New Self-Excited Synchronous Machine with Tooth
Concentrated Winding
Gurakuq Dajaku1) and Dieter Gerling2), IEEE
1
2
FEAAM GmbH, D-85577 Neubiberg, Germany
Universitaet der Bundeswehr Muenchen, D-85577 Neubiberg, Germany
e-mail: gurakuq.dajaku@unibw.de
Abstract— This paper presents a novel and cost effective
self-excitation method for the current excited synchronous
machines where the high winding space harmonics are used for
excitation of the rotor field winding. According to this method,
the new machine design is characterized with simple tooth
concentrated windings for both the stator and the rotor, and it
doesn’t need any additional auxiliary winding in the stator and
its corresponding power supply and control unit compared with
the conventional brushless excitation system. Using the new
machine concept, an 18-teeth/10-poles machine for HEV
applications is designed and investigated. The obtained results
for the electromagnetic torque and the rotor field current
response demonstrate the applicability of the new technique.
Keywords: Tooth concentrated winding, synchronous
machine, self-excited, low manufacturing costs, high
efficiency.
I.
INTRODUCTION
In the last years permanent magnet (PM) synchronous
machines are challenging other electric machines in different
industry and automotive applications due to their high
energy density, compact size, high efficiency, and wide
speed operation range. However, the main disadvantages of
this machine types are the high costs and the fault tolerant
problems. The permanent magnet material is the most
expensive components in the electric machine and their price
in the lasts years is always going increasing. On the other
side different possible fault tolerance problems, such as short
circuit case, limits the viability of the PM machines in some
specific application. Therefore, the current excited
synchronous AC machines are looking to be a good
alternative solution for the future.
By the current excited synchronous AC machines the
field winding which is supplied with a DC current is used to
generate the stationary magnetic field in the rotor. The direct
current required for field excitation is furnished by the
excitation system. Generally, two methods are commonly
utilized for the application of the direct current to the rotor of
a synchronous motor:
1. Brush-type systems apply the output of a separate DC
generator (exciter) to the slip rings of the rotor,
2. Brushless excitation systems utilize an integral exciter
and rotating rectifier assembly that eliminates the need
for brushes and slip rings. The brushless excitation is
called often as self-excitation.
G. Dajaku is Senior Scientist with FEAAM GmbH, D-85577 Neubiberg,
Germany (e-mail: Gurakuq.Dajaku@unibw.de).
D. Gerling is Full Professor at the University of Federal Defense Munich,
Institute for Electrical Drives, D-85577 Neubiberg, Germany (e-mail:
Dieter.Gerling@unibw.de).
A. Brush-type excitation
Before brushless excitation machines, an excitation was
traditionally implemented using slip-rings with carbon
brushes. The distinguishing feature of the brush-type
machines is that stationary brushes are used to transfer the
DC exciting current to the rotating generator field. Current
transfer is made via rotating slip rings (collector rings) that
are in contact with the brushes, Fig 1. Each collector ring is a
hardened-steel forging that is mounted on the exciter shaft.
Two collector rings are used on each exciter, each ring is
fully insulated from the shaft and each other. The inner ring
is usually wired for negative polarity, the outer ring for
positive polarity. The main disadvantage of brush-type
excitation is that the slip-ring excitation equipment is
significantly larger and expensive. Additionally, slip rings
need regular maintenance and the carbon dust from the
brushes is harmful for the machine.
Fig. 1: Brush-type excited current synchronous machine.
B. Brushless excitation system
The self-excitation method for the current excited
synchronous AC machines is not new and is widely used by
the wind power generators. Fig. 2 shows the basic concept
for the brushless excitation method. Based on the
self-excitation concept the air-gap harmonics are used to
supply the rotor field winding. Several patents are granted
according to this excitation method [1] – [3]. The concepts
presented in these inventions are the same however they
differ only on the realization; the stator contains two
winding systems; the main three-phase winding which is
generally a distributed overlapped winding, and an
additional winding (auxiliary or field winding) to generate
high harmonics in the air-gap. Since the both stator windings
are located in the same stator core, they require a large stator
volume for making place for the both winding systems.
