chapter 2 brushless dc motor...28 2.2.1 stator similar to an induction motor, the bldc motor stator...
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CHAPTER 2
BRUSHLESS DC MOTOR
2.1 INTRODUCTION
A motion system based on the DC motor provides a good, simple
and efficient solution to satisfy the requirements of a variable speed drive.
Although dc motors possess good control characteristics and ruggedness, their
performance and applications in wider areas is inhibited due to sparking and
commutation problems. Induction motor do not possess the above mentioned
problems, they have their own limitations such as low power factor and non-
linear speed torque characteristics (Ramu Krishnan 2009). With the
advancement of technology and development of modern control techniques,
the Permanent Magnet Brushless DC (PMBLDC) motor is able to overcome
the limitations mentioned above and satisfy the requirements of a variable
speed drive. The permanent magnet machines have the feature of high torque
to size ratio. They possess very good dynamic characteristics due to low
inertia in the permanent magnet rotor. Permanent magnet machines can be
classified into dc commutator motor, Permanent Magnet Synchronous Motor
(PMSM) and Permanent Magnet Brushless DC (PMBLDC) motor.
The permanent magnet dc commutator motor is similar in
construction to the conventional dc motor except that the field winding is
replaced by permanent magnets. PMBLDC motors are generated by virtually
inverting the stator and rotor of PM DC motors. The ‘DC’ term does not refer
to a DC motor. These motors are actually fed by rectangular AC waveform.
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The PMSM and PMBLDC motors have similar construction with poly-phase
stator windings and permanent magnet rotors, the difference being the method
of control and the distribution of windings. The PMSM motor has
sinusoidally distributed stator windings and the controller tracks sinusoidal
reference current. The PMBLDC motor is fed with rectangular voltages and
the windings are distributed so as to produce trapezoidal back emf (Kenjo &
Nagamori 1985). The advantages of using brushless DC motor are as follows,
� High Speed Operation - BLDC motors can operate at speed
above 10,000 rpm under loaded and unloaded conditions
� Responsiveness and Quick Acceleration - Inner rotor BLDC
motors have low rotor inertia, allowing them to accelerate,
decelerate, and reverse direction quickly
� High Reliability - BLDC motors do not have brushes, have
life expectancies over 10,000 hours
� High Power Density - A good weight/size to power ratio
2.2 COMPONENTS OF BLDC MOTOR
Figure 2.1 shows the structure of BLDC motor that are the ideal
choice for applications that require high reliability, high efficiency and high
power to volume ratio (Chang-liang Xia 2012). Generally, BLDC motors are
considered to be high performance motors that are capable of providing large
amounts of torque over a vast speed range. Figure 2.2(a) and Figure 2.2(b)
show the cross sectional view of DC and BLDC motors which implies that the
derivative of the most commonly used DC motor, the brushless DC motor
share the same torque and speed performance curve characteristics.
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Figure 2.1 Structure of Brushless DC Motor
Figure 2.2 Cross Sectional View of Motors
The coils are attached to the stator and the commutation is
controlled by electronics. Commutation times are provided either by position
sensors or by coils Back Electromotive Force (emf) measurements. Brushless
DC motors usually consist of three main parts: a Stator, a Rotor and Hall
Sensors.
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2.2.1 Stator
Similar to an Induction motor, the BLDC motor stator is made up
of laminated steel stacked up to carry the windings as shown in Figure 2.3.
Windings in a stator can be arranged in two patterns, i.e. a star pattern (Y) or
delta pattern (∆). The major difference between the two patterns is that the Y
pattern gives high torque at low speed and the ∆ pattern gives low torque at
low speed. This is because in the delta configuration, half of the voltage is
applied across the winding that is not driven, thus increasing losses and in
turn, efficiency and torque.
Figure 2.3 Stator in a BLDC Motor
Cross sectional views of slotted and slotless BLDC Motors are
shown in Figure 2.4(a) and Figure 2.4(b). An advantage of the brushless
configuration in which the rotor is inside the stator is that more cross-
sectional area is available for the power or armature winding. At the same
time the conduction of heat through the frame is improved. A slotless core has
lower inductance, thus it can run at very high speed. Because of the absence
of teeth in the lamination stack, requirements for the cogging torque also go
down, thus making them an ideal fit for low speed too (when permanent
magnets on rotor and tooth on the stator align with each other then, because of
the interaction between the two, an undesirable cogging torque develops and
causes ripples in speed).
