Design Considerations For Dual Winding Permanent
Magnet Synchronous Machines
2
1
2
E. Mese1, M. Tezcan1, M. Ayaz , Y.Yasa , K. Yilmaz
(1)
Yildiz Technical University, Dept. of Electrical Engineering, Istanbul,Turkey
emese@yildiz.edu.tr, mutezcan@yildiz.edu.tr, yasa@yildiz.edu.tr
(2)
Kocaeli University, Technical Education Faculty, Kocaeli, Turkey
murat.ayaz@kocaeli.edu.tr, kayel@kocaeli.edu.tr
Abstract— This paper introduces design and analysis
performed for a dual winding electric machine. Windings of
the electric machine are concentrated type so that electrical
and magnetic isolation is maintained. For this reason, the
proposed structure could be a considerable choice for fault
tolerant applications. As required by some applications
(electric accessory drive system (EADS) for hybrid electric
vehicles), with proposed technique, simultaneous motoring and
generating operations can be implemented in a single housing
of electric machine. Design considerations of the proposed
electric machine are outlined in the paper. Also a comparison
between distributed winding topology and concentrated
winding topologies is performed. Finally experimental results
discussed.
Keywords: Permanent Magnet Synchronous Machine,
Concentrated Winding, Dual Winding Electric Machine, Fault
Tolerant Operation, Hybrid Electrical Vehicle Applications
I.
INTRODUCTION
Permanent magnet synchronous machines (PMSM)
with concentrated winding topologies have recently become
popular due to simpler and more compact structure as well
as some operational advantages such as wider flux
weakening range and higher torque output [1,2,8].
Furthermore magnetic isolation feature between coils, which
naturally comes with concentrated winding topology,
presents some opportunities such as better fault tolerance
[3,4] and multi tasked operation as required by some
accessory drive system applications in HEVs. Simultaneous
motor and generator operation is such a multi-task
functionality and it can be achieved by magnetically isolated
dual winding system. This paper presents the evolution of
dual winding structures in an effort to find out the best
winding configuration [5,6] with magnetic isolation so that
a proper multitasking is performed. The winding
configurations under consideration are distributed, doublelayer concentrated and single-layer concentrated windings.
Section II summarizes accessory drive system for hybrid
electric vehicles. Section III discusses baseline study with a
conventional type dual winding electric machine. Section IV
presents the proposed dual winding structure. Section V
presents other design considerations which should be taken
into account for a true magnetic isolation. Section VI
discusses experimental results.
II.
APPLICATION DETAILS
Intended application for the dual winding machine is
accessory drive system (ADS) in HEVs. Continuous
provision of mechanical and electrical power to the
accessory loads and to the battery for zero and nonzero
engine speeds is the main idea behind ADS. Electric
machine, which is integrated into ADS, should run as motor
and generator simultaneously at zero engine speed.
Whereas, at nonzero engine speed, engine itself takes over
motoring duty out of electric machine and let it run in
generator only mode. Detailed description of operating
modes of the ADS is as follow:
Mode 1: In this mode, engine speed is zero, motor
should operate at 1500 rpm and provide 6 kW mechanical
power. In this mode, electric machine should also run as
generator to provide charging power to the 12 V battery.
The required power at the generator output is 3 kW at 12
volt battery voltage.
Mode 2: Electric machine should operate only as a
generator in this mode. No motoring is required since this
task is taken over by the engine. As a generator, the machine
should provide 3 kW at 12 volt battery voltage between
1500 rpm and 6000 rpm motor shaft speed.
Several approaches can be taken to implement both
motor and generation action in ADS system so that
continuous operation is assured regardless of engine speed.
Perhaps, the simplest one is to utilize two electrical
machines: one is dedicated for the motoring and the other
one is dedicated for the generating [9]. However, using two
separate machines will increase packaging size, rotational
inertia, and mass.
