design and performance analysis of a dual stator
TRANSCRIPT
Design and performance analysis of a dual stator multiphase inductionmotor using finite element method
M SOWMIYA1,* and S HOSIMIN THILAGAR2
1Department of Electrical and Electronics Engineering, College of Engineering Guindy, Anna University,
Chennai 600 025, India2Department of Electrical and Electronics Engineering, College of Engineering Guindy, Anna University,
Chennai 600 025, India
e-mail: [email protected]; [email protected]
MS received 7 June 2020; revised 30 October 2020; accepted 8 March 2021
Abstract. A new Dual Stator Multiphase Phase Induction Motor (DSMIM) configuration is proposed and
described in this paper. Steady state equivalent circuit of DSMIM is developed and its performance parameters
are calculated. The efficiency curves plotted under different excitation modes of the machine assists in operating
it at a better efficiency for wider load range. Thus, the proposed configuration can be utilized in Electric Vehicles
(EV) to efficiently support a sudden load change during acceleration, cruising and uphill driving patterns.
A Finite Element Method (FEM) based design of the machine is done using MAGNET software. The elec-
tromagnetic performance of DSMIM is analyzed through the flux patterns obtained at different excitation modes.
Its electromechanical performance is studied by analyzing the current, torque and speed developed under load
driven conditions. The article also proposes an ideology behind the mechanical arrangement of DSMIM for
fabrication. The simulation results are included to illustrate the performance of DSMIM. The results are
compared and validated with the analytical results obtained through equivalent circuit approach.
Keywords. DSMIM; Equivalent circuit; Efficiency curves; EV; FEM; Electromagnetic and electromechanical
performance.
1. Introduction
Induction motors (IM) are the giant utilization in many of
the industrial applications. It is acclaimed for its com-
mendable features such as simplicity, ruggedness and reli-
ability. Significant efforts have been taken to suppress the
shortcomings of an IM by various modifications in its
design and constructional arrangement [1, 2]. Certain
noticeable modifications made IMs extensively used in
traction application [3, 4]. In spite of the fact that a traction
motor should sufficiently develop enough starting torque to
overcome vehicle inertia, it is also vital that it should
operate at a better efficiency for wider load range [5–9]. A
span of research in improving the starting torque of squirrel
cage induction motors sprang up with the development of
deep bar and double cage induction motor configurations.
Both exhibited an appreciable performance in developing a
high starting torque due to the distributed leakage reactance
in its rotor core but failed in performing at a better effi-
ciency for wider load range [10, 11]. This aspect pushed
deep bar and double cage induction motors behind other
traction motors such as Permanent Magnet Synchronous
Motor (PMSM), Switched Reluctance Motor (SRM) and
Brushless DC motor (BLDC) [12, 13]. PMSM, BLDC and
SRM inherently enjoy the features of high power density
and high torque to current ratio which are considered to be
the primary necessities of a traction motor [14, 15]. These
features also made them as the strong competitors in trac-
tion industry. PMSM and BLDC motor require rare earth
materials for its construction. The difficulties faced with
unavailability and unstable cost of the rare earth materials
stirred research towards a magnet free configuration. SRMs
are considered to be a better suited magnet free configu-
ration in traction application but it greatly suffers from
torque ripple and acoustic noise [16–19]. Such undesir-
ability once again directed research towards the renovation
of IMs in order to acquire its beneficial utilization in trac-
tion application.
A Multiphase Induction Motor (MIM), whose number of
phases is usually greater than three, was conceived with the
necessity of high power and reliability demands. It gained
popularity with the growth and expansion of power elec-
tronic drives [20, 21]. Its stator windings are shifted sym-
metrically by 360 o/ number of phases and it could operate
with a reduced current per phase without increasing the
voltage per phase. It is also featured with lower dc link*For correspondence
Sådhanå (2021) 46:67 � Indian Academy of Sciences
https://doi.org/10.1007/s12046-021-01603-6Sadhana(0123456789().,-volV)FT3](0123456789().,-volV)
current harmonics, reduced torque pulsations and increased
torque to current ratio with reference to its three-phase
counterpart. Increased fault tolerance with the failure of one
or two phases is a major aspect for its wider utilization in
submarines, rail traction, avionics and automobiles [22, 23].
