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, and remains
at that temperature continuously for 15 seconds, a warning will
be logged in the controller, and the VFD compressor speed is
reduced by 10 rps and monitored for 15 additional seconds.
The unit controller will continue to reduce the VFD compressor
speed by 10-rps increments every 15 seconds until the
discharge temp drops below 220°F. If the compressor exceeds
250°F continuously for 3 minutes or reaches 275°F the
compressor enters a state of standby for restart. The Inverter
compressor HiDLTStandbyTmr = 100 minutes.
The root cause may be connected to several issues (low
charge, blocked condenser coil, condenser fan failure,
malfunctioning TEV, etc.) and service is required.
The VFD compressor discharge Temp has an upper limit of
275F. The VFD compressor will be shut down if discharge temp
reaches 275F.
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 85°F. The
discharge temperature of the VFD compressor can be viewed
at the unit controller display.
Low Superheat
If the compressor discharge superheat falls below 20°F for
10 consecutive run minutes, the VFD compressor speed is
increased by 10 rps increments up to a 60 rps operating speed.
Superheat is monitored at each speed for 5 minutes and if
superheat remains below 20°F, the VFD compressor speed is
increased an additional 10 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.
High Superheat
If the VFD compressor discharge superheat rises to 85°F for 15
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 85°F the VFD compressor speed is decreased
an additional 10 rps. When compressor discharge superheat
level is at 85°F 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
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 13: VFD Compressor Winding Resistance
Compressor Model
Voltage
Winding Resistance
(Ohms)
VZH-088B-X
208-230V
0.03
460V
0.10
575V
0.10
VZH-117*
208–230V
0.02
460V
0.08
575V
0.08
* Size 020 only
IM-1125-13 • REBEL ROOFTOPS
36
www.DaikinApplied.com
Refrigeration System
