The 5th International Power Engineering and Optimization Conference (PEOCO2011), Shah Alam, Selangor, Malaysia : 6-7 June 2011
Predicting the Localized Flux Distribution in Three Phase Induction Motor between Different Stator Slot Size S. Nor Shafiqin, N.H. Halim, I.Daut, Y.Yanawati, M. Asri, M.Dina, N.Gomesh, I.Pungut, M.Abdullah, M.N. Syatirah Electrical Energy and Industrial Electronic System School of Electrical System Engineering, Universiti Malaysia Perlis (UniMAP) email:
[email protected],
[email protected]
conductor a voltage proportional to the rate of change of the flux linkage. Therefore, as long as the conductor on the rotor experience the time varying magnetic field established by the stator currents, voltages are induced across the conductor. The stator magnetic field originates from the stator currents, while the rotor magnetic field originates from the induced rotor currents. The motor synchronous speed is dictated by the stator winding excitation frequency and the number of magnetic poles for which the motor is wound. A pair of poles, which rotate with flux vectors, exits where flux flowing in opposite direction in the stator back iron, the steel region radially away from the stator slots [2]. In this paper, predicting the localized flux distribution in three phase induction motor stators between different stator slot sizes is presented.
Abstract A three phase induction motor differences of stator slot size is investigated in terms of its localized flux distribution. The search coil induced voltage method is used to analyze the flux distribution in the stator core. For both stator models, the maximum flux density is found near the tooth tip and minimum towards the outer region of the stator core. By saying so, this investigation shows that if there are differences in the stator slot size, the values of flux density differs as well. Keywords: Induction motor, Stator slot, flux density, flux distribution. 1. Introduction Induction machine is the most used of all electric motors. It is generally easy to build and cheaper than corresponding dc or synchronous motors. The induction motors is rugged and require little maintenance [1]. The ac induction consists of stationary member, called the stator and the rotating member, called the rotor. AC power is used to energize the stator windings. Magnetic flux is produces when the electric current flows in a conductor. A magnetic field map describes the strength and direction of flux. The stator in a motor consists of many conductors in slots, most of which may accommodate conductors excited by more than one phase. In an AC motor, the currents vary sinusoidally with time causing the magnetic field to also vary sinusoidally with time. When magnetic flux flows through a coil of wire, it links that conductor. )DUDGD\¶V ODZ RI LQGXFWLRQ VWDWHV WKDW D FKDQJLQJ magnetic field linking a conductor induces in the
2. Experimental Method Two models of the three phase induction motor stators between different stator slot size used in this measurement which is slot 6mm and slot 8mm, as shows in Figure 1 and Figure 2. The number of the stator slot for both model are 24 slots. The outside and inside diameters were 180 and 68mm, respectively and the tooth length was 13mm. The core material was 0.35mm thickness non-oriented silicon steel sheet, and the total number of lamination was 30. The stator winding was a single layer winding and the number of turn in each stator slot was equal to be 60 turns. The exciting frequency is 50Hz. The 0.1mm wire search coils is used to measuring the tangential and radial components. The coils were threaded through 0.3mm diameter holes sufficiently small to avoid disturbing the flux distribution to any practical extent [3]. The drilled process as shown in Figure 3. The search coil positions are designated at stator tooth tip, stator tooth
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center, stator tooth root and stator yoke, as shown in Figure 4 and Figure 5. The distance between the holes is 10mm. The orthogonal search coils were used to detect orthogonal components of flux density using an established method [4]. Figure 6 shows the equipment set up for measuring localized flux density under voltage excitation at 1.0, 1.5 and 1.8T. The value of the induced voltage is directly proportional to the change of flux and also the area enclosed by the search coil. Therefore the flux density waveform can be calculated by integrating the voltage induced in the coil, B
1 NA
³ Vdt
(1)
Figure 3: Drilling Process
This induced voltage will be calculated as the equation, Vind = 4.44 × B × f × A × N
(2)
Where B is the flux density, f is the frequency, A is the cross sectional of the surface of yoke and N is the number of winding turns.
