
If I had three phase compressors the starting torque would have been larger enough but on single phase there's no much torque.
Any ideas?

[frank]

Looking at the manual, there is a parameter setting for eev start opening and duration.
At compressor run signal, the valve opens immediately to the set value for the duration before starting to control sh.
This may solve your wait period for equalisation.

[davidk]

The problem is, despite I can set a minimum open ratio or open at compressor run signal, after "PST" error it closes the valve. The error self clears when system is colder or compressor starts and pulls pressure from evaporator. Maybe my problem is why pressure transducers measure goes wrong despite correct refrigerant setting...

[frank]

To stop the PST error, could you raise the max setpoint in 40063?

I can't see any way in the manual to introduce a delay function to the compressor 'run' signal, which would give the EEV open start function time to eqalise pressures. Perhaps you could insert a 'delay on' timer into the compressor circuit?

[davidk]

As long as I have this error (PST) the valve would stay closed. It starts operating again after pressure drop when compressor running.
I'm afraid modifying Pressure Max Range would mess the reading. For example 50% of sensor voltage would mean a different pressure if I change the range.

[seanf]

Could you switch the input from the pressure sensor over to a resistor, to give the controller a false reading when needed?

[davidk]

This would add complexity to my design. Of course, it could be done but I was hoping to a solution that doesn't involve more wiring. Still, I could put a relay coil onto the compressor and its switches to a trimmer resistor.

[seanf]

Could you use a fixed orifice or capillary tube that bypasses the EEV, just big enough to give the equalisation during the minimum compressor off time?

Not a very useful suggestion I know, but dosnt involve wiring.

[frank]

Could you change to pump down control?

[davidk]

What behavior I'm expecting in pump down mode? I guess this mode allows the unit to run by disabling low refrigerant protections.

[al]

Are you turning the eev controller off once temperature is satisfied? If so, set a delay time on PST error and force valve open 100% at start up, if error still occurs, are your system sizings correct? is the compressor modulating?