Another approach, which is the topic of this paper, is to
use dual winding concept in the stator core. The essence of
this approach is to put two winding sets (3kW – 6kW) into
the stator such that they will share stator slots properly as
well as the rotor while they act as two independent electric
machines.
III.
BASELINE STUDY WITH DIFFERENT MACHINE
STRUCTURES
A. Conventional Type Distributed Dual Winding Electric
Machine
A baseline study has been performed to see the
drawbacks of the conventional dual winding machine. In
conventional distributed type dual winding electric machine,
one set is dedicated for motor operation and the other one is
dedicated for the generator operation. Winding sets are
electrically isolated. However magnetic isolation exists
because of the way two sets are wound. The degree of the
magnetic coupling between the winding sets can be
minimized by properly adjusting phase angle (30ºe) between
sets as shown in the Fig. 1. But complete decoupling
between the sets can never be guaranteed.
Figure 2. Motor torque variation for different load levels of the generator
(n=1500rpm).
After some Finite Element Analysis (FEA) simulations,
existence of magnetic coupling between two winding sets
was observed. In Fig. 2, the effect of generator side loading
on the motor side torque output is shown, where generator is
connected to an uncontrolled rectifier. Even if a severe
interaction between fundamental components of generator
and motor magnetic fields is not observed, higher order
harmonics of generator magnetic field adversely affect
motor magnetic field. As a result high torque ripple at the
motor output and strong dependency of average torque on
the generator load level occur.
B. Concentrated Dual Winding Machine Structures
Concentrated winding coils are wound around single stator
tooth as shown in Fig. 3 (a) and (b). Motivation for
concentrated winding is to provide magnetic isolation
between two sets of stator winding. It can be shown that
each concentrated coil around any stator tooth has an
independent magnetic circuit [1] and it does not interfere
magnetically with any other coils around the stator.
B.1. Double Layer Dual Winding Topology
The same stator geometry as in distributed winding case
is used for double layer concentrated winding. Some basic
parameters for stator and rotor geometry as well as winding
information are given in Tables I-II-III.
Figure 1. Distributed dual winding electric machine
Compared to the distributed winding, generator side
loading has weaker adverse effect on motor torque as shown
in Fig. 4.
(a)
(b)
Figure 3. (a) Concentrated double layer dual winding electric machine (b)
Concentrated single layer dual winding electric machine
Figure 5. Connection diagram of concentrated single layer dual winding
machine
Figure 4. Motor torque variation for different load levels of the
concentrated double layer dual generator winding (n=1500rpm).
B.2. Single Layer Dual Winding Topology
In this configuration, coils are wound around single
stator tooth as shown in Fig. 5 and one slot is occupied
solely by one coil side. Finite element modeling with
Maxwell 2D Transient has been used to show how
concentrated winding approach is effective in decoupling
two winding sets magnetically. Similarly, generator winding
current magnitude is varied to observe how motor operation
is influenced by generator winding current variation. Note
that higher amplitude of the generator phase currents do not
show appreciable increase or decrease in the motor average
torque output. Furthermore torque ripple amplitude did not
change significantly as shown in Fig. 6. In another test,
motor current magnitude is varied to see its influence on
generator output voltage. In Fig. 7, it can be seen that any
increase or decrease in the motor current does not cause any
increase or decrease in the generator back Electromotive
Force (EMF). This proves that motor winding current has no
effect on the generator winding magnetic circuit.
Accordingly generator winding back EMF as an indicator of
generator output voltage is constant regardless of motor
loading.
Figure 6. Motor torque variation for different load levels of the
concentrated single layer dual generator winding (n=1500rpm).
Figure 7. Generator back EMF variation for varying motor current
(n=1500rpm).
Major design parameters of distributed, double layer
concentrated and single layer concentrated electric
machines, shown at three tables below;
TABLE I.
GEOMETRICAL DIMENSIONS
TABLE III.
