A

Project Report
On
BRUSHLESS DC MOTOR

Submitted to
M. S. B. T. E. (MUMBAI)

Submitted by

Mr. Date Yogesh Namdeo 1814640100

Mr. Buchude Vaibhav Dadasaheb 1814640104
Mr. Gade Yogesh Namdeo 1814640097
UNDER THE GUIDANCE OF: Prof Khese S. B.
In partial fulfillment of requirement for the award of
Diploma in Electrical Engineering

Academic Year: 2020-2021

DEPARTMENT OF ELECTRICAL ENGINEERING

S.S.M. ADSUL POLYTECHNIC COLLEGE,
CHAS, AHMEDNAGAR-414 005 (M.S)
 MAHARASHTRA STATE BOARD OF TECHNICAL
EDUCATION

Certificate of Completion
(By respective head of the department and head of the institute)

This is to certify that, the project Production of brushless dc motor

Mr. Date Yogesh Namdeo

Mr. Buchude Vaibhav Dadasaheb

Mr.Gade Yogesh Janardhan

Is a bonafide work completed under my supervision and guidance in partial fulfillment

towards completion of Diploma in Electrical Engineering from Sau. Sundarbai
Manik Adsul Polytechnic, Chas, Ahmednagar with institute code 1464.

Signature Signature
Head of the Department Head of the Institutes

ACKNOWLEDGEMENT
I take this opportunity to acknowledge the constant encouragement and continuous help given to

Me my all staff members I would also like to convey my thank HOD of Electrical Department

Prof. Khese S.B. for his timely support.

I would also like to extend my sincere thanks to principal of our college Prof. Gadakh R.S. for

His continuous encouragement and motivation.

I would also like to thank all the teaching and non-teaching staff of Electrical Department for

Helping us to achieve this goal.

I am also thankful to those who directly or indirectly helped me for completing this industrial

Training.
 ABSTRACT

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 com-

mutated. 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 con-

struction, working principle, characteristics and typical

applications of BLDC motors. Refer to Appendix B:

“Glossary” for a glossary of terms commonly used

when describing BLDC motors.

Brushless DC motors are the latest choice of researchers due to their high efficiency,

silent operation, compact size, high reliability and low maintenance requirements. These motors are
preferred for

numerous applications; however, most of them require sensorless control of these motors. The operation of
BLDC
motors requires rotor-position sensing for controlling the winding currents. The sensorless control would
need estimation

of rotor position from the voltage and current signals, which are easily sensed. This paper presents state of
the art

BLDC motor drives with an emphasis on sensorless control of these motors.

Keywords: Permanent magnet machines, brushless machines, BLDCM and sensorless control, low cost
controllers
BRUSH LESS DC MOTOR FUNDAMENTAL

INTRODUCTION

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. Refer to Appendix B: “Glossary” for a glossary of terms
commonly used when describing BLDC motors.

CONSTRUCTION AND OPERATING PRINCIPLE

BLDC motors are a type of synchronous motor. This means the magnetic field generated by the
stator and the magnetic field 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, 3-phase motors are the most popular
and widely used. This application note focuses on 3-phase motors.

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 (as shown in Figure 3). 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 are
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 are distributed
over the stator periphery to form an even numbers of poles.
There are two types of stator windings variants: trapezoidal and sinusoidal motors. This
differentiation is made on the basis of the interconnection of coils in the stator windings to give the
different types of back Electromotive Force (EMF). Refer to the “What is Back EMF?” section for
more information.
As their names indicate, the trapezoidal motor gives a back EMF in trapezoidal fashion and the
sinusoidal motor’s back EMF is sinusoidal, as shown in Figure 1 and Figure 2. 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.

