JOHNSON CONTROLS
223
SECTION 8 - MICROPANEL
FORM 201.23-NM2
ISSUE DATE: 09/25/2020
8
VSD Transmitted Operating Parameters
VSD operating parameters will be transmitted to the
Chiller Control Board over the RS-485 communica-
tions link between the 2 boards. These values will be
displayed on the control panel display. The data and
display format are outlined in the Table 14 on page
223.
VSD SAFETIES (FAULTS)
VSD operating conditions are monitored by both soft-
ware algorithms and hardware circuitry. Both types ex-
ist as a result of the need for both extremely fast protec-
tion requirements such as a short circuit condition or a
slow reacting trip such as a slow rising overload condi-
tion. To eliminate nuisance unit trips, the sensor inputs
for the VSD’s operating parameters are averaged four
times before “Software” generated unit/system fault
trips from the VSD Logic Board are initiated. These
faults cause single compressor or total unit controlled
“ramped” shutdown. Other parameters that are not fed
to the VSD Logic Board microprocessor are protected
by “Hardware” generated fault trips. Hardware trips
involve electronic circuitry that measures voltages or
currents and activate level sensitive comparators con-
nected to programmable gate arrays on the VSD Logic
Board FPGA (Field Programmable Gate Array). These
safeties operate extremely fast and provide “immedi-
ate” shutdown, because they are not dependent upon
software program loops that operate in seconds or frac-
tions of a second. Outputs from the gate arrays provide
a digital signal to indicate whether a safety threshold
has been reached.
DC Bus Voltage Sensing and Scaling
Full DC Bus voltage and ½ DC Bus voltages are sensed
for up to 2 DC Buses. 2 and 3 compressor chillers share
a common DC Bus, while 4 compressor chillers utilize
2 DC Buses (1/3 and 2/4). The DC Bus is wired to the
DC Bus Isolation Board, the voltage is divided down
through a resistance voltage divider, and the reduced
voltage is fed to the VSD Logic Board for safety moni-
toring.
Current Sensing and Scaling
Individual current transformers on each leg sense three
phases of output current on each compressor. These
signals are buffered, divided by 2, and filtered by an
RMS to DC converter. The highest of the currents in
the three phases of each compressor leaving the RMS
converters is then sent to an A-D converter scaled,
monitored by the VSD Logic Board overload and high
current protection circuitry, and sent to the Chiller
Control Board for display as the compressor current.
In order to set the motor overload level (determined by
the setting of the OVERLOAD ADJUST potentiom-
eter on the VSD Logic Board), the voltage level on the
wipers of the four OVERLOAD ADJUST potentiom-
eters is continuously sensed by the VSD Logic Board
for current protection and sent to the Chiller Control
Board for both display purposes and for current limit-
ing control. This parameter is the 105% FLA value.
TABLE 14 - VSD OPERATING DISPLAY PARAMETERS
DATA
DISPLAY
FORMAT
Highest Phase of Compressor Motor Current in Amperes RMS (per Compressor) XXX Amps
VSD Output Frequency XXX.X Hz
Motor Overload Setting (105% FLA potentiometer setting) in amperes RMS (per Compressor) XXX Amps
DC Bus Voltage in DC Volts (maximum of 2) XXX Volts
VSD Internal Ambient Temperature XXX.XºF (or ºC)
IBGT Power Assembly Power Module Highest Baseplate Temperature (maximum of 2) XXX.XºF (or ºC)
Pre-Charge Enable Signal (maximum of 2) On or OFF
VSD cooling Fan/Pump On or OFF
Compressor Run Status (maximum of 4) On or OFF
Immediate Fault shutdowns are often accompanied
by audible motor backspin due to equalizing of the
differential between discharge and suction when the
compressor is turned off while rotating at high speeds.
This should not cause concern and will not damage the
chiller.
Each fault outlined in the descriptions that follow will
indicate whether it is a hardware or software generated
fault. It will be noted the “ramped” shutdown results
in minimal compressor backspin and noise associated
with backspin. “Immediate” shutdowns will result in
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