Proposed solutions for electric vehicles range from the simple single-motor drive
coupled to one axle through a mechanical differential, to more complex
solutions, such as four in-wheel motors, which ask for electronic torque
vectoring. Main reasons for having more than one electric machine are: reduction
of the rated power of each motor, which most likely leads to simplification and
cost reduction of all the electric drive components; increased reliability of
the overall traction system, enhancing fault tolerance ability; increase of the
degrees of freedom which allows for control strategy optimization and efficiency
improvement. In particular, electrical machines efficiency generally peaks at
around 75% of load and this usually leads to machine downsizing to avoid
operation in low efficiency regions. The same output performance can be achieved
by using two or more electrical machines, rather than only one, of smaller size
and running them at unequal load - one of the machines at higher load and the
other(s) at lower load.
In this paper, the performance of an electric vehicle with multiple electric
machines is analyzed to assess the potential of overall drive train efficiency
increase. In particular, the powertrain drive system comprises of multiple novel
compact and high-efficiency high-speed electrical machines and a three-stage
constant ratio high-speed gearbox, which can be adapted to different
configurations of the powertrain. In particular, the gearbox has a modular
design which allows connecting it to one single electrical machine and the axle
or to two electrical machines and the axle. Therefore, two different integrated
drivetrain modules can be realized: a module composed by two motors connected to
one single gearbox and a drive module composed by one motor and one gearbox.
Different combinations of these modules within the powertrain will be compared
in terms of overall drivetrain efficiency in order to assess the potential of
the proposed solutions against the simplest single-motor-drive powertrain.