0 60 120 180 240 300 360 60

Phase A-B

Phase B-C

Phase C-A
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.
FIGURE 1 :TRAPEZOIDAL BACK EMF

FIGURE 2: SINUSOIDAL BACK EMF

FIGURE 3: STATOR OF A BLDC MOTOR

Stamping with Slots

Stator Windings
Rotor:-
The rotor is made of 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 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. Also, these alloy magnets improve the 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. Continuous research is going on to
improve the flux density to compress the rotor further.
Figure 4 shows cross sections of different arrangements of magnets in a rotor.
FIGURE 4: ROTOR MAGNET CROSS SECTIONS

Hall Sensors:-
Unlike a brushed DC motor, the commutation of a BLDC motor is controlled electronically. To
rotate the BLDC motor, the stator windings should be energized in a sequence. It is important
to know the rotor position in order to understand which winding will be energized following the
energizing sequence. Rotor position is sensed using Hall effect sensors embedded into the
stator.
Most BLDC motors have three Hall sensors embedded into the stator on the non-driving end of
the motor.
Whenever the rotor magnetic poles pass near the Hall sensors, they give a high or low signal,
indicating the N or S pole is passing near the sensors. Based on the combination of these three
Hall sensor signals, the exact sequence of commutation can be determined.
FIGURE 5: BLDC MOTOR TRANSVERSE SECTION

Stator Windings

Hall Sensors Rotor Magnet S

Accessory Shaft Rotor Magnet N

Driving End of the Shaft

Hall Sensor Magnets

Figure 5 shows a transverse section of a BLDC motor with a rotor that has alternate N and S
permanent magnets. Hall sensors are embedded into the stationary part of the motor.
Embedding the Hall sensors into the stator is a complex process because ant misalignment
in these Hall sensors, with respect to the rotor magnets, will generate an error in
determination of the rotor position. To simplify the process of mounting the Hall sensors
onto the stator, some motors may have the Hall sensor magnets on the rotor, in addition to
the main rotor magnets. These are a scaled down replica version of the rotor. Therefore,
whenever the rotor rotates, the Hall sensor magnets give the same effect as the main
magnets. The Hall sensors are normally mounted on a PC board and fixed to the enclosure
cap on the non-driving end. This enables users to adjust the complete assembly of Hall
sensors, to align with the rotor magnets, in order to achieve the best performance.
Based on the physical position of the Hall sensors, there are two versions of output. The
Hall sensors may be at 60° or 120° phase shift to each other. Based on this, the motor
manufacturer defines the commutation sequence, which should be followed when controlling
the motor.
See the “Commutation Sequence” section for an example of Hall sensor signals and further
details on the sequence of commutation.

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 field generated by the stator coils and the permanent magnets. Ideally, the peak
torque occurs when these two fields are at 90° to each other and falls off as the fields move
together. In order to keep the motor running, the magnetic field produced by the windings
should shift position, as the rotor moves to catch up with the stator field. What is known as
“Six-Step Commutation” defines the sequence of energizing the windings. See the
“Commutation Sequence” section for detailed information and an example on six-step
commutation.

TORQUE/SPEED CHARACTERISTICS
Figure 6 shows an example of torque/speed characteristics. There are two torque parameters
used to define a BLDC motor, peak torque (TP) and rated torque (TR). (Refer to Appendix A:
“Typical Motor Technical Specification” for a complete list of parameters.) During continuous
operations, the motor can be loaded up to the rated torque. As discussed earlier, in a BLDC
motor, the torque remains constant for a speed range up to the rated speed. The motor can be
run up to the maximum speed, which can be up to 150% of the rated speed, but the torque
starts dropping.

