The ACH550 range of ac variable speed drives is designed for buildings and in particular HVAC (heating, ventilation and air conditioning) applications.  One of which is cooling tower applications, and the diagram below shows a typical control system with an ACH550 drive.

Fan Speed Control

The cooling tower fan speed is typically controlled to regulate the water temperature of the return condenser water from the cooling tower. The ACH550 process PID controller compares the desired temperature (the Setpoint ) with the actual temperature (the Feedback) from a temperature sensor and adjusts the fan speed to maintain the temperature at the desired level. The Setpoint is typically entered using the Operator Control Panel or a serial communications command.

A built-in PID controller with more than twenty programmable parameters is standard in the ACH550. The PID controller can be configured to control a process variable such as pressure, flow, or fluid level by adjusting the motor speed to correct the process variable.

Bypass Valve Control

The ACH550 External PID Controller can be used to control a valve to regulate the flow water bypassing the cooling tower when the desired temperature can be maintained without full flow through the cooling tower. In this mode of operation, the cooling tower fan speed will have dropped to the minimum speed and stopped.

The ACH550 has a second PID controller that is similar to the controller described above but completely independent. The External PID Controller is intended to control a process that is not related to motor speed. This PID loop is typically used to control a chilled water valve or cooling tower bypass valve or any device requiring a 4-20 mA signal control.

Summer / Winter Settings

A dry contact input or a serial communications command changes drive settings to suit seasonal conditions.  Winter operation may include operating a valve to divert water from the chiller to a plate and frame heat exchanger.

Vibration Protection

The Critical Speed Lockout function can be set to avoid operation at speeds that might cause excessive vibration.  A vibration switch can be connected as is a dry normally closed contact safety interlock input that operates as a permissive start interlock.

Freeze Protection

A Supervision Function can be set to monitor the temperature feedback signal. If the water temperature approaches freezing, the drive provides a contact closure to turn on a sump heater or can modulate a bypass valve. An alarm (dry contact) can be provided to the Building Management System.

Building Management System

The Building Management System (BMS) can monitor and control all aspects of the ACH550 through serial communications.

Serial communications is a powerful mode of operator and supervisory control interface that provides a number of benefits compared to other types of interface. The ACH550 is equipped with an (RS) EIA-485 serial communications port that supports a full range of drive monitoring and control functions through serial communications.

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