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CHAPTER 1
INTRODUCTION
1.1
HISTORY OF BRUSHLESS DC MOTORS
Brushless DC motors (BLDC) are an invaluable part of industry today. Use of
these motors can save nearly any industry a great deal of time and money under the right
circumstances. The BLDC motor actually represents the end, or at least the most recent
end result, of a long evolution of motor technology. Before there were brushless DC
motors there were brushed DC motors, which were brought on in part to replace the less
efficient AC induction motors that came before. The brush DC motor was invented all
the way back in 1856 by famed German inventor and industrialist Ernst Werner Von
Siemens. Von Siemens is so famous that the international standard unit of electrical
conductance is named after him. Von Siemens studied electrical engineering after
leaving the army and produced many contributions to the world of electrical
engineering, including the first electric elevator in 1880. Von Siemens's brush DC motor
was fairly rudimentary and was improved upon by Harry Ward Leonard, who nearly
perfected the first effective motor control system near the end of the 19th century. In the
year of 1873 Zenobe Gramme invented the modern DC motor. This system used a
rheostat to control the current in the field winding, which resulted in adjusting the output
voltage of the DC generator, which in turn adjusted the motor speed. The Ward Leonard
system remained in place all the way until 1960, when the Electronic Regulator
Company's thyristor devices produced solid state controllers that could convert AC
power to rectified DC power more directly. It supplanted the Ward Leonard system due
to its simplicity and efficiency.
1.1.1 Advent of Brushless DC Motors
Once the Electronic Regulator Company maximized the efficiency of the
brush DC motor, the door was opened for an even more efficient motor device.
Brushless DC motors first made the scene in 1962, when T.G. Wilson and P.H. Trickey
unveiled what they called "a DC motor with solid state commutation." Remember that
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the key element of brushless DC motors as opposed to brush DC motors is that the
brushless DC motor requires no physical commutator, a revolutionary difference. As the
device was refined and developed, it became a popular choice for special applications
such as computer disk drives, robotics and in aircraft. In fact, brushless DC motors are
used in these devices today, fifty years later, so great is their effectiveness. The reason
these motors were such a great choice for these devices is that in these devices brush
wear was a big problem, either because of the intense demands of the application or, for
example, in the case of aircraft because of low humidity. Because brushless DC motors
had no brushes that could wear out, they represented a great leap forward in technology
for these types of devices. The problem was that as reliable as they were, these early
brushless DC motors were not able to generate a great deal of power.
1.1.2 Modern Brushless Dc Motors
That all changed in the 1980s, when permanent magnet materials became readily
available. The use of permanent magnets, combined with high voltage transistors,
enabled brushless DC motors to generate as much power as the old brush DC motors, if
not more. Near the end of the 1980s, Robert E. Lordo of the POWERTEC Industrial
Corporation unveiled the first large brushless DC motors, which had at least ten times
the power of the earlier brushless DC motors. Today, there are probably no major motor
manufacturers that do not produce brushless DC motors capable of high power jobs.
Naturally, NMB Tech offers a wide variety of brushless DC motors for you to choose
from, in sizes from 15mm in diameter to 65mm in diameter, Watt’s output range from
0.7 to maximum of 329.9. Industries with motor needs have relied on brushless DC
motors for nearly fifty years, and there is every reason to believe that they will continue
to do so for decades to come. Take a look at some brushless DC motors today.
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1.2
INTRODUCTION OF BLDC MOTOR
Brushless Direct Current (BLDC) motors are one of the motor types rapidly
gaining popularity. BLDC motors are used in industries such as Appliances,
Automotive, Aerospace, Consumer, Medical, Industrial Automation Equipment and
Instrumentation. As the name implies, BLDC motors do not use brushes for
commutation; instead, they are electronically commutated. BLDC motors have many
advantages over brushed DC motors and induction motors. A few of these are:
Better speed versus torque characteristics
High dynamic response
High efficiency
Long operating life
Noiseless operation
Higher speed ranges
In addition, the ratio of torque delivered to the size of the motor is higher,
making it useful in applications where space and weight are critical factors. In this
application note, we will discuss in detail the construction, working principle,
characteristics and typical applications of BLDC motors.