Due to these entrancing features, MIM grabbed its attention
in Electric Vehicle (EV) applications. Later the concept of
load sharing and torque producing capability headed its
importance in EV. In order to satisfy this, a Dual Stator
Winding Induction Motor (DSWIM) which includes a
secondary stator winding in addition to the existing primary
stator winding was proposed. The two sets of insulated
stator winding are wound in the same stator core and are
excited separately by two inverters depending upon the load
demand. This configuration is facilitated with an effective
torque and speed control [24] but it suffers from saturation
of the entire stator core even at lesser load demand i.e. even
at the excitation of any one of the two stator windings. This
results in improper stator core utilization, development of
more iron loss and efficiency deterioration [25].
This paper proposes a novel Dual Stator Multiphase
Induction Motor (DSMIM) which stands apart from other
traction motors in its constructional arrangement. It is
developed with the aim of incorporating the benefits of
MIM and DSWIM in a single configuration. It consists of
two separate stator cores namely the inner stator core and
the outer stator core. Each core involves an individual set of
multiphase stator winding wound separately over its slots.
The outer stator is of higher power rating which supports a
higher load demand and the inner stator is of lower power
rating which supports a lesser load demand. As the load
demand increases more than the rated load demand of the
individual stators when excited separately then both the
stator could be excited simultaneously. To facilitate this
selective excitation, the machine is featured with a stator
selection switch. This switch assists in shifting the excita-
tion between the two stators or bring both stators under
excitation based on the load demand. Thus, with two
individual stator arrangement this configuration satisfies the
concept of load sharing and high torque production. It
paves way for proper stator core utilization by localized
core saturation at minimal load conditions which helps in
minimization of iron losses. A hollow squirrel cage rotor is
housed between the two stators. The configuration is
expected to generously include the advantages of MIM to
make it more specifically applicable in places where better
efficiency and reliability are demanded. The proposed
DSMIM is claimed to be a well potent traction unit in EV
application since it makes the vehicle to operate with a
better efficiency at its various load changing conditions like
acceleration, cursing and uphill movement. This improves
the overall energy efficiency of the vehicle and in turn
improves its fuel economy and drive range.
Prior to the discussions on design and fabrication it is
customary to develop the equivalent circuit of DSMIM to
study, analyze and predict its static or dynamic behavior.
Steady state equivalent circuit approach is a well-estab-
lished conventional method to analytically arrive at the
machine parameters and performance characteristics [26].
This kind of analysis helps out in identifying the perfor-
mance of the machine before being designed or fabricated.
Finite Element Method (FEM) is the most popular and an
extensively used advanced numerical method that assists in
structural, thermal, vibration, electromagnetic and elec-
tromechanical analysis of any complex geometry. The
simple and result oriented approach towards analyzing the
electromagnetic and electromechanical performance of a
machine with FEM based software such as MAGNET,
ANSYS, COMSOL etc. made FEM a versatile tool in
machine designing [27, 28].
The paper is organized to present the development of
steady state equivalent circuit and efficiency curves for
DSMIM along with an enlightenment to its utilization in
EV in Section 2. Design of DSMIM and an ideology for its
mechanical arrangement is presented in Section 3. FEM
based simulation analysis is carried out in MAGNET
software to study and analyze its electromagnetic and
electromechanical performances in Sections 4.1 and 4.2
respectively. The simulation results depicted at different
excitation modes are validated with the analytical results
obtained through equivalent circuit approach in Section 4.
2. Equivalent circuit analysis of DSMIM
Equivalent circuit approach is a traditional tool used in
predicting the performance of any electrical machine. The
constructional approach adapted for building the equivalent
circuit of a MIM is similar to that of a conventional three
phase induction motor [29, 30]. The steady state equivalent
circuit of DSMIM is given in figure 1. This circuit is
developed such that all parameters of inner stator and rotor
are referred to the outer stator. The outer and inner stators
are represented as two stator impedances Zs1 and Z0s2
Figure 1. Steady state equivalent circuit of DSMIM.
