performance analysis of axial field permanent … · magdy khaled, mohamed kamal ahmed, m elwany...

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International Journal of Electrical Engineering & Technology (IJEET)

Volume 10, Issue 4, July -August 2019, pp. 31-42, Article ID: IJEET_10_04_004

Available online at http://www.iaeme.com/IJEET/issues.asp?JType=IJEET&VType=10&IType=4

ISSN Print: 0976-6545 and ISSN Online: 0976-6553

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PERFORMANCE ANALYSIS OF AXIAL FIELD

PERMANENT MAGNET GENERATOR

Magdy Khaled, Mohamed Kamal Ahmed, M Elwany and Mahfouz Shalaby

Electrical Power and Machines Department, Faculty of Engineering,

Al-Azhar University, Cairo, Egypt

ABSTRACT

Axial Field Permanent Magnet (AFPM) synchronous generators are broadly

connected for the little size power wind turbine this is ascribe to its powerful thickness

per unit load than the conventional synchronous generator. It is a circle molded rotor

and the stator structures. In this paper, examine the execution of both AFER-NS and

AFIR-S generators having double side's slotted stator core is accomplished to improve

the output generation characteristics. The electromagnetic plan examination was done

to expand the power density. The enhancements design structure optimizations of the

rotor pole-arc ratio of permanent magnet are proficient to increase output power and

to decrease cogging torque. The output performances of AFER-NS and AFIR-S

generators are associated with each other.

Keywords: (AFPM), AFER-NS, AFIR-S

Cite this Article: Magdy Khaled, Mohamed Kamal Ahmed, M Elwany and Mahfouz

Shalaby, Performance Analysis of Axial Field Permanent Magnet Generator,

International Journal of Electrical Engineering & Technology, 10(4), 2019, pp. 31-42.

http://www.iaeme.com/IJEET/issues.asp?JType=IJEET&VType=10&IType=4

1. INTRODUCTION

The Axial Flux Permanent Magnet (AFPM) synchronous machines have been developing in

fame and have gotten an expanding measure of consideration in direct drive wind control

application [1]. It is creating magnetic flux in the axial direction, rather than the radial

direction. There are two sorts of AFPM machines, AFER-NS (Axial Field External Rotor-

Non-Slotted) and AFIR-S (Axial Field Inner Rotor-Slotted). Contrasted and RFPM machines,

the upsides of AFPM machines can be outlined as pursues:

simple winding,

low cogging torque and noise (in slotless machine),

short axial length,

higher torque/volume ratio”.

In any case, the hindrances of AFPM machines in examination with RFPM machines are

as per the following:



lower torque/mass proportion

larger external distance across, extensive measure of PM and basic precariousness (in slotless

machine)

difficulty to keep up air gap in extensive distance across (in opened machine)

difficulty underway of stator core (in opened machine).

The opportunity and potential of AFPM machines for large-scale direct-drive wind

turbines have been considered, and some varied structures of AFPM machines with surface-

mounted PM have correspondingly been offered in some references [2,3,4].

The non-slotted single stator double rotor is a typical structure of AFPM machines, which

is often referred to as a Torus machine, as displayed in Figure . (1) [5]. The two rotor plates

are made of mild steel and have surface-mounted PM to deliver an axially directed magnetic

field in the machine air gaps. The machine stator includes a non-opened toroidally twisted

strip-iron center that conveys a three-phase winding in a toroidal manner by methods for

concentrated coils.

Figure 1 Axial Field External Rotor-Non Slotted AFER-NS

The non-slotted, toroidal-stator AFPM generator has been likewise examined with a few

points of interest, for example, the lightness, the smallness, the short hub length, the

reasonable coordination with the motor and others by Spooner and Chalmers [5] and Wu et al.

[6,7], the machine's short axial length will in general give it a high capacity to weight

proportion. Parviainen [8] has interduce an analytical method to achieve the preliminary

design of a surface-mounted, low-speed, slotted AFPM machine with one-rotor two-stator

configuration, as exposed in Figure . (2).

The performance and structure amid the low-speed radial flux and axial flux PM machine

keep on similarly associated in the power range from 10 to 500 kW at 150–600 rpm [8].