1.3
CONSTRUCTION AND OPERATING PRINCIPLE
BLDC motors are a type of synchronous motor. This means the magnetic field
generated by the stator and the magnetic fields generated by the rotor rotate at the same
frequency. BLDC motors do not experience the “slip” that is normally seen in induction
motors. BLDC motors come in single-phase, 2-phase and 3-phase configurations.
Corresponding to its type, the stator has the same number of windings. Out of these, 3phase motors are the most popular and widely used. This application note focuses on 3phase motors. The brushless motor, unlike the DC brushed motor, has the permanent
magnets glued on the rotor. It has usually four magnets around the perimeter. The stator
of the motor is composed by the electromagnets, usually four of them, placed in a cross
pattern with 90o angle between them. The major advantage of the brushless motors is
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that, due to the fact that the rotor carries only the permanent magnets, no need of
winding connections to be done with the rotor, no brush-commutator pair needs to be
made. This is how the brushless motors took their name from. This feature gives the
brushless motor great enhancement in reliability, as the brushes wear off very fast.
Moreover, brushless motors are more silent and more efficient in terms of power
consumption.
1.4
MATHEMATICAL MODELLING OF BLDC MOTOR
Brushless DC motors usually driven by balanced three phase waveforms.
Equivalent circuit of each phase consists of a winding inductance, a resistance and
induced back-emf voltage due to the rotation of the rotor. Per phase equivalent circuit of
a BLDC motor is shown in Figure 1.1.
Figure 1.1 Per Phase equivalent circuit of Brushless DC motor
Using the equivalent circuit electrical equation can be obtained as in equation (1.1)
V Ri L
where,
V is applied phase voltage,
i is phase current,
e is back emf voltage and
L is phase inductance.
di
e
dt
(1.1)
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As stated before, Brushless DC motors are usually fed with three phase balanced voltage
waveforms. The voltage equations for the three phase BLDC motor are written in matrix
form in equation (1.2), [25] and [27].
Va R 0 0 I a L M
V 0 R 0 I d 0
L
b b dt Vc 0 0 R I c 0
0
0
M
L
0 I a ea 0 I b eb M I c ec (1.2)
where,
Va , Vb , Vc are applied phase voltages,
I a , I b , I c are phase currents,
L is phase inductance,
R is phase resistance and
M is mutual inductance
Produced electromagnetic torque can be obtained by using the output power of
electrical motor. Electrical output power of electrical motor is defined by production of
three phase back emf voltages and phase currents. Form the mechanical point of view,
power can be represented as output torque multiplied by the angular speed. Using these
two definition electromagnetic torque is defined in equation (1.3).
Te (ea I a eb Ib ec I c ) / w
(1.3)
where, w is mechanical speed of motor and
Te is electromechanical torque
Mechanical relation between the speed and the torque are represented in equation (1.4).
dw P Te Tload
dt 2 J
where,
w
d
dt
(1.4)
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Tload is load torque,
J is rotating inertia,
is rotor mechanical position and
P is number of pole
All the equations stated above are represented in stationary referenced frame. All
electrical quantities (voltages and currents) are changing with the electrical frequency of
the rotation. From the control point of view it is difficult to control the variables
changing with time. To make the control easy these equations can be represented in
synchronously rotating frame with stator. If all quantities are represented in that frame
all variables become constant values and these make the system easily controlled.
Figure 1.2 Stator and Rotor Reference Frames
Figure1.2 shows the stationary frame and the synchronously rotating frame. d-axis is the
axis of permanent magnet flux and the q-axis is the perpendicular axis to the d-axis.
Voltage and current quantities can be transformed into the d-q axis quantities using
Clark-park transformation matrices. Equations for transformations are stated in equation
(1.5) – (1.6).
ia iqs 2 cos( e ) cos( e 2 / 3) cos( e 2 / 3) i 3 sin( ) sin( 2 / 3) sin( 2 / 3) i b e
e
e
ds i c
(1.5)
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Va Vqs 2 cos(e ) cos(e 2 / 3) cos(e 2 / 3) V sin( ) sin( 2 / 3) sin( 2 / 3) Vb e
e
e
ds 3 V c
(1.6)
Equivalent of equation (1.2) in d-q axis is represented as a set of equation (1.7).
