GRD Journals- Global Research and Development Journal for Engineering | Volume 5 | Issue 5 | April 2020 ISSN: 2455-5703
Reduction of Cogging Torque in BLDC Motor using Finite Element Analysis [FEA] Mr. J. Muruganandham Department of Electrical and Electronics Engineering Sri Manakula Vinayagar Engineering College, India S. Deva Department of Electrical and Electronics Engineering Sri Manakula Vinayagar Engineering College, India
P. Santhoshkumar Department of Electrical and Electronics Engineering Sri Manakula Vinayagar Engineering College, India
S. Vengadapathy Department of Electrical and Electronics Engineering Sri Manakula Vinayagar Engineering College, India
S. Yuvan Kumar Department of Electrical and Electronics Engineering Sri Manakula Vinayagar Engineering College, India
Abstract There are several motors are being used in industrial applications in which BLDC motor plays a vital role due to its advantage of higher proficiency, better speed control and high torque ripple. Major reason for high torque ripple is due to cogging torque effect. To minimize the cogging torque, by slot shift opening method in stator part of BLDC motor. Our proposed motor design, 1mm notches have been applied on the stator poles of the BLDC motor using Finite Element Method [FEM]. This can increase overall efficiency of BLDC motor and also it increases the torque characteristics of BLDC motor and decreases the noise and vibration produced by the cogging torque of BLDC motor. Keywords- Cogging Torque, Finite Element Method [FEM], notches, torque ripple
I. INTRODUCTION BLDC motors are widely used now a days in electric drives. Many applications and design variations in BLDC Motor are increasing day by day because of its enormous advantages. The BLDC Motors are generally preferred in industries due their excellent features such as high torque density, compact size, high efficiency, less maintenance, better controlling at wide range of speeds and higher lifetime when compared to conventional motors. BLDC Motors have many benefits over induction motor and conventional DC motors such as no brush frictions to decrease the valuable torque, lower rotor inertia because of permanent magnet, better speed versus torque characteristics, noiseless operation. Unfortunately, one of main disadvantage of BLDC motor is the cogging torque, causing the undesirable effect in the motor i.e. vibrations and audible noises. The cogging torque appears from the rotor permanent magnet interacting with the steel teeth of the stator. This effect is undesirable in the operation of the motor as it causes torque ripple and motor jerking.
II. PROBLEM FORMULATION A. Inefficient commutation: The problem exist in the BLDC motor is inefficient commutation, because of Phase Offset Error and Delay Error. B. Complex Wiring: Brushless motors are not so simple. As mentioned, a brushless motor requires to be controlled to an ESC, which controls the flow of current to each electromagnet. Fortunately, pre-fabricated ESCs can essentially be purchased bundled with brushless motors. C. Magnetic Losses: Brushless motors have a more sophisticated set of electromagnets. More electromagnets mean more conductive coil material, which is often one of the most expensive components of a motor and this results in Magnetic loses such as Eddy current and Hysteresis Losses. D. Cogging Torque: The major problem existing in BLDC motor is Cogging torque. Higher values of cogging torque results in higher torque ripple of the BLDC motor. Cogging torque is a shifting torque due to contact between the magnetic field of the rotor and stator opening.
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Reduction of Cogging Torque in BLDC Motor using Finite Element Analysis [FEA] (GRDJE/ Volume 5 / Issue 5 / 009)
Every magnets in the stator and rotor of the motor will turns over the stator teeth, the reluctance proficiency by the magnet under the slot opening creates a change in reluctance for the magnetic flux in the motor which results in generation of Cogging Torque. The Cogging can be signified by, 1 dR Tcog = − ∅2g 2 dθ Where, Tcog is the Cogging torque. ∅g is the air-gap flux. θ is the rotor position. R is the air-gap reluctance.
