University of L’Aquila
Department of Industrial and Information
Engineering and Economics
Permanent Magnet-assisted Synchronous
Reluctance Motors for Electric Vehicle applications
A. Ometto, F. Parasiliti, M. Villani
9 th International Conference
“Energy Efficiency in Motor Driven Systems” EEMODS’15
Helsinki, September 15 th – 17 th 2015

Electric Vehicles represent the most viable solutions to solve the problems associated with the traditional internal combustion engine motors and different typologies of electric motors are proposed.
Moreover, the progress in power electronic makes it possible to realize direct-adjustable-speed drive machines with a wide operating speed range.

The strong demand of high performance electric motors for automotive application requires the use of:
 innovative and efficient design procedures, by specific tools and optimization processes;
 accurate choice of the materials and electrical steels; in order to fully satisfy the hard specifications and constraints in terms of performance, encumbrance, weight, reliability and cost.

The main requirements of electrical machine for traction are:
• high torque and power density;
• wide speed range;
• high efficiency over wide torque and speed range;
• wide constant power operating capability;
• robustness and reliability;
• reasonable cost.

Main requirements
High Torque
Torque
Power
High Power
0 base speed
High speed speed

Types of EVs Motors
Induction motors
They are widely accepted for EVs because of their low cost, high reliability, and freedom from maintenance.
PM motors
Most EVs use PM synchronous motors and they are becoming more and more attractive and can directly compete with the induction drives. The advantages of
PM motors are their inherently high efficiency , high power density and high reliability .

PM Rotor geometries (interior PMs)
MP
PM
PM
V-shape PM
The key problem is their relatively high cost due to PM materials.

The recent increase of rare-earth PMs cost has led the manufacturers to choice “ low-cost ” motors. This has oriented the designers to investigate alternative solutions without penalizing the motor performance
“Magnetless” motors or motors with low-cost PM


Synchronous Reluctance motors (SRM)
PM-assisted SRM

These motors with multi-barriers rotor structures have been obtained a great interest in brushless AC drives.
Advantages :
 no winding and PM in the rotor (“cold” rotor),

 low inertia, good acceleration performance,

 good flux weakening operation, low manufacturing cost.
Disadvantages :

 low power factor; torque ripple.

Flux-barrier
Flux barriers Rotor electr. steel
Iron bridge
Saliency ratio k s
= L d
/L q

5

8
The torque produced by the SRM is due to the anisotropy of the rotor.
The number of rotor flux barriers affects the anisotropy, so as this number increases → the reluctance torque component increases.

Prototypes of SRMs (by UnivAQ)
2 barriers
4 barriers
Laminated rotors with flux barriers can be manufactured with normal punching tools at very low cost.

d-q axis theory can be used to analyze the electromagnetic performance of the SRM.
T

3
2 p
 
L d

L q
I
 d
I q

Reluctance Torque
The Torque of motor can be varied by means of an accurate control of the d-q axis currents (
“ Vector control ”).

Voltage equations (R

0):
V d
V q
  
 
L q
I q
L d
I d

L q
I q q-axis
V

L d
I d
I q

I q d d-axis
I d
The voltage vector exhibits a large phase difference from the current vector and this means that the power factor ( cos

) is low !

In order to improve the operating performance of the SRM
(torque density, power factor) it is useful to add proper quantity of permanent magnets into the flux barriers of the rotor core, and particularly cheaper PMs, such as Ferrite . In this case, the motor is called PM-assisted SRM .
The PM-assisted SRM produces a torque

20÷30% higher respect to the SRM (without PM).
The amount of the Ferrite placed in the rotor core is limited by the geometry of the rotor and manufacturing cost which is considered as one of the design constraints.

The use of the PMs in the flux barriers allows to reduce the q-axis flux (without affecting the d-axis one) and then to improve the torque and power factor.
Conventional with PM
3

PM-assisted SRMs become attractive for EVs applications:
Low cost of Ferrite;
- Easy to handle;
- High efficiency;
- High power density;
- Good power factor
( size of the Inverter ).