Vqs Ri qs L
di qs
Vds Ri ds L
dt
eq
di ds
ed
dt
(1.7)
eq wed wr (isd Ld m )
ed weq wr (isq Lq )
where,
Ld is d-axis inductance,
Lq is q-axis inductance,
m is rotor flux linkage due to permanent magnets,
d is d-axis flux linkage,
we is rotor electrical speed and
e is rotor electrical position.
Electromagnetic torque representation Te in d-q axis is given in equation (1.8)
Te 1.5
3
P miqs ( Ld Lq )iqsids
2
(1.8)
TYPES OF BRUSHLESS MOTORS
Brushless DC motors can be classified by rotor structural design and rotor flux
direction. Classifications based on rotor structures are shown in Figure1.3. In surface
mounted rotor motors magnets are mounted to the surface of rotor. This process is
relatively easy and low cost process. Magnets are easily skewed in this type and skewing
helps to reduce the torque oscillations (cogging torque) [26]. Also since magnets are on
the surface of rotor, air gap can be large and effective and saliency effect is minimized.
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Lq and Ld inductances are equal and this makes the reluctance torque minimized (1.8).
The main drawback of this type is that magnets can detach from the rotor at high speeds.
Figure 1.3 Rotor structures of BLDC motors (a) Surface Mounted Magnets,
(b) Interior Mounted Magnets and (c) Buried Magnets
In interior mounted magnet of motors, magnets are inserted inside the rotor
rather than bounding the surface. This makes a robust design and motor can operate at
high speeds. But due to differences in d and q axes inductance reluctance torque exists in
this type of motors. Electrical properties of buried magnet motors are nearly same with
the interior mounted magnet motors. In buried magnet motors fluxes can go through the
motor shaft and to prevent this flux, nonmagnetic shafts should be used [28]. BLDC
motor can also be classified in terms of flux directions. Usually BLDC motors are used
in Radial Flux (RF) topology. These motors are used in servo applications. Motor axial
length is longer and inertia of the rotor is kept small in order to have small response time
to load changes. Axial Flux (AF) motors differ from the other types of the motors due to
the flux direction and magnet construction shape of the motor. In (RF) motors, the flux
lines goes through the radial direction from the rotor. In (AF) motors, flux goes through
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the axial direction. The Radial and Axial flux motors are shown in the Figures 1.4 and
1.5.
Figure 1.4 Radial Flux Brushless DC Motors
Figure 1.5 Axial Flux Brushless DC Motors
Axial flux motors can be designed by placing the rotor outside the stator. With
this type of design disc type loads can be coupled with the motor [70]. Sometimes motor
is completely inserted into load (i.e. power transmission components [29]). These
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motors are widely used for low torque servo applications. These types of motors are
used where small axial space and large radial space are needed. Main drawback of axial
flux motors are presence of two air gaps (In RF type motors there is only one air gap).
Mechanical design should be carefully done in AF motors.
1.6
STATOR
The stator of a BLDC motor consists of stacked steel laminations with windings
placed in the slots that are axially cut along the inner periphery. Traditionally, the stator
resembles that of an induction motor; however, the windings are distributed in a
different manner. Most BLDC motors have three stator windings connected in star
fashion. Each of these windings is constructed with numerous coils interconnected to
form a winding. One or more coils are placed in the slots and they are interconnected to
make a winding. Each of these windings is distributed over the stator periphery to form
an even numbers of poles.
There are two types of stator winding variants: trapezoidal and sinusoidal
motors. This differentiation is made on the basis of the interconnection of coils in the
stator windings to give different types of back Electromotive Force (EMF). In addition
to the back EMF, the phase current also has trapezoidal and sinusoidal variations in the
respective types of motor. This makes the torque output by a sinusoidal motor smoother
than that of a trapezoidal motor. However, this comes with an extra cost, as the
sinusoidal motors take extra winding interconnections because of the coils distribution
on the stator periphery, thereby increasing the copper intake by the stator windings.
Depending upon the control power supply capability, the motor with the correct
voltage rating of the stator can be chosen. Forty-eight volts, or less voltage rated motors
are used in automotive, robotics, small arm movements and so on. Motors with 100
volts, or higher ratings, are used in appliances, automation and in industrial applications.