III. METHODS TO REDUCE COGGING TORQUE Virtually all the techniques used against cogging torque also reduce the motor counter-electromotive force and so reduce the resultant running torque. There is no cogging torque in the slotless and coreless permanent magnet motor. The cogging torque reduced related to the stator structure has four methods as follows: 1) Proper thickness of stator tooth tips: When these stator tooth tips are became tiny, then they leads to magnetic saturation so there is increasing in cogging torque. The thickness of stator teeth should be the same width of slot opening. 2) Slots opening shift: The slot opening width affects the cogging torque. By reducing the slot opening width there will be reduction in permeance variation between teeth of stator. Thus, the cogging torque will decreases. 3) Increasing the number of slots/poles: When the number of slots/poles are nearby to 1, then the number of slots becomes more crucial. Higher the number of slots will affects the decreasing of cogging torque. 4) Addition of dummy slots: It makes the splitting of teeth overhang to regulate the permeance variation and to reduce the cogging torque. It has common effect as the double number of slots. Such that, the frequency also would be double. 5) Adding magnetic slot wedges: The openings of stator slots are closed by wedges which is made up of soft magnetic composite materials.
IV. PROPOSED METHOD TO REDUCE COGGING TORQUE: There are various kinds of methods can be adopted for cogging torque reduction; some of the techniques makes the changes in the current wave form, introduced into the stator windings. The cogging torque will emerges from the contact of rotor magnets and stator slots of the motor, there are many techniques for magnet and slot design. The best one is choosing the magnet pole span, an integer multiples of slot pitch, this method reduces the cogging torque associated with the BLDC motor. Slot opening shift is one of the most effective methods. Slot opening shift is a most desired method in which it also has some disadvantages like back-EMF distortion in BLDC motors. Manufacturing of large machines with conductors-bar is another disadvantage of this method. This analysis will mainly focus on the design of stator poles shifting for cogging torque reduction.
V. DESIGN AND ANALYSIS OF BLDC MOTOR USING FEM Table: 1 Dimensions of Proposed BLDC Motor PARAMETER MATERIAL USED Stator outer diameter CR10: Cold rolled 1010 steel Stator inner diameter CR10: Cold rolled 1010 steel Length of Stator CR10: Cold rolled 1010 steel Rotor core diameter CR10: Cold rolled 1010 steel Shaft diameter Stainless Steel No. of poles on stator CR10: Cold rolled 1010 steel No of rotor magnets NdFeB: Neodymium Iron Boron Air gap between stator and rotor Aa Copper: 5.77e7 Siemens/meter
DIMENSION 12 cm 10.4 cm 2.5 cm 7.8 cm 0.762 cm 12 8 0.2 cm 120 Turns