Fig. 2: Brushless excited current synchronous machine.
To overcome the drawbacks of the conventional selfexcited synchronous machine, in this paper a novel brushless
synchronous machine is presented. Fig. 6 show the basic
block circuit diagram for the new machine concept. The new
machine design is characterized with simple tooth
concentrated windings for both the stator and the rotor, and
it doesn’t need any additional auxiliary winding in the stator
and its corresponding power supply and control unit. The
three phases ABC stator winding generate simultaneously
the main working air-gap harmonic which is responsible for
the electromagnetic torque, and also a specific high
harmonic which is used for excitation of the rotor winding.
The rotor consists of two windings; the excitation winding E
and the field winding F. When the rotor rotates, in the rotor
excitation windings E1 to E5 there are induced
electromagnetic forces the magnitude of which depends on
the magnetic field of the air-gap high harmonics. The
induced electromagnetic forces in the rotor excitation
winding are rectified after by the diode bridge circuit, so that
a direct current If flows in the field winding F. To avoid the
high harmonics effect on the field winding, a special
connection for the winding coils for the field winding is
chosen.
To improve the magneto-motive force (MMF) winding
performances of the FSCW regarding to power losses and
noise problems several methods and techniques are
developed and investigated in the past. References [4 to 8]
show different methods for the reduction of winding suband high harmonics such as using magnetic flux barriers in
the specific stator core locations [4] using multi-layer tooth
concentrated windings [5, 6], or increasing the number of
stator slots and using two winding systems [7, 8]. Fig. 3(a)
and (b) show the winding layouts for the conventional 12teeth/10-poles and the new 18-teeth/10-poles concentrated
windings, respectively however, Fig. 4 compares the MMF
winding harmonics. As well shown from the MMF
spectrum, with the new winding the sub- and high MMF
harmonics which are responsible for the rotor losses and also
the low modes radial forces are mostly reduced down to zero.
The investigations carried out on these methods show
enormous improvements on the PM machine performances
such as reduction of the sub- and high MMF harmonics more
than 60%, reduction of radial force modes of low order,
reduction of the machine losses (magnet losses, iron losses
and so on), improves the cooling capability (direct cooling of
coil windings), reduce the slot proximity effects, and so on.
a)
b)
Fig. 3: a). The conventional 12-teeth /10-poles winding, b) The new 18teeth /10-poles winding [7].
12-Teeth/10-Poles
18-Teeth/10-Poles
1
MMF [ p.u. ]
These winding must be good insolated to each other which
leads to further decreasing of the copper filling factor and
increasing also the stator manufacturing costs. Further, the
stator field winding requires an additionally power supply
device or rectifier bridge. On other side, the rotor consists
also of two complex winding systems that in general
increase the total costs of these machine types.
0.8
0.6
II. CONCENTRATED WINDINGS
0.4
Recently, fractional slot concentrated windings (FSCW)
are widely used for the design of different PM machines for
several industry applications. The use of concentrated
windings offers the advantage of short and less complex
end-winding, high slot filling factor, low cogging torque,
greater fault tolerance, and low manufacturing costs.
However, the magnetic field of these windings has more
space harmonics, including sub-harmonics. For the PM
machines, the torque is developed by the interaction of a
specific high stator space harmonic with the permanent
magnets. In other side, the rest of others sub- and high
harmonics, which rotate with the different speed and also in
opposite directions, lead to undesirable effects, such as
additional stator and rotor iron losses, eddy current loss in
the magnets, and noise and vibrations, which are the main
disadvantages of these winding types.
0.2
0
0
5
10
15
Harmonic Order
Fig. 4: Comparison of the MMF space harmonics.