The material used for the construction of the stator in the brushless direct current motor
is CR10: Cold rolled 1010 steel. Its magnetic permeability is 2.2T and electric
permittivity is 1. The construction of the stator of a BLDC motor is shown in Figure 1.6.
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The performance and characteristics of the material used in BLDC motor are briefly
explained in Appendix 2.
Figure 1.6 Stator of BLDC Motor
1.7
ROTOR
The rotor is enclosed or inserted by permanent magnet and can vary from two to
eight pole pairs with alternate North (N) and South (S) poles. Based on the required
magnetic field density of the rotor, the proper magnetic material is chosen to make the
rotor. Ferrite magnets are traditionally used to make permanent magnets. As the
technology advances, rare earth alloy magnets are gaining popularity. The ferrite
magnets are less expensive but they have the disadvantage of low flux density of a given
volume. In contrast, the alloy material has high magnetic density per volume and enables
the rotor to compress further for the same torque. Also, these alloy magnets improve the
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size-to-weight ratio and give higher torque for the same size motor using ferrite
magnets.
Neodymium (Nd), Samarium Cobalt (SmCo) and the alloy of Neodymium,
Ferrite and Boron (NdFeB) are some examples of rare earth alloy magnets [32].
Continuous research is going on to improve the flux density to compress the rotor
further.
1.8
THEORY OF OPERATION
Each commutation sequence has one of the windings energized to positive power
(current enters into the winding), the second winding is negative (current exits the
winding) and the third is in a non-energized condition. Torque is produced because of
the interaction between the magnetic fields generated by the stator coils and the
permanent magnets. Ideally, the peak should shift positions, as the rotor moves to catch
up with the stator field which is known as “Six-Step Commutation” defining the
sequence of energizing the windings.
1.9
COGGING TORQUE
Cogging torque is also called detent torque, and it is one of the inherent
characteristics of Permanent Magnet motors. Theoretically, cogging torque is caused by
the reluctance change between the stator teeth and magnetic poles on the rotor, and it is
mainly the magnetic poles corners, not the whole magnetic poles, which create the
cogging torque. Cogging torque is influenced by a variety of design factors of BLDC
Motor. Among the factors, air gap length, slot opening, and magnetic poles pitch play
important roles. Cogging torque drastically influences the control precision of PM
motors used in speed and position control systems. In these control systems, usually
Permanent Magnet Brushless DC motors (PMBLDC) and PM synchronous motors
(PMSM) are employed. A bigger cogging torque can affect the speed, disturb the
position of the system and in the precise control applications, such effects are undesired.
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Cogging torque is a magnetic locking that occurs in the motor which results in
increase of noise, vibration in motor and hence preventing the motor to rotate smoothly.
Finally, it affects the enhancement of the motor performance and in serious cases, a
mechanical resonant may occur so that serious destruction is caused. To significantly
reduce harmful cogging torque, PM motor has become one of the most interesting
research topics in the motor design and application fields. The cogging torque
calculating methods and reduction measures will be discussed in the next section.
1.10
COGGING TORQUE REDUCTION METHODS
With the development of high-performance PM material, PM motors are widely
used in speed and position control systems. However, in slotted motors, cogging torque,
which may affect the control accuracy, is produced as a result of the interaction between
PMs and armature. Some advantages of this motor can be listed as their high
performance, high relation of torque/volume, capability at high speed applications and
electronic driven commutation. These motors have also some drawbacks in addition to
all these advantages.
Thus technique for reducing cogging torque plays an important role in motor
design, and many studies have been carried out on the prediction and reduction of
cogging torque. The various techniques have been adopted to reduce cogging torque
such as magnetic poles design, skewing, and dummy slots [1] and [8]. The CAD of
radial flux surface-mounted magnets were easy to fabricate and was applied successfully
[2]-[5]. Asymmetric magnets and shifting angles were applied to reduce harmonics of
cogging torque [3].
