Comparison between radial and axial permanent

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Comparison between radial and axial permanentmagnet generators for low speed application.

Velington de Aquino Neumann

Universidade Federal do Rio Grande do SulPorto Alegre, [email protected]

Roberto Petry Homrich

Universidade Federal do Rio Grande do SulPorto Alegre, [email protected]

Abstract -The work aims to define a topology for the design of

an electromechanical energy conversion system, which shall consist

of an electromagnetic device that will use the vertical oscillation of

a buoy at sea as a source of primary energy to feed the system a

flashing buoy signaling. A comparison is presented between radial

and axial topologies considering its constructive features.

Keywords — Permanent Magnets Machines; Axial Machines;

Radial Machines; Wave Energy.

I. I NTRODUCTION

The economy of the state of Rio Grande Sul, in the south ofBrazil, is highly concentrated in the metropolitan area of PortoAlegre. Due to its geographic location, the access to the sea isthrough a lake, called Lagoa dos Patos, which connects themetropolis to the port of the city of Rio Grande, about 320 Kmaway from there.

Currently, signaling the navigation channel between PortoAlegre and the seaport of Rio Grande is accomplished throughsignaling buoys that use photovoltaic systems with batteries to

power the lighting flashing system. The signaling buoys, whichare equipped with automotive batteries, are often targets of predation by vandals who steal batteries for their own use. Thisarticle is part of a project that aims to develop a system ofsignaling power that is reliable, robust and not attractive tovandals since it is compact and does not have its visiblecomponents in the buoy. This paper is organized as follows:Section II presents the basic concepts of the topologies ofelectrical machines and gives special attention to the coggingtorque of slots machines. Section III presents the comparison oftopologies, taking into account the dimensions of the proposeddevices. Section IV concludes the paper with the choice oftopology that will be designed and built.

II. TOPOLOGIES OF PERMANENT MAGNET MACHINES

In general, an electric radial machine operates with a radialmagnetic flux produced by windings or permanent magnets.The use of one or another is normally determined by the natureof the required application, the economic viability andtechnology involved in the manufacture [1]. The variety of

electrical machines types that are currently available is proportional to the technology advance. In the latest decades, a plenty of machines have been developed [2]. The currentcomputational processing capability make possible the designand spatial simulation using the Finite Elements Method forelectromagnetic field and magnetic circuits, and it enables thedevelopment and the application of PM machines, so thatthese kinds of machines can be a more interesting option thanthe conventional ones, with field coils [3].

A. Axial Machines

The axial flux machine, as called, comes from the fact that,in these machines, the air-gap magnetic flux is in the axialdirection. The lamination of the stator cores of the axial fluxmachines, due to the axial direction of magnetic flux in the airgap is stacked in the radial direction, difficulting themanufacture process.

Some applications of axial flux machinery, as wheelmotors (in-wheel motors) for cars and electric bikes, fans, windgenerators, marine propulsion, video recorders and disc playersare used today. The main features that lead to axial machine

application, in such cases, are the space saving, direct-drivesource, high power and high torque.

B. Radial Machines

The radial flux electrical machines were invented in 1837and they have used almost exclusively since its invention [4].In this configuration, the magnetic flux is perpendicular to thedirection of rotation of the rotor, which can be internal orexternal. The constructive feature, particularly considering thestator lamination, is a topology less expensive and hence itreached a widespread use [5].

C. Arrangement of conductors in the stator

Where the drive speed is too low, the devices’s

operation can be damaged. The conductors layout analysis instator is divided into stators with or without slots. Logically,the slotless stators have a smaller magnetic gap and thus, theycan generate higher voltages with the same volume of permanent magnets of similar slotted stators. Otherwise, the

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slotted stators present cogging torque, as it can be seen in thisstudy, where the drive speed is very low, the operation of thedevice becomes less efficient.

According to [6], in a permanent magnet machine, thecogging torque is generated by the interaction between themagnetic flux of the rotor (permanent magnets) and thevariation of the magnetic permeability of the air gap, due tothe geometry of the stator slots. The cogging torque isinconvenient for the machine operation. It is particularlyimportant in machines that work at low speeds, because at

high speed, the moment of inertia of the rotor causes thecogging torque to be less important.

Always keeping the focus on a project basis, fourtechniques can be simulated to reduce the cogging torque. Thefractional number to the ratio between the number of slots andthe number of poles is the only procedure that is adopted andthen with the other three techniques, which are not uniformdisplacement between the magnets, the special design of themagnets and the phase shift between opposing magnets. Thesemeasures are because the maximum value of the coggingtorque does not occur simultaneously, thereby causing thesmaller average cogging torque.

