Powering Freight & Transportation - Power Systems Design

58
Powering Powering Freight Freight & Transportation
& Transportation
One Step Closer to
the Birds
Glider uses Vicor battery-controlled electrical
propulsion to take to the air
Gliding is one of the most exhilarating sports in the world. It offers a direct experience of air, wind and
weather in a seemingly limitless space, and comes closest to man’s vision of the freedom of a bird’s
flight. Crucial to this type of flying is the need to gain height in order to reach a thermal. Traditionally, a
winch or a tow plane is used to pull the glider to sufficient altitude so that the fun of gliding in search of
a thermal can begin. The requirement to return to the home airfield without additional support limited the
use of gliders, and made careful flight planning a necessity.
By Marco Panizza, European Applications Engineer, Vicor Europe, Germany
Auxiliary propulsion systems
brought an increase in range and
flexibility, and made it possible
to start gliders without additional towing
tools. A conventional solution was
using combustion motors. However, the
performance of a combustion motor de-
creases with its operating altitude. Combustion-powered
propulsion systems
must be oversized in order to deliver the
desired power at all operating altitudes,
and impose an important weight and
noise burden on the glider. Additionally
combustion engines generate substan-
Figure 1: The Antares 20E glider in climb. The aircraft is driven by a propeller powered
by an electric motor on a hinged carrier beam.
tial heat, and must be allowed to cool
down before being retracted into the
fuselage.
The design team at Lange Flugzeugbau
in Zweibruecken, Germany wanted
to set pilots free by adding an electric
engine for self-powered takeoff
and climbing. The company
used Vicor modules to design
the Antares 20E, the first glider
to receive the prestigious EASA
(European Aviation Safety
Agency) type certification for
an electrical propulsion system,
and one of only three electrically
powered gliders on the market.
The glider’s easy to use retractable
electric propulsion solution
is more reliable, quieter, and
produces less vibration than
traditional combustion engines.
Offering high performance
independent of operating altitude,
electric motors provide
the safest and most convenient
choice for power. By integrating
the entire battery charging
circuitry inside the plane, Lange
produced a completely self-reliant
electrically-powered glider
that can make long-distance
Power Systems Design October 2008
flights, and be recharged at any airfield.
The need for good aerodynamic
performance, however, imposes strict
limits on the weight of the entire system,
particularly that of batteries and charging
circuitry.
Design challenges
One of the challenges in the design
of the glider was the integration of the
battery charging subsystem. The entire
system had to be self-reliant in order to
enable long-distance multi-lap flights
without the need for an external charging
unit at the airfield. The propulsion
motor has a nominal power of 42kW
and operates on a voltage of 288V. The
battery system has to provide enough
power for five minutes of operation or
about 3000m of climbing altitude, which
translates into an overall battery capacity
of 11kWh. Charging has to use a
normal single-phase mains connection,
and a full charging cycle needs to be
completed overnight.
The glider uses Li-Ion battery cells
which require tightly controlled operating
conditions to deliver a consistent,
reliable power output. In order to offer
maximum capacity, the cells must be
operated between 20 and 40°C, so
temperature sensing and cell heating
had to be implemented. Additionally all
cells must be kept on the same charging
state, requiring cell voltage monitoring
circuitry to ensure that all cells
have identical cell voltages. Before any
charge cycle, all cells are discharged to
have the same cell voltage with a tolerance
of only ±20mV ensuring that all
cells will be charged evenly. In practice,
charging is several activities that must
be performed in sequence:
• Cell voltage monitoring and selective
discharging until all cells have the same
voltage within the specified tolerance
• Cell heating until the desired operating
temperature is reached
• Battery array charging until the total
voltage reaches 288V
In addition, the charging electronics
have to fit into the overall system of the
glider. This means that the circuitry has
to meet rigorous, weight, heat management
and size specifications.
www.powersystemsdesign.com
Powering Freight & Transportation
Choosing a charging
module vendor
Since total glider weight has a direct
impact on its flying performance, all
components in the aircraft had to be optimized
for lowest possible weight. Commercially
available chargers that met the
specifications would weigh about 10kg
including mains front-end and cabling –
much too heavy for integration in a glider
with a maximum total takeoff weight of
660kg. As Lange’s design engineers
could not find ready-made charging
units that met the weight requirements,
a new charging subsystem had to be
developed.
The charging subsystem essentially
consists of a mains front-end and a high
voltage, high-power DC-DC converter
with programmable output voltage.
Due to space and weight limitations,
the circuits had to offer high efficiency,
as a major factor in the overall weight
Figure 2: Charging system block diagram. The front-end and the power section
are housed in two separate cabinets and are mounted in the glider’s fuselage.
The front-end consists of the EN1C21 Vicor modules while the power section
comprises Vicor Module V300B48T250BL Vicor DC-DC converters. The power
section is connected to the battery array, which is installed inside the wings.
59