3. Electric drive
Two approaches were investigated to minimize the weight of
the electric drive system.
(1) Water cooled high-speed motors provide low machine diameters
and low weights in combination with low motor currents at
a given operating point. As a consequence, low motor currents
and low copper mass for wiring are necessary for high power
applications. Unavoidably, high-speed systems show a considerable
drive gear with high transmission ratios due to kinematic
and kinetic restrictions. As long as low-voltage battery packs are
used, in addition to the heavy water cooling system and electronic
components such as boost converters, this will increase
cost and weight.
(2) Convective or fan air-cooled systems having a reduced gear
head might be an alternative solution. The absence of water
and the heavy gearbox, and missing boost converters result in
weight benefits for these low-voltage, high-current direct-drive
systems. This is contradictory to the increase in mass, because
of the increase of wiring and machine diameters.
The system overview of the realized electrical drive train for the
student racing car is shown in Fig. 8(a). The motor is driven by an
inverter to adjust speed or torque, and is fed by a DC link connected
to the battery packs. In braking phases, the motor works as a generator,
and recuperates mechanical energy back to a storage pack.
Thereby, an optional chopper circuit can activate a braking resistor
to limit the DC link voltage. The storage pack may consist of batteries
and supercapacitors, which limit voltage peaks and absorb short
time charge–discharge cycles [7]. There are two approaches for this
hybridization, whereby the external parallel hybrid (EPH), as an
external hardwired parallel connection of batteries, and capacitors
seems to be most promising [8]. For the motor, different types can
be chosen, although only a limited number may be applicable for
the project.
3.1. Electric motor
For a drive train in the racing car, different combinations of electric
motor, inverter and gear can be found. Traditionally, the electric
motor marks the starting point in comparing different solutions.
Selecting the components focuses on volume and weight of the
total system. Nevertheless, different aspects, such as cost and availability,
cannot be neglected, although they play a minor role in the
selection process. In this paper, the number of different motors has
been limited to the most common DC and AC motors practically
used:
• Induction Machine or Asynchronous Machine (ASM)
• Permanent Magnet Synchronous Machine (PMSM)
• Brushless DC Machine (BLDCM)
• Brushed DC Machine (BDCM)
• Switched Reluctance Machine (SRM)