Moreover, five different topologies of AFPM machines, a double-stator slotted type, a

double-rotor slotted type, a single-sided AFPM with stator equilibrium, a single-sided AFPM

with rotor equilibrium and a non-slotted single-stator double-rotor (Torus machine), have

been examined and accompanying with RFPM machines by Chen et al. [2]”.

Performance Analysis of Axial Field Permanent Magnet Generator


Figure 2 Axial Field Inner Rotor-Slotted AFIR-S double-stator single-rotor configuration

According to [2], the two-sided AFPM machine is superior to the one-sided AFPM

machine; however, one-sided constructions use less copper and have a lower conduction loss.

The Torus construction is simple; however, it needs extra magnet weight since of the

existence of the additional air gap for cooperative stator windings.

As the power rating increases, both air gap and air gap reluctance turn out to be higher,

consequently that this structure is more appropriate for small power rating wind generators. In

accumulation, the possible application of soft magnetic composite (SMC) material practical to

the low speed, direct-drive axial flux PM wind generator was as well considered by Chen et

al. Relative design studies were showed on different configuration PM generators with both

lamination core and SMC core [9].

2. ELECTROMAGNETIC STRUCTURE ANALYSIS OF THE (AFPM)

GENERATOR

To examine the electromagnetic structure design and the design enhancement of AFIR-S

generator for wind turbine initially, the electromagnetic structure design investigation is

cultivated to expand the power density of the recommended AFIR-S generator. Besides, the

design improvement of the shaft circular segment proportion and the perpetual magnet skew

point of rotor to build the yield created voltage and to decrease the cogging torque. At long

last, the output performances of AFER-NS and AFIRS generator are contrasted and one

another.

Figure (3) indicates two structure kinds of AFPM generators. Figure . (3a) is the structure

of the by and large utilized traditional AFER-NS generator.

It has two outer plate rotors at the two sides and has the non-slotted stator without stator

core. The winding turns are twisted in the plastic bobbin and shaped by the epoxy. AFER-NS

has been utilized for small wind turbine due to straightforward structure, wide speed range

and essentially no cogging torque creation.

Nonetheless, its produced voltage is low attributable to extensive reluctance by the long

air-gap. It can likewise cause the power transformation productivity drop and the cost

increment of circuit setup in AC/DC converter and DC/AC inverter. Figure . (3b)

demonstrates the structure of AFIR-S generator proposed in this section.

The rotor having permanent magnets is located between the stators having the opened

steel core. This structure of AFIR-S generator can enhance the output power density per the



volume measurement as a result of the lower air-gap reluctance by slotted steel core of stator.

Be that as it may, it might cause the expansion of axial length and cogging torque.

(b)

Figure 3 Basic configuration of (a) AFER-NS and (b) AFIR-S generator.

Particularly, the cogging torque is imperative to the small wind turbine in light of the fact

that the cut-in wind speed is subject to the cogging torque quality so as to begin the turn of

turbine edge. Furthermore, it causes the acoustic noise and the vibration on the machine [10-

12]. Along these lines, it must be cautiously considered in the design of the wind turbine.

Figure (4) demonstrates the winding design of AFER-NS generator and AFIR-S generator. In

the two structures, the concentrated winding is connected because of low end-turn winding

misfortune and the simple assembling process. Be that as it may, it might cause high cogging

torque particularly in the AFIR-S generator.

(a) (b)

Figure 4 Winding configuration; (a) AFER-NS (b) AFIR-S generator.

The flux flow way of each AFPM generator is appeared in Figure . (5). In AFER-NS

generator, the transition of rotor magnet flux flows, starting with one rotor then onto the next

one through the long air-gap without the stator core. Be that as it may, AFIR-S generator has

the moderately short air-gap because of the interior rotor and slotted core of stators as in

Figure . (5b). It implies the expansion of air- flux density and the generated output voltage.



Figure 5 Flux movement track; (a) AFER-NS and (b) AFIR-S generator.

3. ELECTRO MAGNETIC TORQUE AND EMF FOR AFPM

MACHINE

The magnetic flux waveform per pole excited by the rotor magnet could specified by [12]:

(1(

wherever Φpole is flux per pole, αi is the ratio of the average value to the peak value of the

magnetic flux density in the air-gap, p is the number of magnetic poles, Do is outer diameter

of magnet, Di is inner diameter of magnet and Bmg is maximum flux density in the air-gap.

