EXTERNAL ROTOR SHAPE ESTIMATION OF AN
INDUCTION MOTOR BY FEM ANALYSIS
Bogdan VÎRLAN
Gheorghe Asachi Technical University
of Iaşi
Alecsandru SIMION
Gheorghe Asachi Technical University
of Iaşi
Leonard LIVADARU
Gheorghe Asachi Technical University
of Iaşi
Adrian MUNTEANU
Gheorghe Asachi Technical University
of Iaşi
Ana-Maria MIHAI
Gheorghe Asachi Technical University
of Iaşi
Sorin VLĂSCEANU
Gheorghe Asachi Technical University
of Iaşi
REZUMAT. Optimizarea unui motor electric pentru
pentru a fi utilizat întrîntr-o altă
altă aplicaţ
aplicaţie decât
decât pentru cea care a fost ini
iniţial
ţial proiectat, implică
cunoaş
cunoaşterea în totalitate a structurii
structurii circuitului electric cât şi
şi magnetic. Această lucrare prezintă o analiză comparativă a formei
crestăturilor rotorice în construc
construcţ
onstrucţia unui motor asincron cu rotor exterior, utilizâ
utilizând metoda elementului finit. Având la bază un model
experimental al
al cărui caracteristică mecanică este cunoscută, prin studiu de câmp ss-a ajuns la o formă
formă optimă
optimă a crestă
crestăturii din punct
de vedere al performanţ
performanţelor de funcţ
funcţionare a maş
maşinii.
inii.
Cuvinte cheie:
cheie bare înalte, crestături rotorice, crestături înclinate, element finit, rotor exterior.
ABSTRACT. An electric motor optimization for use in others
others applications,
applications, than the one for which it was initially
initially designed, involves
totally knowledge of electric circuit and magnetic structure.
structure. This paper presents
presents a comparative analysis of rotor slots shape in case
of an external rotor induction motor,
motor, using FEM based simulation. Based on an experimental model whi
which
hich mechanical
characteristics are known, field study is used to estimate
estimate the rotor geometry.
geometry.
Keywords:
Keywords deep bar, rotor slots, skewed slots, finite element, external rotor.
1. INTRODUCTION
The discussion about the electric motor must start
with the nature of the application.
When it comes to electric motor optimization it
must a complete investigation is required. This
involves knowing the electric circuit and magnetic
structure of the machine. If the stator is always
accessible, the rotor geometry is unknown for
squirrel cage induction motor. In this regards a
simulation stage is mandatory in order to determine
mainly construction features. All this is possible if
the motor characteristics are known. For a better
estimation of the motor geometry, is important to
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Buletinul AGIR nr. 4/2011 ● octombrie-decembrie
27
match
the
mechanical
characteristics
from
experimentally model with the mechanical
characteristics form the simulation model.
2. INITIAL MOTOR DESIGN
Before starting the estimation of the geometry, is
mandatory to know the initial electric motor application.
In this case, a three-phase induction motor with
external rotor is proposed for optimization. The initial data
of the motor is presented in Table 1.
Table 1. Motor data
Nominal data
Input power (∆/Y)
Input current (∆/Y)
Effiency
Power factor
Rated speed (∆/Y)
Value
780/550
1,32/0,9
82
0,83
1340/1000
It can be observed that the external rotor
configuration is partially unknown. All the information
that can be easily determined are the section of the
squirrel cage end ring (112 mm2), the material which is
made of (aluminum) and of the rotor slots inclination
(15 geometrical degrees).
Also important is the housing of the motor that is
made by brittle aluminum. Initial the wings were built
from the same material but they were removed during
test.
Therefore is indicated to use an approximation
method of the rotor structure. The finite element
method is a useful way to determine the rotor shape.
MU
W
A
%
rpm
In Fig. 1 is presented the analyzed induction
motor with external rotor. This motor operates as
a fan with wings attached on rotor surface.
Fig. 2 shows views of the stator and rotor structure.
The main geometrical parameters of the motor are
presented in Table 2.
Table 2. Main geometrical parameters of the motor
Inner stator diameter
Outer stator diameter
Inner rotor diameter
Outer rotor diameter
Length of the magnetic circuit
Number of stator slots
Number of rotor slots
Number of stator coil turns
40 mm
106 mm
106,4 mm
146 mm
70 mm
24
30
240
Fig. 1. External rotor motor.
Fig. 2. Stator and rotor structure.
3. EXPERIMANTAL RESULTS
The most important characteristic for an induction
motor is the mechanical one. This was obtained after
laboratory tests for different supply voltage (line
voltage). The achieved results are presented in Fig. 3.
Fig. 3. Mechanical characteristics for different voltage values.
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Of great importance are the transient conditions. In
this case, the start-up process is used for approximation.
In Fig. 4. and Fig. 5. are presented, the rotor speed and
current evolution for the start up process for no-load
operating.
The surface of the rotor slots can be obtained
knowing the sectional areas of the end-ring (expression
1).
Si =
Sb
 2π
1.5 ⋅ sin
 Z2
(1)



After calculation, the sectional area of one-rotor
slots is 34 mm2.