20
III. NOVEL SELF-EXCITED SYNCHRONOUS MACHINE WITH
CONCENTRATED WINDING
Different from the optimization methods mentioned
above where different techniques are used for reduction of
MMF winding unwanted harmonics, in [9] a self-excited
synchronous machine concept is presented where the FSCW
are used for the stator winding and the high harmonics are
used for the excitation of the rotor field winding. In
following the new 18-teeth/10-poles winding is considered,
however, of course the new self-excited concept is useful
also for any conventional concentrated winding.
As well shown from Fig. 4, the new 18-teeth/10-poles
winding is characterized with the 5th and the 13th MMF space
harmonics. Therefore, according to [9] for an 10-poles
self-excited synchronous machine the 5th harmonic is used as
working harmonic, however, the 13th high harmonic is used
to excite the rotor field winding. Figs. 5 and 6 illustrate the
new self-excited concept according to the 18-teeth/10-poles
winding. The rotor contains the excitation winding E and the
field winding F, and the diode rectifier in between. The
excitation and the field winding are realized using tooth
concentrated coils wounded around the same rotor teeth as
illustrated in the Fig. 5. The winding coils for the rotor
winding are connected according to the winding schemas
given in the figures 7 and 8. Further, the power supply
device and the control unit are used for supplying and
controlling the electric machine in a proper way. The
generator and motor operation mode is possible.
Fig. 5: Self-excitation concept for the 18-teeth/10-poled machine.
Fig. 6: Block circuit diagram for the new self-excited synchronous machine.
A. Reduction of harmonics effect on the rotor field winding
Usually three-phase current excited synchronous machines
commonly use single- or double layers, overlapping,
distributed windings with q ≥ 2 ( q – is the number of coils
per pole per phase). This winding configuration results in
more sinusoidal magneto-motive force (MMF) and
electromotive force (EMF) distributions, and hence, good
machine performances. However, it is characterized with
many drawbacks such as large end-winding length,
overlapping coils, high number of stator slots per pole and
low slot filling factor which are related with higher
manufacturing and material costs, high copper losses and
other end-winding parasitic effects. In other side,
fractional-slot concentrated windings, or modular windings,
have been gaining a lot of interest in Permanent Magnet
(PM) synchronous machines. This is due to the several
advantages provided by this type of windings which are
mentioned in previous section.
Different from the distributed windings, FSCW do not
produce high quality traveling fields (sinusoidal MMF). As
is shown from Fig. 4, several space harmonics of closely the
same magnitude are produced that travel with different speed
and in different directions referred with the working MMF
harmonics. In the case of the synchronous machine with
wounded rotor these harmonics induces currents and
generate parasitic torque components and high rotor losses
compared to a conventional distributed winding. Therefore,
to avoid the harmonics effect on the rotor field winding in
the new machine design the winding coils for the field
winding are connected in series as is illustrated in Fig. 7. As
will be shown in the following analysis, by series
connection, even the high harmonics induce flux
components in each coil they are cancelled in the resulting
induced flux-linkage.
As mentioned above, the air-gap flux density components
due to the MMF high harmonics rotate with the different
speed (asynchronous speed) referred to the rotor
synchronous speed. Analogous to the asynchronous
machines the slip of the ν th harmonic is
ν
ω − ωR
ν
s = νS
= 1 −ν (1 − s )
(1)
ωS
with,
ν
where, ωS is the stator electrical angular frequency, ωR is
the rotor electrical angular frequency.
The induced flux-linkage in the rotor field winding (one
winding coil) for the case when coil pitch is equal with the
rotor pole pitch is
ν
ψ ( t ) = νψˆ ⋅ cos ( ν s ⋅ ωS t )
(2)
1
ν
ψˆ = 2 ⋅ r ⋅ l ⋅ ν Bˆ ⋅ ν ξψ
Fig. 7: Ten poles rotor field winding.
E1
E2
E3
E4
E5
E1
E2
E3
Fig. 8: Rotor excitation winding.
