Performance Comparison Analysis of a Squirrel-cage Rotor Induction Motor with Different Rotor Structures

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1 Sensors & Transducers 2014 by IFSA Publishing, S. L. Performance Comparison Analysis of a Squirrel-cage Rotor Induction Motor with Different Rotor Structures 1 Jun Wang, 1 Huijuan Liu, 1 Cai Chen, 2 Weinong Fu 1 School of Electrical Engineering, Beijing Jiaotong University, Beijing, , China 2 Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong 1 Tel.: @bjtu.edu.cn Received: 18 August 2014 /Accepted: 20 October 2014 /Published: 31 December 2014 Abstract: A new squirrel-cage rotor structure induction motor (IM) that has a auxiliary permanent magnet (PM) rotor and improved cage-rotor structure is presented to enhance the inherent power factor of the IM, and a prototype motor is fabricated by modifying the rotor of a conventional three-phase four-pole squirrel-cage induction motor. The flux linkage, flux distribution, power factor, core loss and efficiency of the novel and conventional rotor structure induction motor are comparative analyzed. The 2D transient finite element analysis method is used for computation the characteristics of the two induction motors. Copyright 2014 IFSA Publishing, S. L. Keywords: Squirrel-cage rotor induction motor (IM), High power factor, High efficiency, Permanent magnet (PM), FEM. 1. Introduction Three-phase induction motors are most widely used in industry, since they are simple and robust, inexpensive to produce, and easy to maintain [1-3]. However, induction motors have the demerits of lagging power factor and they need reactive power from a grid to establish the exciting magnetic field, especially evident at light load or no load condition [4]. The low power factor can lead to increase of losses in both the motor and the power network, and then the improvement of the power factor is effective not only against the decrease of the current loss, but against the decrease of voltage drop and installation capacity [5]. The most commonly used method to improve the power factor is to connect shunt capacitors on the terminals of the stator windings. It increases the power factor as seen from the external to the paralleled motor and capacitors [6]. However, the overall weight and size of the motor driving system are increasing, and the leading current in light-load conditions will bring about the abnormal rising of the terminal voltage [7]. In addition, the inherent power factor of the motor is still poor. In other words, the magnitude of the current in the stator windings is the same and the copper loss inside the motor cannot be effectively reduced. It has been a dream of researchers to develop a high power factor induction motor. In all electric motors, a magnetic field is required as a medium to convert electrical energy into mechanical energy. If the magnetic field is generated by reactive power drawn from the grid, the power factor of the machine will be low. It indicates that if the magnetic field is generated by permanent magnets 209

2 (PMs), then reactive power is not required from the grid, which leads us to present a new induction motor with auxiliary magnetic field to achieve high power factor. As early as 1926, Punga and Schon presented a structure to improve the power factor. Between the stator and the rotor, a ring-shaped intermediate rotor carrying the d-c exciting winding runs freely at synchronous speed [8, 9]. In 1959, Douglas replaced the D-C exciting winding with permanent magnets (PMs) [10]. Because the intermediate rotor has no iron core, the magnetic reluctance is very large. It needs a very large amount of PM materials. The stator windings and rotor windings have a very large leakage flux. Therefore the overall performances are poor and no prototype was fabricated and hence there were no experimental supporting results. This structure was also applied to a single-phase induction motor [11]. In 1999 a structure with free-rotating magnets inside a rotor was presented [12]. Although it pointed out a new possibility to improve the power factor, it has fatal disadvantages. We have simulated the operation of this motor using time-stepping 2-D FEM which supports the simulation of a motor having two rotating rotors. Our simulation shows that actually the PMs in the inner rotor cannot improve the power factor. The magnetic flux produced by the PMs is shortcircuited by the yoke of the rotor iron core as shown in Fig. 1. It cannot reach the main air-gap and do not link with the stator windings. Because of additional mechanical, frictional and windage losses of the PM rotor, the efficiency of this motor might be lower than that of conventional motors. Based on the study of different existing methods for improving the power factor of induction motors, and our numerous simulation results to test our many initial ideas, we propose an induction motor having a new rotor structure with internal PM rotor and improved cage-rotor structure (as shown in Fig. 2). (a) Motor proposed by reference [12] (b) The proposed motor in this paper Fig. 1. Flux distributions of the two motors. Fig. 2. Structure of the proposed IM in this paper. The paper focuses on the performances analysis of a new squirrel-cage rotor structure induction motor (IM) which has an auxiliary permanent magnet (PM) rotor and improved cage-rotor structure. The flux distribution, power factor, core losses, and efficiency of the proposed and conventional IMs have been 210