The torque fashioned by the stator current and the rotor magnet is considered as [12, 13].

(2(

anywhere m1 is the number of phases, Ia is line current, N1 is the number of turns per

phase, kw is winding factor, Bg is magnetic flux density in air-gap and kd is defined as ratio of

the inner diameter to outer diameter of magnet. The factor kd influences maximum torque and

torque density. It was confirmed that the maximum torque is obtained at:

√ (3(

The RMS value of the induced EMF in each phase Ef is gotten as [13].

√

(4(

wherever ns is the rotation speed of the motor per second. From (1) to (4), the output

power PR can be found as:

∫

(5(

wherever η is the efficiency of this machine, EPK is the peak value of emf and IPK are the

peak value input current. The proposal parameters of each AFPM generator are as in Table

(1). As denoted in Table (1), the design assessment was completed under the same volume



dimensional situations, e.g. outer diameter, inner diameter, total axial length, air-gap length

and winding turns of each AFER-NS and AFIR-S generator.

Table (2) displays the initial design results of each type‟s generator. Under the same

electromagnetic volume dimension, the output power of AFIR-S generator is nearly two times

of the conservative AFER-NS generator. It is due to the reduction of reluctance of flux paths

between stator and rotor in AFIR-S generator having stator core steel. Nonetheless the

cogging torque is mostly generated by the stator core slot and rotor magnet flux in AFIR-S

generator. It should be minimized to obtain low cut-in speed of wind turbine.

Table 1 Parameters of AFER and AFIR-S generator.

Parameter AFER-NS AFIR-S

Outer-diameter (mm) 211 211

Inner-diameter (mm) 122 122

Axial-thickness of magnets (mm) 5 5

Total-axial length (mm) 30 30

Air-gap length (mm) 1.0 1.0

Number of Poles p 20 20

Number of stators 1 2

Number of rotors 2 1

Number of phases (m1) 3 3

Number of coils/phase (N1) 10 10

Number of slots (Ns) - 30

Number of turns (turns) 16 16

Coil diameter (mm) 1.02 1.02

Permanent magnet grade (NdFeB) 39UH(Br -1.2T) 39UH(Br -1.2T)

Core steel grade S18 S18

Table 2 Initial design results of AFER-NS and AFIR-S generators


Rated speed (rpm) 600 600

Rated torque (Nm) 11.4 23.5

Input power(W) 716 1417

Output voltage (Vrms) 24 60

Output current (Arms) 8.7 7.6

Output power (W) 634 1380

Cogging torque (Nm pk-pk) - 7.2

4. COGGING TORQUE MINIMIZATION

The cogging torque is delivered by the air-gap reluctance variety between stator tooth, slots

and the flux of rotor magnet as the rotor turns. It is evaluated by the computation of absolute

energy stored variety noticeable all around air-gap as for the rotor position development.

Cogging torque Tcog can be composed as:

(6(

where, φg is the air-gap flux, R is the air-gap reluctance and θr is the precise rotor position.

Cogging torque can be a significant issue in AFIR-S generator not at all like AFERNS

generator not having the slotted steel core. Figure . (6) demonstrates the structure answers for

limit cogging torque at AFPM machines [13].



Figure 6 Design resolutions to minimize cogging torque in AFPM machines.

It tends to be accomplished by two habits; the structure adjustments of the stator side and

the rotor side. In any case, the design change of stator impacts the assembling procedure, for

example winding, stacking, etc. It causes the issue of assembling cost and profitability.

In the rotor design, the structure change of pole-arc segment and skew edge of rotor

magnet are the basic answers for limit cogging torque. The skew of magnet can't be connected

frequently due to the assembling issue of magnet. Then again, the magnet pole-arc segment

structure enhancement, as appeared in Figure . (7), can be a decent structure arrangement.

Figure 7 “Pole-arc ratio to pole-pitch of rotor magnets.”