PM Machine Types
Geometrical
Dimensions
Stator Outer
Diameter (mm)
Rotor Outer
Diameter (mm)
Conventional
Distributed
Winding
Concentrated Concentrated
Double
Single
Layer
Winding
Winding
240
240
240
138
138
138
Axial Length (mm)
120
120
120
Stator Slot Number
24
24
24
Pole Number
4
22
22
Coil Pitch
6
1
1
Air gap width (mm)
1
1
1
Magnet height (mm)
5
5
5
TABLE II.
ELECTRICAL PARAMETERS FOR MOTOR MODE
PM Machine Types
Electrical
Parameters
For Motor
Operation
Conventional
Distributed
Winding
Phase number
3
3
3
Rated Power (kW)
6
6
6
Rated Torque (Nm)
38.2
38.2
38.2
Rated Phase RMS
Current(A)
22.81
26.76
26.87
157
157
157
4.71
0.98
0.85
134.656
31.9287
33.5252
81
5.1
5.5
15.362
11.802
15.617
18.02
29.32
21.35
3.86
3.55
3.40
1.198
1.288
1.288
1.966
0.44
0.44
ω (rad/s)
Per phase ind.
(Lad=Laq) (mH)
Per Phase winding
resistance (mΩ)
End Winding
Rezistance (mΩ)
Maximum Output
Power (kW)
Torque Angle
(degree)
Current Density
(A/mm2)
Stator Teeth Flux
Density (Tesla)
Stator Yoke Flux
Density (Tesla)
Concentrated Concentrated
Double
Single
Layer
Winding
Winding
ELECTRICAL PARAMETERS FOR GENERATOR
MODE
PM Machine Types
Electrical Parameters Conventional Concentrated Concentrated
For Generator
Distributed
Double
Single
Winding
Layer
Winding
Operation
Winding
Phase number
Rated Power (kW)
Rated Torque (Nm)
Rated Phase RMS
Current(A)
ω (rad/s)
Per phase inductance
(Lad=Laq) (mH)
Per Phase winding resistance
(mΩ)
End Winding Rezistance
(mΩ)
Maximum Output Power
(kW)
3
3
20.95
3
3
21.13
3
3
21.47
169.287
143.837
161.17
157
157
157
0.083
0.039
0.043
2.51701
1.19603
1.50906
1.51
0.18
0.25
11.790
5.026
5.704
Power Angle (deg)
10.6
37.24
31.99
Power Factor
Current Density (A/mm2)
Stator Teeth Flux Density
(Tesla)
Stator Yoke Flux Density
(Tesla)
0.69
4.02
0.84
3.32
0.71
3.86
1.123
1.44
1.44
1.966
0.44
0.44
IV. OTHER DESIGN CONSIDERATIONS FOR A DUAL
WINDING CONCENTRATED ELECTRIC MACHINE
Investigations performed during previous sections
indicate that single layer concentrated winding
configuration provides decoupled operation for dual
winding electric machines. Although single layer
concentrated winding has much less coupling compared to
the other winding topologies (distributed and double layer
concentrated), detailed analysis showed that absolute zero
decoupling would not be possible. The arrangement of
phase coils over the stator slots has significant effect on
decoupling performance. Fig. 8 shows different options for
configuring coils in a single layer concentrated topology.
Figure 8. a) Type-1, b) Type-2, c) Type-3
Normally, back emf value of the coils A2-B2-C2 is
expected to be zero but effect of the coils A1-B1-C1 flux
lines cause some induced voltage on coils A2-B2-C2. This
voltage waveform is shown in Fig. 9.
Figure 11. Induced voltage waveforms of A2-B2-C2 due to A1-B1-C1
excitation for Type-3 winding configuration.
Figure 9. Induced voltage waveforms of A2-B2-C2 due to A1-B1-C1
excitation for Type-1 winding configuration.
Based on back emf voltage waveform in Fig. 9, induced
voltage value is around 100 mV. As simulation results show
that mutual coupling in type-1 winding configuration is very
small.
