Download Claw pole synchronous generator for small electric systems

FACULTY OF ELECTRICAL ENGINEERING
Nicolae Florin JURCA
Claw pole synchronous generator for small
electric systems
-PHD THESIS(abstract)
Scientific advisor.
Dr. Karoly BIRO
The INTRODUCTION presents the general context in which this thesis is elaborated in
and then it presents briefly the chapters of this work. A new approach of the claw pole
machine will be presented here. Each chapter of the thesis deals with a stage in the
process of building and testing the machine used in the power conversion system, each
one being closed with conclusions.
CHAPTER I of this thesis is dedicated to the presentation of several types of
synchronous machines with permanent magnet and the advantages of using them in the
small size power generation systems. Different topologies suitable for building such a
machine are presented. Prototypes of claw pole synchronous machines (existing in the
research centers) and the principle of their operation are also exposed. Then, there are
described the most common configurations of this type of generator used in power
generation microsystems – see Figure 1.13. and Figure 1.14.
magnet
magnets
Fig. 1.13 Rotor configuration and claw pole generator
magnetic field map
Fig. 1.14 Claw pole synchronous generator
in modular construction
Several aspects regarding the materials used in building these generators are presented in
the last part of CHAPTER I.
CHAPTER II presents the sizing algorithm developed for this type of generator.
The two major aspects of this procedure are followed: determining geometrical
dimensions of the rotor core, of the stator and of the permanent magnet, and the second
aspect – selecting the type of the stator winding. The base of the design algorithm is the
equivalent magnetic circuit; its constituent elements were determined. Using the finite
element method, the sizing stage results verification was approached.
For this type of generator, the generated rated voltage is directly proportional with
the rated speed of the magnetic field of the permanent magnet. In order to obtain an
efficient generator, the magnet sizes have to be determined so they supply a satisfactory
excitation field E value under the saturation limit, especially under the saturation limit of
the rotor core.
In this type of generator approach, the most difficult aspect of the problem is the
sizing of the rotor poles. These poles have an important role in closing the magnetic field
lines because they assure their convergence from magnet to air gap. Because of the high
concentration of magnetic lines in the transversal section of poles, the sizes should be
chosen carefully in order to avoid the rotor core saturation. Based on the previously
stated aspects, the premise in building the machine is the voltage produced by the
generator dependent on the permanent magnet surface area and on its working point. The
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main geometrical sizes of the generator are determined based on the power of the
generator:
Dis  3
60  S n  2 p
60  S n
, li 
2
  C  nN  
k ca   2  Dis  n N  A p  B
(2.1), (2.3)
where the main notations are: l i - the stator length, Dis - the interior diameter of the
stator, S n - apparent power, p - number of pairs of poles, nN - rated speed, B - average
value of the flux density in the air-gap.
Having the main sizes of the generator determined (its sizes are adjusted in order to be
within the standard values, and, as a result, the average value of the flux density in the
air-gap is recalculated), the dimensions of the rotor poles are dimensioned - figure 2.2.
Fig. 2.2 The claw pole rotor – structure and notations
 The rotor pole width – according to bibliographical indications [P1], [M1], it is
recommended:
b pol  (0.6  0.8)   rot
(2.6)
where b pol - the rotor pole width in air gap (figure 2.2).
The formula for the width of the pole base is determined using the formula (2.5):
  R flansa
b1 pol 
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If the distance between the two poles, h pg , has been determined based on the formula
h pg = (5  10)   [P1], then the width of the pole can be modified; thus, the final value of
b pol is obtained.
 The length of the rotor pole [P1], [H1]:
l pol  l rotor  (0.05  0.15)  1rotor

The height of the upper part of the pole:
hspg  (2  4)  
2
(2.7)
(2.8)
The value should be chosen as small as possible because the flux density has a
low value in this section.
After setting of the main formulas for generator geometrical sizes determination,
the corresponding verifications are needed. One of the most efficient verification methods
implies the computation of the equivalent magnetic circuit of the generator; in this case,
the magnetic field in air gap is checked in order to determine whether its value is close to
the one chosen during the geometric dimensioning phase.
