EXTERNAL ROTOR SHAPE ESTIMATION OF AN INDUCTION MOTOR BY FEM ANALYSIS Bogdan VÎRLAN Alecsandru SIMION Leonard LIVADARU Adrian MUNTEANU Ana-Maria MIHAI Sorin VLĂSCEANU 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 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 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. 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. 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. 1. External rotor motor. Fig. 3. Mechanical characteristics for different voltage values. 28
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). Sb Si = 2π 1.5 sin Z 2 (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. 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 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. Bar. 1 Bar. 2 Bar. 3 Bar. 4 Fig. 7. Mesh. Fig. 8. Mechanical characteristics for the simulation model. Fig. 9. Start-up process for the simulation model. 30
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. 31
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, 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 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. 32