For this estimation has been chosen four different
shapes of the rotor slots. From the experimental model,
the mechanical characteristic presents a high starting
torque. In practice the high starting torque is produced
by deep bar or double squerrl cage rotor construction.
The geometry rotor with double squirrel cage this time
is unapproachable because this will bring higher
production costs.
The estimated shapes of the rotor slots are presented
in fig. 6.
Fig. 4. Rotor speed.
Bar. 1
Bar. 2
Bar. 3
Bar. 4
Fig. 6. Shapes rotor slots.
Fig. 5. Absorbed line current.
4. SIMULATION STAGE
This stage has been made in multilayer steady state
and transient magnetic simulation. This is a special
approach for the analysis of axially skewed
topologies. The machine is divided into pieces
along the axial length and the FEM analysis
operates only on the chosen sectional areas.
Usually, the software is than capable to calculate
the resultant. For our analysis, the motor has been
divided into 5 slices.
The mesh for this four rotor structures are presented
in fig. 7. In this simulation has been kept the same mesh
for the stator. The differences appear around the rotor
slots.
The main important results for this simulation are
mechanical characteristics. These results are compared
with the characteristics obtained for the experimental
model.
The high torque obtained in the mechanical
characteristics from the experimental model is not
presented in the simuation. This is the consequence of
the special rotor construction that is unknown.
It is very important to see that is the most
appropriate mechanical characteristic from the
simulation model compared with the characteristic
achieved on the real machine.
The analysis should be done especially on the
operating side, between the nominal torque and critical
torque. In this case the most appropriate mechanical
characteristics are obtained in the simulation of the
motor with Bar. 1 and Bar. 2 geometry (Fig. 8). These
differences between other solutions are obtained from
the bar inductance value of these which are presented in
Table. 3. Transient process offers information about the
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start-up process. Speed variation is important to know
how fast the start-up is. The most appropriate
characteristic is that for model with Bar. 3 type. While
the experimental model starts in 0,55 seconds the motor
model with Bar. 3 start in 0,5 seconds (Fig. 9). This
characteristic, for Bar. 3 type is accepted and because of
the slope of the speed. This rapid start is consequence
of the position of the rotor in the moment of the
beginning of the transient process.
Bar. 2
Bar. 1
Bar. 3
Bar. 4
Fig. 7. Mesh.
Fig. 8. Mechanical characteristics for the simulation model.
Fig. 9. Start-up process for the simulation model.
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Table 3. Inductance value and current rotor bar (s = 0,02)
Bar Type
Inductance (H)
Current (A)
Bar. 1
2,68·10-6
26,38
Bar. 2
2,74·10-6
26,75
Bar. 3
2,66·10-6
26,11
Bar. 1
Bar. 3
Bar. 4
2,71·10-6
26,65
Bar. 2
Fig. 10. Start-up line current.
Bar. 4
The current values presented in Fig. 10, have smaller
value during the start-up process for the simulation
model. This is the consequence of the rotor position at
the moment when the motor start-up. In this case, the
rotor bar is placed perpendicular on the magnetic field
created by the stator.
If the rotor position is changed, the start-up process
presents higher stator current (Fig. 11).
Also very important is the flux density color map.
For the Bar. 3. geometry that sims to match very much
with the experimental model the flux density color map
shows superior values for the rotor yoke (Fig. 12). This
is the consequens of the adopted geometry (deep bar).
Fig. 11. Line current, changing the initial rotor position.
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CONCLUSIONS
A FEM analysis can be used as a non distructive
method of induction motor rotor shape estimation but
with certain approximations.
The number of involved parameters that has to be set
in the simulation process implies a high risk of error.
After simulation an appropriate configuration of the
rotor geometry was estimated.
REFERENCES
Fig. 12. Flux density color map (finial version).
[1] Wayne Beaty H., Kirtley Jr., Electric motor handbook, McGraw-Hill, 1998, ISBN 0 – 07 – 035791 – 7.
[2] Repo A.-K., Niemenmaa A., Arkkio A. Estimating circuit
models for a deep-bar induction motor using time harmonic
finite element analysis. Proceedings – International Conference
in Electrical Machines, Crete, Greece, September 2006, No. 614,
pp. 6.
[3] Lee. K., Berkopec W.E., Jahns T.M., Lipo T.A., Influence of
deep bar effect on induction machine modeling with gammacontrolled soft starters, Applied Power Electronics Conference
and Exposition, 2005. APEC 2005. Twentieth Annual IEEE.
[4] James L. Kirtley Jr., Designing Squirrel Cage Rotor Slots with
High Conductivity, Massachusetts Institute of Technology
Cambridge, Massachusetts, 02139, USA, 2000.
[5] Mihai A-M, Simion Al., Livadaru L., Virlan B., Ghidus G.,
Study on the influence of the rotor slot shape upon the
performance developed by the induction motor with deep bars
using FEM analysis, EPE 2010, Volumul II, pp.II-201-204.
Fig. 13. Final structure.
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