Fig. 1. Winding and magnets shapes adopted in axial generators: a) fractionalreason, b) Displaced permanent magnets, c) Permanent magnets with special

design and d ) Lagged permanent magnets.

Table I shows some of the simulations considering thecases above. As the purpose of the device is to generatevoltage, beyond cogging torque, the induced voltages in eachturns of winding are taken into account. The table I shows theresults of simulations where different design for cogging torquedecrease.

TABLE I. SIMULATION OF COGGING TORQUE AND VOLTAGE TURN

Simulation Cogging (Nm) V/turn (mV)

a 1,224 6,070b 0,790 6,328c 0,990 9,437d 0,932 4,480

The lowest value for the cogging torque occurs with thedisplacement of the magnets and the induced voltage per turndecreases with the winding and magnets shapes from a to d,table I. Even lowering the value of cogging torque, the design

adopted cannot cancel it and therefore, due to the type of drive,the model without stator slots was chosen.

II. COMPARISON BETWEEN TOPOLOGIES

A. Construtive characteristics

The power converter is composed of two identicalgenerators, mechanically connected by a system of gears that

forces the movement of the rotors in opposite directions inorder to cancel the lateral movement of the float. The signalingsystem is powered at 12 V and consumes 4.5 W. Therefore, themajor obstacle is the value of voltage supply because of thelow speed drive.

The comparison takes into account the dimensions of thegenerators. In a radial topology, the effective length of theconductors is in the axial direction of the generator while in anaxial topology the effective length of the conductors is in itsradial direction. Therefore, the radial generator has an axial

length greater than the axial generator.

Fig. 2. Proposed topologies: a) Axial Generator b) Radial Generator

The generators are composed of 18 coils and 12 poles in anarrangement NN as shown in Fig.3.

Fig. 3. Path of the magnetic flux generators

Initially, the core thickness was 20 mm and the height was50 mm in both the radial and axial generator. The height of thecore is the dimension that is perpendicular to the magneticflux from the magnets. To maintain the constant total volumeof core, the inner radius of the core is changed, interferingdirectly in the number of turns per coil.

a b

Fig. 4. Stator cores: a) axial generator e b) radial generator

Where Rin is the internal radius of the core, hn is theheight of the core and the core thickness is Esp. Fig. 5 shows agenerator radially symmetric, three of which are representedeighteen coils.

a b c d

a b



Fig. 5. Symmetric radial generator

Fig. 6 shows the induction line on the core, illustrated inFig. 5, to the radial generator with a core thickness of 20 mmand a height of 50 mm.

Fig. 6. Induction in the radial generator core

The average amount of induction in a radial line on the coreis 1.43 T and an axial line of 1.39 T, which is a normal valuefor operating.

For an axial generator the induction is evaluated on aradial line on the core, as shown in Fig.7.

Fig. 7. Symmetric axial generator

Fig.8 shows the induction in the core line, illustrated inFig.7, for the axial generator with a core thickness of 20 mmand a height of 50 mm.

Fig. 8. Core induction for the axial generator

The average amount of induction in a radial line on thecore is 1.26 T and an axial line of 1.28 T.

B. Calculating the number of turns

In the radial generator, the effective length of the radial airgap is defined by the radial thickness of the coil. Fig. 9 shows,in highlight, the inside area of coil, used for calculating thenumber of turns.

Fig. 9. Coil radial generator

In the axial generator, the height of the coil, define theeffective length of the air gap, as shown in Fig.10.

Fig. 10. Coil axial generator

The minor area is used for the number of turns evaluated.

C. Same volume

In this analysis, we consider generators with the samevolume but different heights and diameters in order tomaintain the same effective lengths of the conductors, thesame thickness of the permanent magnets and thus, the samemagnetic induction core. The values of terminal voltage of

generators are shown in Table II.

TABLE II. GENERATED VOLTAGE BY GENERATOR WITH THE SAME VOLUME

Topology V/turn (V) Ntc (turn) Vc (V)

Axial 18.75.10-3 226 4.24Radial 20.12.10-3 314 6.32



The voltage per turn induced in the radial generator ishigher than in the axial generator. The number of turns percoil of the generator is also larger radially. Thus, the inducedvoltage by coil and phase of generator are larger in the radialthan in the axial generator.

D. Same diameter

Considering the outer radius of the generators as 95 mmand height as 50 mm, the inner radius of the core of generator

is 50 mm and 40.5 mm, in radial and axial dimensionsrespectively. As the inner radius of the core is directly proportional to the number of turns of the coils, the coilsobviously in a radial generator may have more turns andconsequently will generate a higher value of terminal voltage.