Flanşă (2)
Flanşă (1)
Fig. 2.5 The magnetic field line direction in the claw pole synchronous generator and the
corresponding magnetic circuit
5   
R gp R ax
( Re1  R gp )  ( Rax  Re 2 )
(2.29)
In formula (2.29) it can be noticed that all the reluctances of the equivalent circuit
have to be determined based on the below formulas:
1 l
(2.30)
R 
 S
  0  r
(2.31)
where  - magnetic permeability of the environment,  0  4    10 7 - magnetic
permeability of the vacuum,  r - relative permeability of the environment compared to
vacuum, S - area of the active zone of the volume, l - length of the active zone.
The value obtained using the formula (2.48) is compared with the one chosen in
the dimensioning phase (0.1 T); if B and Bc values are almost equal, then the results
obtained in the two stages presented above (dimensioning method and equivalent
magnetic circuit method) can be verified with the finite element method using a field
calculation software.
In order to obtain the most accurate results, and because of the intricate geometry
of the generator, the finite element method has been applied for a quarter of the machine
and respecting the symmetry and periodicity conditions; thus, the results obtained for a
quarter of electrical machine correspond to the whole machine – figures 2.19 and 2.20.
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0.25
0.2
0.15
0.1
B [T]
0.05
0
-0.05
-0.1
-0.15
-0.2
-0.25
0
10
20
30
40
[o]
50
60
70
80
90
Fig. 2.20 Map of the magnetic flux density
of the rotor core
Fig. 2.19 Distribution of magnetic field in airgap
The value of magnetic field in air gap obtained using the finite element method is
around 0.16 T, which is very close to the value chosen in the dimensioning phase. These
results validate the dimensioning algorithm and confirm the efficiency of using the finite
element method in the process of shaping the electric machines.
CHAPTER III presents the way the second variant of the generator was
dimensioned, a generator with an energetic performance higher than the one of the
previous variant. The same stator is used, but the rotor is modified in order to obtain the
highest possible value of the voltage and low leakage flux values, and to reduce the
saturated zones and the volume of the material. In order to meet the first condition (high
voltage), it is analyzed the replacement of the Alnico magnet with another type of magnet
with high magnetic performance. The other three conditions impose the modification of
the claw pole geometry. The most convenient method to modify the geometry, with
results very easy to be visualized, is the finite element method, Flux 3 being used (in this
case) thanks to the aspects showed in the second chapter. Following the phases presented
above, the desired configuration was obtained; it is presented in the figure below.
0.4
0.3
0.2
B[T]
0.1
0
-0.1
-0.2
-0.3
-0.4
0
Fig. 3.13 Map of the magnetic flux density
of rotor core
30
[o]
60
90
Fig. 3.14 Distribution of magnetic flux density
in air-gap
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2
(3.1)
    p  k w  w 1  82.05 [V]

The phase voltage (82.05 V) (see formula 3.1) is much higher than the phase
Ef 
voltage obtained when using Alnico magnets (57 V).
CHAPTER IV presents the process of building the machine and the experimental
results. Based on the results obtained in the dimensioning phase and on those obtained
from the finite element method, it is created the prototype of a synchronous claw pole
generator with only one stator and two different rotors; the two different rotor
configurations have different permanent magnets - figures 4.11, 4.12. After the assembly
of the generator, the experimental stand has been constructed in order to test the
generator - figures 4.13, 4.14.
Fig. 4.11 Overview of the two rotors - NdFeB Fig. 4.12 Overview of the two rotors - Alnico
permanent magnet
permanent magnet
Fig. 4.13 Electrical scheme
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Fig. 4.14 Experimental stand
90
60
60
40
30
20
Tensiune [V]
Tensiune [V]
The scope of the first measurements are made in order to validate the results
obtained in Flux 3D for the two structures. Based on the data presented in the previous
chapters, the value of the phase voltage is 80.05 V for the rotor with NdFeB magnet and
57 V for the configuration with Alnico magnet. For this, the claw pole synchronous
generator is speeded up at nominal speed for both configurations; the voltage waveforms
are recorded using the data aquisition system at a 10 kHz frequency, the results being
processed in Matlab. The voltages obtained in this case are presented in figure 4.15.