FIGURE 6: TORQUE/SPEED CHARACTERISTICS

Peak Torque
TP

Torque
Intermittent
Torque Zone
Rated Torque
TR
Continuous
Torque Zone

Rated Speed Maximum

Speed
Speed

Applications that have frequent starts and stops and frequent reversals of rotation with load
on the motor, demand more torque than the rated torque. This requirement comes for a
brief period, especially when the motor starts from a standstill and during acceleration.
During this period, extra torque is required to overcome the inertia of the load and the
rotor itself. The motor can deliver a higher torque, maximum up to peak torque, as long as it
follows the speed torque curve. Refer to the “Selecting a Suitable Motor Rating for the
Application” section to understand how to select these parameters for an application.
COMPARING BLDC MOTORS TO OTHER MOTOR TYPES
Compared to brushed DC motors and induction motors, BLDC motors have many advantages
and few disadvantages. Brushless motors require less maintenance, so they have a longer life
compared with brushed DC motors. BLDC motors produce more output power per frame size
than brushed DC motors and induction motors. Because the rotor is made of permanent
magnets, the rotor inertia is less, compared with other types of motors. This improves
acceleration and deceleration characteristics, shortening operating cycles. Their linear
speed/torque characteristics produce predictable speed regulation. With brushless motors,
brush inspection is eliminated, making them ideal for limited access areas and applications
where servicing is difficult. BLDC motors operate much more quietly than brushed DC motors,
reducing Electromagnetic Interference (EMI). Low-voltage models are ideal for battery
operation, portable equipment or medical applications.

TABLE 1: COMPARING A BLDC MOTOR TO A BRUSHED DC MOTOR

Feature BLDC Motor Brushed DC Motor
Commutation Electronic commutation based on Hall Brushed commutation.
position sensors.
Maintenance Less required due to absence of Periodic maintenance is required.
brushes.
Life Longer. Shorter.
Speed/Torque Flat – Enables operation at all speeds Moderately flat – At higher speeds,
Characteristics with rated load. brush friction increases, thus reducing
useful torque.
Efficiency High – No voltage drop across brushes. Moderate.
Output Power/ High – Reduced size due to superior Moderate/Low – The heat produced
Frame Size thermal characteristics. Because BLDC by the armature is dissipated in the
has the windings on the stator, which air gap, thus increasing the
is connected to the case, the heat temperature in the air gap and
dissipation is better. limiting specs on the output
power/frame size.
Rotor Inertia Low, because it has permanent Higher rotor inertia which limits the
magnets on the rotor. This improves dynamic characteristics.
the dynamic response.
Speed Range Higher – No mechanical limitation Lower – Mechanical limitations by the
imposed by brushes/commutator. brushes.
Electric Noise Low. Arcs in the brushes will generate
Generation noise causing EMI in the equipment
nearby.
 Cost of Building Higher – Since it has permanent Low.
magnets, building costs are higher.
Control Complex and expensive. Simple and inexpensive.
Control A controller is always required to No controller is required for fixed
Requirements keep the motor running. The same speed; a controller is required only if
controller can be used for variable variable speed is desired.
speed control.
TABLE 2: COMPARING A BLDC MOTOR TO AN INDUCTION MOTOR
Features BLDC Motors AC Induction Motors
Speed/Torque Flat – Enables operation at all Nonlinear – Lower torque at lower
Characteristics speeds with rated load. speeds.
Output Power/ High – Since it has permanent Moderate – Since both stator and
Frame Size magnets on the rotor, smaller size can rotor have windings, the output
be achieved for a given output power. power to size is lower than BLDC.
Rotor Inertia Low – Better dynamic characteristics. High – Poor dynamic characteristics.
Starting Current Rated – No special starter circuit Approximately up to seven times of
required. rated – Starter circuit rating should be
carefully selected. Normally uses a
Star-Delta starter.
Control A controller is always required to No controller is required for fixed
Requirements keep the motor running. The same speed; a controller is required only if
controller can be used for variable variable speed is desired.
speed control.
Slip No slip is experienced between stator The rotor runs at a lower frequency
and rotor frequencies. than stator by slip frequency and slip
increases with load on the motor.
Table 1 summarizes the comparison between a BLDC motor and a brushed DC motor. Table 2
compares a BLDC motor to an induction motor.
COMMUTATION SEQUENCE

