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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.