CenTraVac Liquid Chillers - 11/97 - Page 34

Conventional "relay logic" circuits have, been replaced by software imbedded in the CenTraVac ^® microprocessor controller. The functions of the microprocessor are divided into six standard modules. Optional modules are available for those applications that require additional control capability. Optional communication interface modules are available for alternative control sources. All modules communicate with each other on the interprocessor communication bus (IPC).

All information is available and all setpoint/setup adjustments can be accomplished at the operator interface. An optional remote display permits the operator to monitor and operate the chiller from a remote location.

The six standard modules consist of a chiller module, a circuit module, a starter module, a stepper module, a purge module and local display module.

The chiller module is the master of the chiller. It communicates commands to other modules and collects data/status/ diagnostic information from other modules over the IPC.

The chiller module performs the leaving evaporator fluid temperature and limit control algorithms arbitrating capacity against any operating limit the chiller may find itself working against. The circuit module is assigned inputs and outputs associated with the refrigerant and lubrication circuits.

The starter module provides control of the starter when starting, running and stopping the motor. It provides interface to and control of wye-delta, across the line, primary reactor, auto transformer, solid-state starters and Trane Adaptive Frequency ™ drive. The starter module also provides protection to both the motor and the compressor in the form of running overload, phase reversal, phase loss, phase unbalance, momentary power loss and compressor surge. All diagnostics are communicated across the IPC to the human interface.

The stepper module is designed to drive the stepper motor inlet guide vane actuator and other flow control devices within a system. This module receives from the chiller module the direction and distance to drive the inlet guide vanes and then generates the appropriate signals to operate the stepper motor.

The purge module provides control of the purge including all the inputs and outputs to control the purge, to optimize both purge and chiller efficiency, and to communicate purge diagnostics to the human interface.

Before anything can begin, 115 volt (50 or 60 Hz) power is applied to the control panel. In that several control source devices may coexist, the operator determines which device has priority via the operator interface. All control settings at that control source are then in effect (i.e. active setpoints).

A control source is the device that determines setpoints and whether the chiller is auto/off (such as local control panel, remote control display, 4-20 ma external device, Tracer ^® , generic BAS).

For this sequence of operation it will be assumed that the control source has signaled the chiller to be in Automatic (i.e. when there is a load present, the chiller will turn on and when the load disappears, the chiller will turn off). It is also assumed that no diagnostic has occurred either prior to start-up or during run time and that no "special" applications exist.