OuFigure 7 shows an example of Hall sensor signals with respect to back EMF and the phase
current. Figure 8 shows the switching sequence that should be followed with respect to the Hall
sensors. The sequence numbers on Figure 7 correspond to the numbers given in Figure 8.
Every 60 electrical degrees of rotation, one of the Hall sensors changes the state. Given this, it
takes six steps to complete an electrical cycle. In synchronous, with every 60 electrical degrees,
the phase current switching should be updated. However, one electrical cycle may not
correspond to a complete mechanical revolution of the rotor. The number of electrical cycles to
be repeated to complete a mechanical rotation is determined by the rotor pole pairs. For each
rotor pole pairs, one electrical cycle is completed. So, the number of electrical cycles/rotations
equals the rotor pole pairs.
Figure 9 shows a block diagram of the controller used to control a BLDC motor. Q0 to Q5 are
the power switches controlled by the PIC18FXX31 microcontroller. Based on the motor voltage
and current ratings, these switches can be MOSFETs, or IGBTs, or simple bipolar transistors.
Table 3 and Table 4 show the sequence in which these power switches should be switched
based on the Hall sensor inputs, A, B and C. Table 3 is for clockwise rotation of the motor and
Table 4 is for counter clockwise motor rotation. This is an example of Hall sensor signals having
a 60 degree phase shift with respect to each other. As we have previously discussed in the “Hall
Sensors” section, the Hall sensors may be at 60° or 120° phase shift to each other. When
deriving a controller for a particular motor, the sequence defined by the motor manufacturer
should be followed.
Referring to Figure 9, if the signals marked by PWMx are switched ON or OFF according to the
sequence, the motor will run at the rated speed. This is assuming that the DC bus voltage is
equal to the motor rated voltage, plus any losses across the switches. To vary the speed, these
signals should be Pulse Width Modulated (PWM) at a much higher frequency than the motor
frequency. As a rule of thumb, the PWM frequency should be at least 10 times that of the
maximum frequency of the motor. When the duty cycle of PWM is varied within the sequences,
the average voltage supplied to the stator reduces, thus reducing the speed. Another advantage
of having PWM is that, if the DC bus voltage is much higher than the motor rated voltage, the
motor can be controlled by limiting the percentage of PWM duty cycle corresponding to that of
the motor rated voltage. This adds flexibility to the controller to hook up motors with different
rated voltages and match the average voltage output by the controller, to the motor rated
voltage, by controlling the PWM duty cycle.
There are different approaches of controls. If the PWM signals are limited in the
microcontroller, the upper switches can be turned on for the entire time during the
corresponding sequence and the corresponding lower switch can be controlled by the
required duty cycle on
PWM.
The potentiometer, connected to the analog-to-digital converter channel in Figure 9, is for
setting a speed reference. Based on this input voltage, the PWM duty cycle should be
calculated.

Closed-Loop Control
The speed can be controlled in a closed loop by measuring the actual speed of the motor.
The error in the set speed and actual speed is calculated. A Proportional plus Integral plus
Derivative (P.I.D.) controller can be used to amplify the speed error and dynamically adjust
the PWM duty cycle.
For low-cost, low-resolution speed requirements, the Hall signals can be used to measure
the speed feedback. A timer from the PIC18FXX31 can be used to count between two Hall
transitions. With this count, the actual speed of the motor can be calculated.
For high-resolution speed measurements, an optical encoder can be fitted onto the motor,
which gives two signals with 90 degrees phase difference. Using these signals, both speed and
direction of rotation can be determined. Also, most of the encoders give a third index signal,
which is one pulse per revolution. This can be used for positioning applications. Optical
encoders are available with different choices of Pulse Per Revolution (PPR), ranging from
hundreds to thousands

FIGURE 7: HALL SENSOR SIGNAL, BACK EMF, OUTPUT TORQUE AND PHASE
FIGURE 8: WINDING ENERGIZING SEQUENCE WITH RESPECT TO THE HALL SENSOR

