YORK INTERNATIONAL
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Movement of the slide valve is accomplished by forcing
pressurized oil against a piston which drives the slide
valve open to load the compressor. Electrical pulses open
a solenoid valve, allowing oil pressure to move the pis-
ton which operates the slide valve. The pulses originate
at the micro and are a function of the required load on
the chiller in response to leaving water temperature.
The micro must repetitively pulse the slide valve to
maintain a constant load. This is required because dis-
charge pressure is always present on the other end of
the slide valve, trying to force it closed.
To unload the compressor, a second solenoid is
opened, which vents oil pressure away from the pis-
ton. By releasing the pressure on the piston, the dis-
charge pressure is able to push the valve toward the
closed or unload position.
It will be noted that slide valve movement is greater at
high discharge pressures. This is due to higher oil pres-
sure which is directly proportional to discharge pres-
sure. Higher oil pressures exert more pressure on the
slide valve causing greater movement as the slide valve
is pulsed with oil. This condition is normal. Also normal
and a result of compressor design is non-linear move-
ment at various points in the slide valve travel as the
slide valve is pulsed. Closely monitoring the movement
will show that the slide valve will move easier at the
ends and in the center of its travel range.
When the compressor is shut down, a check valve re-
leases oil from the piston. Since no discharge pres-
sure is available to push the valve closed, the slide
valve will maintain the position it was in when the com-
pressor stopped. To assure the compressor starts un-
loaded, the micro sends an unload signal to the slide
valve on start-up. In some cases, oil pressure may not
be high enough at start-up to move the slide valve. This
means the compressor could start-up partially loaded.
This will seldom occur and will not hurt the motor due
to the extra torque built into the design.
STARTER
Two types of compressor motor starting are available:
Across-the-line and optional Wye-Delta Closed Tran-
sition Starter. The Across-the-line starters will utilize
one contactor per compressor.
The optional Wye-Delta starter utilizes 4 contactors, a
time delay relay, and wire wound power resistors for
each compressor. See Fig. 18.
The Wye-Delta start allows inrush current to be limited
to approximately 33% LRA for the first 15 seconds with
current increasing to normal running current when the
Delta connection is completed.
When the micro initiates a start signal to run a com-
pressor, the 1CR (SYS 1) or 2CR (SYS 2) relay is en-
ergized. At the same time, the 1TR (SYS 1) or 2TR
(SYS 2) 15 second time delay relay is energized and
begins timing. The transition of the 1CR (SYS 1) or
2CR (SYS 2) contactor also energizes 1S (SYS 1) or
2S (SYS 2) normally open auxiliary interlock contacts
after approximately 16ms which in turn energizes the
1M (SYS 1) or 3M (SYS 2) motor contactor after ap-
proximately another 16ms. This completes the WYE
connection of the motor start. At the same time, the
normally closed 1S or 2S auxiliary interlock open, pre-
venting 2M (SYS 1) or 4M (SYS 2) from energizing.
The WYE connection of the motor start is enabled for
approximately 15 seconds. When the 1TR or 2TR timer
times out after 15 seconds, the interlock contacts ener-
gize 1A (SYS 1) or 2A (SYS 2) TRANSITION contac-
tor which puts the resistance networks across each wind-
ing of the WYE connection. At the same time, the nor-
mally closed 1A or 2A auxiliary interlock contacts de-
energize 1S or 2S after approximately 16ms. The nor-
mally closed auxiliary interlock contacts on 1S or 2S will
close energizing 2M or 4M after approximately 16ms.
With the 1M or 3M contactors already held in by the 1M
or 3M auxiliary contacts and 2M or 4M energized, the
normally closed auxiliary interlock contact on 2M or 4M
contactor holding in the 1A or 2A TRANSITION contac-
tor opens removing the resistor network and completing
the DELTA connection of the WYE-DELTA start.
FIG. 18
POWER PANEL
(WITH WYE-DELTA STARTING)
27963A
