UJET VOL. 2, NO. 2, DEC Page 8

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1 UMUDIKE JOURNAL OF ENGINEERING AND TECHNOLOGY (UJET) VOL. 2, NO. 2, DEC 2016 PAGE 8-15 FINITE ELEMENT ANALYSIS OF A 7.5KW ASYNCHRONOUS MOTOR UNDER INTERMITTENT LOADING. Abunike, E. C. and Okoro, O. I. Department of Electrical and Electronic Engineering, Michael Okpara University of Agriculture, Umudike, Abia State. ABSTRACT This paper explores dynamic behaviors of asynchronous motor under intermittent loading based on finite element models and solutions in time domain. Transient Finite Element Analysis (FEA) of the motor in an on and off loading conditions is performed at time intervals of 0.2s for a 2D model. Dynamic characteristics such as speed, torque, phase currents and flux distributions of the motor were studied. It was observed that at the 37 Nm rated load, the motor recorded a negative speed when loaded intermittently at 0.2s intervals. The maximum load for which positive speed was observed is 32 Nm. The other characteristics were in acceptable ranges for both loads. Key words: Finite Element Analysis (FEA), induction motor, intermittent loading, magnetic flux distributions and movement band. 1. INTRODUCTION Induction machines are asynchronous speed machines operated as motors or generators. The induction motor is the workhorse of modern industry and has practically replaced the DC motor (Akpama and Okoro, 2015) and (Chikuni et al.,2008). This is because of its ruggedness, low cost of purchase and minimal maintenance requirements. It consists of a stator with slots in which the windings are placed and a rotor, which is constructed like a squirrel cage or has windings around rotor cores. In squirrel cage motors, the rotor slots consist of copper or aluminium bars forming the conductors. Each bar is welded to the two end rings and a number of these bars short-circuited in this way resemble a squirrel cage (Chikuni et al.,2008). The running operations at any given time are crucial for the production safety and that is why a special attention is paid to such investigations in many research papers. The application of induction machines in the industrial, commercial and domestic sectors are numerous and as such research work in this regard has become endless (Akpama and Okoro, 2015). Corresponding Author: Abunike E. C. abunikeweta@gmail.com FEA is a technique based on the magnetic field analysis that takes into account the magnetic circuit geometry, spatial distribution of stator windings and rotor bars, existing slots around air gap, and nonlinear behavior of ferromagnetic materials. By the introduction of the FEA, numerical field calculation approach has become a strong alternative in electrical machine design and analysis and many performance outcomes (current-speed, torque-speed, efficiency-output power characteristics etc) can be assessed (Huseyin, 2011). Intensive knowledge of Finite Element Method could be found in (Silvester and Ferrari, 1996), (Jabber, 2004) and (Arkkio, 1987). Intermittent loading is a loading condition which involves stopping and starting of the motor often over a period of time. Often times, in production lines, the asynchronous motors are operated in an intermittent mode and accurate model for predicting the behavior of this motor is very important. In order to elaborate an accurate methodology, a deep analysis of the motor during intermittent loading needs to be carried out. In this context a lot of interesting and helpful information can be obtained by using modern numerical investigation tools based on finite element method in order to comparatively analyze the motor. Such investigations that could not be done in the past due to the significant limitations of the computers performances in terms of memory size and CPU speed are now available and could provide useful information in this area. UJET VOL. 2, NO. 2, DEC Page 8

2 2. TRANSIENT ANALYSIS OF ASYNCHRONOUS MOTOR USING FEA The beauty of finite element analysis is that both the physical and mathematical modeling is required so that the problem domain is accurately described. FEA has well defined procedure that must be followed and the steps are outlined as: description of the geometry, material definition, assigning the boundary conditions, assigning excitations, indication of the parameters, mesh operation, analysis and post-processing (Maxwell, 2013). In modeling of the motor, the position of the moving parts of the motor must be taken into account, because the magnetic forces depend on the position of these moving parts. These positions, in turn, influence the magnetic field within the motor. Therefore, for full and suitable modeling, a link between the fields and motion must be established. At this end, only three-phase voltage applied to the terminal of the motor are required as known input value and phase current is evaluated as unknown value. The governing equation for two dimensional (2-D) FE analysis is given by (Lee et al., 2004): x 1 A 1 A da ) ( ) x y y dt ( (1) Where, µ is the permeability, A is the component of magnetic vector potential, σ is the conductivity of the materials, and J o is the existing current density of the stator winding. The voltage equation per each phase is: V a J o dia d a I ara Le (2) dt dt Where V a, I a, R a, ϕ a, and L e are the input voltage, the current, the resistance, the flux linkage of each phase and the end-coil inductance, respectively. It should be noted, L e is calculated by using RMxprt toolbox in Ansys Maxwell. 2.1 FINITE ELEMENT MODEL and the corresponding finite element mesh that includes information like 579 elements of the band, 123 elements of the bar are presented in figure 1. In order to analyze the motor properly, the 2D electromagnetic field computation domain could not be reduced by exploiting the geometrical or physical symmetries. The stator windings and the rotor bars-end rings are made respectively with copper and aluminum materials. The stator armature contains 48 slots and the rotor armature 44 slots. The boundary condition (vector potential) is specified on the outer edges of the stator of the motor. Figure 1: Geometry and mesh of the computation mesh Based on the works of (Nwangwu, 2015), the magnetic field equation solved by Maxwell2D is: v A J s A v H t Where v is the material reluctivity, J s is the current density, and H c is the permanent magnet field strength. The terms in the right hand side of (3) include current density source term (Js), eddy current term (σ A t), electric vector potential term (σ v), permanent magnet source term ( H c ), and mechanical movement term (σv A) respectively. The nature of the problem determines which term(s) to include in the analysis. (3) c v A The complete field model of the induction motor is shown in figure 1. The 2D electromagnetic field computation domain represented by a cross section through the induction motor UJET VOL. 2, NO. 2, DEC Page 9

