2003 design of a new axial flux permanent magnet generator for hybrid electric vehicles

8/14/2019 2003 Design of a New Axial Flux Permanent Magnet Generator for Hybrid Electric Vehicles

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Design of a New Axial Flux Permanent Magnet Generator

for Hybrid Electric Vehicles

P. Van TichelenVito

Boeretang 200

2400 Mol

BelgiumTel: +32 14 335883

Fax: +32 14 [email protected]

E. PeetersVito

Boeretang 200

2400 Mol

BelgiumTel: +32 14 335923

Fax: +32 14 [email protected]

This paper discusses the development of a new axial flux

permanent magnet generator that was specially designed to

achieve simple robust construction with a low weight-to-power

ratio, high efficiency and a controlled output voltage and

frequency over a wide range of speeds. By using a set of standard

components and changing only a few key design parameters, a

whole series of generators can be designed with various system

parameters (power, efficiency, weight). A model based on

analytical and finite element calculations is presented togetherwith the test results of a mono-phase prototype that was built.

Based on the calculation model, various machines were designed

for application in electric vehicles. The effects of different system

parameters such as efficiency, weight-to-power ratio and

dimensions of the machine were evaluated for a series hybrid

electric van that was built at Vito.

Key words: permanent magnet, generator, HEV.

I. INTRODUCTIONThe use of permanent magnet (PM) machines has become attractivefor hybrid electric vehicles (HEV) because the available permanent

magnet materials have high coercive field strength and temperatureresistance, and are price competitive [4,7]. In addition, the requiredpower electronic converters for output power control have undergonea major evolution.

For use with an internal combustion engine (ICE), flywheel or

pancake generators can be compactly integrated and many designsare presented for HEVs with Integrated Starter Generators (ISG) [3,12]. Most permanent magnet motors and generators have a radialmagnetic field to interfere with the stator windings but in the case of

flywheel generators, axial flux machines, in which the magnetic fieldis parallel to the rotational shaft, are possible too [3,4,11]. There are

also many alternatives for the design of axial flux or disk-type PMmachines: with or without armature slots, with internal or external

PM rotors, and with surface-mounted or buried permanent magnets[3,11].

Vito developed a new axial flux machine with U-shaped stator coilsand interior cylindrical PMs. This was named AXIFUS, for AXIalFlux generator with U-shaped Stator coils (fig. 1). The main

advantage of this design is simple and robust construction combined

with a high power-to-weight ratio and efficiency. The generator usesstandard components for the permanent magnets (cylindrical) andstator silicon steel (U-cores). The winding of the stator coils is simpleas standard U-cores are used. Once a set of standard components is

selected, new machines can easily be redesigned for other system

requirements (power, efficiency, weight) simply by varying a numberof key design parameters (e.g. number of U-cores, length of U-core,

etc.). Because of its simple construction the machine is also well-suited for both small-scale and mass production.

A calculation model was made based on a combination of analyticaland finite element calculations. To prove the feasibility of theconcept, a mono-phase prototype was built and tested. Measuring the

results of tests performed on the prototype has shown that thecalculated design parameters correspond quite well with the

measured ones.

As hybrid electric vehicles have more components than theirconventional opponents and this increases the weight of the vehicle, i t

is important to reach a balanced trade-off between the weight,dimensions and total vehicle efficiency. Therefore, the effects ofdifferent system parameters such as efficiency, weight-to-power ratioand maximum dimensions on the output of the machine were

evaluated with a simplified city driving cycle model for a series

hybrid electric van (Hevan) that was build at Vito [9]. Variousmachines with different system parameters were designed andevaluated for Hevan.

II. AXIFUSCONCEPT DESCRIPTION AND DEFINITION OF KEY DESIGNPARAMETERS

Figure 1. Assembled AXIFUS generator

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Figure 2. AXIFUS rotor with permanent magnets buried behind flux

distribution plates

The AXIFUS[1,10] disc permanent magnet generator (fig.1) conceptis built with very simple basic components. Table 1 illustrates theproperties of the standard components. Cylindrical permanent magnet

NdFeB material is used in the rotor. The magnets are buried insidethe rotor (fig.2) behind two flux distribution plates, one for each pole

type. The rotor structural material is none magnetic, e.g. epoxy hardpaper. These flux distribution plates (fig.2) contain a laser-cut patternto lower eddy current losses. The stator (fig.1) contains U-coresilicon steel (table 1). The U-core silicon steel is standard available

transformer construction material and contains the stator coil formerswith the copper windings. Coil winding is simple to perform and isadapted to the required voltage output of the machine. The U-cores

are mounted on a support plate. A three-phase machine can also beconstructed when all U-cores are connected magnetically. This can be

useful if the generator must also be able to operate as a motor (e.g. inintegrated starter/generators (ISG) for hybrid vehicles). The machinecan be constructed with two stators at each side of the rotor - this willbe called a double stator machine (DSM). The DSM is magneticallythe most effective design. Another possibility is to have a single

stator machine (SSM) (see fig.1). An SSM rotor has a magnetic ring

at one side to turn the magnetic field and is therefore less efficientthen a DSM. This SSM design can be interesting if stator cooling isdifficult - e.g. when mounted on a internal combustion engine (ICE).It is also possible to construct stacked machines with two or more

rotors. This can be useful if the outer diameter becomes too large butwill effect the total weight of the machine. Using this concept it iseasy to design various machines that fit different system requirementsfor power, dimension, weight and efficiency. Redesign can be

achieved with the same set of standard components (table 1) and by

varying only some of the key machine design parameters defined intable 2.