67 Page 2 of 11 Sådhanå (2021) 46:67
respectively. The air gap between the outer stator and the
rotor is represented by the magnetizing reactance jXm1 and
the air gap between the inner stator and the rotor is repre-
sented by the magnetizing reactance jX0m2. These two
windings are connected in parallel with the rotor impedance
Z0r through the control switches S1 and S2. The control
switches facilitate the excitation of the inner and outer
stator separately. Based on the switching arrangement three
different modes of excitation can be achieved in DSMIM.
Mode I correspond to outer stator excitation with switch S1turned on and S2 turned off. Mode II correspond to inner
stator excitation with switch S2 turned on and S1 turned off.
Mode III correspond to dual stator excitation with both the
switches turned on. It is also interesting to note that under
dual excitation mode the equivalent circuit appears as two
parallel connected motors with a common rotor. Thus, the
machine is designed in such a way that it could be ener-
gized by either of the stators or both simultaneously
depending upon the power and efficiency demands.
The equivalent circuit during mode I and mode II is
similar to the conventional steady state equivalent circuit of
a five-phase induction motor. Therefore, the torque devel-
oped under these modes is given by equation (1).
Te ¼ 5
xsI2
0r
R0r
sð1Þ
The equation of rotor current Ir0 under dual excitation
cannot be the same as that in a conventional five phase
induction motor. Hence the developed equivalent circuit is
simplified with an interest of determining the equation of
rotor current through Thevinin’s approach. Under Mode III
the two control switches are kept closed. This modifies the
equivalent circuit as in figure 2. Further it is simplified into
Thevenin’s equivalent circuit as shown in figure 3. From
figure 3 the rotor current Ir0 can be calculated as in equation
(2)
I0r ¼
Vth
Zth þ Z0r
ð2Þ
Where, the values of Vth and Zth are given by equation
(3) and equation (4) respectively.
Vth ¼ VsðjXm1jjjX 0m2Þ
ðZs1jjZ 0s2Þ þ ðjXm1jjjX 0
m2Þð3Þ
Zth ¼ Zs1jjZ 0s2
� �þ jXm1jjjX 0
m2
� �ð4Þ
Load impedance ZL is equal to the rotor impedance Z0r
and it is given by equation (5).
Z0r ¼ R
0r þ jX
0r ð5Þ
On substituting the equations of Vth, Zth and Z0r in
equation (2) the rotor current Ir0 under dual excitation can
be represented by equation (6).
I0r ¼
VthðZs1 þ Z0s2Þ
Zs1Z0s2ð1þ X0
rÞð6Þ
Therefore, the torque developed by the DSMIM under
dual stator excitation is obtained through equation (7)
Te ¼ 5
xs
V2thðZs1 þ Z
0s2Þ2
ðZs1Z 0s2ð1þ X 0
rÞÞ2R
0r
sð7Þ
Equation (7) exhibits the dependency of developed tor-
que upon the parameters of both outer and inner stator
windings. It is also important to note that the torque
developed in mode III can be enhanced only by a syn-
chronized five phase dual stator excitation because such
excitation increases flux density and aids in acquiring
rotating fluxes that lie in phase to each other. These fluxes
go hand in hand to develop the net torque. In other words,
net torque Te developed by DSMIM under dual excitation
will be the vector sum of the torque Te1 developed by the
outer stator excitation and the torque Te2 developed by the
of inner stator excitation as represented in equation (8).
Te ¼ Te1 þ Te2 ð8ÞTherefore, during mode III it is essential to have a syn-
chronized five phase excitation at the dual stator windings
of DSMIM. This could be achieved by two identical five
phase voltage source inverters with separate DC inputFigure 2. Equivalent circuit of DSMIM under dual stator
excitation.
Figure 3. Thevinin’s equivalent circuit of DSMIM under dual
stator excitation.
