Two-phase transverse flux machine with disc rotor for high torque low speed HIEKE Sebastian
application
Two-phase transverse flux machine with disc rotor for high torque low speed
application
Sebastian Hieke, Mario Stamann, Andrii Masliennikov, Aleksei Duniev,
Dmytro Lagunov, Roberto Leidhold Andrii Yehorov
Otto-von Guericke-Universität National Technical University
Universitätsplatz 2 Kharkiv Polytechnic Institute
39104 Magdeburg, Germany Kyrpychova str. 2
Phone: +49 0391-67-54898 63101 Kharkiv, Ukraine
Fax: +49 0391-67-12481 Phone: +38 057-707-65-14
Email: sebastian.hieke@ovgu.de Email: x-maslennikov@yandex.ua
URL: http://www.ovgu.de URL: www.kpi.kharkov.ua
Keywords
Transverse flux motor, Efficiency, Renewable energy systems, Design, Permanent
magnet motor
Abstract
This paper presents a special two-phase transverse flux machine with disc rotor. Because of the noncom-
plex construction and by using simple 2D flux path and 3D printing methods the manufacturing costs are
hold low. A design criterion is proposed for maximizing the torque density in the low speed range. To
prove the concept and analyse the potential of this construction a first direct driven prototype was built.
The experimental results confirm the viability of this proposal and provide the required information for
further enhancements.
1 Introduction
The present work deals with a special design of transverse flux machines. Transverse flux machines
(TFM) is a kind of synchronous machines with a particular concept for the flux path. It consists of a
rotor with permanent magnets and a stator coil, which is enclosed by a toothed steel structure (Figure 1).
stator tooth bi bW 2
3
A 4W hW bM
1 
rotor disc coil
magnet
Fig. 1: Complete model of the transverse flux machine with disc rotor and model of one tooth: 1-tooth,
2-iron back, 3-coil, 4-permanent magnet
The TFM is capable of a very high torque per volume and mass in the low speed range [1]. Strong
ripple torque and a low power factor at full load are major drawbacks of such machines. Nevertheless
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Two-phase transverse flux machine with disc rotor for high torque low speed HIEKE Sebastian
application
its construction can be simple. Especially, the winding consists only on a ring coil per phase, unlike
conventional machines with distributed or concentrated windings, which require several coils per phase.
Furthermore, the teeth are made of conventional electrical steel sheets and its U-form allows a simple
manufacturing.
What we aim at with this paper is to design a TFM generator with the simplest possible construction
for use in renewable energy generation. For example, hydrokinetic power stations can be used to gain
energy from river streams as off-grid generation in sparse populated regions, where power grids are not
available. Figure 2 shows a testing plant of a small horizontal water wheel with a diameter of 0.75m. It
is used for drinking water purification. This is a candidate application for the TFM.
Fig. 2: testing plant of a small horizontal river water wheel for drinking water purification
For electric power generation a high efficiency generator is needed, because the maximum mechanical
power of the water wheel is limited by the stream speed of the river. Most efficient energy conversion
can only be reached by using a direct driven generator without gearbox. Therefore this paper deals with
the design of an unconventional generator, which is working at low speed with high torque and high
efficiency. To reduce the payback period of the power station, also production costs of the generator have
to be as low as possible.
Several scientific articles were published about machines with a disc rotor. The form factor (high diam-
eter to length-ratio) of such machines is suited for direct drive applications. These are mostly axial flux
machines, using concentrated windings and surface mounted magnets [2]. In [3] the axial flux machine
is proposed for application in wind power plants, as a low speed gearless generator. In [4] a TFM with
disc rotor is presented, which is based on the switched reluctance principle. However, the complexity
is quite high, requiring six windings and three discs. Another prototype of a TFM with only one disc is
introduced in [5]. In that topology the flux actually does not closes through the disk but radially through
rings attached to the disc. A drawback of that proposal is that the flux of the stator cores has a 3D flux
path, which makes the realization more difficult.
