Rotating Field Type Axial Flux Permanent Magnet ...The rotating magnetic field AFPM generator...

Rotating Field Type Axial Flux Permanent Magnet Synchronous Generator

Shubham Negi

U.G. student, Department of Mechanical Engineering,

Dev Bhoomi Institute of Technology, Dehradun

Uttarakhand, India

[email protected]

Nitish Kumar Saini

Assistant Professor, Department of Mechanical Engineering


Uttarakhand, India

[email protected]

Km Deepmala

Assistant Professor, Department of Mechanical Engineering


Uttarakhand, India

[email protected]

Adita Saini

Assistant Professor, Department of Electrical Engineering

Dev Bhoomi Institute of Technology, Dehradun,

Uttarakhand, India

[email protected]

Abstract

This paper presented the design of axial flux permanent

magnet (AFPM) generator having high compactness in axial

direction in terms of number of stator in single unit. Winding

is made by giving shape to the big circular loop of multiple

turns. This shape forming butterfly structure over the stator

which offering the easy construction of core which reduces

flux leakage. The performance of rotating magnetic field type

single phase AFPM generator is study for different

parameters. Consideration is also given on variation of emf

with the change in axial distance between the stator and rotor.

The performance test of this generator is carried practically by

constructing a prototype consist half of unit that is one rotor

and one stator. The rotor consist four circular magnet arranged

on surface alternatively facing to the stator, resulting

formation of sinusoidal magnetic field.

Keywords: AFPM – Axial Flux Permanent Magnet,

Generator unit, Sinusoidal, RFPM – Radial Flux Permanent

Magnet

Introduction

The objective is to design the more compact and high power

density generator which led to axial flux permanent magnet

generator. As today in most of application radial flux

generator is in use which produce significant power but at the

same time they are non-compact which make them unsuitable

for certain application especially when focus is on renewable

energy sources in terms of installation and its cost. This

objective if achieve will help in reducing energy wastage and

better conversion of mechanical power in to electrical power

and at the same time due to compactness connection of more

than one generator to the single source is possible. Today the

objective of the world is to reduce energy wastage because of

increasing pollution and exhausting of most fossil fuel

reservoirs. Around 80% of world electrical energy production

is obtained from the fossil fuel. So the most basic reduction in

pollution can be achieve by use of renewable sources which

require effective design of electrical machine converting one

form of energy in to electrical energy.

AFPM generator design shown in this paper have high

compactness in axial direction because of the use of single

rotor providing magnetic flux to two stator in single unit.

Rotor use here contain circular magnets which are arranged

alternatively on rotor to make alternative poles. The

alternative arrangement of this circular magnet on rotor causes

production of rotating magnetic field which interact with the

stators consist winding present on both side of rotor. The

performance of this design AFPM generator is find out by

testing it practically. For testing purpose half of the unit is

taken under consideration also without use of steel core.

Literature review-

Dr K R Pullen et al. 1996, shows a paper in which

they analyse the potential of axial flux permanent magnet

generator in a gas turbine engine based hybrid automotive.

Research result shows axial flux PM generator due to its some

characteristics over RFPM found to be effective which are

high power density, compact size and series connection of

multiple unit.

Eduard Muljadi et al. 1999, presented paper in

which the axial flux PM generator is found to be more

effective for wind turbine due to its low weight and compact

size. The main focus of this paper is to study and

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manufacturing of toroidal stator winding based AFPM for

wind turbine. The analysis of this design generator is carried

out in finite element analysis whose results are verify

practically. The voltage characteristics of this generator is

plotted. The efficiency of toroidal winding based AFPM

comes to be 75%. Results of this research in terms of

manufacturing is that toroidal winding based AFPM generator

are easy to construct and can easily be mass produced.

F. Chimento et al. 2004, presented a paper in which

they discuss about the development of low speed AFPM

generator for wind turbine. Working of this generator at low

speed modification is carried out in design e.g. making it slot

less. Also this design based generator directly connect to the

wind turbine without gear reduction. Also magnetic, thermal

and electrical characteristics of this design is analysed in FEA

to make this generator able to run at low speed.

Pragasen Pillay 2005, presented a paper in which

study is on the effect of presence of soft magnetic composite

as compared to electrical steel core. The analysis is carried in

FEA and then practically by constructing prototype. The

results are that the soft magnetic composite core have less

reasonable effect on performance as compared to electrical

steel but the advantage of SMC are that they are easy in

construction as compared to laminations of electrical steel.

