hitachi screw puzzle!

[mgtet]

I have a 3 circuit hitachi machine running on R407c a while ago we had a failed compressor on one circuit, can't remember model number but it is 500 series modulating inifity control.
The rebuilt compressor started to trip out on its internal, checked out and found all pressures ok, superheats and sub cooling ok. When system was running fully loaded it was fine but when unloaded the bell end of the compressor, not the suction pipe, just the compressor gets hot to touch 50degc+ as does the body at the internal protection connections. Initially i thought that the screws were passing but as i said when fully loaded it was fine. The remanufacturing company came in and replaced the compressor with a remanufactored one and took the apparently faulty one for inspection, they say they could see no reason for the fault.
I went back to site to check and it is doing the same thing!!!:rolleyes: Oil charge is good, 6litres, solenoid coils are on correctly, ie bottom unloaded started, middle unload, top load. It only seems to happen when the compressor is fully unloaded- around 30amps, when it loads up a little 40amps+ it is fine, has me stumped any ideas greatfully recieved.

[GHAZ]

hi it could be short of gas,or running unloaded to long 33%,you need about -2c saturated vapour at compressor in suction and a minimium of 12c discharge superheat, i think the model you have is the scz 5 series which have a internal metal gasket between the screw pack and the compressor body, for it to work the infinite capacity control if missing it wont work properly, the sch and scz models look identical from outside ,but to make sure the scz has cappillary going from the unloading solnoid side to opposite side just behind the terminal plate

[GHAZ]

hi i missed the operating of unloaders earlier , start/stop solnoid (a) on, (b) off, (c) off. loading up (a) off, (b) off, (c) on. unloading (a)off, (b) on, (c) off. and loaded constantly all the solnoids coils are off, hope this might help