2 Proposal
In this paper a two-phase transverse flux machine with a disc rotor for high torque low speed applica-
tions is proposed. Figure 1 shows a model of the machine with two coils enclosed by the teeth. The rotor
consists of a circular massive steel disc and surface-mounted magnets for each phase. The disc does not
need to be laminated due to the low frequency at low speed and expectable slight eddy current losses.
As there is one disc-side required for each phase of the TFM, a three phase machine would require two
discs. Consequently, it was chosen to use only two phases for a simpler design with only one disc.
Furthermore, the design of the teeth is kept in the configuration as proposed by Weh [6]. Thereby the
magnetic flux is limited to only two dimensions, unlike usual configurations that require flux in three di-
mensions. For this reason the tooth can be made of laminated steel, rather than soft magnetic composites
needed for a three dimensional flux path [7]. The teeth are U-shaped as shown in figure 1.
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Two-phase transverse flux machine with disc rotor for high torque low speed HIEKE Sebastian
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To allow a simple modular production of the machines of different ratings (speed, torque) the tooth shape
is kept constant. That is bi, bw and hw in figure 1. Just the stack height bm is variable, which together
with the additive-manufactured parts allow to flexibly arrange the number of pole pairs depending on the
area of application, rated torque and rated speed.
Table I: Parameters of the TFM
Quantity Symbol Value
Rated torque T 10 Nm
Speed n 100 rpm
Height H 180 mm
Axial length l 110 mm
Width W 180 mm
Radius r 73 mm
Rated Power P 100 W
Air-gab length g 1 mm
Magnet thickness hm 2 mm
Pole pairs p 32
Number of turns Nt 280
Coil resistance R 4.4 Ω
Coil Inductance L 76 mH
Not only the magnetic circuit should be of simple construction, also the mechanical components, like
the bearing plates, will be kept as simple as possible. Therefore additive manufacturing is used for most
parts of the machine, which are not moving and not in the magnetic flux path. Some precedence can be
found in [8], where an axial flux machine using composite plastic materials is investigated.
Figure 3 shows the disassembled machine (left), where the rotor and both stators parts can be appreciated,
as well as the assembled machine (right). In figure 4 the machine is shown mounted on the test rig. The
stator cases are 3D-printed with a fill factor of 50% to ensure adequate stability. As can be seen, there
are different recesses and slots, the former is for saving materials the latter is to plug in the U-Cores and
the bearing. The ring-coil is placed inside the U-Cores. This clearly shows the advantage of such coils,
which can be winded in a separated step and thereafter placed and fixed with an epoxy resin into the case.
Fig. 3: Prototype of the two-phase TFM with disc rotor
The use of plastic materials together with additive manufacturing methods allows a highly flexible pro-
duction, which offers the possibility to produce cheap generators for a small number of units, especially
for prototyping. This can also be a disadvantage due to higher tolerances in the geometry, which in turns
lead to increasing cogging torque, increasing harmonics of the back-EMF and therefore to more ripple
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Two-phase transverse flux machine with disc rotor for high torque low speed HIEKE Sebastian
application
torque. However, several scientific articles have shown that it can be effectively be reduced by means of
an active (control base) torque ripple compensation as can be found in [9] and [10]. Based on the method
in [10], the torque ripple of the proposed TFM with disc rotor was minimized, as detailed in section 4.
Fig. 4: Prototype of the two-phase TFM with disc rotor on the test rig
The whole structure of the stator is made of PLA except teeth and coils, as shown in figure 3. Cer-
tainly, this affects the power rating of the machine due to the maximal allowed temperature and material
strength. Conventional rotary machines with air cooling operate with a current density of 3 - 6 A/mm2.
The prototype presented in this paper uses a maximum current density of 3 A/mm2, because of the low
maximal temperature allowed by the frame material.