3 investigated by using 2D transient finite element analysis method. The motor can be operated at optimized power factor with high efficiency by changing the source voltage or supply frequency in various load conditions. 2. The Configuration of Proposed Induction Motor As shown in Fig. 2, the proposed IM has two rotors and one stator, and the structure of the stator is the same as that of a conventional induction motor. The outer rotor comprises of a hollow cylindrical structure enclosing the inner rotor. The inner rotor comprises PMs for producing a rotating magnetic field in the main air-gap to supply a substantial part of its excitation energy. The number of PMs is equal to the pole number of the motor. If the rotor s magnetic reluctance is too large, the stator flux will not reach to the rotor and link with its conductors. If the rotor s magnetic reluctance is too small, the flux produced by the PMs will be short-circuited so that it will not produce magnetic field in the main air-gap and the power factor cannot be improved. The slot shape, dimensions and magnetic barrier of the squirrel-cage should be properly optimized. The squirrel cage bars and the iron can be a solid body to strengthen the mechanical structure. The slots are skewed to reduce higher-order slot harmonics. In the proposed IM, the outer rotor starts to rotate by the induction torque. When the outer rotor rotates, the eddy current is generated by the inner rotor, and the rotating torque is generated in the inner rotor. By try-and-error, the proposed IM main dimensions are determined with the design specifications listed in Table 1, and the conventional IM main dimensions are also listed in Table 1 for comparison. Table 1. Main dimensions. Conventional IM Proposed IM Stator OD (mm) Stator ID (mm) Cage Rotor OD (mm) Cage Rotor ID (mm) PM Rotor OD (mm) \ 64 PM Rotor ID (mm) \ 30 PM Thickness (mm) \ 5.5 Stator slot number Rotor slot number Pole number 4 4 Stack Length (mm) Simulation Results by FEM Two-dimensional (2D) transient finite element method (FEM) is employed to compute the performances of the proposed IM and the conventional IM, the advantages of the transient FEM is that the realistic conditions of lamination geometries, core materials, and winding connections are all considered collectively and faithfully [13, 14]. As shown in Table 1, a three-phase, four-pole squirrel cage induction motor is used as a typical example Flux Density Distribution In IM, when stator windings are excited by threephase symmetrical sinusoidal AC currents, the rotating magnetic field will be generated in the machine, and the back EMF voltages will be induced in three-phase stator windings. If the mechanical load is coupled with the motor shaft, then the motor will output mechanical power and is running in loaded condition. In the finite element analysis, we have directly calculated the flux distribution and the winding flux linkages of two types of IM. Fig. 1(b) shows the flux distributions of the proposed motor in this paper. Owning to the effect of the magnetic barriers in the squirrel-cage rotor, the flux produced by the PMs can reach the main air-gap and link with the stator windings, which is not the same as the motor of the reference [12] which is shown in Fig. 1. The proposed IM will decrease the value of magnetizing current because of the existence of permanent magnets which can provide the air-gap flux. Therefore it is possible to improve power factor and efficiency for the proposed IM. Flux density distributions of two types of IM which is running in loaded operation are shown in Fig. 3(a) and Fig. 3(b) respectively. In Fig. 3, it very important to note that the magnetic saturation in the conventional IM is worse than that of the proposed IM, although the maximum flux density of the two IMs is approximately 1.8~1.9 T. Owing to the different rotor structure of the two IMs, the magnetic field in the proposed motor is generated by PMs and the magnetizing current drawn from grid and that of the convention IM is only generated by the magnetizing current drawn from the stator current. Considering the magnetic saturation of two IMs shown in Fig. 3, we can derive that the magnetizing current of the proposed IM is smaller than that of conventional IM, and power density of the proposed IM is higher than that of the conventional IM, which is important for the improvement of power factor and efficiency. Fig. 4 shows the flux linkage waveforms of two motors running in same loaded condition. The profile of the mutual flux linkage of two motors is sinusoidal curves, which indicates that the induced speed voltage will be sinusoidal. Meanwhile, we can also find that the flux linkage magnitude of the proposed IM is smaller than that of the conventional IM, then the magnetizing current of the proposed IM will smaller than that of the conventional IM. 211