CONFIGURAZIONI
(
?
SRM
Coppia
Torque Nm
?

Torque %
?
?
( cos
?
) cos
 Ripple %
0.711
4.0%
SRM
0.867
4.5%
PM_ass
0.904
4.0%
PM_ass

T

3
2
[( d

) q d q
 
I mag d
] q-axis

(L q
I q
-
 mag
)
V

L d
I d
I q

I d-axis
 mag
I d
The PM allows to reduce the angle between voltage and current vectors and this increases the power factor respect to conventional SRM.

The design of PM-assisted SRM for EVs requires the use of innovative and efficient design procedures, by using specific tools and optimization processes, in order to fully satisfy the specifications and the constraints on the encumbrance .
Optimization procedures + Finite Element Analysis
Objective: • max Torque density
• max Efficiency
• combinations of more Obj.

Design Optimization procedure by FEA x2 x1 x3 x8 x7 x6 x5 x4 x9 x10
Design variables (X)
Optimized design
Preliminary design
Optimization
Algorithm
X k
F(X)
FEA
F(X k
)
Minimum ?
k = k+1

Design of PM-assisted SRM for EV: case study
Specifications
DC voltage supply
Base speed
Torque @ base speed
Output Power
Max speed
Torque @ max speed
Axial core length
Outer stator diameter
Stator winding
PM-Ferrite
Cooling
V rpm
Nm kW rpm
Nm mm mm
500
4000
200
83.8
12000
60
100
240 flat-wire
Br=0.35 T; Hc=270 kA/m
Liquid-cooled

Stator winding with flat wires (harpins)
For this application
(→ high torque density motor) the stator winding with flat wires has been chosen.
This solution requires rectangular slots.
Stators with flat wires
Advantages : - high “slot fill factor” (up to 0.80÷0.85);
- reduction of winding overhang;
- high quality process.

Details of stator core with flat wires
In this case, the phase resistance should be calculated taking into account the “proximity and skin-effects” that heavily depend on the frequency and flat-wire size.
In co-operation with:

Cross-section of the optimized PM-assisted SRM
6 pole - 54 slots
• flat wires
• slot fill factor = 0.80
The iron bridges in the rotor core have been careful sized since they have impact on the motor performance and rotor robustness. Moreover, resin can be inserted in the flux barriers in order to improve the robustness of the rotor structure against the centrifugal forces at high speed.

Choice of the electrical steel
High performance motor requires a right choice of the electrical steel and this is an important step during the sizing procedure. The requirements on electrical steels are:
low losses;
high permeability.

Different commercial non-oriented fully-processed materials have been tested and compared using the manufacturers data.
400-50 AP
530-50 AP
330-50 AP
800-50

Comparison of different electrical steels
800-50
Torque Nm
Speed rpm
Frequency Hz
Output Power kW
Phase current Arms
AC Joule losses W
Core losses W
Efficiency %
Power factor
Bteeth ; Byoke T
164
2337
735
95.4
0.87
1.82; 1.60
530-50 AP 400-50 AP 330-50 AP
200
4000
200
83.8
161
2258
620
95.6
161
2258
553
95.7
163
2317
423
95.7
0.89
0.89
0.88
1.82; 1.60
1.83; 1.60
1.83; 1.61
The electrical steel 400-50 AP is the most suitable choice because combines low specific losses with high permeability and the motor presents good performance in terms of efficiency and power factor; the 400-50 AP has been preferred for this specific application.

Performance of the PM-assisted SRM
• T
CU
• T
PM
= 90 °C
= 70 °C
Phase current
Torque
Output Power
AC Joule losses
Power factor
Efficiency
Arms
Nm kW
W
%
4000 rpm
161
200
83.8
2258
0.89
95.7
12000 rpm
161
64
80.4
2574
0.86
94.6

200 Nm, 4000 rpm
Flux density
64 Nm, 12000 rpm
(T)

Torque Nm
Torque and Power vs. Speed rpm
Power kW
CPSR rpm

Comparison with IPM synchronous motor
The proposed PM-assisted SRM has been compared with a PM synchronous motor with Interior PM (NdFeB-N38SH) in order to evaluate the differences in terms of performance, weight and costs.
PM-assisted
SRM
IPM
Ferrite
6 pole, 54slots
NdFeB

The comparison has been carried out considering the same overall dimensions and winding. In particular the two motors have:
 the same stator lamination (diameters and n. of slots);
 the same air-gap ;
 the same number of turns and wire size;
 the same electrical steel (400-50 AP);
 the same temperatures of the winding and PMs.

