Rev. Roum. Sci. Techn.– Électrotechn. et Énerg. Vol. 61, 1, pp. 3–7, Bucarest, 2016

Électrotechnique et électroénergétique

1 Icpe, Splaiul Unirii 313 Str., 030138, Bucharest, Romania, E-mail: bogdan.varaticeanu@icpe.ro 2 “Politehnica” University of Bucharest, Romania

PERFORMANCE COMPARISON BETWEEN SURFACE, SPOKE AND INTERIOR PERMANENT MAGNET MACHINES DESIGNED

FOR TRACTION APPLICATIONS

BOGDAN DUMITRU VĂRĂTICEANU1,2, PAUL MINCIUNESCU1, CONSTANTIN NICOLESCU1

Key words: Electric motor, Permanent magnet (PM), High speed, Interior PM machines, Finite element (FE) methods, Traction.

Three types of high-performance permanent magnet (PM) machines have been designed and analyzed in order to determine the ideal candidate for high speed traction application. The surface PM machine, spoke PM machine and interior PM machine were compared in terms of performances and capabilities at high speed operation. Finite element analysis was used for accurate performances prediction. For the sake of accuracy, the amount and type of PM and the stator structure are maintained the same in the design process for all three machines.

1. INTRODUCTION

The objective of this work is the design and comparison of three types of high power density, high efficiency, synchronous permanent magnet (PM) machines that will be part of the powertrain of an Electric Vehicle (EV). These three types of PM machines are: surface PM (SPM) machine, spoke PM (SpPM) machine and interior PM (IPM) machine. The performances of these PM machines are based on the requirements of FreedomCAR 2020 which is a US Department of Energy program for developing more energy efficient and environmentally friendly transportation technologies.

The required set of specifications for the PM machines is based on the automotive industry demands and is summarized in Table 1 [1, 2]. The challenging set of specifications require: continuous power of 30 kW over 20–100 % speed range, peak power of 55 kW for 18 seconds at 2800 rpm, base speed of 2800 rpm, maximum speed of 14000 rpm, minimum efficiency of 95 % over 20 % to maximum power and nominal voltage of 325 V dc.

Table 1 Summary of key machine performance requirements

of FreedomCAR program

Requirement Value Condition Continuous power 30 kW Peak power 55 kW for maximum 18 s Base speed 2800 rpm Maximum speed 14000 rpm Dc bus voltage 325 V Maximum phase current 400 A Peak line to line back EMF < 600 V at 2800 rpm Torque ripple < 5 % from peak torque

Efficiency > 95 % from 20 % rated power un to maxi-mum speed

The PM machines characteristics are calculated using an analytical method. This method is very fast for preliminary sizing and design and is closely related to the classical theory of machines, including the circuit theory [3]. The finite element method (FEM) is used to consider the nonlinear magnetic behavior of stator and rotor core and for a good performance prediction. FEM has proved to be the most powerful and widely used numerical method in electric machine design and analysis. This numerical

method is used to evaluate the PM machines final design. Demanding excessive computational time and resources, finite element analysis is not time efficient during all design procedures.

This paper presents and compares three PM machines which have been designed to meet the FredomCAR 2020 requirements. These PM machines are analysed in order to determine which one is the most attractive choice for the automotive industry. Due to the requested performances the machines need to have high torque, high efficiency and the capability to operate in a wide speed range.

2. DESIGN PROCEDURE

The first motor designed was the surface PM machine. The motor structure was optimized regarding high torque production in association with reduced torque ripple and low cogging torque. The effects of winding configuration were studied. Concentrated windings are known to generate a magneto-motive force (MMF) with a higher number of harmonics [4]. At high-speed operation these harmonics increase the eddy currents induced in the stator and rotor core as well as in the rare-earth PMs. In order to avoid high losses, a distributed winding was chosen. Due to the machine requirements, in this case 24 stator slots (Z) and 10 magnetic poles (2p) were chosen due to the advantages of this com-bination [5]. The cogging torque in this type of machine is very small because of the fractional ratio of slot number to pole number and the fact that the least common multiple between the number of slots and poles is large, while their greatest common divider is 1.