3 3. MATERIALS AND METHOD The design and analysis of the proposed motor was performed on the Rmxprt and Maxwell2D platform. The design parameters are presented in table 1. Table 1: Specifications of the Proposed Asynchronous Motor. Parameters Number of poles 4 Values Number of phases 3 Outer diameter of stator (mm) 210 Inner diameter of stator (mm) 148 Air gap length (mm) 0.7 Axial length (mm) 250 Outer diameter of rotor (mm) Inner diameter of rotor (mm) 48 Number of stator slots 48 Number of rotor slots 44 Rated voltage, V 380 Rated frequency, Hz 50 Rated power, kw 7.5 Stacking factor 0.92 Slot type 2 The modeled motor was analyzed using Finite Element as detailed machine parameters of stator and rotor dimensions, number of poles and phases, air-gap length, axial length, stacking factor etc were fully utilized. Transient analysis was implemented for loads of 37 Nm and 32 Nm, full voltage (220 V), direct on line starting of the designated motor. 4. SIMULATION RESULTS AND DISCUSSION The three-phase asynchronous motor performance characteristics of speed, torque, phase currents, magnetic flux lines, magnetic flux density magnitude, magnetic flux density vector and vector potential were monitored during the intermittent loading of the considered machine. The motor is loaded with torques of 37 Nm(load torque) and 32 Nm at start, and was operated at 0.2 second intervals, with the motor switched on and off between the intervals. The motor performance was observed for 1 second. The magnetic flux lines plot of the motor at 1 second after the introduction of the load (37Nm) over the motor cross section is shown in figure 2. Figure 2: Flux lines at 1 second The magnetic flux density magnitude and the magnitude of vector potential can be observed after the load has been applied as seen in figures 3 and 4. As expected, 4 pole field, light and heavy magnetic induction regions can be distinguished. Since sequential field maps are available after a simulation, rotating field and winding MMF forms can be inspected to have a better idea of a specific winding and rotor slot designs. UJET VOL. 2, NO. 2, DEC Page 10

4 Figure 3: Magnitude of magnetic flux density Figure 4: Magnitude of vector potential Figure 5: Magnetic flux density vector showing the direction of concentration. UJET VOL. 2, NO. 2, DEC Page 11

5 Moving1. Torque[Nm] Figure 6: Magnetic vector potential vector lines characteristics Figure 5 shows the flux density vector. The flux is indeed concentrated in the iron cores and air-gap as expected. The vector potentials vector lines links all the slots of the modeled motor as graphed in figure 6. The transient solver was also used to study the behavior of the motor during the intermittent mechanical movement. A movement band is created in the model that encloses the rotor and part of the air-gap. The electromagnetic torque characteristic at loads of 37Nm and 32Nm is shown in figure 7. Expectedly, the torques were observed to decay to zero at the off-states and oscillate about 37Nm and 32 Nm respectively at start-up. The electromagnetic torque observed was expected as the motor is constantly in transient mode. Moving1.Torque [NewtonMeter]@ 37 Nm Moving1.Torque [NewtonMeter]@ 32 Nm Time[s] Figure 7: Relationship between the torque and time under the intermittent 37Nm and 32Nm. The speed performance characteristic of the motor under the intermittent loading was also observed. With a little oscillation around zero speed, it went to an unacceptable speed range for a load of 37Nm; thereby driving the machine into generating mode. This phenomenon can be partly explained by the fact that the motor transients were not allowed to settle before the load was removed. The range of speed during the on and off states when a load of 32Nm was introduced were found acceptable within the rated speed of 1360 rpm and 1500 rpm respectively. UJET VOL. 2, NO. 2, DEC Page 12

