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Unit Controls
INTRODUCTION
The YORK MicroComputer Control Center is a micropro-
cessor based control system designed to provide the
entire control for the condensing unit. The control logic
embedded in the microprocessor based control system
will provide control for the discharge air temperature or
suction pressure, as well as sequencing, system safe-
ties, displaying status, and daily schedules. The
MicroComputer Control Center consists of four basic
components: 1) microprocessor board, 2) transformer,
3) display and 4) keypad. The keypad allows program-
ming and accessing setpoints, pressures, temperatures,
cutouts, daily schedule, options, and fault information.
Remote cycling, demand limiting and temperature reset
can be accomplished by field supplied contacts.
Compressor starting/stopping and loading/unloading de-
cisions are performed by the Microprocessor to maintain
either the discharge air temperature or system suction
pressure. These decisions are a function of temperature
or system suction pressure deviation from setpoint.
A Master ON/OFF switch is available to activate or de-
activate the unit.
MICROPROCESSOR BOARD
The Microprocessor Board is the controller and decision
maker in the control panel. System inputs such as pres-
sure transducers and temperature sensors are connected
directly to the Microprocessor Board. The Microproces-
sor Board circuitry multiplexes the analog inputs, digi-
96
UNIT CONTROLS
YORK MILLENNIUM CONTROL CENTER
00065VIP
tizes them, and scans them to keep a constant watch
on the unit operating conditions. From this information,
the Microprocessor then issues commands to the Relay
Outputs to control contactors and solenoids for discharge
air temperature or system suction pressure control, and
to react to safety conditions.
Keypad commands are acted upon by the micro to
change setpoints, cutouts, scheduling, operating require-
ments, and to provide displays.
The on-board power supply converts 24VAC from the 1T
transformer to a +12VDC and +5VDC regulated supply
located on the Microprocessor Board. This voltage is
used to operate integrated circuitry on the board. The
40 character display and unit sensors are supplied power
from the micro board 5VDC supply.
24VAC is rectified and filtered to provide unregulated +30
vdc to supply the flow switch, PWM remote temperature
reset, and demand limit circuitry which is available to be
used with field supplied contacts.
The Microprocessor Board energizes on-board relays to
output 120 VAC to motor contactors and solenoid valves
to control system operation.
UNIT ON/OFF SWITCH
A UNIT ON/OFF switch is just underneath the keypad.
This switch allows the operator to turn the entire unit
OFF if desired. The switch must be placed in the ON
position for the chiller to operate.
YORK INTERNATIONAL
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