TABLE I PROPERTIES OF STANDARD COMPONENTS

Component Property Value

Permanent

Magnets

diameter 40 mm

height 10 mm

material NdFeB

remanence 1.2 T

U-core type UI60

width 60 mm

maximum height 80 mm

stacked thickness 25.2mm

material DIN41302 (0.35mm)

TABLE II VARIABLE KEY MACHINE DESIGN PARAMETERS

Parameter Symbol Typical Values

Air gap between rotorand stator

h 1.5-3 mm

Number of U-cores #U >6

Height of U-cores hu 54-80 mm

Double-sided or single-

sided stator

DSM or

SSM

DSM

Mono-phase or 3-phase

machine

- mono

Number of stacked rotors #Ro 1-3

III .CALCULATION MODEL

In order to facilitate and accelerate the design process, a calculationmodel was developed based on a combination of analyticalcalculations and finite element calculations [1]. Analytical

calculations are solved in excel and finite element calculations inFEMLAB.It is possible to enter the system design parameters, suchas efficiency, maximum power, no-load voltage and the dimensions,and calculate by an iterative process the required key machine designparameters (table 2). When geometric parameters such as air-gap, U-

core or PM dimensions of the machine change, other finite elementparameters must be entered in the analytical model.

The air gap flux density of the permanent magnet machine iscalculated with the demagnetization characteristic of the permanent

magnet material and the geometry of the air gap. Thedemagnetization curve of the permanent magnets is given by theremanence flux density Br and the coercive force BHC. UsingAmperes law and a straight line approximation for the

demagnetization characteristic of the magnets, the following relation

determines the operating point of the magnets (Hm, Bm) at no-load:

( )

+

=

m

r

m

a

rm

a

m

lAA

BA

A

B *

*

(1)

Aa is the surface of the air gap, Amthe surface of the magnets, rthe

relative permeability of the magnet material, the length of the airgap, and lmthe length of the magnets. The flux density in the air gap(Ba) and in the stator core (Bs) can be calculated by using themagnetic flux law:

m

a

ma B

A

AKB **

= (2)

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smaller than the measured value. The difference between thecalculated and measured value of the no-load voltage is larger: thecalculated value is respectively 14%, 13.5%, 12.3% and 11.9%

smaller than the measured value. This manifests itself in figure 3 inthe shift of the calculated stator current by maximum power output to

higher values compared to the measured ones. There is thus goodagreement (+/- 15%) between the calculation model and theprototype. Deviations are most likely the result of the simplifications

introduced in the calculation model.

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 5 10 15 20 25

Stator current (A)

Outputpower(W)

3000rpm (meas)

3000rpm (AC)

3300rpm (meas)

3300rpm (AC)

3600rpm (meas)

3600rpm (AC)

4000rpm (meas)

4000rpm (AC)

Figure 5. Measured (meas) and analytically calculated (AC) values of the

output power of a machine with an air gap of 3 mm as a function of the stator

current for different rotor speeds and for a resistive load

V. HYBRID VEHICLE DRIVING CYCLE MODEL

With the calculation model, various AXIFUS machines weredesigned for Hevan, the converted hybrid city van, developed by theinstitute (fig.6) [8].The impact of various designs on vehicle performance was calculated

through a simple driving cycle model based on data collected fromtypical city driving cycles [8].The impact on fuel consumption is calculated with the followingformulae. All parameters used are also explained in table 4:

Etot(Wh/km) = Eairres + Em*M*(100-B)/100 (8)

where E = energy, B = recoverable energy fraction duringbraking

Eice(Wh/km) = 100*Etot*((100-Fbatt)/100+Fbatt/batt)/gen (9)

where Fbatt = fraction of energy passing through the battery.

Figure 6. Hevan at EVS18 in Berlin (2000).

VI. AXIFUSDESIGN ALTERNATIVES FOR HEVAN HYBRID VAN

Four AXIFUS configurations were calculated for Hevan andcompared with a standard implementation, based on typical data foran industrial synchronous generator with rectifier (table 4). Typical

data was used for the ICE. For the drive cycle, a typical city drivecycle [8] was used with an average speed of 15 kph. Axifus1fast isthe prototype configuration. Axifus2fast has a longer U-core and

therefore reduced copper losses and higher weight. Axifus4fast hasan increased air gap, although this design is less optimized and hashigher weight and lower efficiency. Axifus3slow is designed for a

slow-running (1500 rpm) ICE with increased efficiency. The resultsare summarized in table 4 and are related to the drive cycleparameters (in this case typical city use). If space is availableAxifus3slow gives the best results for Hevan.

VII. CONCLUSIONS

A new axial flux permanent magnet generator concept is presentedand can be used for the disc of flywheel-type generators. This

concept offers the flexibility needed to meet a wide range of systemrequirements (power, efficiency, weight, volume) and can be realizedwith standard components. Because of its simple construction, themachine is also well-suited to both small-scale and mass production.

A calculation model has been developed and verified with aprototype. Based on the calculation model for AXIFUS and a drive

cycle model for hybrid vehicles, various AXIFUS configurationswere evaluated.

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2003 design of a new axial flux permanent magnet generator for hybrid electric vehicles

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