I was on a site today with a sab202 that didn't pull as much amps as it should due to a bad calibration of the cap slide.



Someone had tampered with the calibration in the unisabII and adjusted the min and max points.

so, I calibrated the cap slide at the mechanical 0% with the screws on the potentiometer in front, removed the increase solenoid on the Vi and started the compressor.
Checked that the Vi remained in 0% (1.3) and then ran the capacity slide up to max by using a permanent solenoid (with care mind you )
With the suction valve I got full movement and had 270 amps for a minute (280 max) before I set the 100% point.

I took the capacity back down to 0% and did the same steps on the Vi slide.

During the test below I was at a suction pressure of approx -38 - -42C (0- -0,3 bar) and discharge of 25-29C (9-10 bar) (NH3)

Now I started the compressor in auto as they would normally do, and the slide moved from mech 0% to around 20-25% and set that as regulator 0% as it should.

the Vi slide moved to 35% (2.5)

The cap increased to around 70% where it should lock with the Vi slide, and then the cap slide went down to 50% as the Vi slide followed up to 100% (4.5).
When the Vi hit 100%, the increase solenoid is permanently on to keep it in position.

However, I never got the amps up to more than 210-220 amps, while according to the site machinist (who I believe), at this suction pressure, the compressor should be working on the amp limit and increase decrease on the motor protection in the unisab because the motor is a bit to small.

I went back to manual and checked my calibrations and both slides hit 0 and 100 % when the should.

so I started it up in auto again and let it go up to 100% (50% on the cap slide and 100% on the Vi slide), then I put my permanent solenoid on the cap slide increase solenoid, and since the cap piston has a larger surface than the Vi piston, It slowly pushed the cap slide towards the Vi slide, moving them both, and when the indicator closed on 2.5 Vi (75% of the total movement on the cap slide) the amps started increasing, and at around 1.9 Vi (85% of the total movement on the cap slide) we were stable at 270 amps because now the suction pressure was down to -42C.

I have never opened a SAB202 and haven't seen the slides with my own eyes, but according to the drawings they are similar to those on a howden, so in my mind, there is no reason why the amps should increase when I moved the slide manually, unless they weren't fully locked in the first place, and when I moved the cap slide it hit a point where the pressure behind the cap piston and Vi piston won over the spring pressure between them.

As I see it now, we are looking at a mechanical problem rather than a control problem, I was thinking maybe the cap piston seal was leaking, and the spring was pushing the slides apart, but if the piston is open to the rotors on the backside, it should increase capacity.


Any thoughts?