Since cogging torque is emerged by the connection of the edges of magnet poles and the

stator slots, cogging torque is subject to the magnet pole-arc structure. The pole-arc segment

configuration may impact the flux of rotor magnet, and it likewise impacts the output

characteristics. Along these lines, the exchange off plan is required [12]. The magnet shaft

circular segment proportion αp of rotor magnet is characterized to as eq. (7):



(7)

where, τp is the rotor pole pitch and τm is the magnet pitch. The optimal magnet pole-arc

ratio αpo for the smallest cogging torque in the radial field machine can similarly be practical

for AFPM machines as in eq. (8) [11].

(8)

wherever, N = Nc/2p, p is the pole pair number, Nc is LCM of the rotor pole numbers and

the stator slots numbers Ns. In the proposed AFIR-S generator, αpo is calculated as 0.677.

Practically, the optimum value of αp would be a diminutive larger than the considered number

of eq. (8) because of the leakage flux of the stator [12]. The trial and error method are applied

to find the optimum value of pole-arc ratio equivalent to the smallest cogging torque

generation. The cogging torque and output characteristics are studied under the pole-arc angle

is wide-ranging by the step of 10 deg. from 180 deg. To 100 deg”.

Figure 8 Cogging torque characteristics of the AFIR-S generator rendering to the pole-arc to pole-

pitch ratio.

Figure 9 Cogging torque (peak-peak) of AFIR-S generator rendering to pole-arc ratio.

Figure (8) displays the cogging torque characteristics of the AFIR-S generator according

to the pole-arc ratio. And the peak to peek amplitude of cogging torque is exposed in Figure .

(9). Consequently, we can see that the optimum value of pole-arc ratio can minimize the

cogging torque in AFIR-S generator.



5. PERFORMANCE COMPARISON OF AFER-NS AND AFIRS

GENERATOR

Figure (10) displays the examples of AFER-NS generator and AFIR-S generator having the

same electromagnetic volume. Figure . (11) demonstrations the air-gap flux density

distribution at the no-load state. The air-gap flux density of AFIR-S generator is improved by

53% than that of AFER-NS generator.

Figure 10 Prototypes of AFER-NS generator and AFIR-S generator having the similar

electromagnetic volume.

In the AFIR-S generator, the failure of air-gap flux is happened by the magnet dead zone

of rotor pole and the slot of stator core. The main deep of air-gap flux is caused by stator slot,

and the failure of air-gap flux during high flux density area is produced by the magnet dead

zone of rotor pole. Figure. (12) shows the no-load generated voltage of AFIR-S generator. As

exposed in Figure . (12), the generated voltage of AFIR-S generator is improved by 43.18

[Vrms] associated with AFER-NS generator and, the generated voltage waveform of AFIR-S

is trapezoidal waveform rendering to the air-gap flux waveform.



Figure 11 Air-gap flux distribution characteristics at the no-load complaint.

Figure 12 Generated voltage characteristics at the no-load complaint.

The output performances calculation results are like as Table (3). In supposition, the

output power of AFIR-S generator can be enlarged to approximately 2 times of the

predictable AFER-NS generator at the same volume dimension.

Even though the output power is enlarged to over than 2 times, the efficiency is also

enhanced to 91.2 %. And the cogging torque is similarly minimalized to under 5 % of rated

torque, it is adequate low level in the design of wind turbine generator [12]”.



Figure 13 Comparison of the generated voltage and the output current characteristics at the full load

condition.

Table 3 Performance comparison at rated load and the same electromagnetic volume dimension.


Rated speed (rpm) 600 600

Rated torque (Nm) 11.9 22.5

Input power(W) 747.7 1414

Output voltage (Vrms) 24 60

Output current (Arms) 9.0 7.1

Output power (W) 642.6 1288.7

Efficiency (%) 85.9 91.2

Cogging torque (Nm) - 0.795

Cogging torque / Rated torque (%) - 3.5

Torque ripple (%) 3.4 1.87

6. CONCLUSION

This paper offerings the strategy analysis and the performance evaluation result of two types

of AFPM generator. By this strategy study, the recommended innovative structure AFIR-S

generator can be designed to have higher output power density per unit volume dimension and

efficiency compared with conventional AFER-NS generator.



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performance analysis of axial field permanent … · magdy khaled, mohamed kamal ahmed, m elwany...

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