York YCUL0020, Installation, Operation And Maintenance Manual

Page: 109 / 178

JOHNSON CONTROLS
109
Form 150.63-NM9
Issue date: 5/20/2021
7
Section 7 – Unit controls
00065VIP
Introduction
The YORK Millenium MicroComputer Control Cen-
ter is a microprocessor based control system designed
to provide the entire control for the liquid condensing
unit. The control logic embedded in the microproces-
sor based control system will provide control for the
suction pressure or discharge air temperature, as well
as sequencing, system safeties, displaying status, and
daily schedules. The MicroComputer Control Center
consists of four basic components:
1. Microprocessor Board
2. Transformer
3. Display
4. Keypad
The keypad allows programming and accessing set-
points, pressures, temperatures, cutouts, daily sched-
ule, options, and fault information.
Remote cycling, demand limiting and chilled liquid
temperature reset can be accomplished by field sup-
plied contacts.
Compressor starting/stopping and loading/unload-
ing decisions are performed by the Microprocessor to
maintain suction pressure or discharge air temperature.
These decisions are a function of temperature devia-
tion 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 deci-
sion maker in the control panel. System inputs such
as pressure transducers and temperature sensors are
connected directly to the Microprocessor Board. The
Microprocessor Board circuitry multiplexes the ana-
log inputs, digitizes them, and scans them to keep a
constant watch on the unit operating conditions. From
this information, the Microprocessor then issues com-
mands to the Relay Outputs to control contactors, so-
lenoids, etc. for Temperature Control and to react to
safety conditions.
Keypad commands are acted upon by the microboard
to change setpoints, cutouts, scheduling, operating re-
quirements, and to provide displays.
The on-board power supply converts 24 VAC from the
1T transformer to a +12 VDC and +5 VDC regulated
supply located on the Microprocessor Board. This volt-
age is used to operate integrated circuitry on the board.
The 40 character display and unit sensors are supplied
power from the microboard 5 VDC supply.
24 VAC is rectified and filtered to provide unregulat-
ed +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, solenoid valves,
etc. to control system operation. 120 VAC is supplied
to the optimal T3 Transformer, which supplies 12 VAC
to the bridge diode module. The Bridge Diode Mod-
ule rectifies the voltage to -12 V unregulated The +12
V unregulated voltage supplies power to the Remote
Temperature Reset Circuit Board.