Carrier RTU Open v3 - To Use an Auxiliary Control Power Transformer

Installing the RTU Open
RTU Open v3 CARRIER CORPORATION ©2019
Installation and Start-up Guide All rights reserved
9
To use an auxiliary control power transformer
If you use a separate control power transformer, it is essential that the auxiliary transformer and the rooftop
transformer are in-phase. You
must
verify this prior to connecting the auxiliary transformer to the RTU Open.
Follow these steps:
1 Verify the available primary voltage at the rooftop equipment.
2 Remove power from the rooftop equipment and install the appropriate auxiliary transformer. Follow the
manufacturer’s installation instructions.
3 Ground one leg of the auxiliary transformer’s secondary wiring.
4 Apply power to the rooftop equipment. Measure the potential between the rooftop equipment control power
and auxiliary transformer's secondary hot (non-grounded) legs. If the voltage measured is less than 5 volts,
the transformers are in-phase; proceed to step 7. If you measure a voltage greater than 24 Vac, then the
phases are reversed.
5 Correct the phase reversal by either of the following methods:
○ Remove the ground from the secondary at the auxiliary transformer and connect it to the other
secondary
○ Reverse the primary wiring at the auxiliary transformer
6 Repeat step 4 to rewire.
7 Remove connector assembly from RTU Open's
J1
connector.
8 Connect the auxiliary transformer wires to
J1
wires
1
(24 Vac) and
3
(Gnd).
9 Apply power to the transformer.
10 Measure the voltage at the RTU Open’s
J1
-
1
and
3
to verify that the voltage is within the operating range of
21.6–26.4 Vac.
11 Attach harness to RTU Open's connector
J1
. See illustration below.
NOTE
The harness connectors are keyed and must be oriented properly for correct installation.
12 Verify that the
Power
LED is on and the
Run
LED is blinking.