High-Performance Electric Motor for Motor Sport
Application
Giuseppe Volpe;
Fabrizio Marignetti
Department of Electrical and
Information Engineering
University of Cassino
Cassino, Italy
James Goss;
Mircea Popescu;
David A. Staton
Equipmake Limited
Norwich, United Kingdom
Motor Design Limited
Ellesmere, United Kingdom
Abstract—The objective of this paper is to introduce a new
high performance electric motor for a high speed and torque
motor sport application. A compact radial flux machine, with
double layer concentrated winding and interior permanent
magnets has been designed to achieve high torque with a low
weight. The motor is made with 18 copper coils, 6 per phase, prewound and placed in the slots, which are directly cooled by
forced oil. The motor is also equipped with a shaft spiral groove
cooling system for magnet temperature control.
Keywords—Brushless Motors; Permanent Magnet Motors;
Traction Motors; Motor Drives; High Performance; Design
Optimizaion;
I.
Ian Foley
INTRODUCTION
The sustainability is a concept becoming increasingly
important, with most of the attention given to the future air
quality. One of the biggest contribution to the air pollution and
to the global greenhouse effect is given by the emissions from
fossil fuel combustion [1]. With this in mind combined with a
passion for the world of motorsport, in 2014 the FIA Formula
E Championship started. During the first season, cars and
powertrains had to be the same for each of the participating
teams. For the second season cars remained the same, but the
powertrain technology has been opened up in season 20152016. Seven manufacturers have produced new motor, inverter
and gearbox solutions [2]. During the third season (20162017) Mahindra, Faraday, Andretti have chosen a six phase
motor with two or three speeds gearbox; Virgin, Venturi,
Jaguar, Audi opted for one motor with two or three speeds
gearbox; Renault has adopted a single speed gearbox
powertrain with one motor [3]. The use of a different speed
gearbox allows greater flexibility, while using a low speed
motor. However, each shift involves a small loss of time, the
regeneration of the battery stops and it increases the overall
weight of the car. Based on these considerations a single speed
solution is considered (Fig.1). Using a single speed solution,
the electrical machine has to be able to speed up to 10.00012.000RPM and to deliver high torque.
In racing cars, motors operate over dynamic drive cycles
which deliver peak performances for a short time. High
torque, low weight are required, which means high torque to
weight ratio. In these applications, a high torque/weight ratio
value for PM machines, able to run beyond 6000RPM is
around 15 Nm/kg (YASA 400) [4]. There are few low speed
machine (YASA 750 [5], Protean [6]) with a torque to weight
ratio greater than 20 Nm/kg but they can’t be used in a high
speed application without a gearbox.
Different motor configurations are used in conventional
automotive applications, where the main design objective is
not the optimization of the torque/weight ratio but the
performance/cost. In these applications the torque/weight ratio
value for PM machines is typically less than 10 Nm/kg
(Nissan Leaf 6.52 Nm/kg; Toyota Prius 7.84 Nm/kg) [7-11]
and for induction machine it is difficult to achieve 5 Nm/kg
(TeslaS)[12].
In the FIA Formula E each driver has two cars at their disposal
with 200kW available throughout. In race mode, the
maximum power is restricted to 170kW but the three winning
FanBoost drivers each receive an extra 100kJ of energy to be
used in a power window between 180kW and 200kW [13].
For these reasons a 200kW, high speed, interior permanent
magnet motor with high torque density, low weight has been
designed, manufactured and tested.
Fig. 1. Single speed Powertrain. [14]
978-1-5386-1317-7/17/$31.00 ©2017 IEEE
II.
DESIGN OVERVIEW
The machine design process is based on the target peak torque
of 450Nm, peak power of 200kW, maximum speed of 1000012000rpm. The choice of magnets configuration, number of
slots and poles are explained and validated trough FEA
analysis. Electro-magnetic and thermal results are reported,
accompanied by efficiency maps.