A. Cogging torque Calculation: To minimize the cogging torque, by slot shift opening method in stator part of BLDC motor. Our proposed motor design, 1mm notches have been applied on the stator poles of the BLDC motor using Finite Element Method [FEM]. This can increase overall efficiency of BLDC motor and also it increases the torque characteristics of BLDC motor and decreases the noise and vibration produced by the cogging torque of BLDC motor. The total magnetic flux (φt) that includes the winding flux (φw) and the PM flux (φPM) φđ?‘Ą= φđ?‘¤ + φđ?‘ƒđ?‘€ = đ?‘ đ?‘¤ đ?‘–đ?‘¤đ?‘…đ?‘¤+đ?‘…đ?‘Žđ?‘”+đ?‘…đ?‘ƒđ?‘€ + φđ?‘&#x;đ?‘…đ?‘ƒđ?‘€đ?‘…đ?‘¤+đ?‘…đ?‘Žđ?‘”+đ?‘…đ?‘ƒđ?‘€
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Reduction of Cogging Torque in BLDC Motor using Finite Element Analysis [FEA] (GRDJE/ Volume 5 / Issue 5 / 009)
= đ?‘ đ?‘¤ đ?‘–đ?‘¤đ?‘…đ?‘ đ?‘˘đ?‘š+ φđ?‘&#x;đ?‘…đ?‘ƒđ?‘€đ?‘…đ?‘ đ?‘˘đ?‘š where Nw is the number of turns of a winding, iw is the energized current, Rw is the reluctance of a winding, Rag is the reluctance of the air gap, RPM is the reluctance of a PM, đ?›—đ?’“ is the magnetic flux linking the winding due to a PM, and Rsum is the reluctance sum of the winding, the air gap, and the PM. The self-inductance of a winding can be written as đ??żđ?‘†= đ?‘ đ?‘¤ 2đ?‘…đ?‘¤+đ?‘…đ?‘Žđ?‘”+đ?‘…đ?‘ƒđ?‘€= đ?‘ đ?‘¤ 2đ?‘…đ?‘ đ?‘˘đ?‘š Wc(đ?‘–đ?‘¤,θ)= Wđ??ż + Wđ?‘Š + Wđ?‘ƒđ?‘€ = 12đ??żđ?‘†2đ?‘–đ?‘¤2+ 12đ?‘…đ?‘ đ?‘˘đ?‘šĎ†đ?‘Žđ?‘”2+ đ?‘ đ?‘¤đ?‘–đ?‘¤Ď†đ?‘ƒđ?‘€ Where WL is the co-energy stored in the self-inductance, WPM is the co-energy stored due to the isolated PM, Ww is the co-energy due to joint flux in the winding, and đ?›—đ?’‚đ?’ˆ is the airgap flux. Torque equation can be obtained by differentiating the coenergy equation with respect to the rotation angle θ đ?‘‡ = đ?œ•đ?‘Šđ?‘?đ?œ•đ?›ł |đ?‘–đ?‘¤=đ?‘?đ?‘œđ?‘›đ?‘ đ?‘Ąđ?‘Žđ?‘›đ?‘Ą = 12đ?‘–đ?‘¤2đ??żđ?‘†2đ?‘‘đ?›łâˆ’ 12φđ?‘Žđ?‘”2đ?‘…đ?‘ đ?‘˘đ?‘šđ?‘‘đ?›ł+ đ?‘ đ?‘¤ đ?‘–đ?‘¤Ď†đ?‘ƒđ?‘€đ?‘‘đ?›ł The first and third term in the above equation are zero without input energized current, i.e., iw = 0. Therefore, the cogging torque Tcog can be obtained by considering the interaction between the rotor PMs and slots and poles of the stator as given by đ?‘ťđ?’„đ?’?đ?’ˆ = - đ?&#x;?đ?&#x;?đ?›—đ?’‚đ?’ˆđ?&#x;?đ?‘šđ?’”đ?’–đ?’Žđ?’…đ?œ đ?‘…đ?‘ đ?‘˘đ?‘š=đ?‘…đ?‘¤+đ?‘…đ?‘Žđ?‘”+đ?‘…đ?‘ƒđ?‘€ đ?‘…đ?‘Žđ?‘” ≍ đ?‘…đ?‘¤+đ?‘…đ?‘ƒđ?‘€ đ?‘ťđ?’„đ?’?đ?’ˆ = - đ?&#x;?đ?&#x;?đ?›—đ?’‚đ?’ˆđ?&#x;?đ?‘šđ?’‚đ?’ˆđ?’…đ?œ Air gap reluctance ( ) and air gap flux (đ?›—đ?’‚đ?’ˆ) is directly proportional to cogging torque (đ?‘ťđ?’„đ?’?đ?’ˆ) ď€ Cogging torque can be minimized by reducing the air-gap reluctance (đ?‘šđ?’‚đ?’ˆ) or air gap flux (φg) đ?‘šđ?’‚đ?’ˆ = đ?’?Îź. đ?‘¨ đ?’?=length of the air gap A = Area of air gap Îź = permeability ď€ In our method, cogging torque (đ?’ˆ) can be reduced by varying the change of air gap reluctance. If Length of air gap increases (l), then Air gap Reluctance increases (đ?‘šđ?’‚đ?’ˆ) đ??‹đ?’‚đ?’ˆ = đ?&#x;?đ?‘šđ?’‚đ?’ˆ đ?‘šđ?’‚đ?’ˆ is inversely proportional to đ??‹đ?’‚đ?’ˆ If đ?‘šđ?’‚đ?’ˆ increases, đ??‹đ?’‚đ?’ˆ decreases, đ?‘ťđ?’„đ?’?g decreases ď€ Change of air gap reluctance is varying by creating novel air gap profile on stator slots, thereby we have reducing the cogging torque of brushless dc motor. ď€ We have improved the torque and speed characteristic of motor by reducing the cogging torque of brushless DC motor. Thereby, we have increased the overall efficiency of motor.