FIGURE 9: CONTROL BLOCK DIAGRAM

 TABLE 3: SEQUENCE FOR ROTATING THE MOTOR IN CLOCKWISE DIRECTION WHEN
VIEWED FROM NON-DRIVING END
H all put Phase nt
Sequence Sensor Curre
Active PWMs
# In
A B C A B C
1 0 0 1 PWM1(Q1) PWM4(Q4) DC+ Off DC-
2 0 0 0 PWM1(Q1) PWM2(Q2) DC+ DC- Off
3 1 0 0 PWM5(Q5) PWM2(Q2) Off DC- DC+
4 1 1 0 PWM5(Q5) PWM0(Q0) DC- Off DC+
5 1 1 1 PWM3(Q3) PWM0(Q0) DC- DC+ Off
6 0 1 1 PWM3(Q3) PWM4(Q4) Off DC+ DC-
TABLE 4: SEQUENCE FOR ROTATING THE MOTOR IN COUNTER-CLOCKWISE DIRECTION
WHEN VIEWED FROM NON-DRIVING END
WHAT IS BACK EMF?
When a BLDC motor rotates, each winding generates a voltage known as back Electromotive
Force or back EMF, which opposes the main voltage supplied to the windings according to
Lenz’s Law. The polarity of this back EMF is in opposite direction of the energized voltage. Back
EMF depends mainly on three factors:
• Angular velocity of the rotor
• Magnetic field generated by rotor magnets
• The number of turns in the stator windings

EQUATION 1:
Back EMF = (E) ∝ NlrBω

where:
N is the number of winding turns per
phase,
l is the length of the rotor,
r is the internal radius of the
rotor, B is the rotor magnetic
field density and ω is the
motor’s angular velocity
Once the motor is designed, the rotor magnetic field and the number of turns in the stator
windings remain constant. The only factor that governs back EMF is the angular velocity or
speed of the rotor and as the speed increases, back EMF also increases. The motor technical
specification gives a parameter called, back EMF constant (refer to Appendix A: “Typical
Motor Technical Specification”), that can be used to estimate back EMF for a given speed.
The potential difference across a winding can be calculated by subtracting the back EMF
value from the supply voltage. The motors are designed with a back EMF constant in such a
way that when the motor is running at the rated speed, the potential difference between
the back EMF and the supply voltage will be sufficient for the motor to draw the rated
current and deliver the rated torque. If the motor is driven beyond the rated speed, back
EMF may increase substantially, thus decreasing the potential difference across the winding,
reducing the current drawn which results in a drooping torque curve. The last point on the
speed curve would be when the supply voltage is equal to the sum of the back EMF and the
losses in the motor, where the current and torque are equal to zero.
 Sensorless Control of BLDC Motors
Until now we have seen commutation based on the rotor position given by the Hall sensor.
BLDC motors can be commutated by monitoring the back EMF signals instead of the Hall
sensors. The relationship between the Hall sensors and back EMF, with respect to the phase
voltage, is shown in Figure 7. As we have seen in earlier sections, every commutation sequence
has one of the windings energized positive, the second negative and the third left open. As
shown in Figure 7, the Hall sensor signal changes the state when the voltage polarity of back
EMF crosses from a positive to negative or from negative to positive. In ideal cases, this
happens on zero-crossing of back EMF, but practically, there will be a delay due to the winding
characteristics. This delay should be compensated by the microcontroller. Figure 10 shows a
block diagram for sensorless control of a BLDC motor.
Another aspect to be considered is very low speeds. Because back EMF is proportional to the
speed of rotation, at a very low speed, the back EMF would be at a very low amplitude to
detect zero-crossing. The motor has to be started in open loop, from standstill and when
sufficient back EMF is built to detect the zero-cross point, the control should be shifted to the
back EMF sensing. The minimum speed at which back EMF can be sensed is calculated from the
back EMF constant of the motor.