3 Geometry and force optimisation
The number of pole pairs and consequently the pole pitch is only in a close range alterable to get an
optimal yield of torque per volume. Figure 5 shows the torque characteristic of the disc rotor TFM for a
constant diameter and MMF, depending on the number of pole pairs, as computed by the finite element
method (FEM). By a given air gap, a small pole pitch leads to high leakage flux, while a wide pole pitch
decreases dB/dϑ in the air gap, and consequently the torque. Where B is the flux density and ϑ is the
angular position of the rotor.
25 # = 1000 A
20
15
10
5 10 15 20 25 30 35 40
number of pole pairs
Fig. 5: Torque dependent on number of pole pairs computed by FEM; MMF is 1000A
The rating of the torque can be improved by further modifications of the magnetic circuit. The winding
sections between two U-Cores are linked with the leakage flux of the exposed magnets. It is important
to note, that this flux is in opposite direction than the flux in the U-Cores. Thus it appears that a negative
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torque [N "m]
Two-phase transverse flux machine with disc rotor for high torque low speed HIEKE Sebastian
application
torque component is produced, reducing the total torque of the machine [11]. To avoid this effect an
additional I-Core is placed between two U-Cores. Consequently, the leakage flux of the exposed (i.e.
inverted) permanent magnets is not linked with the windings any more and the back-EMF and torque
increases. Both machines versions i.e. without and with I-cores are investigated in the next section.
I-Core
U-Core
stator coil
Fig. 6: Magnetic circuit modification by adding I-cores between the U-cores for increasing the EMF and
torque
4 Results
A first prototype was built in two versions to validate the proposal, as shown in figure 3. Figure 7 shows
the position-derivative of the flux-linkage calculated from the measured back-EMF and speed for both
versions according to (1).
1.2 without I-Core 1.2
0.8 with I-Core
0.6
0.4
0 0
-0.4
-0.6
-0.8
-1.2 -1.2
0 1 2 3 4 5 6 7 8 9 10 11 12 -1.2 -0.6 0 0.6 1.2
d*
# [rad] , [V "selec d#mech rad ]
Fig. 7: Position-derivative of the flux linkage as function of the position i.e. the EMF per speed unit
The position-derivative of the flux-linkage can also be considered as the back-EMF per speed unit, which
is a characteristic of the machine. Both curve characteristics are notably different from each other. On
the one hand the version without I-Cores gives a near to sinusoidal wave form, contrary to the second
version with a near triangular waveform. The magnet flux of the permanent magnet is shorten when
facing of the I-Cores and concentrated when facing the U-cores providing a sharper transition, which
leads to the near triangular waveform.
dΨ dΨ dϑ dΨ
ui = = = ·ω (1)dt dϑ dt dϑ
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d*, [V "sd# rad ]elec
d*- [V "sd#mech rad ]
Two-phase transverse flux machine with disc rotor for high torque low speed HIEKE Sebastian
application
Where ui is the induced voltage, Ψ is the flux linkage and ω is the angular speed. The increase in the
RMS value clearly shows the advantage of the prototype with I-Core, for instance the calculated RMS
values are given in table II. Comparison of the torque per-unit volume shows that the modified machine
produced 1.5 times the torque per unit volume than the machine without I-cores. Consequently, it is
possible to reduce the current for an equivalent output power and therefore the copper losses decrease.
Table II: Comparision of
Quantity ( Symb)ol Value
dΨ rms V · s
RMS of derivative of the flux linkage without I-Cores α 0.476
(dϑelec) rad
dΨ rms V · s
RMS of derivative of the flux linkage with I-Cores α 0.723
dϑelec rad
The torque ripple at 10 rpm of the proposed TFM is shown in figure 8. It is around 17.5 % of the
rated torque. For analysis purpose, the frequency spectrum is given in the same figure. By comparing
the torque variation at full load for both machine types, it can be recognised that the second prototype
has a slightly reduced torque ripple. A valuable improvement in minimizing the torque ripple for both
machines is gained by compensating the frequency components twofold, fourfold and sixfold of the TFM
supply frequency [10], thereby decreasing the torque ripple from around 15-17.5 % to 2.5-5 %.
