CONTROL TRANSFORMER MEASUREMENT PROCEDURE

StepExpected Result/Action
NOTE: Steps 16 and 17 check control transformer operation.
16. Measure the voltage at the secondary windings of the control transformer.
Turn power off. Set up the VOM/DMM to measure AC control voltage on a
range that is higher than the highest voltage expected. (Refer to the unit
schematic or manufacturer's service literature.) Connect VOM/DMM across
control transformer secondary winding, os shown in Figure SP-7-7, View A.
Turn power on, then measure the control transformer secondary voltage.
Turn power off and disconnect VOM/DMM.
If measured voltage across secondary winding is within ±10% of the voltage
indicated by the schematic or manufacturer's service literature, the control
transformer is good. Troubleshoot control circuit components and wiring common
to both the heating and cooling modes of operation.
If secondary voltage is low, it can affect the operation of relays and other control
circuit components. Check for low primary voltage per step 17. If primary voltage
is good, replace transformer.
If there is no voltage at the secondary winding, perform step 17.
17. Measure the voltage at the primary winding of the controi transformer.
Turn power off. Set up the VOM/DMM to measure AC voltage on a range that
is higher than the highest voltage expected. (Refer to the unit schematic or
manufacturer's service literature.) Connect VOM/DMM across the control
transformer primary winding, as shown in Figure SP-7-7, View B. Turn power
on, then measure the control transformer primary voltage.
Turn power off and disconnect VOM/DMM.
If no voltage, or low voltage, is measured at the primary winding, measure the
equipment input voltage per step 1 (single-phase system) or step 2 (three-phase
system). If the input voltage is good, troubleshoot the load circuit wiring between
the power supply and the control transformer primary winding.

pic1 92 CONTROL TRANSFORMER MEASUREMENT PROCEDURE


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