With this method of commutation, the Hall sensors can be eliminated and in some motors, the
magnets for Hall sensors also can be eliminated. This simplifies the motor construction and
reduces the cost as well. This is advantageous if the motor is operating in dusty or oily
environments, where occasional cleaning is required in order for the Hall sensors to sense
properly. The same thing applies if the motor is mounted in a less accessible location.
SELECTING A SUITABLE MOTOR RATING FOR THE APPLICATION
Selecting the right type of motor for the given application is very important. Based on the load
characteristics, the motor must be selected with the proper rating. Three parameters govern
the motor selection for the given application. They are:
• Peak torque required for the application
• RMS torque required
• The operating speed range

Peak Torque (TP) Requirement

The peak, or maximum torque required for the application, can be calculated by summing the
load torque (TL), torque due to inertia (TJ) and the torque required to overcome the friction
(TF).
There are other factors which will contribute to the overall peak torque requirements. For
example, the windage loss which is contributed by the resistance offered by the air in the air
gap. These factors are complicated to account for. Therefore, a 20% safety margin is given as a
rule of thumb when calculating the torque.

EQUATION 2:

∴ TP = (TL + TJ + TF) * 1.2

The torque due to inertia (TJ) is the torque required to accelerate the load from standstill or
from a lower speed to a higher speed. This can be calculated by taking the product of load
inertia, including the rotor inertia and load acceleration.

EQUATION 3:
TJ = JL + M * α where:
JL + M is the sum of the load and rotor inertia and α is the required acceleration

The mechanical system coupled to the motor shaft determines the load torque and the
frictional torque.
RMS Torque Requirement (TRMS)
The Root Mean Square (RMS) torque can be roughly translated to the average continuous
torque required for the application. This depends upon many factors. The peak torque (TP),
load torque (TL), torque due to inertia (TJ), frictional torque (TF) and acceleration,
deceleration and run times.
The following equation gives the RMS torque required for a typical application where TA is
the acceleration time, TR is the run time and TD is the deceleration time.
EQUATION 4:

TRMS = √ [{TP2 TA + (TL + TF)2 TR +

(TJ – TL – TF)2 TD}/(TA + TR + TD)]

Speed Range
This is the motor speed required to drive the application and is determined by the type of
application. For example, an application like a blower where the speed variation is not very
frequent and the maximum speed of the blower can be the average motor speed required.
Whereas in the case of a point-to-point positioning system, like in a high-precision conveyer
belt movement or robotic arm movements, this would require a motor with a rated
operating speed higher than the average movement speed. The higher operating speed can
be accounted for the components of the trapezoidal speed curve, resulting in an average
speed equal to the movement speed. The trapezoidal curve is shown in Figure 11.
It is always suggested to allow a safety margin of 10%, as a rule of thumb, to account for
miscellaneous factors which are beyond our calculations.

Maximum
Speed

Average Motor Speed

Speed

TA TR TD

Time

FIGURE 11: TRAPEZOIDAL SPEED CURVE

TYPICAL BLDC MOTOR APPLICATIONS
BLDC motors find applications in every segment of the market. Automotive, appliance, industrial
controls, automation, aviation and so on, have applications for BLDC motors. Out of these, we can
categorize the type of BLDC motor control into three major types:
• Constant load
• Varying loads
• Positioning applications

Applications With Constant Loads

These are the types of applications where a variable speed is more important than keeping the
accuracy of the speed at a set speed. In addition, the acceleration and deceleration rates are not
dynamically changing. 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.

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 a 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 a 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.
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 could be 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, as shown in Figure 11. The load on the motor may vary during all of these phases,
causing the controller to be complex. These systems mostly operate in closed loop. There could be
three control loops functioning simultaneously: Torque Control Loop, Speed Control Loop and
Position Control Loop. Optical encoder or synchronous resolvers 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. Computer Numeric
Controlled (CNC) machines are a good example of this. Process controls, machinery controls and
conveyer controls have plenty of applications in this category.