TABLE I.
pairs has been chosen. With this combination is possible to
obtain a balanced winding with a fundamental winding
coefficient of 0.945. A double layer winding is chosen, as this
slot/pole configuration is not feasible with a single layer
concentrated winding. The use of a concentrated winding
gives lower copper volume for the end winding, this leads to
less Joule losses, higher efficiency and less heat generation
[16]. The winding is made of 12 turns, coil throw of 1 with
two parallel paths shown in Fig.3.
HIGH PERFORMANCE MACHINE TARGET
Parameter
Value
Maximum Torque (Nm)
450
Maximum Speed (rpm)
>10000rpm
Maximum Current (Arms)
350-400
Output Peak Power (kW)
200-250
A. Magnets Configuration
The configuration of magnets has been studied to achieve high
torque density, using the least possible rare earth material. A
spoke rotor configuration has been chosen. The advantages of
a spoke rotor configuration can be explained through
comparison with a more typical IPM rotor geometry. Fig. 2
shows a comparison between the IPM rotor in the 2004 Honda
Accord [15] and a spoke rotor equivalent.
Fig. 3. Winding pattern.
C. Geometry details and Predicted Performance
The designed machine, based on the above considerations is
shown in Fig.4. To accurately predict the behavior in different
operational points 2D and 3D machine models are created.
Using Finite Element Analysis (FEA) and analytical models
the machine is simulated. Open circuit and on load flux
density at 4000RPM with a maximum current of 380Arms are
shown in Fig.5 Fig.6. Simulation results have been used for
stator and rotor dimension optimization, with a special
attention given to magnets demagnetization.
Fig. 2. Magnets configuration: a) IPM (left) b) Spoke (right).
In both configurations the stator is the same with the same
winding, type and pattern. The number of rotor poles is
unchanged, the only difference is the magnets position. The
spoke machine has been optimized to deliver the same torque
as the IPM machine. Considering the same input current of
250 Apk the delivered torque by both machines is around 135
Nm. It is possible to obtain the same maximum torque with a
magnets weight reduction of 25%. The reason of this behavior
can be seen in the flux lines path. In the first configuration,
considering an open circuit test, some of the flux is used to
saturate the bridge. In the second configuration, the magnet is
used more effectively, there is no leakage path for the magnet,
the flux in the airgap increases and more flux lines
concatenate the stator coils.
B. Electro-magntic configuration
Number of slots, poles and type of winding are fundamental
parameters to take into account for obtaining high
performance. A configuration made with 18 slots and 8 pole
Fig. 4. High-Performance machine radial view.
Fig. 5. Flux Density distribution at 4000RPM in open-circuit conditions.
Due to magnets disposition and compact axial length a nonnegligible magnet axial-end leakage is expected. To
understand the impact of this, different open circuit 3D
simulations are performed (one of them is shown in Fig.7). A
reduction of 10% of magnets remanence is used in 2D
simulations, to obtain a good agreement between 2D and 3D.
For reducing the magnets losses, a radial segmentation is used
in addition to an axial segmentation to minimize the eddy
current losses. Geometry details and dimensions are given in
Table II.
Fig. 6. Flux density distribution at 380Arms, 4000RPM.
TABLE II.
HIGH PERFORMANCE MACHINE PARAMETERS
Parameter
Value
Torque ratio per weight (Nm/kg)
15.5
Maximum Speed (rpm)
12000
Maximum Current (Arms)
380
Output Peak Power (kW)
230
Rated Efficieny (%)
+92
Stator OD (mm)
300
Stator Bore (mm)
220
Airgap(mm)
1
Axial active length (mm)
70
Rotor poles
16
Stator slots
18
Weight (kg)
30
D. Efficiency Map
The efficiency maps are calculated to obtain the maximum
performance. This is obtained with an optimal split of the
current in the d-q axis, hence changing the phase advance in
each condition. For this reason the Maximum Torque Per Amp
(MTPA) method is implemented in the calculation software
[17]. The efficiency map is reported in Fig.9. Maximum motor
efficiency of 97% is obtained between 4000rpm and 8000rpm
within a current range of 150-250Arms.
