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Selecting The
LAG COMPRESSOR DIFFERENTIAL OFF %
Selection of the LAG COMPRESSOR DIFFERENTIAL
OFF % under the PROGRAM key allows the operator to
select the range over which the lead and lag compressor
load shares equally. This affects the loading sequence at
low loads. Once the lag compressor is brought on and
loads up to a % FLA equal to the lead compressor, if
demand allows, Load sharing of the compressors will
begin. Load sharing will continue until loading of the com-
pressors drops to the programmed differential % below
the LAG COMPRESSOR START POINT %.
For example: If a LAG COMPRESSOR START POINT
of 70% is selected, and a LAG COMPRESSOR DIF-
FERENTIAL OFF % of 50% is selected, the lead and lag
compressor will both load share until the % FLA of the
lead compressor drops to 20% (70% - 50% = 20%). At
this point, the micro will unload only the lag compressor
until it reaches a point at which the micro determines it is
fully unloaded and the load does not require its contin-
ued operation. The micro will then cycle the lag compres-
sor off. It will also cycle off as described previously, if
the micro determines that chilled liquid temperature is
dropping too low.
The larger the differential programmed, the more efficient
the use of the evaporator. For efficiency purposes, 50%
is recommended. Cycling of the lag compressor will also
be minimized since equalized unloading operates for a
longer period of the loading/unloading scheme.
A small % differential will increase cycling and lower ef-
ficiency slightly. For example: If 10% LAG COMPRES-
SOR DIFFERENTIAL OFF % is selected with a LAG
COMPRESSOR START POINT % of 70%, the lag com-
pressor will start when the lead % FLA reaches 70%.
The lag loading will be brought up to equalize the lead of
70% if demand requires. Both compressors will increase
in loading proportionately as demand increases. When
the load drops, both compressors will unload to 60% (70%
- 10% = 60%). At this point, only the lag compressor will
be unloaded as demand decreases. It will unload to a
point at which the micro determines it is fully unloaded. If
load does not require its continued operation, the com-
pressor will cycle off.
A minimum % FLA point is built into the micro to only
allow load sharing to operate to a minimum of 20% FLA
of the lead compressor. Therefore, the LAG COMPRES-
SOR START % MINUS THE LAG COMPRESSOR DIF-
FERENTIAL OFF % cannot be less than 20% or an OUT
OF RANGE message will appear.
To program the LAG COMPRESSOR DIFFERENTIAL
OFF % under the "PROGRAM" key, key in the desired
value and press the "ENTER" key. The new value will be
102
entered into memory. This value is not password pro-
tected and can be programmed anytime the PROGRAM
key is pressed. Shown below is an example of this dis-
play:
L A G
C O M P R E S S O R
D I F F E R E N T I A L
The micro will accept a range of programmable values
between 0 - 50% for this operating control, provided the
LAG COMPRESSOR START POINT % minus the LAG
COMPRESSOR DIFFERENTIAL OFF % does not equal
less than 20%.
CONTROL SCHEME 2
Loading And Unloading Utilizing % FLA And Con-
densing Temperature To Predict Slide Valve Position
This control method uses a slightly more complex scheme
to control loading/unloading and requires slightly more
programming. This scheme was derived from a computer
model and actual tests under controlled conditions to
plot slide valve position versus motor current and con-
densing temperature (CTP). The chiller sensors involved
in the control scheme are the motor current C.T.'s and
discharge pressure transducers. Actual condensing tem-
perature is derived from system discharge pressure.
Slide valve position calculation requires that the micro
be programmed to know a specific chiller's RLA at 100%
load at design conditions. This is required for each indi-
vidual refrigerant system. The system Motor Current =
100% RLA value will vary from chiller to chiller and should
be programmed according to the model number and volt-
age from Table 5.
Access the password protected values and key in the
current that equals 100% RLA and press the ENTER
key. The new value will be entered into memory and the
display will advance to the next programmable value.
Repeat for the other system.
The displays for each system programmable value will
appear as shown below:
S Y S T E M
M O T O R
A M P S
S Y S T E M
2
A M P S
To compute slide valve position, the DESIGN COND
TEMP. must be entered under the PROGRAM key. The
Condensing Temperature is entered as °F / °C equating
to 100% load at design conditions. The temperature to
be programmed will vary from chiller to chiller and should
be programmed according to Table 6.
O F F
5 0 %
C U R R
=
1 0 0
% F L A
M O T O R
C U R R
=
1 0 0
% F L A
YORK INTERNATIONAL
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