Current Transformer and Current Input
RTHC-SVD01B-EN 27
The 30% unbalance trip criteria is active down to 10% RLA for the
chiller. In addition to the ultimate 30% unbalance trip criteria, the
RLA of the motor shall be derated by resetting the Active Current
Limit Setpoint based on the Current Unbalance. The UCP2 pro-
vides a means to disable the RLA Derate in the Field Startup
Menu.
The 30% phase unbalance trip criteria shall be non-defeatable for
the purpose of eliminating critical components from the compres-
sor overload circuitry and providing ultimate protection for the
motor.
Both the derate and the 30% phase unbalance protection shall be
active in all running modes.
The Starter module will shut down the compressor if a phase cur-
rent unbalance of 30% is detected by the current transformers for
a period of 90 seconds. The phase unbalance diagnostics will be
displayed.
6. Current Limit. This is a multistage control that acts to prevent
chiller shutdown when the compressor motor current approaches
the cutout setpoint by reducing chiller capacity. Chiller capacity is
reduced via the following steps, providing increasingly aggressive
action as the trip point is approached:
Limit requests for capacity increases. (10% to 2% below Current
Limit Setpoint). Limit Loading
Prohibit requests for capacity increases. (2% to 0% below CLS).
Hold
Initiate stepped capacity reduction. (0% to 2% above CLS).
Unload
Initiate maximum capacity reduction. (more than 2% above CLS)
The intent of this control is to provide maximum capacity while
preventing chiller shutdown due to high compressor motor cur-
rents.
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