The 2D finite element analysis is applied successfully to surface mounted PM
motors [5], [8], [13]. The radial field topology has applied FEM in optimization of PM
motors [6]-[7]. Eccentric and uniform pole surface designs are proposed to make a
sinusoidal magnetic flux density in air gap in order to reduce cogging torque [8]. The
response surface method also uses the multi-quadric radial basis function to interpolate
the objective function in design of rotor [8]-[10]. Hyper cube sampling strategy is
applied to optimize a magnetic poles shape of the large scale permanent magnet motor
[10]. The influence of stator tooth width on cogging torque is analyzed both theoretically
14
and experimentally [11]. Recently, some researchers proposed modifications in the air
gap profile to reduce cogging torque and improve starting torque [12]. An integrated
optimization process to minimize cogging torque in permanent magnet (PM) motors by
a simple Gradient Descent method and the design techniques of non-uniformly
distributed magnets and teeth are presented to illustrate the optimization process [14].
Many methods have been developed to minimize the cogging torque. Some of
them in the literature are using the shape of lamination [15-16], using the auxiliary slots
using air gap profiles [17], skewing the rotor magnets, step skewing implemented by
FEA, stator slots skewing [18]-[19], adapting to different combinations of slot numbers
and pole numbers [20] and adapting Iso-diametric magnet [21]. A FEA method based on
a combination of electromagnetic fields and circuit equations for magnetic field
modeling and torque prediction are presented in decreasing the level of cogging torque
without skewing [22].
Some core shapes that reduce cogging torque is designed by using genetic
algorithm [23]. The radial field topology has applied FEM in optimization of PM motors
[12]. The predefined slot shapes reduce the cogging torque, which is an evolution
strategy for the optimal design process to determine the slot size [24]. The Optimization
of Two Design Techniques for Cogging Torque Reduction Combined with Analytical
Method by a Simple Gradient Descent Method are discussed in [14].
The existing methods for cogging torque reduction are: changing the shape of
lamination [1]-[2], using auxiliary slots [3]-[4], skewing the rotor magnets [5] or the
stator slots [6], adapting different combinations of slot numbers and pole numbers [7].
The first four methods increase the complexity of the motor construction. The last
method is effective, but it limits the choice of slots numbers. Based on Fourier
expansion and energy method, the analytical expression of cogging torque is derived,
which can be used to analyze the effects of design parameters on cogging torque [75][80].
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1.11
LITERATURE REVIEW:
Mohammad Islam et al., (2004) have verified the techniques by designing the
magnetic poles design, skewing, dummy slots and step skewing for reducing the cogging
torque. Parag Upadhyay and Rajagopal., (2005) have done the analysis in radial-flux
surface mounted magnet, air gap flux density, slot electric loading, winding factor,
stacking factor, stator current density, slot space factor, magnet fraction and slot
fraction. Yubo Yang., (2006) has implemented the optimization method for improved
domain elimination, finite-element method and analytical method.
Lijian Wu et al., (2007) have designed and analyzed the surface mounted
permanent-magnet (PM) motors and auxiliary grooves are applied on the PM poles. Fei,
W., and Luk, P.C.K., (2007) have designed the axial magnet pole pairing method. KyuYun Hwang et al., (2007) have analyzed the rotor pole design in spoke-Type Brushless
DC Motor by Response Surface Method (RSM).
Pan Seok Shin et al., (2008) have proposed a new algorithm for the shape
optimization of a large-scale BLDC motor to reduce the cogging torque. In the
algorithm, an adaptive RSM using the multi-quadric radial basis function is employed to
interpolate the objective function. In the adaptive RSM, an adaptive sampling point
insertion method is developed utilizing the design sensitivities computed by using the
finite-element method to get a reasonable response surface with a relatively small
number of sampling points. An adaptive response surface method with Latin hypercube
sampling strategy is employed to optimize a magnet pole shape of large scale brushless
BLDC motor to minimize the cogging torque.
Jiang Xintong et al., (2009) have verified the tooth width and slot/pole match on
cogging torque corresponding to both uniformed teeth and non-uniformed teeth. The
theoretical analysis on the basis of analytical expressions of cogging torque is deduced
with energy method. Yubo Yang et al., (2009) have illustrated the asymmetry magnets,
in which the magnetic poles are non identical in that the irregularities cause reduction in
cogging torque. Lin, D., et al., (2009) have implemented the design techniques for
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reducing the Cogging torque in surface-mounted PM motors. The surface mounted PM
motor is one of the classifications of PM motors used for high performance.