Figure 10. Induced voltage waveforms of A2-B2-C2 due to A1-B1-C1
excitation for Type-2 winding configuration.
Based on back emf voltage waveform shown in Fig. 10,
the peak induced voltage due to mutual coupling is around 4
V. Compared to type-1 winding configuration; type-2
winding has more coupling effect.
Based on back emf voltage waveforms, induced voltage
values are under 75 mV. Compared to the other winding
configurations; type-3 has the least mutual coupling
between coils.
Comparison between different configurations shows that
Type-1 and Type-3 seems performing better than Type-2.
Although Type-1 winding arrangement has little mutual
coupling between coils, back emf value of A2-B2-C2 and
average torque drops significantly.
Mutual coupling between winding sets generates
undesirable torque pulsation. The implication of this is
undesired induced torque due to excitation of the other
winding set. In order to quantify the significance of
coupling on the torque output, a simple test is performed.
A1-B1-C1 (motor windings) is excited with 47 A peak
sinusoidal current for each phase without any magnets. In
the mean time, induced back emf across A2-B2-C2
(generator windings in Fig. 10) is observed and stored.
Then, this waveforms are multiplied by sinusoidal phase
current with peak value of 250 A (max. generator output
current) and zero degree phase shift with associated phase
back emf. The multiplication is repeated for each phase and
the results are added together. The sum is divided by
synchronous speed to find out torque pulsation effect of the
winding set A1-B1-C1 on A2-B2-C2 (1). Fig. 12 shows the
induced torque only due to coupling between winding sets
[6,7].
T=
E A I A + E B I B + EC I C
ωS
(Nm)
(1)
The application for which this electric machine is designed
requires 47 A peak phase current for A1-B1-C1 set whereas
250 A peak phase current for A2-B2-C2 set for rated
operation. Hence, the test is conducted by using these
current values.
In order to show magnetic decoupling between motor
and generator winding sets of the proposed PMSM, some
tests were conducted with experimental setup in Fig. 13. At
first, no load motor back EMF waveform at 750 rpm shaft
speed has been recorded as shown Fig. 14. After that,
generator windings were loaded with balanced wye
connected resistive load (resistance of single phase load is
0.576 ohm) and motor back EMF waveform at 750 rpm
shaft speed has been recorded as shown in Fig. 15.
Figure 12. Torque ripple for Type-2 A2-B2-C2 winding where A1-B1-C1
is excited (A2-B2-C2 is unexcited).
V.
EXPERIMENTAL RESULTS
An experimental setup was built to test performance of
the proposed electric machine. Fig. 13 shows preliminary
setup of the experimental system. Dual winding PMSM was
driven by a squirrel cage induction motor. A rotary torque
sensor between induction motor and dual winding PMSM
measures torque level dynamically. Four-quadrant
adjustable speed drive system controls induction machine so
that both motoring and generating operation would become
possible. Induction machine in the experimental setup
represents mechanical load in accessory drive system as
well as internal combustion engine of the vehicle. In Mode
1, induction machine imitates mechanical accessory loads
and runs as generator. In Mode 2, it imitates internal
combustion engine and runs as motor. On the other side of
the experimental system, there exists a resistive load bank
which imitates electrical accessory loads in the vehicle.
Generator side of the dual winding PMSM will be
ultimately connected to resistive load bank through either a
thyristor controlled rectifier or uncontrolled rectifier/DC-DC
converter set [10]. However, at this stage of the preliminary
setup, generator side of the proposed PMSM is loaded with
an uncontrolled rectifier and resistive load bank. A bulk DC
link capacitor also exists at the output of the rectifier.
