CTV-PRB004-EN
25
Low-Voltage Starter Types
Low Voltage—Unit-Mounted Adaptive Frequency Drive
TheTrane Adaptive Frequency Drive is a refrigerant-cooled, microprocessor controlled design.The
AFD is used in lieu of a constant-speed starter and is currently available for use with 460 or 480 volts
only. Adaptive Frequency is a trademarked term for the Trane
®
variable-speed drive, using
proprietary control logic and made to Trane specifications.
About the Trane AFD
The AFD is unit-mounted and ships completely assembled, wired, and tested from the factory.The
AFD controller is designed to interface with the chiller controller. It adapts to the operating ranges
and specific characteristics of the chiller.The optimum chiller efficiency is created by coordinating
the compressor-motor speed with the compressor inlet guide vanes.The chiller controller and the
AFD controller work together to maintain the chilled-water setpoint and avoid surge. If surge is
detected, the chiller controller’s surge-avoidance logic in the chiller controller makes the proper
adjustments to move the operating point away from surge.
How it works
The frequency drive regulates output voltage in proportion to output frequency to maintain ideal
motor flux and constant torque-producing capability. Or put simply, a variable-speed drive controls
load-side frequency and voltage to adjust the compressor motor speed. The AFD is a voltage-
source, pulse-width modulated (PWM) design. It consists of three primary power sections as
shown in
Figure 17
: the active rectifier, the DC bus, and the inverter.
Rectifier (active).
Takes incoming AC power, filters it with an LCL filter (not shown), and then
converts it to a fixed DC voltage. The insulated-gate bipolar transistor (IGBT) active rectifier
significantly reduces the amount of line-side harmonic levels and the amount of ripple on the DC
bus. The active rectifier also has some traditional post-generation filtering capabilities to further
smooth out remaining line-side harmonics.
DC bus.
Capacitors store the DC power provided by the rectifier until it is needed by the inverter.
Inverter.
Converts the DC voltage into a synthesized AC output voltage.This synthesized output
controls both the voltage and the frequency.The synthesized output waveform consists of a series
of pulses, hence the “pulse” in PWM.
Starting sequence
Trane AFDs are programmed to start the compressor motor using low frequency and low voltage,
thereby minimizing the inrush current. The motor is then brought up to speed by gradually
increasing both frequency and voltage at the same time.Thus, current and torque are much lower
during startup and motor acceleration than the high current, high torque associated with across-
the-line or even reduced-voltage starters; refer to the inrush current vs. time graph (
Figure 6, p. 18
).
Figure 17. AFD power sections
M
Rectifier DC Bus Inverter
Determines line-side harmonics
Determines load-side harmonics
M
Rectifier DC Bus Inverter
Determines line-side harmonics
Determines load-side harmonics
CTV-PRB004.book Page 25 Sunday, December 18, 2011 6:39 PM