Mohammed Fazil., (2010) has proposed a new air-gap profile for single-phase
PMBLDC motor to improve starting torque and to reduce cogging torque. Mohammed
Fazil., (2011) has implemented the Non-linear dynamic modeling of single phase
PMBLDC motor.
Daohan Wang et al., (2012) have verified the simple gradient descent method to
reduce the cogging torque in BLDC motor. The presented optimization method can be
easily achieved in motor design. The design techniques of non-uniformly distributed
magnets and teeth are presented to illustrate the optimization process. First, with the
assistance of an analytical motor deduced, the initial solution and feasible domain of the
optimization can be easily identified. Then a simple Gradient Descent method is
combined with finite element analysis to perform the optimization within the identified
feasible domain.
Upadhayay and Rajagopal., (2013) have designed a surface mounted BLDC
motor using magnet pole shaping techniques for cogging torque reduction. Hong Seok
Kim et al., (2013) have proposed the optimization of an anisotropic ferrite magnet shape
and magnetization direction to maximize back-EMF of interior permanent magnet
BLDC motor. Kang et al., (2013) have mathematically derived the frequency equations
of back electromotive force, cogging torque and unbalanced magnetic force of a BLDC
motor due to unevenly magnetized permanent magnet. The mathematical equations are
experimentally validated by comparing with the measured cogging torque and
unbalanced and back electro motive force of a hard disk drive spindle motor.
Chang Seop Koh (1997) have verified the magnetic pole shape optimization of
permanent magnet motor for reduction of cogging torque. Cogging torque reduction
methods are discussed and applied to the optimum design of a small BLDC motor.
Because the cogging torque has a close relation with the distribution of the
magnetization, the magnetizing system for permanent magnets is analyzed numerically
by using the time-stepping finite element method. Based on the remanant magnetic flux
densities, the cogging torque is computed by using finite element analysis.
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Yang, Y.P., et al., (2004) have designed and controlled the axial flux BLDC
motor for electric vehicles and they have done the multi objective optimal design and
analysis. The resulting axial-flux permanent-magnet motor has high torque-to-weight
ratio and motor efficiency and is suitable for direct-driven wheel applications. Because
the disk-type wheel motor is built into the hub of the wheel, no transmission gears or
mechanical differentials are necessary and overall efficiency is thereby increased and
weight is reduced.
Caricchi, F., et al., (2004) have implemented experimental study on reducing
cogging torque and no-load power loss in Axial-Flux Permanent Magnet Motors
(AFPM) with slotted winding. The AFPM topology is suited for direct-drive
applications, due to its enhanced flux-weakening capability. AFPMs having slotted
windings are the most promising candidates for use in wheel-motor drives. Based on the
above, this paper deals with an experimental study devoted to investigate a number of
technical solutions to be used in AFPMs having slotted windings in order to achieve
substantial reduction of both cogging torque and no-load power loss in the motor.
Aydin, M., et al., (2006) have verified the torque quality and comparison of
internal and external rotor axial flux surface-magnet disc motors. Pulsating torque
components of permanent magnet motors and pulsating torque minimization techniques
are discussed for axial flux surface-magnet disc-type PM motors. The pulsating torque
analysis describing general instantaneous electromagnetic torque equation and torque
ripple factor is briefly provided in order to analyze torque ripple component.
Breton et al., (2000) studied and presented the influence of motor symmetry on
reduction of cogging torque in PMBLDC motors. They have used two methods to
reduce cogging torque, both based on the analysis of the rotational symmetry in a
PMBLDC motor. The first one seeks to lower the symmetry in the flux of a conventional
motor with asymmetrical magnet distribution. The second one uses auxiliary slots to
increase the frequency of cogging. Analytical calculations have been made to predict the
harmonic spectra of the motors. Numerical calculations by the finite-element method
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have been carried out. The results support the analytical conclusions. The frequency of
the torque pulsation is increased, as predicted.
Nicola Bianchi and Silverio Bolognani (2000) have implemented the design
techniques for reducing the cogging torque in surface-mounted PM motors. Several
techniques are adapted in designing surface-mounted permanent-magnet motors in order
to reduce the cogging torque. This paper describes various classical and innovative
techniques, giving a theoretical justification for each one of them. A simple original
motor of the cogging torque mechanism and a Fourier analysis are introduced. As a
result, it is highlighted that some techniques are not always utilizable, and some of them
may even be deprecatory when not used correctly. In addition, effects of cogging torque
elimination on back electromotive force are discussed.