ω S = ωS / ν
E4
E5
ν
ν
where, ξψ represents the coupling factor between the ν th
stator MMF harmonic and the rotor field winding,
⎛ν π⎞
ν
ξψ = sin ⎜ ⋅ ⎟
(3)
⎝ pR 2 ⎠
The above expression shows that for pR = 5 (ten-poles rotor)
and for ν = 13 the coupling factor is about 80%.
Let be considered now the case when coils of the
field winding are connected in series. According to eq. (2),
the induced flux-linkage in the k-th winding coil can be
written as
⎛
π ⎞
ν
ψ k ( t ) = νψˆ ⋅ cos ⎜ ν s ⋅ ωS t + ( k − 1) ⋅ν
(4)
⎟
pR ⎠
⎝
From eq. (4) and according to Fig. 7, the resulting fluxlinkage represents the sum of all induced fluxes in each coil
winding and can be determined as,
2 pR
2 pR
⎛
π ⎞
ν
ψ ( t ) = ∑ νψ kb ( t ) = ∑ νψˆ ⋅ cos ⎜ ν s ⋅ ωS t + ( k −1) ⋅ν ⎟
(5)
pR ⎠
k =1
k =1
⎝
Therefore, a possible realization of the rotor excitation
winding with coupling factor equal to one is illustrated in
Fig. 9. Of course, it is also possible to vary the coil span for
the rotor excitation winding in such a way that to reduce the
effect of other high harmonics. Further, different from the
field winding, to ensure an induced voltage due to the high
harmonics, only the symmetrical coils of the excitation
winding can be connected in series, however, the coil groups
should be connected directly to the diode rectifier as is
illustrated in Figs. 6 and 8.
Solving the above relation, the resulting flux-linkage in rotor
field winding is
⎛
3⎞ π ⎞
⎛
ν
ψ ( t ) = 2 pR ⋅ νψˆ ⋅ ν ξ ∑ψ ⋅ sin ⎜ ν s ⋅ ωS t + ⎜ 2 pR − ⎟ ⋅ν
⎟
(6)
2 ⎠ pR ⎠
⎝
⎝
ν = (2 g − 1) ⋅ pR and g = 1, 2, 3,....
with
ν
sin (ν ⋅ π )
⎛ν π⎞
⋅ sin ⎜
⎟
⎛ π ⎞
⎝ pR 2 ⎠
sin ⎜ν ⋅
⎟
⎝ pR ⎠
⎧ 1 for ν = (2 g − 1) ⋅ pR ,
⎪
=⎨
⎪0 for ν ≠ (2 g − 1) ⋅ p ,
⎩
R
ξ ∑ψ =
(7)
g = 1, 2, 3,....
Fig. 9: Rotor excitation winding with winding factor equal to one regarding
to the 13th air-gap flux density harmonic.
g = 1, 2, 3,....
According to the above relations (6) and (7) it is shown that
for the case when the all coil windings are connected in
series only the working harmonic is induced in the rotor
winding, however, the other unwanted harmonics
(ν ≠ (2 g − 1) ⋅ pR ) , even they are induced on each rotor coil,
are cancelled in the resulting induced flux-linkage.
B. Increasing the coupling effect for the excitation winding
According to eq (3), the coupling factor for the rotor
excitation winding and for the case when the coil span-angle
is α x cab be written as,
ν
⎛
ξψ = sin ⎜ν ⋅
C. Hybrid rotor design
Another possible design for the new self-excitation
synchronous machine is to use permanent magnets (PM) in
the rotor teeth (in small amount) to improve the machine
performances in low speed. The following Fig. 10 shows a
hybrid rotor topology with permanent magnets inset in the
rotor top teeth. Of course, other rotor topologies are also
possible such as using the magnets in the rotor yoke beside
the slot bottom and magnetized in the circumferential
direction, Fig. 11.
αx ⎞
(8)
2 ⎟⎠
⎝
Fig. 8 show the coupling factors vs. coil-span angle for the
5th and the 13th MMF harmonics. We can see here that the
maximal coupling factor for the 13th MMF harmonic can be
realized when the coil-pitch of the rotor excitation winding
is 70° electrical degree.
