JOHNSON CONTROLS
234
FORM 201.23-NM2
ISSUE DATE: 3/9/2015
SECTION 8 - MICROPANEL
LM 34
SENSOR
NEW BOARD - P/N 031-02507-XXX
LD13123
VSD Internal Ambient Temperature Sensing
A National LM34 temperature sensor located on the
VSD Logic Board is used to measure the internal am-
bient temperature of the Control Panel/VSD enclosure.
It has an output voltage that is linearly proportional to
the temperature in degrees Fahrenheit. If the tempera-
ture exceeds the software trip point, the unit will shut
down on a safety. See “High VSD Ambient Tempera-
ture Fault” (Page 241) for details.
Pre-charge
When cooling is required (LCWT is more than SPHL),
leaving chilled liquid temp is greater than the setpoint
high limit), the chiller Control Board will send a
Pre-Charge Enable (2 enables on a 4 comp unit) via
comms to the VSD Logic Board. The VSD’s DC Bus
voltage(s) across the Bus Filter Capacitors will slowly
be increased to the proper level (more than 500VDC)
through firing of the SCR Trigger Board(s) and the as-
sociated pre-charge enable control signal(s). The pre-
charge time interval is fixed at 20 seconds. The pur-
pose of the precharge is to limit current when charging
an uncharged capacitor bank. When uncharged, the
capacitor bank looks like an electrical short. The bus is
brought up slowly by only turning on the SCR’s during
the trailing half of the + and – portion of the incom-
ing AC sine wave. Following is the status message dis-
played while the precharge is taking place.
S YS X VSD DC BUS PRECHARGE
Following successful completion of the pre-charge
interval, the SCR’s on the AC to DC semi-converter
(SCR/Diode Modules) will be gated fully on by the
SCR Trigger Board and the DC Bus will be brought up
to its full potential. After pre-charge has been success-
fully completed, the SCR’s will stay fully on until the
Chiller Control Board turns off the Pre-Charge Enable
via comms.
There will be a Unit Pre-charge Enable for 2 and 3
compressor units and separate System Pre-charge En-
ables for 4 compressor units.
The pre-charge will only take place when all of the fol-
lowing conditions are true, otherwise it is disabled:
•
Daily Schedule is ON.
•
UNIT switch is ON.
•
System Switch(es) are ON.
•
Run Permissive(s) are Enabled.
• Flow Switch indicates flow.
•
LCHLT more than Setpoint High Limit.
•
Unit not faulted / locked out.
Run Mode / Unit Restart
In order to initiate a system run, two conditions must
be met. At least 1 of the 2 systems run signals from the
control panel must be present and at least 1 of the 4
possible Compressor RUN bits must be set in the serial
communications link between the VSD Logic Board
and the Chiller Control Board. Following successful
completion of pre-charge and receipt of the system run
signals, the motor output voltage (% modulation) and
output frequency commands will be determined by the
VSD microprocessor located on the VSD Logic Board.
These two parameters will be sent to the PWM gen-
erator located on the VSD Logic Board for waveform
processing at a rate of once every 10 msec.
The voltage and frequency commands issued by the
VSD microprocessor are determined by the operating
frequency command received on the communications
link from the Chiller Control Board and by the pres-
ent operating frequency of the drive. Upon receipt of
a legitimate run command communication, the VSD’s
output frequency will be increased from 0 Hz to the
operating frequency command from the communica-
tions link.
Summary of Contents for YCIV0157
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