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Figure 2.4 Slotted and Slotless Motor
The main disadvantage of a slotless core is higher cost because
it requires more winding to compensate for the larger air gap. The
magnetization of the permanent magnets and their displacement on the rotor
is chosen so that shape of the back emf (the voltage induced into the stator
winding due to rotor movement) is trapezoidal. This allows the DC voltage of
a rectangular shape, to create a rotational field with low torque ripples. The
motor can have more than one pole-pair per phase. Proper selection of the
laminated steel and windings for the construction of stator are crucial to motor
performance. An improper selection may lead to multiple problems during
production, resulting in market delays and increased design costs.
2.2.2 Rotor
Depending upon the application requirements, the number of poles
in the rotor may vary. Figure 2.5 (a) and Figure 2.5 (b) show the 4 and 8 pole
of the permanent magnet rotor respectively. Increasing the number of poles
give better torque but the cost has to be reduced with the maximum possible
speed (Jang & Lee 2005). Another rotor parameter that makes an impact on
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the maximum torque is the material used for the construction of permanent
magnet, higher the flux density of the material and higher the torque.
(a) Four Pole (b) Eight Pole
Figure 2.5 Permanent Magnet Rotor
The rotor in a BLDC motor consists of an even number of
permanent magnets. The number of magnetic poles in the rotor also affects
the step size and torque ripple of the motor. More poles give smaller steps and
less torque ripple. Any of these PMBLM rotor configurations can be selected
on the basis of application and power rating. The flux density of the rotor is
high due to the construction of permanent magnet, hence there are no losses in
rotor because of no winding present in core.
Figure 2.6 Rotor in a BLDC Motor
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The permanent magnets go from 1 to 5 pairs of poles. The rotor can
vary from two to eight pole pairs with alternate North (N) and South (S)
poles. Based on the required magnetic field density in 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 for 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. The rotor of
brushless DC motor with one and two pair of poles are represented in Figure
2.6(a) and Figure 2.6(b). Also, these alloy magnets improve the size-to-weight
ratio and give higher torque for the same size motor using ferrite magnets.
2.2.3 Hall Sensors
These kinds of devices are based on Hall-effect theory, which states
that if an electric current carrying conductor is kept in a magnetic field, the
magnetic field exerts a transverse force on the moving charge carriers that
tends to push them to one side of the conductor. A build-up of charge at the
sides of the conductors will balance this magnetic influence thus producing a
measurable voltage between the two sides of the conductor. The presence of
this measurable transverse voltage is called the Hall-effect because it was
discovered by Edwin Hall in 1879.
For the estimation of the rotor position, the motor is equipped with
three hall sensors. These hall sensors are placed every 120°, with these
sensors, 6 different commutations are possible. Phase commutation depends
on hall sensor values. Power supply to the coils changes when hall sensor
values change. With right synchronized commutations, the torque remains
nearly constant and high.
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Figure 2.7 Hall Sensor Phase Commutation of BLDC Motor
Figure 2.7 shows the phase commutation of BLDC motor
depending on hall sensor. It is possible to determine when to commutate the
motor drive voltages by sensing the back emf voltage on an undriven motor
terminal during one of the drive phases. The obvious cost advantage of
sensorless control is the elimination of the Hall position sensors. However the
usage of BLDC motor with sensor is applicable for some applications.
2.2.4 Phase Commutation
To simplify the explanation of how to operate a three phase BLDC
motor, a typical BLDC motor with only three coils is considered. As
previously shown, phases commutation depends on the hall sensor values.
When motor coils are correctly supplied, a magnetic field is created and the
rotor moves. The most elementary commutation driving method used for
BLDC motors is an ON-OFF scheme, a coil is either conducting or not
conducting. Only two windings are supplied at the same time and the third
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winding is floating (Jan & Kim 2006). Connecting the coils to the power and
neutral bus induces the current flow. This is referred as trapezoidal
commutation or block commutation.
Figure 2.8 Three Phase Bridge Inverter
Figure 2.8 shows the three phase bridges of inverter to run the
BLDC Motor. To command brushless DC motors, a three phase bridges is
used. For motors with multiple poles the electrical rotation does not
correspond to a mechanical rotation. A four pole BLDC motor uses four
electrical rotation cycles to have one mechanical rotation. The back emf of the
BLDC Motor can drive the inverter by detecting the zero crossing point of the
back emf, then commutate the inverter power switching devices. The two
power switching device turn ON at any instant for 60 degree and the
commutation occurs by next pair conducted for the continuous operation of
Motor.
