Specifications

54
JOHNSON CONTROLS
Form 201.28-NM1.EN.PED/CE (0211)
OPERATION
On power-up, the Chiller Microprocessor Board will
attempt to initialize communications with the VSD
Logic Board. The control panel will request data from
the VSD, which includes the number of compressors and
the VSD software version. Once these data points have
been received by the Chiller Control Board, and have
been successfully initialized, the Chiller Control Board
will not request them again. If the comms connection
fails to occur and a reply from the VSD Logic Board does
take place in 8 seconds, the Chiller Control Board will
prevent the chiller from operating and a fault message
will be displayed.
During normal operation, if the control panel Chiller
Control Board receives no valid response to messages
for 8 seconds, the unit will shut down all compressors on
a Comms fault. The Chiller Control Board will continue
to send messages to the VSD while faulted. The unit
will be inhibited from starting until communications is
established. The fault will automatically reset when the
Chiller Control Board receives a valid response from the
VSD for a data request. Shown below is an example of
a Comms Failure fault message:
UNIT FAULT:
VSD COMMUNICATIONS FAILURE
VSD SAFETIES (FAULTS)
VSD operating conditions are monitored by both software
algorithms and hardware circuitry. Both types exist as
a result of the need for both extremely fast protection
requirements such as a short circuit condition or a slow
reacting trip such as a slow rising overload condition.
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 connected to programmable
gate arrays on the VSD Logic Board FPGA (Field
Programmable Gate Array). These safeties operate
extremely fast and provide “immediate” shutdown,
because they are not dependent upon software program
loops that operate in seconds or fractions of a second.
Outputs from the gate arrays provide a digital signal to
indicate whether a safety threshold has been reached.
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
compressor backspin and a higher noise level based upon
the differential pressure between discharge and suction.
When a VSD fault occurs, the VSD Logic Board captures
VSD data in the onboard battery backed RAM. At the
same time, the VSD Board “Fault Relay” will open,
signaling the Chiller Control Board microprocessor to
save a snapshot of system data. The VSD Logic Board
then transmits the fault data to the Chiller Control Board
microprocessor on the next comms between the two
boards.
If the Chiller Control Board receives the comms
fault indication before the Fault Relay signal, it will
immediately save a snapshot of system data when the
comms fault is recognized. This also enables the micro
to capture fault data if the Fault relay fails. Both the
system and VSD fault data are then stored in the Chiller
Control Board history buffers. Any additional faults that
may occur during shutdown on the first fault or between
the first fault and the next comms will also be stored
and transmitted to the Chiller Control Board along with
the original fault data. This data will be stored as “ALL
FAULT” data.
When the control panel acknowledges a fault (via the
fault acknowledge bit in comms) the fault relay will
be reset (closed) by the VSD Logic Board and the fault
indication flag (in comms) will be reset.
The fault relay will not open when a non-running fault
occurs. In this case, the system will be inhibited from
running until the fault condition is corrected. An inhibit
message will be displayed on the panel display indicating
the system is not allowed to run. Examples of this type
of fault would be the High Internal Ambient fault and
the VSD CT Plug Fault. When the chiller receives the
transmitted fault data via comms, it will save a snapshot
of system data in the history buffer even though the
chiller is not running.