Daikin MicroTech III Operation Manuals page 120

High Discharge Line Temperature Unloading Control
A compressor discharge temperature sensor (Thermistor) is
installed on the VFD compressor as standard . The temperature
is used to measure discharge temperature and superheat
levels at the VFD compressor discharge .
If the compressor discharge temp reaches 250F, the VFD
compressor speed is reduced by 10 rps and monitored for
3 additional minutes . The unit controller will continue to
reduce VFD compressor speed by 10-rps increments until
the discharge temp drops below 250F . If the VFD compressor
discharge temp reaches 250F a warning will be logged in the
controller . If reduced compressor speed does not resolve the
issue, the VFD compressor is shut down and allowed to cool
for 30 minutes . The VFD compressor is then re-started and if
the discharge temperature cannot be held under 250F, the VFD
compressor is locked out . The root cause may be connected
to a number of issues (low charge, blocked condenser coil,
condenser fan failure, malfunctioning TEV, etc .) and service is
needed .
The VFD compressor discharge Temp has an upper limit of
275F . The VFD compressor will be shut down if discharge temp
reaches 275F .
NOTE: The fixed speed compressor circuit does not contain
this feature .
Low/High Discharge Superheat
With the high side pressure transducer and discharge
thermistor, the unit controller is capable of monitoring the
variable speed compressor discharge superheat . This feature
protects the VFD compressor against flood back, undercharged
conditions, overcharged conditions, malfunctioning TEV,
etc . Typical compressor superheat range is 20F to 75F . The
discharge temperature of the VFD compressor can be viewed
at the unit controller display .
Low Superheat
If the compressor discharge superheat falls below 20F for
15 consecutive run minutes, the VFD compressor speed is
increased by 5 rps increments up to a 60 rps operating speed .
Superheat is monitored at each speed for 10 minutes and if
superheat remains below 20F, the VFD compressor speed is
increased an additional 5 rps . When compressor discharge
superheat level is at 20F or above, the compressor speed is no
longer increased . If speed correction resolves problem, the unit
controller will have logged that a speed correction was needed
for superheat in the event log . If increasing speed does not
correct issue, the VFD compressor is locked out and an alarm
is logged in the unit controller .
OM 920-6 • MICROTECH UNIT CONTROLLER
High Superheat
If the VFD compressor discharge superheat rises to 75F for 10
consecutive minutes, the VFD compressor speed is reduced
by 10 rps increments down to a 40 rps operating speed .
Superheat is monitored at each speed for 10 minutes and if
superheat remains above 75F, the VFD compressor speed is
decreased an additional 10 rps . When compressor discharge
superheat level is at 75F or below, the compressor speed is no
longer decreased . If speed correction resolves problem,
the unit controller will have logged that a speed correction was
needed for superheat in the event log . If decreasing speed
does not correct issue, the VFD compressor is locked out and
an alarm is logged in the unit controller .
Manual Control (with VFD Compressor)
For service and troubleshooting the unit controller has capability
to allow the VFD compressor to be operated manually . Refer to
the controller OM-920 for more detail .
The basic manual operations include the following:
• Start or Stop the VFD compressor
• Modulate VFD compressor from 0 – 100% speed
• Ability to energize condenser coil splitter solenoids (Only
for low ambient option)
• Ability to initiate or terminate oil boost sequence
If an electrical issue with the VFD compressor is suspected,
the winding resistance can be checked at the motor terminals .
Table 53: VFD Compressor Winding Resistance
Compressor Model
VZH-088B-X
VZH-088B-X
VZH-088B-X
120
o '
peraTor s
Winding Resistance
Voltage
(Ohms)
208-230V 0 .03
460V 0 .10
575V 0 .10
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