Sådhanå (2021) 46:67 Page 3 of 11 67
sources. Providing a synchronized Pulse Width Modulation
(PWM) to the two inverter switches helps in developing a
synchronized five phase output voltage. Such voltage is free
from even and sub- harmonic waves and its symmetrical
control enables speed control of DSMIM [31].
The electrical specifications considered for the design of
DSMIM are listed in table 1. The traditional analytical
approach involved in a no load and blocked rotor test is
adapted for acquiring the equivalent circuit parameters. The
parameters are calculated for all the three modes of
DSMIM and listed in table 2. Performance characteristics
of the machine shall be studied analytically using the tab-
ulated equivalent circuit parameters.
Torque and efficiency of DSMIM are calculated under
rated load conditions and listed in table 5 in section 4. In
order to explore the maximum efficiency load points, effi-
ciency curves of the machine under all three excitation
modes are plotted as shown in figure 4. The plot is drawn
between percentage efficiency and the per unit values of
output power calculated by considering 1.5 HP as the base
power. It shows that under dual excitation the machine
operates at a maximum efficiency of 85% at 80% full load
i.e. 80% of 1.5 HP. As the load demand drops either the
inner or the outer stator alone is excited and the machine
now runs as a single stator five phase induction motor. A
maximum efficiency of 84% is attained at 90% full load i.e.
90% of 1 HP under outer stator excitation and a maximum
efficiency of 82% is attained at 80% full load i.e. 80% of
0.5 HP under inner stator excitation. Thus, the DSMIM tend
to operate with a higher efficiency at various load points.
In an EV there are three basic driving patterns such as
acceleration, cursing and uphill movement. During accel-
eration the vehicle requires high power to displace itself
from rest. Cruising is the vehicle’s free running condition
over level roads which require only a minimal amount of
power. Power demand of the vehicle reaches to its maxi-
mum when it is subjected to uphill movement. Any traction
motor could sufficiently satisfy these variations in power
demand but how efficiently it does matters. Because an
efficient operation of a traction motor at various load points
highly assist in energy saving and in turn increases the
driving range of EV. The proposed DSMIM configuration
is claimed to operate with better efficiency at different load
points by changing the stator excitation.
Let us now consider the proposed DSMIM being oper-
ated as a traction motor under acceleration, cruising and
uphill movement with load demands assumed to be 0.5 p.u,
0.2 p.u and 0.7 p.u respectively. The selection of stator
excitation during each driving pattern is represented in
table 3.
At T = t1 the vehicle is considered to accelerate with load
demand of 0.5 p.u. From the efficiency curve plot in fig-
ure 4 it is observable that this load demand could be sat-
isfied by either exciting the outer stator or both the stators.
But the machine operates at a higher efficiency only on
exciting the outer stator. At T = t2 the vehicle is considered
to cruise with a minimal load demand of 0.2 p.u. This load
demand can be satisfied by any of the three excitation
modes. But the machine operates at a higher efficiency only
on exciting the inner stator. Thus, the excitation is now
shifted to inner stator alone. At T = t3 the vehicle moves
over uphill with a further raise in load demand up to 0.7 p.u.
This load demand can only be satisfied by dual stator
excitation as the load demand now exceeds the rated load of
the inner and outer stator. Therefore, depending upon the
driving patterns and its load demand, excitation of stator
shall be selected to make the machine operate at a higher
Table 1. DSMIM specification.
Rating Outer stator Inner stator
Rated power (P) 1 HP 0.5 HP
Supply voltage (V) 230 V 230V
No. of phases (n) 5 5
No. of poles (p) 4 4
Rated speed (N) 1440 rpm 1440 rpm
Frequency (f) 50 Hz 50 Hz
Table 2. Equivalent Circuit Parameters of DSMIM.
Parameters (X)Operating modes
I II III
Stator resistance Rs 18.2 37.9 8.2
Stator reactance Xs 52.6 72.1 52.9
Outer magnetizing reactance Xm1 247 – 227
Inner magnetizing reactance Xm20 – 365 230
Rotor resistance Rr0 8.41 13.3 8.41
Rotor reactance Xr0 19.5 30.7 38.5
Figure 4. Efficiency curves of DSMIM under the three excita-
tion modes.