0
0
-30
-20
-60
-40
-90
0.02
0.04
Timp [s]
0.06
a)
0.08
-60
0.1
0.02
0.04
b)
Timp [s]
0.06
0.08
Fig. 4.15 Phase voltages at no load rated speed (750 rpm): a) rotor with NdFeB magnet, b) rotor
with Alnico magnet
From the graph presented above it can be observed that the generated voltage values are
close to the ones obtained through calculations and finite element method. For the
configuration with NdFeB, the generated voltage is 79.8 V, thus a difference of 2.2 V, so
it can be confirmed that the results obtained in Flux 3D are validated. The difference of
2.2 V between the voltages is mainly due to mechanical processing, where some of the
geometric dimensions of poles are slightly different than the ones obtained by the
simulation program. Another cause of the difference between voltages is the computation
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0.1
errors that might appear. In Alnico magnet case, the difference is of 1.5 V and it is due to
same causes exposed above.
Further, it is analyzed the generator behavior at variable speeds and, in order to do
that, the U = f(n) characteristic is drawn – figure 4.17.
1 2 0
NdFeB
1 0 0
U f [V ]
8 0
6 0
ALNICO
4 0
2 0
0
0
2 0 0
4 0 0
6 0 0
n
8 0 0
1 0 0 0
1 2 0 0
[r p m ]
Fig. 4.17 Characteristic U=f(n)
The first test with resistive load is realized by loading the generator with an
electric current of 25% of I (0.25 A).
Fig. 4.20 Electric current, voltage and harmonic content: a) NdFeB configuration, b) Alnico
In Alnico configuration case, the generated voltage is strongly deformed by this value of
0.25 A of the electric current because the value of the magnetic field generated by the
ALNICO magnet is very low, and this leads to an increasing of the influence of the
armature on the generated voltage, whose value decreases from 57 V to 47 V. The
amplitude of the harmonics the is much higher in this case then before, and the 3rd
harmonic has an amplitude of 42% of the fundamental. From the above aspects it can be
concluded that for the Alnico configuration, the increasing of the current over this limit
would lead to an excessive deformation of the generated voltage; the use of a higher load
is not possible without some measures in order to reduce the armature’s reaction. When
using NdFeB magnets the influence of the armature’s reaction is minimum for a current
of 0.25 A. The influence of the armature’s reaction is visible for values of current of 0.75
A and at nominal value of 1 A.
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Fig. 4.23 The generated voltage and harmonic content at nominal load of the generator –
NdFeB configuration
Also in this chapter measures to improve these configurations are adopted. The
chapter ends with a study of the generator in case of supplying c.c to consumers. By
accomplihing these tests, is is noticed the efficiency of the proposed configurations
within the framework of such power microsystems.
A synthesis study (in the first chapter) of the synchronous machine with
permanent magnet in both classical and with claw poles constructions; a
new use of the generator is proposed - generator in power mycrosystem.
Development of a dimensioning algorithm for low size claw poles
synchronous machine with permanent magnet, having, as primary scope,
the determination of the volume of the permanent magnet and also the
determination of the claw poles geometry.
Determination of the equivalent magnetic circuit in no load schema for
the dimensioning algorithm verification.
Achievement of the numeric modelling using the Flux 3D software.
Achievement of a study for the optimization of a rotor of a synchronous
claw pole generator using finite element on the 3D module of the
simulation software.
Achievement, in practice, of the designed stator and two rotors.
Achievement of the experimental testing stands. Achievement of the
experimental measurements for validating the results obtained in the
theoretical stages.
Analysis of the behavior of the generator in different working conditions.
Investigation of the possibility of using the generator as a power supply
for the insulated c.c. consumers.
One considers that, by this work, news ways to a future development of the
generator have been opened. These are related to the increase of the performances by
improving the generator structure; also, it some advanced control methods that can be
implemented for these systems can be adopted.
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