| Features and Benefits Unit Options-Adaptive Frequency™ Drives
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. |