67 Page 4 of 11 Sådhanå (2021) 46:67
efficiency. The selection mechanism is facilitated by a
programmable switch termed as the stator selection switch.
By sensing the load current the switch is programmed to
estimate the load demand and accordingly shift the exci-
tation modes. This illustration justifies that the proposed
DSMIM can be considered as a well potent traction motor
in improving the performance of an EV.
Dynamic performance of DSMIM with high degree of
accuracy can be studied through its design and structural
analysis using Finite Element Method. This method
requires a sound knowledge over the characterization of
machine under study [32]. Physical geometry, choice of
material, winding distribution, electromagnetic property,
nature of instantaneous current and flux distribution is the
key information prerequisite to successfully proceed with
design and construction of the machine.
3. Design and construction of DSMIM
The cross-sectional quadrant view of DSMIM is shown in
figure 5. Its construction includes two electrically isolated
stators with a hollow squirrel cage rotor housed in between.
There are two air gaps, one between the outer stator and the
rotor and the other between the inner stator and the rotor.
Under the excitation of any one of the stators, this config-
uration exhibits an operation similar to that of a conven-
tional five phase induction motor. A traditional design
procedure is adopted in designing the proposed configura-
tion. The rated power of the DSMIM considered in the
design is 1.5 HP. The inner stator is constructed with a
lesser diameter and material than the outer stator and hence
it is considered that the motor delivers a rated mechanical
power output of 0.5 HP, 1.0 HP and 1.5 HP with the inner,
outer, and dual excitations respectively.
The two stator cores are constructed by laminated sheets
of Si steel stacked together with a stack factor of 0.9.
Choice of specific electric loading, magnetic loading, L/sratio and winding factor are fixed for both the stators sep-
arately. An optimal value is chosen for magnetic and
electric loading as it greatly influences the total iron and
copper losses in the machine. The L/s ratio is fixed on the
choice of achieving a better efficiency [33]. By fixing the
flux densities at various parts of the magnetic circuit the
stator core, stator slot, rotor core, rotor bar and end ring
dimensions are calculated and listed in table 4.
Based on the above tabulated design parameters, the five-
phase dual stator induction motor is designed using
MAGNET software. The design procedure involved is
discussed in the following sections.
3.1 Design of dual stator
The inner and outer stator model developed in MAGNET is
shown in figure 6. The space between inner and outer stator
is utilized for fixing the hollow squirrel cage rotor
Table 3. Excitation Mode Selection in DSMIM under EV Driving Patterns.
Time interval T (Sec) EV driving pattern Load demand P0 (p.u) Excitation
t1 Acceleration 0.5 Outer stator
t2 Cruising 0.2 Inner stator
t3 uphill Movement 0.7 Dual stator
Figure 5.. Dual stator five phase induction motor drive.
Table 4. Design parameters of DSMIM.
Parameters Outer stator Inner stator
Stator bore diameter (mm) 146 115
Gross iron length (mm) 61.9 61.9
No. of stator slots 40 40
Stator teeth width (mm) 4 3.5
Stator slot depth (mm) 13 16
Stator core depth (mm) 18.5 14.5
Slot opening (mm) 0.2 0.2
Air gap length (mm) 0.35 0.3
Rotor outer diameter (mm) 145.3
Rotor inner diameter (mm) 115.3
No. of rotor slots 42
Rotor slot pitch (mm) 10.8
Rotor bar length (mm) 80
Bar cross sectional area (mm2) 11 9 2.5
End ring mean diameter (mm) 131
End ring cross sectional area (m2) 12.5 9 6
Sådhanå (2021) 46:67 Page 5 of 11 67
circumvented by inner and outer air gaps. The winding
arrangement patterns of both the stators are the same and it
is represented by the winding distribution diagram shown in
figure 7. Mush winding is chosen for the design. It is a type
of single layer winding preferred for small induction motors
having circular conductors. The number of phases in both
the stators is five and therefore the phase difference
between the windings is 720 and phase spread is 36�. Thecoils are short pitched by one slot with a chording angle of
18�. Both stators involve four poles and ten slots corre-
sponding to each pole.
