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EXTERNAL ROTOR SHAPEEXTERNAL ROTOR SHAPEEXTERNAL ROTOR SHAPEEXTERNAL ROTOR SHAPE ESTIMATIONESTIMATIONESTIMATIONESTIMATION OF AN OF AN OF AN OF AN INDUCTION MOTORINDUCTION MOTORINDUCTION MOTORINDUCTION MOTOR BY FEM ANALYSISBY FEM ANALYSISBY FEM ANALYSISBY 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. REZUMAT. REZUMAT. REZUMAT. Optimizarea unui motor electric penOptimizarea unui motor electric penOptimizarea unui motor electric penOptimizarea unui motor electric pentru atru atru atru a fi utilizat întrfi utilizat întrfi utilizat întrfi utilizat într----o alto alto alto altăăăă aplicaaplicaaplicaaplicaţţţţie die die die decât pentru cea care a fost inecât pentru cea care a fost inecât pentru cea care a fost inecât pentru cea care a fost iniiiiţiţiţiţial proiectat, implică al proiectat, implică al proiectat, implică al proiectat, implică cunoacunoacunoacunoaşşşşterea în totalitate a structerea în totalitate a structerea în totalitate a structerea în totalitate a structurii circuitului electric cât şturii circuitului electric cât şturii circuitului electric cât şturii circuitului electric cât şi magnetic. i magnetic. i magnetic. i magnetic. Această lucrare prezintă o analiză comparativă a formei Această lucrare prezintă o analiză comparativă a formei Această lucrare prezintă o analiză comparativă a formei Această lucrare prezintă o analiză comparativă a formei crestăturilor rotorice în ccrestăturilor rotorice în ccrestăturilor rotorice în ccrestăturilor rotorice în construconstruconstruconstrucţţţţia unui motor asincron cu rotor exterior, utilizia unui motor asincron cu rotor exterior, utilizia unui motor asincron cu rotor exterior, utilizia unui motor asincron cu rotor exterior, utilizâââând nd nd nd metoda elementului finit. Având la bază un model metoda elementului finit. Având la bază un model metoda elementului finit. Având la bază un model metoda elementului finit. Având la bază un model experimental aexperimental aexperimental aexperimental allll ccccăăăărui caracteristică mecanică este cunoscută, prin studiu de câmp srui caracteristică mecanică este cunoscută, prin studiu de câmp srui caracteristică mecanică este cunoscută, prin studiu de câmp srui caracteristică mecanică este cunoscută, prin studiu de câmp s----a ajuns la o forma ajuns la o forma ajuns la o forma ajuns la o formăăăă optimoptimoptimoptimăăăă a cresta cresta cresta crestăăăăturii din punct turii din punct turii din punct turii din punct de vedere al de vedere al de vedere al de vedere al performanperformanperformanperformanţţţţelor de funcelor de funcelor de funcelor de funcţţţţionare a maionare a maionare a maionare a maşşşşiniiiniiiniiinii.... Cuvinte cheieCuvinte cheieCuvinte cheieCuvinte cheie: bare înalte, crestături rotorice, crestături înclinate, element finit, rotor exterior. ABSTRACT. ABSTRACT. ABSTRACT. ABSTRACT. AAAAn electric motorn electric motorn electric motorn electric motor optimizationoptimizationoptimizationoptimization for use in otherfor use in otherfor use in otherfor use in otherssss applicationapplicationapplicationapplications, than the ones, than the ones, than the ones, than the one for which it was for which it was for which it was for which it was initialinitialinitialinitiallylylyly designed,designed,designed,designed, involvesinvolvesinvolvesinvolves totally totally totally totally knowledgeknowledgeknowledgeknowledge of electric circuit and magnetic structureof electric circuit and magnetic structureof electric circuit and magnetic structureof electric circuit and magnetic structure. This paper present. This paper present. This paper present. This paper presentssss a a a a comparative analysiscomparative analysiscomparative analysiscomparative analysis of rotor of rotor of rotor of rotor slotsslotsslotsslots shapeshapeshapeshape inininin case case case case of an external rotor of an external rotor of an external rotor of an external rotor induction induction induction induction motormotormotormotor, using , using , using , using FEM basedFEM basedFEM basedFEM based simulation. Based on simulation. Based on simulation. Based on simulation. Based on anananan experimental experimental experimental experimental model wmodel wmodel wmodel whihihihichchchch mechanical mechanical mechanical mechanical characteristics are characteristics are characteristics are characteristics are known, field study is usedknown, field study is usedknown, field study is usedknown, field study is used to estto estto estto estimate the rotor geometryimate the rotor geometryimate the rotor geometryimate the rotor geometry.... KeywordsKeywordsKeywordsKeywords: 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

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 Value MU Input power (∆/Y) 780/550 W

Input current (∆/Y) 1,32/0,9 A

Effiency 82 %

Power factor 0,83

Rated speed (∆/Y) 1340/1000 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 40 mm

Outer stator diameter 106 mm

Inner rotor diameter 106,4 mm

Outer rotor diameter 146 mm

Length of the magnetic circuit 70 mm

Number of stator slots 24

Number of rotor slots 30

Number of stator coil turns 240

Fig. 1. External rotor motor.

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 mm

2),

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.

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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Buletinul AGIR nr. 4/2011 ● octombrie-decembrie

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.

Fig. 4. Rotor speed.

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 surface of the rotor slots can be obtained knowing the sectional areas of the end-ring (expression 1).

(1) After calculation, the sectional area of one-rotor slots is 34 mm

2.

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.

Bar. 1 Bar. 2 Bar. 3 Bar. 4

Fig. 6. Shapes rotor slots.

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

⋅

=

2

2sin5.1

Z

SS b

iπ


29

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.

Fig. 7. Mesh.

Fig. 8. Mechanical characteristics for the simulation model.

Fig. 9. Start-up process for the simulation model.

Bar. 1 Bar. 2

Bar. 3 Bar. 4

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Table 3. Inductance value and current rotor bar (s = 0,02)

Bar Type Bar. 1 Bar. 2 Bar. 3 Bar. 4 Inductance (H) 2,68·10

-6 2,74·10

-6 2,66·10

-6 2,71·10

-6

Current (A) 26,38 26,75 26,11 26,65

Bar. 1 Bar. 2

Bar. 3 Bar. 4 Fig. 10. Start-up line current.

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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Fig. 12. Flux density color map (finial version).

Fig. 13. Final structure.

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

[1] Wayne Beaty H., Kirtley Jr., Electric motor handbook, Mc-

Graw-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 gamma-

controlled 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.

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