Daikin MicroTech III Operation Manuals page 115

VFD Compressor Protection Unloading Control
There are several unloading control functions that limit the
speed of the VFD compressor to protect it from damage
under abnormal operating conditions . The following functions
are provided:
• High Pressure Unloading Control
• High Discharge Line Temperature Unloading Control
• Low Discharge Superheat Unloading Control
• High Discharge Superheat Unloading Control
• Condenser Coil Splitter Coil Unloading Control
• VFD Compressor Emergency Stop Control
High Pressure Unloading Control
Normal VFD compressor control is limited when a high discharge
pressure conditions occur on the VFD compressor circuit . If the
VFD compressor is operating and the discharge pressure (PTD) is
greater than 525 PSI, a High Pressure Unloading Control Event is
generated and the VFD compressor is slowed every 10 seconds
until either the discharge pressure falls to less than 525 PSI or
remains higher than 575 PSI for 10 minutes . If the discharge
pressure is above 575 PSI for 10 minutes the VFD compressor is
shut off and a High Pressure Problem alarm is generated . If the
OAT is below 45F at the time the VFD compressor is shut OFF
immediately and a High Pressure Problem alarm is generated
anytime the discharge pressure rises above 575 PSI . The alarm
must be manually cleared .
High Discharge Line Temperature Unloading Control
Normal compressor control is limited when a high discharge
line temperature conditions occur . If the VFD compressor is
operating and the discharge Line temperature is greater than
250°F for 15 seconds a High Discharge Line Temperature
Event is generated and the VFD compressor capacity is
reduced every 15 seconds until the discharge Line temperature
falls below 220°F . If the discharge Line temperature is above
250°F continuously for 3 minutes the VFD compressor is
stopped and a High Discharge Line Temperature problem
alarm is generated . The alarm must be manually cleared .
Low Discharge Superheat Unloading Control
Normal compressor control is limited when low superheat
conditions occur . If the VFD compressor is operating and
the discharge superheat is less than 20F continuously for 15
minutes a Low Discharge Superheat Unloading Control Event
is generated and the VFD compressor speed is increased .
The VFD compressor speed is increased every 10 minutes as
long as the discharge superheat remains less than 20°F until
the maximum allowed VFD compressor speed (based on unit
size) is reached . If the discharge superheat remains below
20°F the VFD compressor is shut OFF and a Low Superheat
alarm is generated .
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High Discharge Superheat Unloading Control
Normal compressor control is limited when high superheat
conditions occur . If the VFD compressor is operating and the
discharge superheat is greater than 75°F continuously for 15
minutes a High Discharge Superheat Unloading Control Event
is generated and the VFD compressor speed is decreased .
The VFD compressor speed is decreased every 10 minutes
as long as the discharge superheat remains greater than
75°F until the minimum allowed VFD compressor speed
(based on unit size) is reached . If the discharge superheat
remains above 75°F the VFD compressor is shut OFF and a
High Superheat alarm is generated .
VFD Compressor Emergency Stop Control
If the VFD compressor is enabled (MCB-D03 is closed) and
commanded to run for 30 seconds but the controller fails to
receive the VFD run verification input (EMC-X4 is open) the
VFD compressor enable output (MCB-D03) is cycled OFF for 5
seconds ( a VFD Compressor Emergency Stop Control Event
is logged)and then back ON . The VFD compressor is then
ramped to 45% . If this occurs 3 times in a 30 minute period
the VFD compressor is shut off and a VFD Compressor Fault
alarm is generated .
Condenser Coil Splitter Solenoid Valve Control
Condenser coil splitting is required on both circuits of a
VFD compressor unit to maintain head pressure during low
ambient/low modulating operation . This is accomplished with
a solenoid valve on each circuit controlled by a normally open
digital output (EMC DO1, EMC-DO2) from the MicroTech III
controller . The coil splitter solenoid valve is controlled based
on the average discharge line pressure equivalent saturation
temperature (Tc) determined from the corresponding discharge
pressure (PTD) monitored via an analog input to the MicroTech
III controller as follows:
• The average Tc is determined using a sliding average of
the previous 150 actual Tc values
• The splitter valve on a circuit is closed (energized) when
that circuit is operating and the circuit's average Tc
remains below 83 .0°F continuously for 60 seconds and
the OAT is less than or equal to 80 .0°F .
• Once closed the solenoid valve is re-opened when
the average Tc rises above 105 .0°F continuously for
60 seconds, the OAT rises above 80°F or when all the
compressors on the circuit are OFF .
• When the splitter coil output on the VFD compressor
circuit is closed (EMC-DO2 is energized) normal VFD
compressor control is limited . In this mode of operation
the VFD Compressor Control PI Loop remains active but
the VFD Compressor minimum allowed speed value is
increased and Low Pressure Unloading Event is logged .
115 OM 920-6 • MICROTECH UNIT CONTROLLER
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