In the winding distribution diagram, the five phase coils
are represented by five different colors. In and out in it
represents the entering and leaving ends of the coil that
passes through the slots. The number of stator slots is
chosen as a multiple of number of phases. Also, it is
selected appropriately to fix the weight, cost and opera-
tional characteristics of the motor [34, 35].
Slot depth and slot width that decides the area of the slot
are so chosen to attain a perfectly parallel flux path at the
stator teeth. Semi closed slots are most prevalent in smaller
machines. Since efficiency is chosen as a major concern
both the inner and outer stators of our case study are
designed with semi closed slots.
Length of air gap creates an impact upon the pulsation
losses, magnetic pull, cooling, loading capacity and power
factor of a machine [36]. The outer and inner air gap
lengths in the case study are considered to be 0.35mm and
0.3 mm respectively.
3.2 Design of hollow squirrel cage rotor
The hollow squirrel cage rotor model developed in MAG-
NET is shown in figure 8. It involves a Si steel rotor core
with copper bars embedded within the closed rotor slots.
The rotor bars are short circuited by end rings as in the case
of a conventional type squirrel cage rotor.
Closed rotor slot arrangement helps in achieving a
smooth rotor surface at the air gap. Number of rotor slots is
chosen in proper combination with stator slots to avoid
unpredictable hooks and cusps in torque speed character-
istics [37, 38]. Cross sectional area of the rotor bar and end
ring are so chosen to attain a rotor resistance that meets the
torque and efficiency demands.
3.3 Mechanical arrangement of DSMIM
The mechanical arrangement of DSMIM is not analogous
to a conventional IM. Proper alignment of the rotor core in-
between the two stators with respect to a definite central
axis is the most significant aspect of this design to avoid
eccentricity in standstill and rotary conditions [39, 40].
Also, it is quite delicate to fix the shaft with the hollow
rotor located in between the two stators. Considering these
intricacies, the longitudinal view of the mechanical
arrangement of DSMIM is proposed as shown in figure 9.
It clearly depicts that at drive end, the shaft is clamped
along with a disc like arrangement termed as holder. The
holder is designed with a shaft hole at its center and two
elevated concentric rings along its circumference. The
diameter of the inner and outer ring is so chosen to firmly
Figure 6. Dual stator 2D model.
Figure 7. Mush winding distribution diagram. Figure 8. Hollow squirrel cage rotor 2D model.
67 Page 6 of 11 Sådhanå (2021) 46:67
encase the hollow rotor body. The holder thus establishes a
mechanical contact between the hollow rotor and shaft.
Insulation is made at the inner surface of the rings in order
to electrically isolate the holder from the rotor. During
stator excitation the torque developed rotates the rotor
along with the holder attached to it. This arrangement
smoothens the way for shaft rotation at the drive end. The
holder should also ensure that the position of rotor is fixed
precisely with respect to the central axis so as to maintain
the air gap lengths and avoid eccentricity. It is important to
note that the holder material chosen for the design should
be characterized with less mechanical strength to avoid its
influence over the moment of inertia of rotor.
4. FEM results and discussion
DSMIM considered for the case study is modeled three
dimensionally in MAGNET. Its electromagnetic and elec-
tromechanical performances are obtained through simulat-
ing the model in 2D static and transient solvers. The results
are depicted and discussed in the following sections.
4.1 Electromagnetic performance
Operational analysis of DSMIM and its electromagnetic
performance greatly depends on the number, size and type
of the finite elements chosen. The proposed configuration’s
3D design involves 5, 21,967 triangular meshes created for
the complete 3600 geometry with the default element size
setup in the software. The DSMIM is made to operate in
three different modes by exciting the outer and inner stator
five phase windings separately and concurrently with a five-
phase power supply of 230V rms. The flux patterns
developed and distributed over its 2D geometrical view
during the excitation of the outer, inner and dual excitation
are shown in figure 10, figure 11 and figure 12 respectively.