6 Moving1.Speed[rpm] The phase currents were also monitored during the intermittent loading of the motor. The phase currents with time characteristics at full load are shown in figure 9. It was observed that the phase currents at the on-states of the motor were 15A, higher than the 11.65A rated current of the motor at the same load torque of 37Nm. This can be accounted for by the fact that the motor was constrained to operate in a time interval that is not sufficient to attain a steady state. The characteristic relationship between the phase currents with time at a load of 32Nm is shown in figure 10. Moving1.Speed [rpm]@ 37 Nm Moving1.Speed [rpm]@ 32 Nm Time[s] Figure 8: Speed performance Characteristics of the motor under intermittent loading Phase A Phase C XY Plot 64 Maxwell2DDesign1 Curve Info Current(PhaseA) Setup11 : Transient Current(PhaseB) Setup11 : Transient Current(PhaseC) Setup11 : Transient CURRENT [A] Phase B Time [s] Figure 9: Phase currents with time under the intermittent loading at 37 Nm. UJET VOL. 2, NO. 2, DEC Page 13

7 CURRENTS [A] Phase A Current[A] XY Plot 85 Maxwell2DDesign1 Curve Info Phase A Phase C Current(PhaseA) Setup23 : Transient Current(PhaseB) Setup23 : Transient Current(PhaseC) Setup23 : Transient Phase B Time [s] Figure 10: Phase currents with Time at 32 Nm. Current(PhaseA) [A]@ 37 Nm Current(PhaseA) [A]@ 32 Nm Time[s] Figure 11: Phase A Current with Time at 37Nm and 32Nm As also expected, the phase currents are observed to oscillate about zero at the off-states and oscillate about the motor s rated current of 11.65A at the on-states. The relationship of the phase A current of the motor when loads of 37 Nm and 32Nm were applied intermittently are presented in figure 11. It is observed that the magnitude of the current at both torques is almost the same both for the on and off states operation with little harmonics on the current at the full load of 37Nm. It should be noted that mesh size, time steps and solution types must be chosen appropriately in order to carry out a better analysis. These results can be commented as a validation of transient FEA. In transient FEA, electrical and mechanical behavior of the asynchronous is based on lowest level interaction of electrical current and magnetic field (Hameyer and Belmans, 1999), (Pao et al., 2008). 5. CONCLUSION In this paper, the study of FEA of a 7.5 kw asynchronous motor under intermittent loading has been given. The design and analysis has been performed on the Ansys s Rmxprt and Maxwell platform. A 7.5 kw, 4 pole asynchronous motor UJET VOL. 2, NO. 2, DEC Page 14

8 geometry and parameters have been obtained by Rmxprt and transferred into Maxwell 2D for transient magnetic loading. The FEA has aided the monitoring of the transient performance characteristics of the motor during the intermittent loading where the applied voltage was considered as input to FE computation. The motor can be operated at a maximum load of 32Nm under this condition. The monitored motor characteristics of torque, phase current, magnetic flux density and vector potential showed a better transient performances for the given time interval of 0.2s unlike the speed characteristics (at full load) which resulted in the machine operating at generating mode. Therefore, it is not recommended to operate the machine at the rated load of 37 Nm at the 0.2 s intervals on intermittent basis. fed by Pulse Width Modulated Inverter. IEEE Transaction on Magnets. Vol 40(2): Maxwell Training Manual (2013). Vol 14,pp Nwangwu, E.O. (2015). Magnetic Analysis and Simulation of IPM Synchronous Motor using Maxwell2D Finite Element Software. Proceeding of the ICEPENG. Pp Pao, L.O. (2008). Studies of Mechanical Vibrations and Current Harmonics in Induction motors using FEM. WSEAS Transactions on Systems. Vol. 7(3): Silvester, P.P. and Ferrari, R.L. (1996). Finite Elements for Electrical Engineers. Third Edition, Cambridge University Press, London. REFERENCES Akpama, E.J. and Okoro, O.I. (2015). Modelling Multiphae Induction Machine for Torque Improvement. Umudike Journal of Engineering and Technology. Vol 1 (1): Arkkio, A. (1987). Analysis of Induction Motors Based on the Numerical Solution of the Magnets Field and Circuit Equations. Doctoral Thesis, Faculty of Electrical Engineering, Helsinki University of Technology. Chikuni, E, Okoro, O.I. and Khan, M.T. (2008). Concise Higher Electrical Engineering. First Edition. Juta and Company Ltd. Cape Town, South Africa. Hameyer, K. and Belmans, R. (1999). Numerical Modelling and Design of Electrical Machines and Devices. WIT Press, Parker Street, London. Huseyin, T. D. (2011). Computer Aided Design and Transient Finite Element Analysis of Induction Motor. Recent Advances in Electrical Engineering. Vol.9 (2): Jabber, M.A., Phyu H.N., Liu Z. and Chao B. (2004). Modelling and Numerical Simulation of a Brushless Permanent Magnet DC Motor in Dynamic Conditions by Time-Stepping Technique. IEEE Transanctions on Indsutry Applications. Vol.40 (3): Lee, J.J., Kim Y.K., Nam K.H., Ha J., Hong P. and Hwang D.H. (2004). Loss Distribution of 3-Phase Induction Motor UJET VOL. 2, NO. 2, DEC Page 15

9 UMUDIKE JOURNAL OF ENGINEERING AND TECHNOLOGY (UJET), VOL. 2, NO. 2, DEC 2016