without I-Cores without I-Cores
1.5
12
uncompensated
11 compensated1
10
9 0.5
8
0
0 0.2 0.4 0.6 0.8 1 0 10 20 30 40 50
time [s] frequency [Hz]
with I-Cores with I-Cores
1.5
12
11 1
10
9 0.5
8
0
0 0.2 0.4 0.6 0.8 1 0 10 20 30 40 50
time [s] frequency [Hz]
Fig. 8: Comparison of the torque ripple at 10 rpm
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torque [N "m] torque [N "m]
torque [N "m] torque [N "m]
Two-phase transverse flux machine with disc rotor for high torque low speed HIEKE Sebastian
application
Figure 9 illustrates the control scheme of the realised compensation method. The Park transformation of
stator related variables ua, ub, uc and ia, ib, ic into rotor orientated variables ud , uq and id , iq is performed
through T , using the electrical rotor angle θel for field orientation.
id ua
iref=0 - ud PI ud b
id uc
~ T
iref iq PI u iq q a
- iq ik=1 b
: i
k=n q ic
iqc ,k ,Φk
θel
Fig. 9: Control scheme of ripple torque compensation by feed-forward set-point control in rotor orien-
tated coordinates d and q
Two PI-controllers are used to control id and iq separately and are optimised by the amplitude optimum
criterion. It is not necessary to utilise every frequency component. This should be done dependent
upon the frequency spectrum of the torque ripple. Magnitude îqc,k and phase shift φk have be adjusted
according to load and rotating direction [10].
Figure 10 shows the efficiency of the proposed TFM with and without I-Cores, as a function of the speed
and for a constant torque of 10 Nm. The proposed concept without I-Cores is able to achieve a maximum
efficiency of 80 % at around 100 rpm, when iron loss equals copper loss. Further increasing of the speed
has negative effects due to higher iron losses. In Contrast, the TFM with I-Cores achieves the maximum
efficiency at 40 rpm, due to the reduced copper losses but additional iron losses in the I-Cores. This
prototype uses I-Cores made of massive steel, rather than laminated steel leading to higher iron losses in
the additional cores.
As for comparison, the efficiency for a 120 W , 8 poles induction machine with efficiency class IE4 (EN
60034-30-1) is 62.3 %. Here still a gearbox has to be used to reach the required speed, which will reduce
the overall efficiency even more.
30
80
with I-Core
70 without I-Core
60 20
50
40 10
30
20
0
0 20 40 60 80 100 0 20 40 60 80 100
speed [rpm] speed [rpm]
Fig. 10: Efficiency and iron losses at 10 N ·m
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e/ciency [%]
iron losses [W ]
Two-phase transverse flux machine with disc rotor for high torque low speed HIEKE Sebastian
application
The chosen concepts provides the opportunity for using a ring coil, which could be prepressed in a die
to increase the fill factor and consequently minimize the resistance R of the TFM. Thus copper losses
and the resistance could be reduced by 20 % by increasing the copper fill factor from 60 % to 80 % [12].
This would lead to a slight improvement of the efficiency from 80 % to around 82 % at 100 rpm in the
machine without I-Cores.
Table III: Inductance of the TFM
Quantity Symbol Value
direct-axis Inductance Ld 76 mH
quadrature-axis Inductance Lq 78.4 mH
In order to analyse the self sensing capability of the machine the inductance was measured as function of
the position. From this measurement, the direct-axis inductance Ld and the quadrature-axis inductance
Lq were determined. They show a difference of only 3 %, which in general is to low implement self
sensing methods.
5 Conclusion
A two phase transverse flux machine with disc rotor has been successfully manufactured and tested.
The motor should provide an easy way of manufacturing and a high efficiency at low speed. Both
aims were achieved for two different prototypes. It has been showed, that there is potential for further
improvements, like prepressed windings and I-Cores made of laminated steel sheets to reduce copper
and iron losses.
References
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