4 3.2. Power Factor (a) Conventional IM (b) Proposed IM Fig. 3. Flux density of two motors under loaded condition. Y1 [Wb] Y1 [Wb] XY Plot 5 Maxwell2DDesign1 FluxLinkage(WindingA) FluxLinkage(WindingB) FluxLinkage(WindingC) (a) Flux linkage of the conventional IM XY Plot 5 FluxLinkage(WindingA) FluxLinkage(WindingB) FluxLinkage(WindingC) (b) Flux linkage of the proposed IM Fig. 4. Flux linkage of two motors in the same load. In conventional induction motors, a magnetic field is required as a medium to achieve the electromechanical energy conversion. And the magnetic field is generated by magnetization current drawn from the grid, therefore, the power factor of the conventional IMs will be low, especially the motor is running at no-load or light-load conditions. In the proposed IM, the magnetic field is generated by permanent magnets (PMs) and magnetization current drawn from the grid. Therefore the proposed IM s power factor can be reached at arbitrary level (0~1) in theory, if all of the magnetic field is generated by PMs, the motor is operating at unity power factor. By the finite element analysis method, we have also investigated the power factor of two IMs. Fig. 5(a) and Fig. 5(b) show the inducted voltage waveform and input current waveform of two IMs running on same loaded condition respectively. In Fig. 5(a), the power factor angle of the conventional IM is about 46.8 degrees, and in Fig. 5(b), the power factor angle of the proposed IM is about 30.6 degrees, then the proposed IM s power factor (cos30.6 =0.861) is higher than that of the conventional IM (cos46.8 =0.684). Therefore the proposed IM having a new internal PM rotor and a magnetic barrier cage-rotor is potential to improve its power factor. [V] XY Plot 2 Maxwell2DDesign1 2 InputCurrent(WindingA) [V] (a) Conventional IM with 87Nm load torque XY Plot 2 InputCurrent(WindingA) (b) Proposed IM with 87Nm load torque 2 Fig. 5. Phase difference between inducted voltage and input current. 1-1 Table 2 lists the power factors comparison for two IMs. We can find that the proposed IM s power factors are higher than that of the conventional IM. When the motors are running at no-load or light-load InputCurrent(WindingA) [A] In p u tc u rre n t(w in d in g A ) [A ] 212

5 condition, i.e. at a load of 14 %~47 % rated power, the power factor of a conventional motor is 0.16~0.37, and the proposed motor is 1~ And the power factor of the proposed IM is decreasing with the load torque increasing. This means that when the load torque is increasing, active current drawn from the grid will increase obviously. Y1 [V] XY Plot 3 Maxwell2DDesign1 InducedVoltage(WindingB) InducedVoltage(WindingC) Torque (Nm) Table 2. Power factor comparisons. Proposed IM Conventional IM φ cos φ φ cos φ In the proposed IM, the main flux is generated by the PM and the magnetization current drawn from the grid [15]. According to the IM operation principles, the induced electromotive force (EMF) of the motor is given by E = fnk w φ, (1) where k w is the winding factor, f is the frequency of the power supply, N is the total number of turns per phase, and Φ is the magnetic flux per pole. In Eq. (1), the mutual flux Φ is consisting of Φ 1 generated by PMs and Φ 2 generated by the magnetization current drawn from the grid. Fig. 6 (a) and Fig. 6 (b) show the EMF waveforms of proposed IM and conventional IM running on the same loaded condition respectively. In Fig. 6 (a) and Fig. 6 (b), we can find that the EMF magnitude of the proposed IM is smaller than that of the conventional IM. According to Eq. (1), the part of mutual flux Φ 2 of the proposed IM is smaller than that of the conventional IM, then the reactive current of the proposed IM is much smaller than that of the conventional IM, and the power factor of the proposed IM is higher. Fig. 6 (b) and Fig. 6 (c) show the EMF waveform of the proposed IM when its load varying from 74 Nm to 87 Nm. We can find that the EMF magnitude is decreasing about 100 V. According to Eq.(2), we can derive that the main flux Φ of the proposed IM running at 87 Nm is smaller than that running at 74 Nm, and the magnetization current drawn from the grid is smaller than that running at 74 Nm. Therefore, the proposed IM has great potential to improve the performances, such as the power factor and the efficiency. -80 Y 1 [V ] Y1 [V] (a) EMF of conventional IM with 87 Nm load torque XY Plot 3 InducedVoltage(WindingB) InducedVoltage(WindingC) (b) EMF of proposed IM with 87Nm load torque XY Plot 4-80 InducedVoltage(WindingB) InducedVoltage(WindingC) (c) EMF of proposed IM with 74 Nm load torque Fig. 6. EMF of two IMs in different load conditions Energy Efficiency To maximize energy efficiency of an IM is to minimize various losses including the iron and copper losses for the machine operated at various load conditions. The iron losses are directly calculated by 2D transient finite element method and Fig. 7 shows the curves of iron losses versus time of two IMs running at same condition. In Fig. 7, the average of iron losses of the conventional IM is about 111 W, and that of the proposed IM is about 70 W. Core Loss(W) Proposed IM Core Loss [W] Conventional IM Core Loss [W] Time(s) Fig. 7. Iron losses versus time curve of two IMs. 213