The parameters of the surface PM machine were calculated using an analytical method [6]. General design process for brushless ac machine is valid as well for this type of machine. The electromotive force per phase is:

wphph kfNE Φπ= 2 , (1)

where: Eph – phase electromotive force (EMF), f – frequency, Nph – number of turns per phase, Φ – magnetic flux and kw – winding factor. With Bg the amplitude of the first harmonic of the flux density in the air gap, the magnetic flux is:

Bogdan Dumitru Vărăticeanu, Paul Minciunescu, Constantin Nicolescu 2

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π⋅π

=Φ2

2 MgL

pDB , (2)

Fig. 1 – Stator structure and winding configuration.

where: D – outer rotor diameter, 2p – number of magnetic poles and LM – active axial length.

The EMF per phase becomes:

Mwgphph LDkBnNE 2π= , (3)

where n – speed, with s – the transversal section of the conductor, ku – the copper fill factor, Z – the number of stator slots and As – the slot area, then:

ZAksN suph =6 . (4)

The relationship between the electromagnetic torque and the machine dimensions is established:

nIE

M phπ

=2

3, (5)

where M – electromagnetic torque and I – the phase current. From (3), (4) and (5) the electromagnetic torque becomes:

Mgwsu JLZBDkAkM 42

= . (6)

The Joule losses are:

2/3 22 suwphJ AZJklIRP ρ== , (7)

where: Rph – phase resistance, ρ – the copper resistivity and lw – the length of one turn.

Synchronous motor with rare-earth PMs has high efficiency, high torque densities and high power densities. The amount of PM influences on a large scale the machine’s charac-teristics. This is also applicable for machines in which torque production is given by reluctance component and PMs component [7]. For an equitable design process of the interior and spoke PM machines, the stator configuration (magnetic core and winding), the number of magnetic poles, the type and quantity of PM, and the active axial length are kept unchanged for all three machines. For these two types of machines, the torque production is characterized by two components: the reluctance torque by means of saliency and the PM torque due to the interaction between PM and magnetic field generated by stator winding. The equation of torque production is given by [8]:

,

,23)(

23

qd

qmqdqdmagrel

LL

IpIILLpTTT

>

ψ+−=+=(8)

where: Trel – reluctance torque component, Tmag – magnet torque component, p – number of pole pairs, Ψm – the magnitude of flux that come from magnets, Ld, Lq, Id and Iq are the d and q axis inductances and currents respectively.

The rotor structure for interior and spoke PM machines was designed and optimised regarding shape and number of PM slots in order to obtain high torque with reduced ripple. The quantity of PM was kept unchanged compared with surface PM machine.

The general details of the PM machines are given in Table 2.

Table 2 PM machines details

Value

Dimension/Rating surface PM machine

spoke PM machine

interior PM

machine Number of stator slots 24 Number of magnetic poles 10

Stator outer radius 118.5 mm Stack axial length 80 mm Copper mass 5.2 kg PM mass 1 kg PM remanent flux density 1.2 T

Base speed 2800 rpm Maximum speed 14000 rpm Rated current amplitude 241 A Rated voltage 325 Vdc Number of phase 3 Winding type Distributed Rotor outer radius 71 mm Airgap thickness (physical) 0.75 mm*

*The surface PM machine has a bandage of 1.5 mm over the magnets, resulting a magnetically airgap of 2.25 mm.

For an accurate comparison, the stator structure and the winding configuration are the same for all three machines. In Fig. 1 the stator structure and the winding configuration are shown. Due to the motor periodicity, only half of the stator model is presented.

