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PRODUCT DESIGN
27
CAPACITY CONTROL
During the compression process, there are several pockets
within the scroll that are compressing gas. Modulation is
achieved by venting a portion of the gas in the first suction
pocket back to the low side of the compressor thereby
reducing the effective displacement of the compressor. See
Figure A. Full capacity is achieved by blocking these vents,
increasing the displacement to 100%. A solenoid in the
compressor, controlled by an external 24-volt ac signal,
moves the slider ring that covers and uncovers these vents.
The vent covers are arranged in such a manner that the
compressor operates somewhere around 67% capacity when
the solenoid is not energized and 100% capacity when the
solenoid is energized. The loading and unloading of the two
step scroll is done “on the fly” without shutting off the motor
between steps. See Figure B below. The unloaded mode
default was chosen for two reasons:
FIGURE B
1. It is expected that the majority of run hours will be in the
low capacity, unloaded mode.
2. It allows a simple two-stage thermostat to control capac-
ity through the second stage in both cooling and possibly
heating if desired.
UNLOADER SOLENOID
A nominal 24-volt direct current coil activates the internal
unloader solenoid. The input control circuit voltage must be
18 to 28 volt ac. The coil power requirement is 20 VA. The
external electrical connection is made with a molded plug
assembly. This plug is connected to the Comfort Alert
Module which contains a full wave rectifier to supply direct
current to the unloader coil.