Figure 14. Single phase motor back EMF waveform for no load operation
(Vmax=40 V)
Figure 15. Motor back EMF waveform for loaded operation (Vmax=40V)
Figure 13. Experimental set up for EADS
As observed in the last two figures, loading of the
generator side does not interfere with the motor winding
operation. There is no difference between no load operation
back EMF waveform and loaded operation back EMF
waveform of the motor winding set. Wye connected
resistive load bank draws sinusoidal current out of the
generator windings and this would not be the case in real
application of the proposed machine.
To be more realistic, an uncontrolled rectifier is
connected to the generator output for observing the effect on
generator winding set and similar tests were performed at
1500 rpm shaft speed. In Fig. 16, it can be seen that
distorted current drawn by the load (green trace) causes
distortion at the voltage output of the machine (pink trace).
Motor winding back EMF waveform during no load
operation of the generator winding is also shown in the Fig.
16 with white trace. Yellow trace also shows motor winding
back emf during loaded generator operation with
uncontrolled rectifier having 1516 Watt resistive load at its
output.
Figure 17. Rectifier input/output voltage waveforms (blue/green), rectifier
output current waveform (pink) and average electromechanical torque
waveform (yellow) for dual winding PMSM
Figure 16. Some waveforms from dual winding PMSM experimental
operation where generator side is loaded with uncontrolled rectifier. Motor
back EMF waveform with no load (white trace), Motor Back EMF
waveform with load (yellow trace), Generator Back EMF waveform with
load (pink trace), Generator Output current waveform (green trace).
Similarly there is no difference between no load back
EMF waveform and rectifier loaded back EMF waveform of
the motor winding. Experimental results show that there is
no magnetic coupling between motor and generator winding
sets with uncontrolled rectifier application.
Yellow curve in Fig. 17 shows that mean value of
electromechanical torque for dual winding PMSM shaft for
loaded conditions. Measured voltage data from torque
sensor output is calibrated to show half of the real torque
value. Hence real value of the electromechanical torque is
12.52 Nm at 1500 rpm shaft speed. This figure also shows
the uncontrolled rectifier’s output current waveform (green
trace), output voltage waveform (blue trace) and terminal
voltage waveform of the generator winding (pink trace) at
1500 rpm shaft speed. In addition, Fig. 18 shows induced
shaft torque waveform for 1516 W resistive load.
Figure 18. Induced shaft torque for 0.864 ohm load value
Proposed PMSM was designed to operate as motor at
1500 rpm shaft speed and as generator between 1500 rpm
and 6000 rpm shaft speed. In this preliminary experimental
work, limited generator operation was tested. Motor
operation as well as more extensive generator operation
results will be given in a future publication with more
details. Table IV shows efficiency values of generator side
for two different load levels at 1500 rpm shaft speed. By
recalling speed range (1500 rpm-6000 rpm) and rated power
level (3 kW) of the generator, the values of in Table IV can
be considered as low-speed and partial-load operation data.
ELECTRICAL AND MECHANICAL VALUES FOR
DIFFERENT LOADS ON GENERATOR MODE
TABLE IV.
Shaft
Speed
(rpm)
Rectifier
Load
(Ω)
Input
Power
(W)
Output
Power
(W)
Shaft
Torque
(Nm)
Efficiency
(%η)
1500
1500
1,728
0,864
1196
1965
938
1516
7.62
12.52
%78
%77
VI.
CONCLUSIONS
This paper presents a comparative study among
different dual winding electric machine configurations. FEA
simulations show that single layer concentrated winding
structure is capable of providing better isolation between
two winding sets. Furthermore, the location of coils in the
stator periphery is highly important for minimum coupling.
Otherwise, even single layer concentrated winding structure
would not provide sufficient magnetic isolation. Also this
paper presents detailed results of FEA along with some
analytical justification regarding the magnetically decoupled
structures. Experimental results also prove that single layer
concentrated winding approach provides sufficient magnetic
isolation between coils.
ACKNOWLEDGMENT
This work is being supported by The Scientific and
Technological Research Council of Turkey under contract
number 110E111.
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