Two different IPM motors have been proposed:
IPM_1
IPM_2 with the same stack length of the
PM-assisted SRM; with a reduce stack length (compact design) and the same current of
PM-assisted SRM.

PM-assisted SRM vs. IPM-NdFeB (same stack length)
PM
Stack length mm
Outer stat. Diameter mm
Phase current Arms
Torque @ 4000 rpm Nm
4000 rpm
Output Power
AC Joule losses kW
W
Power factor
Efficiency %
12000 rpm
Torque @ 12000 rpm Nm
Output Power kW
Current density A/mm 2
PM-ass SRM
Ferrite
100
240
161
200
83.8
2258
0.89
95.7
64
80.4
10.1
IPM_1
NdFeB
100
240
150
200
83.8
1945
0.94
96.2
73
91.7
9.4

PM-assisted SRM vs. IPM-NdFeB
PM
Stack length mm
Outer stat. Diameter mm
Phase current Arms
Torque @ 4000 rpm Nm
4000 rpm Output Power
AC Joule losses kW
W
Power factor
Efficiency %
12000 rpm
Torque @ 12000 rpm Nm
Output Power kW
TRV kNm/m 3
PM-ass SRM
Ferrite
100
240
161
200
83.8
2258
0.89
95.7
64
80.4
32.0
IPM_1
NdFeB
100
240
150
200
83.8
1945
0.94
96.2
73
91.7
36.5
IPM_2
NdFeB
83.8
2131
0.90
95.9
91
240
161
200
79
99.3
43.2

Torque Nm
PM-assisted SRM vs. IPM-NdFeB
IPM_2
IPM_1
SRM_Fe rpm
Power kW rpm

Weight and Cost comparison (active materials)
Stack length
Gross iron mm kg
Stator winding kg
PM kg
PM-ass SRM
100
45
6.2
0.92
IPM_1
100
45
6.2
0.93
IPM_2
91
41
5.9
0.85
Cost (*):
Gross iron Euro
Stator winding Euro
PM Euro
Total Euro
40.5
43.4
23.0
106.9
40.5
43.4
111.6
195.5
36.9
41.3
102.0
180.2
- 45%
- 40%
(*) Premium steel = 0.90

/kg; Cu = 7.0

/kg; Ferrite = 25

/kg; NdFeB = 120

/kg

Comments
 The IPM motors have higher power factors and this allows the inverter rating to be reduced.
 At high speed (12000 rpm) the IPM motors exhibit good performance with a power density (and TRV) higher than the synchronous Reluctance motor one.
 The PM-assisted SRM has excellent efficiency, very close to that one of the IPM_2, and good constant-power operating capability.
 The cost reduction for the PM-assisted SRM respect to IPM motors is mainly due to the lower cost of the PM in Ferrite.
 Ferrite PM has got a positive reversible temperature coefficient of coercivity, respect to NdFeB, and this increases the demagnetization strength as the temperature increases, leading to better dynamic performance of car.

Conclusions
The Brushless motors are gaining a growing interest thanks to their power density capability , high efficiency and high reliability . Moreover, the progress in power electronic makes it possible to realize directadjustable-speed drive machines with a wide operating speed range.
The demand of high performance electric motors for automotive applications requires the use of innovative and efficient design procedures, by using specific tools and optimization processes, and accurate choices of the materials and electrical steel .
PM-assisted Synchronous Reluctance ensures good performance with high power density, high efficiency and reasonable cost and then it can be considered a strong potential for powertrains and an efficient alternative to IPMs and Induction motors.