Yicheng Chen et al. 2005, presented paper which

consist the comparison of seven different configuration

generators. Various parameters of generators such as power,

angular speed and number of poles are analysed. The result of

this research was that AFPM generator are more effective due

to high power density. Also, significant consideration should

need to be given in AFPM generator to their dynamic balance

when their size is considerably large. Significance is also

given to use of toroidal winding on stator due to its easy

construction.

M. A. Khan et al. 2006, presented the paper in

which method of construction and analysis of axial flux PM

generator consisting soft magnetic composite/steel stator core

is described. The prototype of this generator is construct and

test in laboratory where the results consisting terminal

voltage, phase current, torque, power and efficiency are

obtained. The method of insulation of machined iron on the

SMC by the use of phosphoric acid is also describe.

Stephane Brisset et al. 2008, presented paper about

optimization of nine phase winding configuration for the

AFPM generator. The 2D FEA analysis is carried out to find

the optimum configuration, result of which is validate by

making model. The purpose of optimization of nine phase

winding configuration is to produce effective interaction of

winding with magnet and to maximize the active power.

D. A. Howey 2009, publishes a paper in which study

is on the suitability of axial flux permanent magnet generator

for Pico Hydropower. It also describe the construction,

working and its effectiveness, in terms of performance as

compared to radial flux permanent magnet (RFPM) generator.

B. Xia et al. 2010, presented paper in which the

design and analysis of dual rotor single stator air cored AFPM

generator is described. The analysis is carried out for a 2D

magnetic field in FEA to produce approximate result of 3D

magnetic field present in actual AFPM generator. The results

obtained from FEA is analysed with results obtained from real

air cored AFPM generator which found to be sufficient

accurate. The result of this research shows that other part of

AFPM generator can be analysed by 2D magnetic field

approach.

K. C. Latoufis et al. 2011, published the paper in

which they describe design and manufacturing process for the

construction of low cost axial flux permanent magnet

generator for rural area wind turbine. The study is on grid

connection and battery charging to maximize performance of

generator along with the low cost.

Juha Pyrhonen et al. 2015, presented a paper in

which the modified design of AFPM describe which have

better cooling characteristics. As AFPM machine not have

much good cooling characteristics due to axial compact

structure. This compactness also make difficulty to provide

mechanism which provide better and efficient cooling. The

design describe in paper involves used of copper bars of 6 or

8mm of total length 70mm whose 60mm length is inserted in

stator teeth and remain 10mm in cooling water jacket. Further

improvement in cooling characteristics obtain by used of high

thermal conductance potting material on one side of end

winding which makes contact with liquid cooling jacket.

Results by both simulation and experiments shows

considerable improvements in cooling characteristics of

AFPM machine.

Design Descriptions

The rotating magnetic field AFPM generator consist following

parts-

Rotor – Rotor is connected to the shaft from which it obtained

power. The rotor consist ring type magnets which are

arranged alternatively to produce rotating magnetic field.

Also, Magnets are mounted in a manner on rotor so that their

rotating magnetic field is produced on both side of rotor. This

enable us to use only one set of magnet on rotor.

Fig. 1. Top view of rotor, hatching region showing

rotor plate. Rotating magnetic field produces on both side of

rotor plate.

Stator – Stator is positioned at both side of rotor, which

consist winding and core. EMF produces in stator winding

when varying magnetic field pass through it.

Winding and Core – Winding, across which emf is obtained

when varying magnetic field is pass through it. It is made in

such a butterfly shape so that it is easily build. It offer another

advantage of using electrical steel core easily on this type of

machine which is otherwise very difficult due to geometry.

Electrical steel used here for its construction is I type core of


Page 114 of 120

Transformer. This I type sheet is bend in circular shape and

then arranged to form circle.

Fig. 2. Core ring made of lamination of electrical steel sheet

for 4 magnets.

Shaft and Bearings – The purpose of shaft here is to transmit

power from external device to rotor. Bearings are used for the

purpose for which it is build that is to minimize friction

between the parts having relative motion which is here are

stator and shaft.

Working:

When the rotor gets power from external source by the help of

shaft it start moving which causes the movement of rotating

magnetic field. Due to this variation of magnetic field over

winding, an emf induced across winding. The field is

sinusoidal in shape and also it have opposite direction of field

lines corresponding to two adjacent gap. When the conductor

is in first gap it have a induce emf which is just opposite to the

emf induced in it when it is in second adjacent gap. The

variation of induced emf up to opposite direction is also

similar to the field i.e. sinusoidal which mean inducing

alternating emf.

Manufacturing details:

On a rotor plate the axial hole are drill at points where magnet

centres are lying. As in case of four magnet, each magnet is

apart from other by 90o, so holes should also be 90 degree

apart. Holes drilled are utilized for fastening of magnet on

rotor by nut and bolt. The fixing of magnet on rotor is

required to balance centrifugal force which otherwise separate

the magnet from rotor when rotor gain sufficient speed. After

magnet attachment rotor is examined for its balancing to avoid

radial vibrations and also for unbalanced moment along axial

directions.