Besides the electromagnetic and thermal design, at high speed (14000 rpm) the most critical aspects of design goes to the mechanical analysis of the rotor structure [9]. It is important that rotor structural integrity to remain undamaged at top speed. The mechanical design of the rotor with surface-mounted magnets requests the usage of a sleeve (bandage) to withhold the PMs to the lamination stack. The operating principle of the retainer sleeve is to generate a compressive force to be applied to the permanent magnets at all operational speed, allowing the torque transmission between the magnets and the other rotor parts. The sleeve can be manufactured from different types of materials, such as high strength steel, fibre composites, titanium, Inconel, etc. The configuration of the rotor with spoke PM provides flux concentration effect. In order to hold and protect PM into the lamination stack, round end slot edges are used. These end slot edges should be able to sustain large operating stresses. For rotor with interior PM, the magnets

3 Surface, spoke and interior permanent magnet machines comparison

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are inserted in a slot below the surface of the rotor. The lamination stack bridges must withstand the centrifugal force produced by both bridges and magnets.

From a mechanical point of view, these narrow regions (retainer sleeve for surface PM rotor, round end slot edges for spoke PM rotor and bridges for interior PM rotor) are subjected to a substantial concentration of mechanical stress that can lead to a rotor failure at high speed.

Surface PM machine has the physical thickness of the airgap equal with the thickness of the other two machines. But due to the sleeve dimension the magnetically airgap of this machine is three time higher compared to the other two machines solution.

The rotor geometry satisfies the electromagnetic design criteria together with mechanical design criteria regarding the mechanical stress in the rotor parts. The stator geometry and winding configuration satisfy the electromagnetic design criteria.

3. NUMERICAL ANALYSIS

The performances of the designed machines were numerically analyzed by using the finite element method (FEM). The structures of the proposed machines were evaluated based on the 2-D FEM [10]. The results are presented in such a way that they underline the differences between the three machine types. The magnetic flux density distribution and the magnetic flux lines in the proposed machines are shown in Fig. 2.

(a)

(b)

(c)

Fig. 2 – Magnetic flux density distribution and magnetic flux lines in: (a) surface PM machine, (b) spoke PM machine,

(c) interior PM machine.

Fig. 3 – Phase back electromotive force.

The distribution of the magnetic flux density confirms the predictions of the analytical estimations. The PMs are parallel magnetised and have alternate polarity for each magnetic poles. The average magnetic flux density in the stator teeth is 1.65 T. At base speed and nominal current operation, the stator and rotor core do not experience the magnetic saturation.

In order to reduce the computation time, a partial model of the machine was used. To create the partial model, periodic conditions were imposed on the periodicity plan. On the periodic boundary the homogeneous Neumann boundary condition δA/δn = 0 is imposed in order to ensure the problem symmetry, where A is the magnetic vector potential. On the exterior air boundary the Dirichlet condition A = 0 is imposed.

3.1. NO-LOAD OPERATION

The predicted performances for PM machines running at no-load condition are examined regarding electromotive force and cogging torque. In Fig. 3 the back electromotive force of one phase at rotor speed of 1000 rpm is shown. The waveforms of the EMF are different for each machine type. For spoke PM machine, the induced voltage waveform has an almost trapezoidal shape. Even if the magnetic airgap for surface PM machine is 2.25 mm and is three times higher compared to the other machines; the induced EMF has the same average value of 23 V for all the motors. However surface PM machine has the most sinusoidal shape of the EMF waveform. For this type of machine the PMs are placed directly in the airgap, and the interaction between the magnetic field generated by the PMs and the winding wires is more easily achieved.

3.2. LOAD OPERATION

The predicted performances for PM machines running at load condition are examined regarding electromagnetic torque, torque ripple and the capability to operate in a wide speed range. In Fig. 4, the electromagnetic torque at a rotor speed of 28000 rpm is shown.

The FEM simulation for torque waveforms were done using the same current amplitude for all three machines. For spoke PM machine and for interior PM machine, the maximum torque will occur when the current is shifted by an angle (current phase angle) relatively to the q axis [11].