As the number of poles chosen for the design equals to
four for both the stators, the flux pattern developed under
all the three modes of operation exhibits four closed paths
each concealing ten stator slots. The direction of the field
flux can be reversed by reversing the excitation to the field
windings. From figure 10 and figure 11 during individual
excitation of the outer and inner stator windings the flux
produced links the rotor and the unexcited stator by passing
through the inner and outer air gaps. This pattern of flux
linkage illustrates an increased reluctance offered to the
flux path. Only the useful flux linked with the rotor assists
in developing the net torque. Also, the pattern significantly
indicates the increased total leakage reactance of the pro-
posed model during individual excitation than a conven-
tional single stator induction motor.
During dual excitation the flux pattern developed is
obtained as in figure 12. It illustrates the formation of four
poles in both the stators with its flux path linking the outer
stator, outer air gap, rotor, inner air gap and the inner stator.
The shaded plot obtained through the simulation also
reveals that the maximum flux density region occurs at the
pole formation region of the stator teeth. The value of
maximum flux density equals to 1.907 wb/m2 which lies
within the maximum flux density limit of the stator core
material used.
4.2 Electromechanical performance
Electromechanical performance study helps in analyzing
the transient characteristics of a machine. The DSMIM
design is investigated under load driven conditions. It is
carried out by providing an external load torque as input to
graphically obtain the corresponding electromagnetic tor-
que and speed developed as outputs. The design is simu-
lated in transient 2D motion solver for 1000 ms under all
three excitation modes. The rated per phase current
Figure 9. Longitudinal view of the mechanical arrangement in
DSMIM.Figure 10. Flux developed due to outer stator excitation.
Sådhanå (2021) 46:67 Page 7 of 11 67
waveform developed under inner and outer stator excita-
tions are depicted up to 500 ms for better clarity.
Under mode I the rated power of DSMIM is 1 HP and
therefore its rated load torque is calculated to be 5 Nm. This
load demand is met by exciting the outer stator alone. The
rated per phase current Ia1 developed in the outer stator
winding is shown in figure 13 and the corresponding torque
developed is shown in figure 14.
From figure 13 the RMS value of the rated per phase
current Ia1 is observed to be equal to 1.02 A. From figure 14
it is noticeable that the torque developed under the outer
stator excitation settles at 600 ms with an average rated
torque value of 5.21 Nm and a torque ripple of 24.6 %.
From simulation results of rated current, torque and speed
the efficiency of DSMIM under outer stator excitation is
calculated to be 82.5 %.
Under mode II the rated power of DSMIM is 0.5 HP and
therefore its rated load torque is calculated to be 2.5 Nm.
This load demand is met by exciting the inner stator alone.
The rated per phase current Ia1 developed in the inner stator
winding is shown in figure 15 and the corresponding torque
developed is shown in figure 16.
From figure 15 the RMS value of the rated per phase
current Ia2 is observed to be equal to 0.52 A. From figure 16
it is noticeable that the torque developed under the inner
stator excitation settles at 650 ms with an average rated
torque of 2.56 Nm and a torque ripple of 15.1 %. From
simulation results of rated current, torque and speed the
efficiency of DSMIM under inner stator excitation is cal-
culated to be 79.7%.
Under mode III the rated power of DSMIM is 1.5 HP and
therefore its rated load torque is calculated to be 7.4 Nm.
This load demand can only be met by simultaneously
exciting the inner and outer stators. The total rated current
developed under this mode will be the addition of the
current developed in the outer and inner stator excitations.
Figure 11. Flux developed due to inner stator excitation.
Figure 12. Flux developed due to dual stator excitation.
Figure 13. Rated per phase current of DSMIM under outer stator
excitation.
Figure 14. Torque developed by DSMIM under outer stator
excitation.
67 Page 8 of 11 Sådhanå (2021) 46:67
The net torque developed under this mode is depicted in
figure 17.