For construction of stator components, I type electrical steel

sheets of transformer are first bend in to circular shape and

then their pack of required thickness (depend upon magnet

size) is made called as core. For keeping the individual sheet

of pack to gather and at desired location over the plate a comb

type structure of mild steel rod is used. As the magnet are

four, so it is better to make four packs of given size electrical

steel sheet for better alignment and to avoid winding position

at joint between two cores.

Prior to installation of core on stator the radial holes are

drilled at desired location over the core packs of required

diameter. This holes are then opened by removing material for

insertion of winding on core. With the increase in number of

magnets pitch radius of magnet need to be increased to avoid

drilled holes merging at inner diameter of core.

Winding is formed by making big circular loop of required

circumference with the required number of turns. This circular

loop is then insert in to the cores holes and given shape. After

fixing it to stator it forms butterfly structure. Varnish is used

at last over the core and winding to provide insulation and to

make set up rigid.

Test model/ prototype description

Model test here consist half unit of rotating field AFPM

generator that is one stator and one rotor. Information of

testing model is given below:

Single phase winding, wire used in winding have a size of 20

gauge (SWG standard based). Each turn of winding have a

length of 0.95m.

Electrical steel core pack of thickness around 8mm used (Here

main purpose is to hold winding instead of enhancing

magnetic field across stator winding). Each electrical steel has

approximate size of 152.2×25×0.55.

Permanent magnet type – Four magnet are used and each

magnet is Ceramic ferrite ring magnet of size 72×32×13mm.

Mild steel circular plate of diameter 300mm is used as rotor

and circular wood plate is used as stator. Pitch radius of

magnet from centre of rotor is 81.5 mm.

Fig. 3. Computer aided drawing of prototype stator with

single phase winding. In drawing, A representing first

terminal of single turn winding, B representing second

terminal of single turn winding, C representing core of

electrical steel and D representing stator circular plate.


Page 115 of 120

Fig. 4. Computer aided drawing of prototype rotor with

alternative poles.

Fig. 5. Computer aided drawing of Unit of rotating field type

AFPM generator consist two stator and one rotor. In drawing,

A representing stator and B representing rotor.

Fig. 6. Actual built prototype stator.

Fig. 7. Actual built prototype rotor.

Fig. 8. Half unit of generator consist one rotor and one stator.

Testing equipment used:

1. Unity multimeter – DT830D

2. Mastech multimeter

3. Cathode Ray Oscilloscope

4. Tachometer

5. Rheostat

Parameters representation:

Eo = Open circuit or no load emf across winding (in V)

Isc = Short circuit current (in A)

V = Load voltage (in V)

I = Load current (in A)

P = Active power (in W)

N = Angular speed of rotor (in rpm)

n = Number of turns

ϰ = Gap between rotor and stator along axial direction (in

mm)

Rp = Pitch radius of magnet from centre of rotor

Practical Results – This curves are drawn by testing of

prototype. For this rotor is connected to chuck of variable

speed drive lathe machine and stator is fixed to tool post of


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carriage. Gap variation is achieve by moving carriage away

and towards rotor. Number of turns variation is achieve by

making four winding set each of 25 turns.

0

1

2

3

4

5

6

7

8

9

0 100 200 300 400 500 600 700

Eo

(in

vo

lt)

ϰ (in mm)

Eo vs ϰ

2 per. Mov. Avg. (n = 25)



2 per. Mov. Avg. (n = 100)

Fig. 9. Variation of no load or open circuit emf (Eo) across

stator winding with the gap (ϰ ) between stator and rotor for

various number of turns (n) at constant angular speed of rotor

(N = 1535rpm).

0

1

2

3

4

5

6

7

8

9

0 250 500 750 1000 1250 1500

Eo

(in

vo

lt)

N (in rpm)

Eo vs N

Linear (n = 25)

Linear (n = 50)

Linear (n = 75)

Linear (n = 100)

Fig. 10. Variation of open circuit EMF (Eo) across stator

winding with angular speed of rotor (N) for various number of

turns (n) at constant gap (ϰ = 30mm) between rotor and

stator.

0

1

2

3

4

5

6

7

8

9

0 10 20 30 40 50 60 70 80 90 100 110

Eo

(in

vo

lt)

n

Eo vs n

Fig. 11. Variation of open circuit EMF (Eo) across winding

with number of turns (n) at constant angular speed (N

=1535rpm) of rotor and at constant gap (ϰ = 30mm) between

rotor and stator.