Bogdan Dumitru Vărăticeanu, Paul Minciunescu, Constantin Nicolescu 4

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The current phase angle for which the average torque has the maximum value is 50 deg for SpPM machine and 30 deg for IPM machine. This current phase angle is necessary to be imposed due to the fact that torque production in such machines is characterized by two components: the reluctance torque due to saliency and the PM torque due to the interaction between PMs and stator currents. The IPM machine has the average torque value of 111 Nm, being the highest value compared to the other two types of PM machines. However for this type of machine the torque has a high ripple. The SpPM machine has the lowest torque ripple with an average torque value of 102 Nm.

Fig. 4 – Electromagnetic torque waveforms.

Fig. 5 – Torque versus speed.

The PM machines designed for traction application have two distinct regions of operation: the constant torque operation below the base speed and the constant power region above the base speed. The base speed is defined as the maximum speed to which the rated torque can be maintained constant, without exceeding the rated voltage [12]. The torque versus speed for the PM machines, simulated at maximum output power conditions are shown in Fig 5. The IPM machine has the highest torque value. The SPM has the highest constant torque region. On the other side, the SpPM machine has the lowest torque. The SPM machine has the lowest flux weakening range and it is not a good choice for application that requires high speed.

In the above figure the maximum output performances is shown, in terms of torque vs. speed characteristic which can

be developed by all three machines when these are supplied with nominal current and nominal voltage. However in the real life operation, above the base speed the current amplitude will be limited, in order that the PM machines do not exceed the rated power of 30 kW.

In Table 3, the simulation results regarding the EMF, cogging torque, torque value, torque ripple and torque vs. speed curve are presented. For low speed operation, the SPM machine has very good performances regarding torque production and torque ripple. The limited flux weakening capabilities do not recommend this machine for high speed applications. An unexpected issue is that the SpPM machine has the lowest performances regarding torque production of all three machines. This is due to the restricted PM mass imposed in the design process. Due to flux concentration effect, the SpPM machine can offer very good performances in applications that do not impose quantity restriction in using PMs. Alternatively IPM machine has very good characteristics regarding torque production and torque vs. speed curve. The electromagnetic torque ripple has a high value, but by using different methods the ripple can be reduced. However in traction application the ripple of the electromagnetic torque is not a very restrictive issue. The flux weakening capabilities recommend SpPM and IPM machines for applications that require a high speed range.

Table 3 Simulation results

Value

Rating surface PM machine

spoke PM machine

interior PM machine

Electromotive force 22.5 V 22.7 V 23.6 V Cogging torque (peak value) 0.09 Nm 0.42 Nm 0.17 Nm

Electromagnetic torque 105 Nm 102 Nm 111 Nm

Torque ripple 1 % 5 % 13 % Base speed 5300 rpm 5000 rpm 4600 rpm Maximum speed 12000 rom > 14000 rpm > 14000 rpm

4. CONCLUSIONS

The performances comparison between surface PM machine, spoke PM machine and interior PM machine is presented in this paper. The stator and winding structure, the quantity and type of PM were kept the same in the design process for all three proposed machines.

The SPM machine cannot be considered an ideal candidate for applications that require extended speed range by using flux weakening control. Nevertheless, for low speed, this type machine has very good performances regarding the sinusoidal waveform of the EMF, torque production and torque ripple.

For high speed traction applications, the most robust candidates are represented by SpPM machine and by IPM machine. The restricted quantity of PM makes the SpPM machine to have slightly weaker performances compared to IPM one. However, both machine types have very good performances at high speed, easily exceeding the imposed speed of 14000 rpm.

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ACKNOWLEDGMENTS

This work was founded by the Sectoral Operational Programme Human Resources Development 2007–2013 of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/134398.

Part of the research results presented in this paper have been obtained in the context of the EC Marie Curie IAPP project DemoTest-EV, http://www.demotest-ev.com/ coordi-nated by the Technical University of Cluj-Napoca and with as partners LMS International, the Universite Libre de Bruxelles and ICPE.

Received on October 17, 2015

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