TABLE II. I NDUCED VOLTAGE BY GENERATOR WITH THE SAME DIAMETER

Topology V/turn (V) Ntc (turn) Vc (V)

Axial 21.67.10-3 300 6.50Radial 21.20.10-3 390 8.27

E. Same external diameter and different heights

The height dimension is considered much smaller than the

diameter, so an increase in this dimension does not imply a problem for the application under consideration. The increaseheight axial generator does not increase the active length ofconductors and therefore does not increase the inducedvoltage. Radial generator, increasing the height of the core,increases proportionally the active length of conductors,thereby increasing the value of the induced voltage.

Table III shows the results of simulations of the radialgenerator with 60 mm in the core height while the axialgenerator is maintained 50 mm height.

TABLE III. GENERATED VOLTAGE BY GENERATOR WITH THE SAME DIAMETER

AND DIFFERENT HEIGHTS

Topology V/turn (V) Ntc (turn) Vc (V)Axial 21.67.10-3 300 6.50Radial 24,13.10-3 390 9.41

The active length of the conductor increases with theincreasing height of the core also increased the inducedvoltage.

F. Armature reaction

The comparison of the armature reaction, for same outerdiameter, between radial and axial generators is shown. Theheight of the radial core generator is greater, about 60 mm.The simulations consider electrical current of 1 A.

TABLE IV. ARMATURE REACTION Topology Iload (A) V/turn (V) %V

Axial 0.00 21,67.10-3 -1.11Axial 1.00 21.43.10-3

Radial 0.00 24.13.10-3 -1.70Radial 1.00 23.72.10-3

The armature reaction is in both cases too small, notsignificant for selecting one of the topologies. For the radialgenerator, the voltage drop by the armature reaction is about1.70% and in axial about 1.11%.

G. Weigth of generators

The weight of the generator is calculated from the volumeof each component and the corresponding specific weight.Here, we consider generators with same volume.

TABLE V. WEIGHT OF COMPONENTS AND COMPLETE

TopologyWeight (kg)

Permanent Magnets Iron Copper Total

Axial 1.01 5.72 1.31 8.04Radial 2.13 6.07 2.09 10.30

It is noticed that the weight of the radial generator isapproximately 28% greater than the axial generator. Withconstant diameter in two topologies and varying the height ofradial core generator, these percentages obviously increase.

III. CONCLUSION

Due to low driving speed of the generator, the cogging

torque of the machines with slots is not the best option and themore interesting is the slotless machine. The position of theactive sides of coils in a radial machine allows the inner radiusof the core is greater than the axial machine, for the sameouter diameter of machines. The number of turns per coil is proportional to the inner radius of the core and the coil-induced voltage is also proportional to the number of turns,then, the coil voltage generated will be greater in a radialgenerator. In both topologies, the outer diameter of themachine is greater than the height, in the axial direction, ascan be seen in Fig. 2. Therefore, the diameter is the dimensionthat limits the size of the machine. In the radial machine, theinduced voltage is proportional to the axial length, different of

an axial machine, in which the voltage is proportional to theradius of the machine. Therefore, if the height of the radialmachine core is increased, the voltage per turn increases indirect proportion and so the terminal voltage. The armaturereaction simulation in both cases show insignificant values todetermine the topology choice. The weight of the radialgenerator is greater than the axial generator. From thesimulation results, the topology chosen for the continuation ofthe project is the radial slot-less. Thus, a radial slot-less, three- phased generator, with twelve poles and eighteen coils will beconstructed.

R EFERENCES

[1]

A.P.B.S, Ferreira. Projecto de uma Máquina de ìmanes Permanentes deFluxo Axial Orientado para os Sistemas de Conversão de EnergiaElétrica. 318P. Tese (Doutorado em Engenharia) – Faculdade deEngenharia da Universidade do Porto, Porto, 2011.

[2] GIERAS, J. F.; WING, M. Permanent Magnet Motor Technology. FirstEdition. New York: Marcel Dekker, Inc, 589p., 2002.



[3] BELAHCEN, A.; ARKKIO, A. Eletrical Machines (ICEM), XIX International Conference on, DOI:10.1109 /ICELMACH.2010.5608241, pp.1-6, 2010.

[4] L.T.R. Loureiro, Um Estudo sobre a Máquina Torus, Tesi de Doutorado,Universidade Federal do Rio Grande do Sul, Porto Alegre, 2008.

[5] T.A.C. MAIA, Projeto e Construção de um Gerador a Ímãs Permanentesde Fluxo Axial para Turbina Eólica de Pequena Potência. Dissertação,UFMG, Agosto 2011

[6] L. Dosiek, Cogging Torque Reduction in Permanent Magnet Machines.IEEE Transactions on Industry Applications, New York, v. 43, n.6,

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Comparison between radial and axial permanent