The most common method to control a motor is by using a motor starter. Trane Adaptive Frequency drives* provide motor control, but they are much more than just starters. They also control the operating speed of the chiller compressor motor by regulating output voltage in proportion to output frequency. Varying the speed of the compressor motor can translate into significant energy cost savings.

Benefits

Reliable, Optimized Compressor Efficiency for Energy Savings

Conventional chillers use inlet vanes to provide stable operation at part-load conditions. Capacity is reduced by closing the vanes while maintaining a constant motor speed. The drive can be used to significantly reduce power consumption by reducing motor speed at low load conditions. Trane patented AFD Adaptive Control ™ logic safely allows inlet guide vane and speed control combinations that optimize part-load performance.

To Avoid Mechanical Stress

Controlled "soft" start with linear acceleration results in limited starting current to eliminate motor stress, reduce power line disturbance and provide a lower power demand on start. Reduced motor speed as a result of reduced chiller load means less current drawn, less heat generated, increased motor winding life. This translates into longer time between compressor maintenance and less downtime throughout the life of the machine.

Application

Certain system characteristics favor installation of an AFD because of energy cost savings and shorter payback. Among them are:

A large number of part-load operating hours annually The following chart based on a CVHE500, 500-ton load at standard ARI conditions, shows that major kW savings occur at part-load conditions, typically below 90 percent load.

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 ripple.
• Inverter — converts the DC voltage into a synthesized output AC voltage. This synthesized output controls both the voltage and frequency.

A fourth element of AFD design is the control logic which is the intelligence for the power section.

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.

*Limited availability for International orders - Please contact International CenTraVac Marketing Group.