Trane ReliaTel - ReliaTel Condenser Fan Control Logic; Condenser Fan Sequencing Control

RT-SVD03G-EN 133
ReliaTel Condenser Fan Control Logic
12.5 – 25 Ton
For units that have two condenser fans, special control logic is designed to provide proper
condenser head pressure control at different ambient conditions. With normal operation, either
one or both fans will be operating depending on the outdoor temperature.
Control logic resides in the RTRM module. When the outdoor ambient drops below 60 degrees the
second condenser fan will be turned off. When the outdoor temperature reaches 65 degrees, the
fan will once again restart.
When only one condenser fan is running, the second fan will “windmill” backwards. This is caused
by air being drawn in through the opening of the fan orifice which bypasses the condenser coil. If
the windmilling is fast enough and the condenser fan is energized, it can actually start and run in
the reverse direction. To prevent this, when there is a call for the second condenser fan to start,
control logic forces the first condenser fan to shut off for seven seconds to allow the second fan
to slow down. After seven seconds, both the first and the second condenser fan will restart.
Because the contactors that provide power to the condenser fan are also the same ones used to
power the compressors, both compressors (if the second compressor is running) will also shut
down during the seven second interval. When the fans restart, the compressor will also restart.
27.5 – 50 Ton
Condenser Fan Sequencing Control
The condenser fans are cycled according to the outdoor air temperature and the number of cooling
steps that are operating. Table 47, p. 133 lists the temperatures at which the A and B Condenser Fan
Outputs on the RTRM switches the fans "Off". The fans are switched back "ON" when the outdoor
temperature rises approximately 5° F above the "Off" temperature.
Figure 75, p. 134 shows the condenser fans as viewed from the top of the unit facing the control
panel. Whenever a condenser fan is cycled back "On", the condenser fan Outputs A and B and the
compressor steps are de-energized for approximately seven seconds to prevent problems with fan
windmill.
Table 47. Condenser Fan/Compressor Sequence
Unit Size Compressor Staging Sequence Condenser Fan Output
O/A Temp.
(°F)
(Tons) Step 1 Step2 Step 3 Output A Output B Fans "Off"
27.5 - 30 CPR 1* CPR 1, 2 N/A
Fan #2 Fan #3
70
90
Fan #2 Fan #3
-10
60
35 CPR 1* CPR 1, 2 N/A
Fan #2 Fan #3
65
85
Fan #2 Fan #3
-20
55
40 CPR 1 ** CPR 2, 3*** CPR 1, 2, 3
Fan #2 Fan #3, 4
50
70
Fan #2
Fan #3, 4
20
60
Fan #2
Fan #3, 4
-30
50