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Table 2.1 Hall Sensor Truth Table
Hall Sensors Values Phase Switches
101 U-V Q1;Q4
001 U-W Q1;Q6
011 V-W Q3;Q6
010 V-U Q3;Q2
110 V-W Q5;Q2
100 W-V Q5;Q4
The strength of the magnetic field determines the force and speed
of the motor. By varying the current flow through the coils, the speed and
torque of the motor can be adjusted. The most common way to control the
current flow is to control the average current flow through the coils. PWM is
used to adjust the average voltage and thereby the average current, inducing
the speed. Table 2.1 shows the operation sequence of a BLDC motor with
Hall Sensors. The proposed scheme utilizes the back emf difference between
two phases for BLDC sensorless drive instead of using the phase back emf.
Figure 2.9 shows the equivalent circuit of a Y connection BLDC motor and
the inverter topology.
Figure 2.9 Circuit Diagrams of BLDC Motor with Inverter
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Figure 2.10 Phase Back EMF of BLDC Motor
The zero crossing points of the back emf in each phase may be an
attractive feature used for sensing, because these points are independent of
speed and occur at rotor positions where the phase winding is not excited.
However, these points do not correspond to the commutation instants.
Therefore, the signals must be phase shifted by 90 electrical degree before
they can be used for commutation. The detection of the third harmonic
component in back emf, direct current control algorithm and phase locked
loops have been proposed to overcome the phase-shifting problem.
Figure 2.10 shows the phase back emf of BLDC motor. The
commutation sequence with back emf difference estimation method is that
positive sign indicates the current entering into the stator winding and the
negative sign indicates the current leaving from the stator winding. At any
instant two stator windings are energized and one winding will be in floating.
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2.3 DYNAMIC MODEL OF BLDC MOTOR
The derivation of this model is based on the assumption that the
induced currents in the rotor due to stator harmonic fields are neglected and
the iron and stray losses are also neglected (Krishnan 2009). Damper
windings are not usually a part of PMBLDCM where damping is provided by
the inverter control. The motor is considered to have three phases even though
the derivation process is valid for any number of phases shown in Figure 2.11.
Equations (2.1), (2.2) & (2.3) implies the voltage equation of the stator
windings.
aan a a a a
diV R i L edt
� � � (2.1)
bbn b b b b
diV R i L edt
� � � (2.2)
ccn c c c c
diV R i L edt
� � � (2.3)
Figure 2.11 Dynamic Model of BLDC Motor
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where,
Van, Vbn and Vcn : phase voltage in volts
ia, ib and ic : phase current in amps
ea, eb and ec : phase voltage back-emf in volts
Ra, Rb and Rc : phase resistance in ohms
La, Lb and Lc : phase inductance in henry
Equation 2.4 is the mechanical equation that relates the machine's
angular velocity to the developed electromagnetic torque, load torque, and
motor parameters.
em m LdT B J Tdt��� � � (2.4)
em t aT k i� (2.5)
a ee k �� (2.6)
where,
Tem : developed electromagnetic torque in Nm
� : rotor angular velocity in rad/sec
B : viscous friction constant in N-m/rad/sec
Jm : rotor moment of inertia in Kg-m2
TL : load torque in Nm
ke : back emf constant
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The voltage equation can be written in Laplace domain as shown in
Equation (2.7)
( ) ( ) ( ) ( )an a a a a aV s R I s L sI s E s� � �
( ) ( )[ ] ( )an a a a aV s I s R sL E s� � � (2.7)
The Laplace transform of Equation (2.6) is
( ) ( )a eE s k s�� (2.8)
The Equation (2.8) is substituted in Equation (2.7), which gives
Equation (2.9)
( ) ( )[ ] ( )an a a a eV s I s R sL k s�� � � (2.9)
From Equation (2.9), phase current can be written as
( ) ( )( ) an ea
a a
V s k sI sR sL
���
� (2.10)
The electromagnetic torque in the Laplace domain are
( ) ( ) ( ) ( )em m LT s B s J s s T s� �� � � (2.11)
( ) ( )em t aT s k I s� (2.12)
Using Equation (2.11), the angular velocity of motor is
( ) ( )(s) em L
m
T s T sB sJ
� ��
� (2.13)
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Using Equation (2.10) and (2.12), it is possible to express the
torque equation as,
( ) ( )( ) an eem t
a a
V s k sT s kR sL
���
� (2.14)
From the above equations it is possible to derive the transfer function
2
( )( )
t
m a
an m a a a t e
m a m a
kJ Ls
V s J R BL BR k ks sJ L J L
��
� �� �� � � �
� �
(2.15)
Equation (2.15) gives the transfer function of BLDC motor, from
that desired performance of the system can be easily achieved.