Figure 4: Search coil positions for stator slot size 6mm
Figure 1: Stator slot size of 6mm
Figure 5: Search coil positions for stator slot size 8mm
Figure 2: Stator slot size of 8mm
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torque [8], it shows that the stator slot size of 8mm is better than stator slot size of 6mm because it has lower flux density and contribute lower localized power loss.
Figure 6: Equipment set up for measuring localized flux density 3. Results and Discussion The magnetic field was analyzed by selected four sites for the investigation of flux density variation which are stator tooth tip, stator tooth center, stator tooth root and stator yoke in both models which is the stator slot size of 6mm and the stator slot size of 8mm. The magnetic flux density vectors in the stator yoke for both models exhibit an elliptical form. The peak value of resultant flux density is decreased towards the outer region of the stator core. This is due to core geometry, which influences the flux and loss distribution [3]. Along the rest of the tooth, the radial component of the flux density is dominant and the flux density in the stator tooth is nearly purely alternating in most of the tooth. The maximum flux density is found near the tooth tip which is the narrowest portion of the tooth aside from the area [5]. This is due to the slotting effects have the most influence in the proximity of the air-gap and the rotating rotor teeth produce large variation of the air-JDS¶s reluctivity [6]. Therefore the flux density polarization is much stronger behind the stator tooth than behind the slot [7]. From Figure 7, Figure 8 and Figure 9, it is shows that the stator slot size of 8mm has lower localized flux density value compare to stator slot size of 6mm measurement. The maximum localized flux density in the stator slot size of 8mm is approximately 0.082T, 0.219T and 0.401 when stator magnetized at 1.0T, 1.5T and 1.8T respectively while the maximum localized flux density in stator slot size of 6mm is approximately 0.123T, 0.237T and 0.420T when stator magnetized at 1.0T, 1.5T and 1.8T respectively. For both model, the minimum peak flux density occurred at the outer regions and the maximum peak flux density occurred at the inner regions of stator core especially at the stator tooth. Since the real motors depend on magnetic flux to produce voltage and
(a)
(b) Figure 7: Fundamental component of peak flux density at stator core model magnetized at 1.0T
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(a)
(a)
(b)
(b)
Figure 8: Fundamental component of peak flux density at stator core model magnetized at 1.5T
Figure 9: Fundamental component of peak flux density at stator core model magnetized at 1.8T
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Mater. 83 (1-3) (1990) 553-554. [5]R.D.Findlay, N. S. (2000). Predicting Rotational Flux Distribution in Three Phase Induction Motor Stators IEEE Transactions on Energy Conversion, pp.375379. [6]T.Marcic, B. S., G.Stumber, M.Hadziselimovic, I.Zagradisnik. (2008). The impact of different stator and rotor slot number combinations on iron loss of a three phase induction motor at no-load. Journal of Magnetism and Materials, 320, 891895. [7]Stranges, N. (2000). An Investigation of Iron Losses due to Ratating Flux in Three Phase Induction Motor Cores. McMaster University, Hamilton, Ontario. [8]J.Chapman, S. (2005). Electric Machinery Fundamentals (Fourth Edition ed.). Australia: Mc Graw Hill.
4. Conclusion Predicting the localized flux distribution in three phase induction motor stators between different stator slot sizes using search coils induced voltage method was proposed. We may conclude from the experiment that the increasing size of stator slot size will give the lower flux density value. The flux for both models, is nearly circularly polarized at the roots of the stator teeth, and is elliptically polarized at the base of the stator slots [5]. Acknowledgment The authors wish to thank School of Electrical Systems Engineering, University Malaysia Perlis (UniMAP) for the technical and financial support. References [1]Hubert, C. I. (2002). ELECTRIC MACHINES Theory, Operation, Application, Adjustment, and Control. Columbus, Ohio: Prentice Hall. [2]Ho, S. (1996). Analysis and Design of AC Induction Motors with Squirrel Cage Rotors. University of New Hamshire, New Hampshire. [3]A.J. Moses, N.Tutkun, Localised losses in stator laminations of an induction motor under PWM excitation, Journal of Materials Processing Technology 161 (2005) 79-82. [4]R.S. Albir, A.J. Moses, Improved DC bride method employed to measured local power loss in electrical steels and armorphous material, J. Magn. Magn.
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