Hansel man, D.C., (1997) have presented the effect of skew, pole count and slot
count on BLDC motor radial force, cogging torque and back EMF. Permanent magnet
brushless motors are increasingly being used in high performance applications. In many
of these applications, the acoustic noise and torque ripple characteristics of the motor are
of primary concern. Because of this, it is important to understand the influence of the
motor geometrical parameters of skew amount, pole count and slot count on the
resulting motor characteristics of radial force, cogging torque and back EMF.
Hwang, C.C., and Wu, S.S., (1998) have implemented reduction of cogging
torque in spindle motors for CD-ROM drive. The effects of the slot and pole number
combination and optimal choice of the width ratio of armature teeth to magnet pole are
investigated. A finite element method based on a combination of electromagnetic field
and circuit equations for magnetic field motoring and torque prediction is presented.
Results show that increasing the number of the least common multiple of slot and pole
gives rise to decrease the level of cogging torque. To obtain smaller amplitude of the
cogging torque, it is also necessary to choose the proper width ratio of armature teeth to
magnet pole.
Ki-jinHua et al., (2000) have designed the optimal core shape design for cogging
torque reduction of BLDC motor using genetic algorithm. In this paper, some core
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shapes that reduce cogging torque are found by using the reluctance network method
(RNM) for magnetic field analysis and genetic algorithms (GAs) for optimization. The
outer rotor type BLDC motor for the DVD ROM driving system is optimized as a
sample motor.
Tae Kyung Chungand Suk Ki Kim (1997) have implemented the optimal pole
shape design for the reduction of cogging torque of brushless DC motor using evolution
strategy. Maxwell stress tensor for torque calculation and an evolution strategy for the
optimal design process to determine the slot size are used. Experimental verification
shows that the proposed algorithm reduces the level of mechanical vibration and noise
substantially.
In this thesis, new design techniques are proposed to reduce the cogging torque
in PMBLDC motor. The performances of the proposed method are compared with the
existing methods in the literature. It is shown that the proposed methods give enhanced
performance in reducing the cogging torque.
1.12
OBJECTIVES AND CONTRIBUTIONS
Brushless DC motor (BLDC) is a motor that develops maximum torque when
stationary and have linearly decreasing torque with increasing speed. Limitations of
brushed DC motors are low efficiency, poor performance, high wear and tear, less
rugged, more complex and expensive control electronics. The BLDC motor has
permanent magnets, which eliminate most of the drawbacks mentioned above. In this
BLDC motor the cogging torque occurs, due to interaction between permanent magnet
and stator slots of the motor. In order to reduce this cogging torque, many researchers
have reported methods based on surface mounted magnets, skewing of PMs, Isodiametric magnetic poles, bifurcation, dummy slots and other slot modifications. In this
thesis, new methods have been developed such as: Semi-circled magnetic poles, Uclamped magnetic poles, Grooving in rotor PMs and T-shaped bifurcation in stator slots.
The performances of proposed methods are evaluated using CAD software and FEA
method and compared with the latest techniques reported in the literature. From the
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results it is found that all the four methods have shown considerable performance
improvement in the existing methods of reducing the cogging torque in BLDC motors.
1.13
ORGANISATION OF THESIS
Chapter-2 deals with the proposed techniques using the reshaping of magnetic
poles namely Semi-circled and U-clamped magnetic poles, which are derived from the
existing Iso-diametric magnetic poles.
Chapter-3
proposes
the
techniques
using
Modified
Symmetrical
and
Asymmetrical and Grooving in magnetic pole designs.
Chapter-4 discusses the existing slot modification techniques such as Single,
Dual Bifurcations and Reduced Stator Slot Width. The performances of the existing
methods have been compared in reducing the cogging torque in BLDC motors.
Chapter-5 gives the detailed derivation of T-shaped bifurcation method of stator
teeth and the superiority of this method is highlighted.
Chapter-6 concludes giving the salient points of the research carried out in this
thesis.