From figure 17 it is observed that, the torque developed
on exciting the outer and inner stators settles at 610 ms at
7.31 Nm. The torque ripple is calculated to be 8.6 %. From
simulation results of rated current, torque and speed the
efficiency of DSMIM under inner stator excitation is cal-
culated to be 81.6 %.
It is observable from the simulation results of torque that
the average torque developed under dual excitation is
greater than the average torque developed under single
stator excitation. It is due to the synchronized five phase
excitation of the dual stators under this mode as discussed
in section 2. Also, it is important to note from the simu-
lation results that the average torque developed under dual
excitation mode is almost equal to the sum of average
torques developed under single stator excitation mode.
The percentage torque ripple during dual excitation mode
is observed to be lesser than in the other two modes. It is
because of the involvement of two sets of five phase stator
winding with a synchronized voltage excitation. The five
stator windings are spatially displaced by 360�/5 = 72�.This displacement neutralizes the fifth harmonic flux wave
produced by each of the five phases winding with one
another. Therefore, the synchronized five phase voltages
provided at the stator windings are devoid of 5th harmonic,
even harmonic and sub harmonic waves and so it greatly
reduces the space harmonic fluxes developed at the inner
and outer air gaps. The hollow rotor mounted in between
the two air gaps is now subjected to a lesser air gap flux
harmonic and hence the developed harmonic rotor current
which is responsible for the production of torque ripple is
also lesser. It is noticeable from figure 12 that the flux
pattern obtained during dual excitation is much smoother at
the air gap and evenly distributed all over the motor
geometry resulting in the development of torque with lesser
ripple as depicted in figure 17.
The average angular speed of DSMIM is obtained by
measuring the angle at which the rotor rotates in a second
and hence it is represented in degree/sec. Approximately 1
degree/ sec = (1/6) rpm. From the electrical specification as
mentioned in table 1 the rated speed developed by the
configuration is 1440 rpm i.e. 8640 deg/sec. Since the
simulation is carried out by applying an external load equal
to the rated load that corresponds to all three modes, the
DSMIM develops a rated speed which is the same at all
three modes of operation as depicted in figure 18. The
speed output settles at a value of 8542 deg/sec i.e. 1423 rpm
at 650 ms with a steady state error of 1.13%.
A comparative analysis is carried out between the rated
average torque and efficiency obtained analytically through
equivalent circuit approach and FEM software. The calcu-
lated torque an efficiency values are listed in table 5.
Figure 15. Rated per phase current of DSMIM under inner stator
excitation.
Figure 16. Torque developed by DSMIM under inner stator
excitation.
Figure 17. Torque developed by DSMIM under dual stator
excitation.
Sådhanå (2021) 46:67 Page 9 of 11 67
The percentage error obtained through comparison of
results is less than 5 %. Validation of the results strikes on
improving the performance characteristics of DSMIM fur-
ther through design optimization. In spite of the promising
features optimization of stack length, conductor area,
number of stator and rotor slots of DSMIM becomes
inevitable to persuade the power and efficiency demands of
future EVs.
5. Conclusion
Equivalent circuit of the proposed DSMIM is developed
and its performance characteristics are determined analyt-
ically. Efficiency curves plotted under all three excitation
modes of the machine exhibit a better efficiency at different
load points. This helps in selection of stator excitation to
operate the machine with a higher efficiency under the EV
driving patterns such as acceleration, cruising and uphill
movement. The configuration is successfully designed
using FEM and an ideology behind its mechanical
arrangement for fabrication is also proposed. Its electro-
magnetic and electromechanical performances were inves-
tigated through the operational analysis carried out in
MAGNET. The simulation results illustrate that the
machine generously exhibits the advantages of lesser torque
ripple as in a MIM and an efficient load sharing as in a
DSWIM. Rated average torque and efficiency of the
machine were calculated through equivalent circuit
approach and the simulation results of FEM. The compar-
ative analysis carried out between the results exhibit an
error of less than 5 %. Thus, the promising features and the
results revealed from simulation flourishes the utilization of
DSMIM in an EV application.
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