Current Transformers - Trane RTHD Troubleshooting Manual

26

Current Transformers

Each compressor motor has all three of its line currents monitored by toroid
(doughnut) current transformers. These currents are normalized with respect to
the Rated Load Amps of the respective compressor and thus are expressed in
terms of % RLA and in Amps. The currents are "normalized" through the proper
selection of the Current Transformer and the unit configuration. CTs are selected
based on the motor RLA. Refer to
The current transformers provide the input for five basic functions of the starter
module:
1 Current overload protection using a programmed % of Rated Load Amps
versus time to trip characteristic. The steady state "must trip" value is 140%
RLA and the "must hold" value is 125% RLA. The starter Module will trip out
the compressor, and the Main Processor (MP) will then shut down the other
compressor on the same refrigerant circuit (if applicable). The appropriate
diagnostic codes will be displayed.
2 Loss of Phase Current. A phase loss due specifically to lack of Current
Transformer inputs is not looked for during transition (which lasts for 3
seconds in software) and is additionally delayed 2.8 seconds. As such, the
diagnostic will not be called out any sooner than 5.8 seconds after transition
even if it occurs immediately at start. Additional, two of the three current
inputs also serve the Phase Rotation circuits. If this circuit (monitored
immediately after start), outputs an inconsistent value to the module (due to
loss of one or both of its inputs), a diagnostic of either Phase Loss (steady
output) or Phase Reversal Protection Lost (varying output) will occur with a
maximum delay of 1.2 seconds after start.
3 Phase Rotation. Screw compressors cannot be allowed to run in reverse
direction. To protect the compressors, the phase rotation is detected by the
current transformers immediately at start up. If improper phasing is detected,
within 1 second of startup, the Starter Module will trip out the compressor,
and the MP will then shut down the other compressor on the same refrigerant
circuit (if applicable). The Phase Rotation diagnostic codes will be displayed.
Phase Rotation is only active for the first second of operation.
4 Phase Unbalance. The Starter Module will shut down the compressor if a
phase current unbalance is detected by the current transformers while the
compressor is running. A 30% unbalance will cause the Starter Module to trip
out the compressor, and the MP will then shut down the other compressor on
the same refrigeration circuit (if applicable). Phase unbalance is based on the
following calculation:
Table 5.
% unbalance = (I0x-Iave) x 100
Iave
RLC-SVD05A-EN