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Design Procedure for Low Cost, Low Mass, Direct
Drive, In-Wheel Motor Drivetrains for Electric and
Hybrid Vehicles
Howard C Lovatt1, Darrell Elton2, Laurence Cahill2, Duc Hau Huynh2, Alex Stumpf2, Ambarish Kulkarni3, Ajay Kapoor3,
Mehran Ektesabi3, Himani Mazumder3, Thomas Dittmar4, and Gary White5
1Commonwealth Scientific and Industrial Research Organisation (CSIRO), 2La Trobe University, 3Swinburne University of
Technology, 4Victorian Partnership for Advanced Computing (VPAC), and 5Automotive Cooperative Research Centre
(AutoCRC)
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Abstract-Direct drive, in-wheel motors are ideal for electric and
hybrid vehicles because the packaging of the drivetrain is so
simple, because drivetrain losses are eliminated, and because
individual wheel control improves handling and safety. In
applications where cost is not a constraint, e.g. solar car racing,
direct drive, in-wheel motors are the norm. In-wheel motors are
also regularly demonstrated in concept vehicles. However, inwheel
motors are not used for production vehicles because of
their high cost and high-unsprung mass. This paper describes a
project that addresses these issues through the use of a novel,
multiple-airgap, axial-flux, switched-reluctance motor with
optimized packaging and low cost electronics. The emphasis of
the paper is on how to design the system as a whole.
I. INTRODUCTION
This paper describes the outcomes from the first stage of a
larger project funded by the Automotive Cooperative
Research Centre (AutoCRC) Visionary Project [1] and the
authors’ institutions. This first stage is the design of the drive
system components. The second stage, which is just
beginning, is the construction of the drivetrain. The third
stage is fitting the drivetrain to a prototype vehicle. In this
paper emphasis is given to the design procedure rather than
the outcome for each individual unit of the drivetrain, though
each unit is described and some details are provided where
relevant to discussing the design procedure.
Direct drive, in-wheel (DDIW) motors are recognized as
ideal for vehicles, from an efficiency, packaging, handling
(other than unsprung mass), and safety perspective. So too are
their drawbacks of cost and mass; e.g. [2]. In cost-insensitive
applications, e.g. solar car racing, DDIW are the norm, e.g.
[3]. In these applications, the drawback of mass can be
eliminated at the expense of increased cost by using ‘exotic’
motor types, e.g. ironless, Halbach magnet array, axial-flux
motors [4]. Concept vehicles have featured DDIW motors,
e.g. [5]. Tellingly production vehicles, even when the vehicle
manufacturer has DDIW technology, have used a single
electric motor driving a differential typically through a fixed
speed reduction gear, e.g. [6]. Therefore developing a low
cost and low mass DDIW motor is challenging and hence its
inclusion in the AutoCRC’s Visionary Project suite.
The project is not yet complete and therefore success
cannot be guaranteed, however the project has progressed
well and preliminary data can be given along with a
description of how the design is formulated in terms of an
optimization problem. It is hoped that articulating our design
formulation will be beneficial to others working on wheel
motors.
The authors speculate that previous DDIW motors are
either/both too expensive or too heavy because of two factors:
1. Inappropriate choice of technology
2. A focus on components instead of the system
The choice of technology is discussed in light of the
specification in the following sections.
Instead of focusing on a particular aspect of the drivetrain,
e.g. the motor, a holistic approach is taken to the whole drive
system in this project. This allows an overall optimum to be
found for the complete system for a given weighting and/or
limits of system cost and wheel mass. The starting point of
such an optimization is the specification, which defines limits
and weightings for design parameters and/or derived
quantities.
Having defined the optimization parameters and limits, the
individual components (motor, wheel, power electronics,
system electronics, and battery pack) can each be designed
and each component is addressed below. Although designed
separately on the basis of the optimization parameters the
individual items are interlinked by the motor component:
1. The wheel has to house the motor and therefore the
motor and wheel are mechanically linked
2. The power electronics control the motor and the
ratings and detailed operation of the controller are
defined by the motor
3. The system electronics coordinate all the units and
the limits of its control are defined by the motor
4. The ratings of the battery pack depend on motor
performance
The start of the design processes is the specification.