1
v=5
v=13
0.8
0.6
Coupling-factor
0.4
0.2
Fig. 10: Self-excitation synchronous machine using additional PM in the
rotor teeth.
0
-0.2
-0.4
-0.6
-0.8
-1
0
20
40
60
80
100
120
140
160
180
alfax [el. degree]
Fig. 8: Flux-linkage coupling factors for the 5th and the 13th MMF
harmonics.
Fig. 11: Alternative hybrid rotor topology.
IV. EXEMPLARY MACHINE DESIGN
Field Current
80
60
If [A]
According to the new machine concept, an 18-teeth/10poles self-excited synchronous machine for electric vehicle
application is designed and analyzed. As typical
requirements for the automotive traction drive application,
the following data are used: maximum DC-voltage
UDC=400V, maximum current Imax = 650Arms maximum
short-time torque Tmax=270Nm @ 5100rpm, and the
available machine volume: DOut=230mm, LStack=150mm. Fig.
12 and Table-1 show the geometry and the main geometrical
data of the investigated machine.
40
20
0
0
60
120
180
240
300
360
time [ms]
Electromagnetic Torque
350
300
T [Nm]
250
200
150
100
50
0
-50 0
60
120
180
240
300
360
time [ms]
Fig. 12: Geometries of the new 18-teeth/10-poles self-excited synchronous
machine.
Fig. 12: Simulation results for Is = 500A and n = 5100 rpm.
TABLE I: MAIN GEOMETRY DATA
150 mm
Outer stator diameter
230 mm
Outer rotor diameter
160 mm
Gap length
1 mm
Turns per phase - Stator
16/6
Turns per phase – Rotor field winding
350
Turns per phase – Rotor field winding
70
Number of rotor poles
30
20
10
0
STEEL 330-35AP
0
0.5
The design and analysis of the investigated machine is
performed using finite elements method (FEM). The
simulations results for the electromagnetic torque and the
rotor field current presented in the following Figs. 12 and 13
are determined for different operation conditions. Firstly, the
machine is investigated for 500A stator current and at 5100
rpm rotor speed. As well is shown, the machine torque for
this operation point is about 290Nm, however, the rotor field
current is 47 A. Afterwards the simulations are performed
for low rotor speed (600 rpm). Since the induced rotor filed
current depends on the rotor speed, during this simulation
the stator phase current is increased up to 650A to have a
machine torque of about 260Nm. Of course, the machine
torque for the low rotor speed further can be increase by
optimizing the reluctance torque component or by selecting a
proper winding relation for the field- and the excitation
winding.
1
1.5
2
2.5
time [ms]
10
Electromagnetic Torque
300
250
200
T [Nm]
Iron Core material
Field Current
40
If [A]
Active length
150
100
50
0
0
0.5
1time [ms]
1.5
2
2.5
Fig. 13: Simulation results for Is = 650A and n = 600 rpm.
V. CONCLUSIONS
This paper presents a new self-excited synchronous
machine concept where the both stator and rotor contains
simple tooth concentrated windings. The three phases stator
winding generate simultaneously the main working air-gap
harmonic which is responsible for the electromagnetic
torque, and also a specific high harmonic which is used for
excitation of the rotor winding. Different from the
conventional brushless excitation concept it doesn’t need
any additional auxiliary winding in the stator and its
corresponding power supply and control unit.
Using the new machine concept, an 18-teeth/10-poles
machine for HEV applications is designed and investigated.
For the considered stator winding the 5th MMF harmonics is
used as working (fundamental) harmonic, however, the 13th
MMF harmonic is used for the excitation of the rotor field
winding. To proof the operation of the new machine concept
different operation conditions are investigated. The obtained
results for the electromagnetic torque and the rotor field
current response demonstrate the applicability of the new
technique.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
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