2202MYJE-MY-C8-N_2018.02.
Chapter 5 Maintenance and Inspection
Compound 2-stage Screw Compressor 3225**C 5.5 Reassembly
5-64
5.5.15 Unloader Indicator
The unloader indicator contains micro-switches, a micro-switch cam, and a potentiometer. These parts
are used to detect the change in the rotation angle of the indicator cam shaft, which converts the axial
position change of the unloader slide valve into rotational position change, convert the change into an
electric signal, and send the signal to the package unit and/or the controller of the refrigeration system.
To check the unloader indicator after inspection, adjustment, or parts replacement, coordination with
the controller side will be required. Even in a case where the compressor is carried out of the
installation site for overhaul, the indicator assembly is often removed from the compressor (to be kept
at the site) and the inspection/adjustment and parts replacement are performed at the site. Thus, this
section provides a detailed procedure, which may be helpful after the reassembly work.
When removing the indicator block or performing inspection/adjustment or parts
replacement, be sure to shut down the control power and use lock-out and tag-out
procedures. If the power is not shut down, there is a risk of electric shock.
The high-stage side of 3225**C is implemented with an indicator assembly that is identical to the one
used in a standard single stage machine (except that the dial and micro-switch cam for the 3225*MC
and the 3225*SC are special to the high-stage of the 3225*M/SC types, for the range of 30 to 100 %).
The low-stage side of 3225**C is implemented with an indicator assembly that is identical to the one
used in 1612**C, specially designed for the low-stage use, with the addition of a bevel gear to convert
the axial direction to the lateral direction, for the indicator and dial.
5.5.15.1 Potentiometer
The potentiometer of the standard-type indicator is a rotary instrument for measurement over a full turn.
It senses the continuously variable position (indicated load of 0 % to 100 %) of the unloader slide valve,
and feeds the sensed position as electric signals to the control side of the package unit or refrigerating
system.
While the expected service life of the potentiometer will significantly vary depending on the installation
environment of the compressor (e.g. corrosive gas atmosphere, moisture, or the like.) and operational
conditions (e.g. frequent partial load operations, frequent start/stop operation, vibration, etc.), the
potentiometer is a consumable part that requires regular replacement according to the situation.
■ Disassembly
Refer to Section 5.4.1.1 in this manual for the disassembly procedure for the potentiometers on the
low-stage and high-stage sides.
■ Inspection
a) Check at the terminal block that the lead wires of the potentiometer are not loosened.
b) Check for any crack or other defects in the soldering of the lead wires of the potentiometer.
c) Manually rotate the shaft of the potentiometer and measure the resistance value using a circuit
tester to check that the resistance value changes smoothly.
5.5.15.2 Micro-switches and Micro-switch Cam
The unloader indicator uses two micro-switches and one micro-switch cam to detect the commanded
0 % (30 % for the high-stage of 3225*M/SC) and 100% capacity control positions of the unloader slide
valve.
If the micro-switch fails or any of these connections becomes loose for some reason, correct position
detection cannot be made, and it causes a problem in the operation control of the compressor.
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