1.0 reduces drastically which enhances system efficiency. Frequency

1.0   Literature review

Wind energy is an inexpensive
renewable energy source which has an ability to make significant contribution
to the electricity utility network. However, there are two problems associated
with the construction of wind power generators which has to be addressed, the
first one being the instability of the wind speed and the second one is
rotating speed of the wind turbine which is low due to the large diameter of
the rotor blades. The technologies have been developed to estimate the variable
speed constant frequency to counter the instability of the wind speed. The
later issue was addressed by using conventional solution of gear box to
increase the speed which helps to reduce the size of generators. Although
solution of gear box has several disadvantages as it generates noise and
vibrations, losses in gear drive is high since it is a mechanical device, ned
of constant lubrication and periodic maintenance (Fengxiang, 2005). The cost of
gear box is also high.

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Direct driven variable speed
permanent magnet machines are being a subject of attention due to various
advantages offered by them such competitive cost and possibility to eliminate
the gear box from wind turbine structure. Energy capture is increased in such
application by using variable speed. Due to removal of gearbox weight and
losses in wind mill reduces drastically which enhances system efficiency.
Frequency of periodic maintenance is also reduced saving finances of the
organization. Although, large numbers of poles are required to construct a
generator due to low rotational speed; it is necessary for the generator that
it should be efficient naturally with competitive cost. To supply power to the
grid, frequency converter is required due to the variable speed scheme.
According to (Fengxiang et al, 2005) small pole pitch can be achieved by
incorporating large pole numbers with permanent magnets. A simple and effective
generator construction is shown in figure 1 of appendix 1 in the form of disc
type axial flux configuration. The stator in the figure shown is a toroidal
wound accommodating rectangular coils which forms an air gap winding. Permanent
magnets are attached to the rotor disc located on both side of stator.

According to (Spooner et al, 1996), the assembly of the permanent
magnet machine is the crucial problem during its construction. No strong forces
are present at the time of assembly as the assembly of the magnet is carried
out individually and iron parts are already located in the position in the
modern assembly practice of permanent magnet machines.

To reduce the assembly problems
of PM generators the modular construction is proposed by to (Spooner et al,
1996). The paper presented by to (Spooner et al, 1996) says that, for the large
grid connected wind turbines, direct coupled, permanent magnet, synchronous
with radial field and multipole machines can be used. The power rating could be
between 100 kW to 1 MW and pole numbers could be between 100 to 300. Employing
modular constructions help to reduce need of detail design, number of tools and
drawings. The modular assembly practice can be utilised in vast ranges of
machines. The standard ferrite magnet blocks are used in the rotor module,
whereas the stator module is formed by single rectangular coil embedded in
simple E-cores. The assembly of the magnetised parts can be arranged easily
which improves the efficiency of the machine with low reluctance. The multipole
permanent magnet is shown in figure 2 of appendix 1 whereas modular arrangement
of magnet is shown in figure 3 of appendix 1 which help to visualise the
difference.

 

1.0   Progress of the work

The progress of the work shown
here is divided in to two parts,

1.     
Design
of permanent magnet machine

2.     
Finite
element analysis of the design

5.1 Design of permanent magnet
machine

During the study of permanent
magnet machine, various important design parameters were studied which is
incorporated in the design of the permanent magnet machine,

Rated
power of the machine- Velocity
of the wind speed and speed ration of driving shaft governs the rated output of
the generator. For this project work, minimum wind speed of 4 km/hour is
considered, the rpm produced by the shaft and output of the generator with
single phase connection will be calculated theoretically and evaluated during
FEA analysis if the design.

Number
of phases and poles- Number
of stator poles decide the number of phases in the machine. The thumb rule is
the number of stator poles are twice than number of phases. It has been found
that during research, torque ripple increases with small numbers of phases
where as cogging torque reduces with large pole numbers. Considering these
constraints, three phase machine is selected. By using electrical engineering
handbook by (Chen, 2004), 24 number of stator poles are selected to reduce the
torque ripples. The rotor pole is selected considering the relation between the
rotor and stator.

