Trane ctv-prc007-en manual Unit, Options, Adaptive, Frequency Drives, High kW Charges, Operation

Unit	Adaptive
Options	Frequency Drives

Condenser water temperature relief of chilled water reset

Compressor lift reduction is required for a chiller application, both to provide stable chiller operation at part-loads and to achieve greater energy savings. Intelligent control to reduce condenser water temperature, or chiller water reset strategies are key to AFD savings in chiller applications.

High kW Charges

Electric utility bills normally include both demand and energy components. The demand or kW portion is established by usage during utility peak hours, by individual peak usage or a combination. This portion may or may not be influenced by installation of an AFD. But the energy or kWh portion will almost certainly be reduced because of the improved efficiency of the chiller plant during part-load conditions throughout the year. The greater the kWh charge, the shorter the payback.

Operation

The Trane AFD controls the speed of the chiller compressor by regulating the output voltage in proportion to the output frequency to provide a nominally constant rate of voltage to frequency as required by the characteristics of the compressor motor. Motor speed is proportional to this applied frequency.

The Trane AFD is a voltage source, pulse- width modulated (PWM) design. It consists of three basic power sections:

•Converter — Semi-conductor bridge rectifier takes incoming AC power and converts it to a fixed voltage DC bus.

•DC bus filter — The converted DC bus voltage contains a significant amount of ripple. The DC bus filter smooths the voltage ripple from the converter with capacitors and a DC link reactor to supply a fixed constant voltage to the inverter section. It also minimizes the electrical harmonics generated by the drive back to the distribution system.

•Inverter — Converts the DC voltage into a sinusoidal synthesized output AC voltage. This synthesized output controls both the voltage and frequency which is applied to the motor.

A fourth element of AFD design is the microprocessor control logic which is the intelligence for the power section. It also includes all feedback sensors required for stability in the system and any required shutdown due to a fault.

Soft Start: Inrush Current and Torque Trane AFD’s are programmed to start the compressor motor from low frequency and low voltage. The motor is brought up to speed by increasing both frequency and voltage at the same ratio. Thus current and torque are much lower during start-up and acceleration than the high current, high torque associated with across-the-line or even reduced voltage starters.

Note that the actual torque developed by the AFD is the total of the torque required by the load and the accelerating torque. The AFD is rated by output current and is limited to a maximum of 100 percent continuous RLA through the chiller control (UCP2). A 100 percent output current capability results in 100 percent torque generated by the motor. In other words, the drive regulates output voltage in proportion to output frequency to maintain ideal motor flux and constant torque producing capability.

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