VI. 2D MODEL OF CONVENTIONAL MOTOR The design of the BLDC motor has been drawn based on the dimensional references obtained from the datasheet. The MagNet simulation software has been used to draw the 2D structure of the BLDC motor.
Fig. 1: MagNet model of the conventional BLDC motor.
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Reduction of Cogging Torque in BLDC Motor using Finite Element Analysis [FEA] (GRDJE/ Volume 5 / Issue 5 / 009)
VII.
2D MODEL OF PROPOSED MOTOR
In the model drawn in the Magnet simulation software we can clearly observe the notches in the stator pole shoes in all the 12 poles of the stator.
Fig. 2: MagNet model of the proposed BLDC motor
Fig. 3: Flux pathway of BLDC motor
The above simulation results are obtained from Magnet analysis, the above result shows maximum flux density distribution in our proposal model compare to the conventional model of brushless dc motor. Percentage of Flux Density Distribution improvement in our proposed design when compare to the conventional design of brushless dc motor, Maximum Flux Density Distribution in Conventional motor: 5.19 Wb/m2 Maximum Flux Density Distribution in Proposed motor: 4.33 Wb/m2 = 5.19 − 4.335.19 x 10 = 19 % improvement in flux density Distribution
VIII. RESULTS AND GRAPH OBTAINED:
Fig. 4: Cogging torque of conventional BLDC motor
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Reduction of Cogging Torque in BLDC Motor using Finite Element Analysis [FEA] (GRDJE/ Volume 5 / Issue 5 / 009)
Maximum value of Cogging Torque = 0.64 Nm.
Fig. 5: Cogging torque of Proposed BLDC motor
Maximum value of cogging torque in proposed BLDC motor = 0.23Nm.
Fig. 6: Torque of conventional BLDC motor
Maximum value of Torque: 1.74 Nm
Fig. 7: Torque of Proposed BLDC motor
Maximum value of Torque: 1.85 Nm
IX. OBTAINED VALUES AND RESULTS A. Percentage of Cogging Torque Reduction in our Proposed Model Percentage of Cogging torque reduction in our proposed design when compare to the conventional design of brushless dc motor, Maximum value of Cogging Torque in conventional design: 0.64 Nm Maximum value of Cogging Torque in proposed design: 0.295 Nm = 0.64 −0.2950.64 × 100 = 54 % reduction in cogging torque. B. Percentage of Torque Improvement in our Proposed Model Percentage of torque improvement in our proposed design when compare to the conventional design of brushless dc motor, Maximum value of Torque in conventional design: 1.74 Nm Maximum value of Torque in proposed design: 1.855 Nm All rights reserved by www.grdjournals.com
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Reduction of Cogging Torque in BLDC Motor using Finite Element Analysis [FEA] (GRDJE/ Volume 5 / Issue 5 / 009)
= 1.855−1.741.85 × 100 = 6.4 % improvement in torque. C. Percentage of Efficiency Improvement in our Proposed Model Percentage of torque improvement in our proposed design when compare to the conventional design of brushless dc motor, Maximum percentage of efficiency obtained in conventional design: 80.3 % Maximum percentage of efficiency obtained in proposed design: 84.47 % = 84.47 – 80.3 = 4.17 % improvement in efficiency. D. Percentage of Torque Improvement in our Proposed Model Percentage of Miles improvement in our proposed design when compare to the conventional design of brushless dc motor, Maximum distance in Miles obtained in conventional motor = 26.48 miles Maximum distance in Miles obtained in Proposed motor = 30.7 miles = 30.7 − 26.4826.48 × 100 = 15.9 % increase in Miles.
X. CONCLUSION In this paper, proposed stator design of BLDC motor decreases vibration and noise by reducing the cogging torque. Influencing of change in slot opening across the stator poles, then the reduction of cogging torque is achieved by Finite Element Method (FEM). Application of this proposed method to an 8-pole, 12-slot machine brings a cogging torque reduction of 54%. The scope of this method is reduction of cogging torque with maximum efficiency and reduce time. The torque ripple, torque peak values and harmonics are reduced to considerable amount in the proposed method.
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