Frame
size- Dimension for all
electrical machines are freeze by International Electro-Technical Council known
as IEC, which comply the ISO regulations. The stator and rotor ratio selected
at this moment is 1:16 but it may change during the course of designing
depending upon the need. The structure size is yet to be finalized although
preliminary selection of the ratio fixes the frame structure.

Air
gap- The probability of cogging
torque increases due to use of permanent magnets therefore to reduce the same
and to increase the flux density air gap will be limited to a range of 0.5 to
1.0 mm.

Machine
specification- Although design of
permanent magnet machine is ongoing, and all the parameters are not fixed yet.
Even though tentative specification of permanent magnet rotor is given below
based on selection through engineering handbook and ratio, just a note, below
specification might change depending on the design requirements,

Table
1: Initial specification of permanent magnet
machine

No

Parameters

Value

1

Rating

3000 W

2

Stator
poles

24

3

Magnet
poles

8

4

Phases

3

5

Poles
per phase

8

6

Length
of air gap

0.5 to
1.0 mm

7

Stator
diameter

0.16 m

8

Rotor
diameter

0.1 m

9

Length
of magnet

0.015 m

10

Length
of back iron

0.016 m

Based on initial specification of
permanent magnet machine, initial CAD model is generated which shown in figure
4 of appendix 1.

5.3 Finite element analysis

The design of PM machines will be
validated in FEA (Finite Element Analysis) software developed in the
university. The FEA software is a powerful tool which help to analyse the
design and allows necessary changes before actual production and assembly. Various
scenarios can be simulated to check the performance of the PM machine.

In this project work, FEA will be
used to check the designed structure of the machine, excitation of the rotor,
material properties and torque produced due to different rotor position as well
as wind current. The step by step approach will be used to find solution of
continuum problem during FEA analysis; for example, elements are created by
dividing the continuum region by using different shapes of elements. It may be
possible that different shapes of element produce similar solution for the
continuum. It has been found during familiarization of software that it is
quickly possible to express material properties, constraints and excitation
although it is comparatively difficult to express. Other important parameters
will be calculated by using solution of system equation, such as,
electromagnetic problems, Components of magnetic flux density are nodal
unknowns. By using these components torque, induction and several other
electromagnetic parameters will be calculated and compared with the design one.

Following assumptions are made to
determine distribution of magnetic field inside the machine. These assumptions
are primary and may changed during actual simulation of the design,

1.     
Since the
magnetic field outside the status stamping is almost negligible hence the
magnetic vector potential line of outer periphery of the status stamping is
treated as zero

2.     
Hysteresis
effects are neglected as magnetic material is isotropic for stator and rotor
stampings

3.     
Components
of Z- directions are Current density (J) and magnetic vector potential (A)

4.     
Distribution
of magnetic field along the generator’s axial direction inside the generator is
constant

5.     
End
effects are considered to be zero

 

2.0   Summary and conclusion

The progress report includes
brief overview of permanent magnet machine with its application in wind
turbine, benefits of the same if used in the wind turbines followed by aim,
objectives and glimpse of literature review which primarily emphasis on construction,
assembly and capacity of permanent magnet. Project management section shows
gnat chart in detail along with its completion status. Progress report on the
other hand shows the completed work so far in design part as well as future
considerations and assumptions for FEA.

Considering the work put together
in progress report, it can be concluded that progress of the report is as per
the schedule and it will be completed according to plan provided in project
management section. Initial design specification for the PM machine is
completed, along with initial CAD drawing for the same. Detail design procedure
is in process and it will be completed by end of week 8. Study of FEA software
by using similar case studies have been carried out. Once the final design of
the PM machine is completed then CAD modelling and FEA simulation will commence
which is around start of week 9. Any alteration required in the design will be
carried out considering the results of FEA. Comparison of initial and final
design along with FEA justification will be provided.

 

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