2.4 TORQUE - SPEED CHARACTERISTICS
There are two torque parameters used to define a BLDC motor,
peak torque and rated torque. During continuous operations, the motor can be
loaded up to rated torque. This requirement comes for brief period, especially
when the motor starts from stand still and during acceleration. During this
period, extra torque is required to overcome the inertia of load and the rotor
itself.
The motor can deliver a higher torque up to maximum peak torque,
as long as it follows the speed torque curve. Figure 2.12 shows the torque-
speed characteristics of a BLDC motor. As the speed increases to a maximum
value of torque of the motor, continuous torque zone is maintained up to the
rated speed after exceeding the rated speed the torque of the motor decreases.
The stall torque represents the point on the graph at which the torque is
maximum, but the shaft is not rotating. The no load speed, ωn, is the
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maximum output speed of the motor (when no torque is applied to the output
shaft). If the phase resistance is small, as it should be in an efficient design,
then the characteristic is similar to that of a shunt DC motor.
Figure 2.12 Torque vs Speed Characteristics of BLDC Motor
The speed is essentially controlled by the voltage, and may be
varied by varying the supply voltage. The motor then draws just enough
current to drive the torque at this speed. As the load torque is increased, the
speed drops, and the drop is directly proportional to the phase resistance and
the torque. The voltage is usually controlled by chopping or PWM. This gives
rise to a family of torque/speed characteristics in the boundaries of continuous
and intermittent operation. The continuous limit is usually determined by heat
transfer and temperature rise. The intermittent limit may be determined by the
maximum ratings of semiconductor devices in the controller, or by
temperature rise. In practice the torque/speed characteristic deviates from the
ideal form because of the effects of inductance and other parasitic influences.
The linear model of a DC motor torque/speed curve is a very good
approximation.
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2.5 BENEFITS OF BRUSHLESS TECHNOLOGY
� Broad operating range: Eliminating the brushes is a definite
plus: It not only extend the motor's service life and reduces
maintenance, but also eliminates the speed restrictions
inherent to "brushed" DC motors. BLDC motors can attain
speeds of more than 60,000 rpm. More importantly, the power
circuit components that are required to convert from
alternating to direct current provide the basis for variable-
speed drive, making BLDC motors well-suited for
applications that require speed control over a wide operating
range.
� Higher efficiency: Using permanent magnets in the rotor
helps to keep the rotor small and inertias low. Without current
flow (and the associated losses) in the rotor, the motor
generates less heat. Whatever heat produced dissipates more
efficiently from the brushless motor's wound stator to the
outer metallic housing through the "brushed" motor's shaft or
rotor-stator air gap.
� Flexible design: The DC power supply permits a motor
design with any number of phases in the stator. Although
three-phase configurations are most common, two and four
phased configurations also are used, energization of coils are
flexible. As an example, two windings can be energized with
the third off at any instant in a three phase BLDC
configuration. Energizing the coils in pairs simplifies the
control design, which lowers first cost, and provides motor
torque about 10 percent more than energizing the windings
sinusoidally.
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Table 2.2 and Table 2.3 show the comparison of BLDC Motor with
Brushed DC Motor and BLDC Motor with Induction Motor. The necessity of
the comparison will extract the performance of the PMBLDC Motor.