6 Due to the stator structures, stator windings and stator currents of the two IMs are the same, and then the copper losses of the two IMs are the same. Table 3 lists the efficiency comparison of two IMs. From Table 3, we can find that the efficiency of the proposed IM is higher than that of the conventional IM, and the proposed IM has the potential to achieve the high efficiency. Table 3. Efficiency comparison. Torque (Nm) η (Proposed IM) η (Conventional IM) Conclusion A new squirrel-cage rotor structure induction motor (IM) that has a auxiliary permanent magnet (PM) rotor and improved cage-rotor structure is presented to enhance the inherent power factor, and a prototype motor is fabricated by modifying the rotor of a conventional three-phase four-pole squirrel-cage induction motor. The internal PM rotor is installed on the motor s shaft and has relative motion against the shaft at the synchronous speed. It can be levitated by magnetic bearings and sealed in a vacuum. Therefore there is no mechanical frictional loss and no windage loss. This paper focuses on the performance comparative analysis of the proposed IM and a conventional IM by using 2D transient finite element analysis method. The flux distribution, power factor, core losses, and efficiency of the proposed and conventional IMs have been investigated. At a load of 14 %~100 % rated power, the power factor of a conventional motor is 0.16~0.69, and the proposed motor is 1~0.86. All results show that the proposed IM is potential to achieve the high power factor and high efficiency. Acknowledgements This work was supported by the National Natural Science Foundation of China (No ). References [1]. A. A. Jimoh and D. V. Nicolae, Controlled capacitance injection into a three-phase induction motor through a single-phase auxiliary stator winding, in Proceedings of the IEEE International Electric Machines & Drives Conference, 3-5 May 2007, Vol. 2, pp [2]. M. C. Muteba, A. A. Jimoh and D. V. Nicolae, Improving three-phase induction machines power factor using single phase auxiliary winding fed by an active power filter, in Proceedings of the International Conference (AFRICON 07), September 2007, pp [3]. A. A. Mahmoud, T. H. Ortmeyer, R. G. Harley, and C. Calabrese, Effects of reactive compensation on induction motor dynamic performance, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-99, Issue 3, May 1980, pp [4]. R. Spee and A. K. Wallace, Comparative evaluation of power factor improvement techniques for squirrel cage induction motors, IEEE Transactions on Industry Applications, Vol. 28, Issue 2, 1992, pp [5]. T. Vinnal, K. Janson, H. Kalda and L. Kutt, Analyses of supply voltage quality, power consumption and losses affected by shunt capacitors for power factor correction, in Proceedings of the Electric Power Quality and Supply Reliability Conference, June 2010, pp [6]. L. F. Ruan, W. F. Zhang and P. S. Ye, Unity power factor operation for three-phase induction motor, in Proceedings of the 3 rd International Power Electronics and Motion Control Conference, 2000, Vol. 3, pp [7]. O. Ojo and M. Vipin, Capacitive compensation of large induction motor drives, in Proceedings of the Twenty-Second Southeastern Symposium on System Theory, 1990, pp [8]. F. Punga, L. Schon, Der neue kollektorlose einphasemotor der firma Krupp, Elektro-technishe Zeitschrifs, Bucharest, Rumania, Vol. 47, Pt. I, No. 29, July 22, pp ; Pt. II, No. 30, 29 July 1926, pp [9]. L. Schon, Die motoren der kruppschen hollen thalbahn lokomotive, Elektrische Bahnen, Berlin, Germany, Vol. 11, No. 3, March 1935, pp [10]. John F. H. Douglas, Characteristics of induction motors with permanent-magnet excitation, IEEE Transactions on Power Apparatus and Systems, Vol. 78, Issue 3, 1959, pp [11]. B. Ackermann, Single-phase induction motor with permanent magnet excitation, IEEE Transactions on Magnetics, Vol. 36, Issue 5, Part: 1, 2000, pp [12]. Y. Shibata, N. Tsuchida, K. Imai, Performance of induction motor with free-rotating magnets inside its rotor, IEEE Transactions on Industrial Electronics, Vol. 46, Issue 3, 1999, pp [13]. S. Niu, K. T. Chau, and C. Yu, Quantitative comparison of doublestator and traditional permanent magnet brushless machines, Journal of Applied Physics, Vol. 105, Issue 7, February 2009, pp. 07F F [14]. A. M. El-Refaie and T. M. Jahns, Optimal fluxweakening in surface PM machines using concentrated windings, IEEE Transactions on Industrial Applications, Vol. 41, Issue 3, May/June 2005, pp [15]. A. Toba and T. A. Lipo, Novel dual-excitation permanent magnet vernier machine, in Proceedings of the IEEE Industry Applications Society Annual Meeting, October 1999, Vol. 4, pp Copyright, International Frequency Sensor Association (IFSA) Publishing, S. L. All rights reserved. ( 214