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59
IM 1058-8 • MAVERICK II ROOFTOP SYSTEMS
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.
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
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 28: VFD Compressor Winding Resistance
Compressor Model
Voltage
Winding Resistance
(Ohms)
VZH-088B-X
208-230V
0 .03
VZH-088B-X
460V
0 .10
VZH-088B-X
575V
0 .10