Table 2.2 Comparison of BLDC Motor with Brushed DC Motor
Feature BLDC Motor Brushed DC Motor
Commutation Electronic commutation based on Hall position sensors Brushed commutation
Maintenance Less required due to the absence of brushes
Periodic maintenance is required
Life Longer Shorter
Speed/Torque Characteristics
Flat – Enables operation at all the speed with rated load
Moderately flat – At higher speed, brush friction increases, thus reducing useful torque
Efficiency High Moderate
Output Power/ Frame Size
High – Reduced size due to superior thermal characteristics. Because BLDC has the windings on the stator, which is connected to the case, the heat dissipation is better
Moderate/Low – The heat produced by the armature is dissipated in the air gap, thus increasing the temperature in the air gap and limiting specs on the output power/frame size
Rotor Inertia Low, because it has permanent magnets on the rotor. This improves the dynamic response
Higher rotor inertia which limits the dynamic characteristics
Speed Range Higher – No mechanical limitation imposed by brushes/commutator
Lower – Mechanical limitations by the brushes
Electric Noise Generation
Low Arcs in the brushes will generate noise causing EMI
Cost of Building
Higher – Since it has permanent magnets, building costs are higher
Low
Control Complex and expensive Simple and inexpensive
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The comparison of the proposed method with Induction motor
shows the advantage of the proposed model with the conventional field and
the Table 2.3 which represents the need of the BLDC motor replacement.
Table 2.3 Comparison of BLDC Motor with Induction Motor
Features BLDC Motors Induction Motors
Speed/Torque
Characteristics Flat – Enables operation at all speeds with rated load
Nonlinear – Lower torque at lower speed
Output Power/
Frame Size
High – Since it has permanent magnets on the rotor, smaller size can be achieved for a given output power
Moderate – Since both stator and rotor have windings, the output power to size is lower than BLDC
Rotor Inertia Low – Better dynamic characteristics
High – Poor dynamic characteristics
Starting Current
Rated – No special starter circuit required
Approximately up to seven times of rated – Starter circuit rating should be carefully selected
Control Requirements
A controller is always required to keep the motor running. The same controller can be used for variable speed control
No controller is required for fixed speed; a controller is required only if variable speed is desired
Slip No slip is experienced between stator and rotor frequencies
The rotor runs at a lower frequency than stator
2.6 TYPICAL BLDC MOTOR APPLICATIONS
BLDC motors find applications in every segment of the market.
Such as, appliances, industrial control, automation, aviation and so on
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(Padmaraja Yedamale 2003). One can categorize the BLDC motor control
into three major types such as
• Constant loads
• Varying loads
• Positioning applications
2.6.1 Applications with Constant Loads
These are the types of applications where variable speed is more
important than keeping the accuracy of the speed at a set speed. In these types
of applications, the load is directly coupled to the motor shaft. For example,
fans, pumps and blowers come under these types of applications. These
applications demand low-cost controllers, mostly Operating in open-loop.
2.6.2 Applications with Varying Loads
These are the types of applications where the load on the motor
varies over a speed range. These applications may demand high-speed control
accuracy and good dynamic responses. In home appliances, washers, dryers
and compressors are good examples. In automotive, fuel pump control,
electronic steering control, engine control and electric vehicle control are
good examples of these. In aerospace, there are number of applications, like
centrifuges, pumps, robotic arm controls, gyroscope controls and so on. These
applications may use speed feedback devices and may run in semi-closed loop
or in total closed loop. These applications use advanced control algorithms,
thus complicating the controller. Also, this increases the price of the complete
system.
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2.6.3 Positioning Applications
Most of the industrial and automation types of application come
under this category. The applications in this category have some kind of
power transmission, which was mechanical gears or timer belts, or a simple
belt driven system. In these applications, the dynamic response of speed and
torque are important. Also, these applications may have frequent reversal of
rotation direction. A typical cycle will have an accelerating phase, a constant
speed phase and a deceleration and positioning phase. The load on the motor
may vary during all of these phases, causing the controller to be complex.
These systems mostly operated in closed loop. There was three control loops
functioning simultaneously: Torque Control Loop, Speed Control Loop and
Position Control Loop. Optical encoder or synchronous resolver are used for
measuring the actual speed of the motor. In some cases, the same sensors are
used to get relative position information. Otherwise, separate position sensors
may be used to get absolute positions.
2.7 SUMMARY
The necessity of the BLDCM in application is based on the
efficiency, reliability requirements for variable speed drives. Comparing to
conventional dc motor, the BLDC Motor is most efficient and less
maintenance due to the elimination of commutator and brushes. To detect the
rotor position, it is essential to provide three Hall sensors which makes
complexity. The BLDCM play a vital role in many applications due to high
torque to weight ratio and it has linear torque speed characteristics. Finally, a
dynamic model is performed to validate the desired performance of the
BLDCM system.
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