Fig. 7. 3D and axial view of flux density and flux lines obtained by 3D open
circuit FEA simulation.
Considering a magnet temperature of 65°C, the maximum
shaft torque delivered by the machine with 380Arms is
479.3Nm within the speed range 0-4000rpm (Fig.8), the
maximum shaft power of 236.8kW is obtained at 5400rpm.
Fig. 9. Efficiency map.
E. Thermal study and Predicted Behaviour
For the thermal study of the machine, the maximum winding
average temperature is set to 160°C and the maximum
magnets temperature is set to 140°C. The main objective is to
analyze the maximum performance that the machine can
deliver without exceeding winding and magnets maximum
temperatures. Three different cases are analyzed: 60 seconds,
120 second duty cycles and the continuous case.
Fig. 8. Shaft torque versus speed with different current values.
Fig. 10. Shaft torque versus speed conidering thermal limitation.
Fig. 12. First manufactured prototype.
The thermal envelope shows that machine continuous
performance is limited, particularly at high speeds. Machine
performance is limited, at low speed, by the maximum
winding temperature, hence the current is set to do not exceed
160°C in the winding. Increasing the speed there is an
increment in machine losses especially in iron and magnet
losses. When the magnets temperature achieves the maximum
set value of 140°C, the input current needs a bigger decrement
than before. An important thing to notice is that the maximum
torque, considering 60 seconds transients, is 476.1Nm which
is very close to the maximum deliverable torque.
Thermal behavior is predicted considering a maximum oil
flow rate in the shaft spiral groove of 10 l/min and a maximum
slot water jacket flow rate of 30 l/min considering an inlet
temperature of 60°C.
B. Test Bench Description
A specific test bench is created for testing the high
performance of the machine. It is equipped with two EVO
Motors [18] as load motors, which produce load torque against
the Motor Under Test (MUT) and to spin up the motor for
performing open circuit and short circuit tests. Between the
load motors and the MUT there is a torque transducer to
measure the torque given by the machine and a gearbox. At
the first stage the gearbox has 1:1 gear ratio, hence it does not
introduce a real ratio, but a different gearbox with a different
gear ratio is used to perform high speed tests. For motor
control a dedicated three phase inverter has been developed,
the whole system is shown in the Fig.13.
III.
EXPERIMENTAL TESTS
A. Manufacture
The proposed machine has been manufactured. It is made with
a segmented stator and a segmented rotor. Prototype rotor,
during the assembling process is shown in Fig.11. Fig.12
shows the built manufactured prototype.
Fig. 13. Test Bench Overview.
C. No-Load and Back EMF Mesurements
The measurement of the Back-EMF is performed at different
speed values. Tests are performed from 500rpm to 10000rpm,
the comparison between experimental results (Test) and
simulation (Model) are reported in Fig.14. The comparison is
carried out using the 2D model modified to take into account
magnet’s 3D leakage.
Fig. 11. Rotor during the assembling process.
IV.
CONCLUSIONS
An innovative high performance machine for motor sport
application is presented. The design and the choices taken are
shown and explained. The performance is predicted using
FEA software and efficiency maps are calculated. The
machine has been then manufactured and tested. The
comparison between simulations and test is carried out and the
results show a good agreement with what has been predicted
and what has been measured.
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Fig. 14. Back-EMF Comparison.
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Fig. 15. Torque Comparison.
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D. Torque Comparison
Different on load tests are performed, in different conditions,
with different current levels and phase advance angles. In
Fig.15 is reported the torque comparison between simulation
(Model) and experimental results (Test). The comparison is
carried out using the 2D model modified to take into account
magnet’s 3D leakage. Tests are performed with ten current
levels from 55Apk to 540Apk (380Arms). The measured
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maximum torque measured is 476.5Nm instead of 479.3Nm
with an error of less than one per cent.
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