0

0.5

1

1.5

2

2.5

3

0 100 200 300 400 500 600 700 800 900

I sc(i

n A

)

ϰ (in mm)

Isc vs ϰ




2 per. Mov. Avg. (n = 100)

Fig. 12. Variation of short circuit current (Isc) in winding

with the gap (ϰ ) between rotor and stator for various number

of turns (n) at constant angular speed (N = 1535rpm).


Page 117 of 120

0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

2.4

2.7

3

3.3

0 250 500 750 1000 1250 1500 1750

I sc(i

n A

)

N (in rpm)

Isc vs N

Linear (n = 25)

Linear (n = 50)

Linear (n = 75)

Linear (n = 100)

Fig. 13. Variation of short circuit current (Isc) with angular

speed (N) of rotor for various number of turns (n) at a

constant gap (ϰ = 30mm) between rotor and conductor.

0

1

2

3

0 10 20 30 40 50 60 70 80 90 100 110

I sc

(in

A)

n

Isc vs n

Fig. 14. Variation of short circuit current (Isc) with number of

turns (n) at constant angular speed (N = 1535rpm) of rotor

and at constant gap (ϰ = 30mm) between rotor and stator.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4

P (

in W

)

I (in A)

P vs I

Poly. (at ϰ = 30mm) Poly. (at ϰ = 80mm)

Poly. (at ϰ = 130mm) Poly. (at ϰ = 180mm)

Fig. 15. Variation of active power (P) with load current (I) for

different gap between rotor and stator at constant angular

speed (N = 1535rpm) and number of turns (n= 100).

0

1

2

3

4

5

6

7

8

9

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4

V (

in v

olt

)

I (in A)

V vs IPoly. (at ϰ = 30mm)

Poly. (at ϰ = 80mm)



Fig. 16. Variation of load voltage (V) with load current (I) for

different gap (ϰ ) between rotor and stator at constant angular

speed of rotor (N = 1535rpm) and number of turns (n = 100).


Page 118 of 120

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4

P (

in W

)

I (in A)

P vs I

Poly. (N = 1535rpm) Poly. (N = 1025rpm)




different angular speed of rotor (N) at constant gap (ϰ =

30mm) between rotor and stator and number of turns (n=

100).

0

1

2

3

4

5

6

7

8

9

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4

V (

in v

olt

)

I (in A)

V vs I





different angular speed of rotor (N) at constant gap (ϰ =

30mm) between rotor and stator and number of turns (n =

100)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3

P (

in W

) I (in A)

P vs I

Poly. (n = 100) Poly. (n = 75)

Poly. (n = 50) Poly. (n = 25)


different number of turns (n) at constant angular speed of

rotor (N = 1535 rpm) and at constant gap (ϰ = 30mm)

between rotor and stator.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3

V (

in v

olt

)

I (in A)

V vs I

Poly. (n = 100) Poly. (n = 75)

Poly. (n = 50) Poly. (n = 25)


different number of turns (n) at constant angular speed of

rotor (N = 1535rpm) and at constant gap (ϰ = 30mm)

between rotor and stator.


Page 119 of 120

Conclusion

1. Rotating magnetic field design type axial flux cut

generator prototype work successfully and possess the

above characteristics. Prototype produce maximum

active power of 4.55W for N = 1535rpm, n = 100 and

ϰ = 30mm. Modification that can further improve its

performance such as reduction of gap between stator

and rotor to minimize reluctance which ultimately

provide more power. If the graph (Fig. 13) of active

power is analysed it is showing that power

corresponding to load current is increasing with

decrease in gap. The magnet used in prototype is made

up of ceramic ferrite which have only one-third

magnetic field of neodymium magnet. By use of

neodymium magnet the more output (approximately

three times) can be achieve from the same

configuration.

2. Rotating magnetic field type AFPM generator utilize

only one rotor corresponding to two stator and also

single set of magnet, this make it more axial compact

generator unit in terms of number of stator.

3. The butterfly shape is given to the winding so that

construction of core is easy. Core for this generator can

be made by laminating the circular ring of different

radius. More power can be obtained from the same

configuration prototype as it used only 8mm thickness

of core instead of required 36mm.

4. After analysing active power graph variation it is found

that peak value of power is shifting to a low value of

load current with decrease in angular speed of rotor

and its opposite is taking place when number of turns

is decrease.

5. The characteristics of this generator is very much

similar to already exist generator. Rotating magnetic

field type AFPM design generator is not well efficient

currently but with a further improvements it can be

used in power production applications.

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Rotating Field Type Axial Flux Permanent Magnet ...The rotating magnetic field AFPM generator...

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