State of the Art on Permanent Magnet Brushless DC Motor Drives

1. Introduction

The use of permanent magnets (PMs) in electrical machines in place of electromagnetic excitation results in
many advantages such as no excitation losses, simplified construction, improved efficiency, fast dynamic
performance, and high torque or power per unit volume
[1-137]
. The PM excitation in electrical machines was used for the first time in the early 19 th century, but was not
adopted due to the poor quality of PM materials. In 1932, the invention of Alnico revived the use of PM
excitation systems, however it has been limited to small and fractional horse power dc commutator machines
[8, 24]
.
In the 20th century, squirrel cage induction motors have been the most popular electric motors, due to its
rugged construction. Advancements in power electronics and
digital signal processors have added more features to these motor drives to make them more prevalent in
industrial installations. However squirrel cage induction motors suffer from poor power factor and efficiency
as compared to synchronous motors. On the other hand, synchronous motors and dc commutator motors
have limitations such as speed, noise problems, wear and EMI due to the use of commutator and brushes.
These problems have led to the development of permanent magnet brushless or commutatorless synchronous
motors which have PM excitation on the rotor [1-30].
Therefore, permanent magnet brushless (PMBL) motors can be considered a kind of three phase synchronous
motor, having permanent magnets on the rotor, replacing the mechanical commutator and brush gear.
Commutation is accomplished by electronic switches, which supply current to the motor windings in
synchronization with the rotor position.

Amongst the available PM materials, Alnico magnets can have flux densities equivalent to soft magnetic irons
but they get easily demagnetized due to lower values of coercive force as compared to ceramic magnets [21].
Ceramic magnets are economical but their maximum energy density product is low due to lower values of
retentivity. Rare earth and samarium cobalt alloys have relatively good magnetic properties, but they are
expensive. Other than polymer bonded rare earth magnets, for example, ferrite and cobalt based metallic
magnets are physically hard and brittle. Therefore, selection of the particular PM material is application
specific; however, Neodymium-Iron-Boron (Nd-Fe-B) rare earth magnets are more in demand because they
provide the highest energy density and higher residual flux density than others [21].

The popularity of PMBL motors are increasing day by day due to the availability of high energy density and
cost effective rare earth PM materials like Samarium Cobalt (Sm-Co) and Nd-Fe-B which enhance the
performance of PMBLDCM drives and reduce the size and losses in these motors. The advancements in
geometries and design innovations have made possible the use of PMBL motors in many of domestic,
commercial and industrial applications. PMBL machines are best suited for position control and medium sized
industrial drives due to their excellent dynamic capability, reduced losses and high torque/weight ratio.

PMBL motors find applications in diverse fields such as domestic appliances, automobiles, transportation,
aerospace equipment, power tools, toys, vision and sound equipment and healthcare equipment ranging from
microwatt to megawatts. Advanced control algorithms and ultra fast processors have made PMBLDC motors
suitable for position control in machine tools, robotics and high precision servos, speed control and torque
control in various industrial drives and process control applications. With the advancement in power
electronics it is possible to design PMBL generators for power generation onboard ships, aircraft, hybrid
electric cars and buses while providing reduced generator weight, size and a high payload capacity for the
complete vehicle.

In view of these requirements of PMBLDCM drives, an attempt is made in this paper to introduce various
aspects of PMBLDCM drives. This paper is organized in nine sections as follows. A state of the art PMBLDC
motor application is reviewed in Section 2. The classification of permanent magnet brushless (PMBL) motors is
presented in Section 3. The construction of PMBLDC motors and controllers for PMBLDC motors are discussed
in Sections 4 and 5, respectively. The position sensorless control methods of PMBLDC motors are discussed in
Section 6. The applications, power quality aspects and future trends of PMBLDCM drives are highlighted in
Sections 7, 8 and 9, respectively. The concluding remarks and recommendations are given in Section 10.

2. State of the Art

PMBLDC motors are generally powered by a conventional three-phase voltage source inverter (VSI) or current
source inverter (CSI) which is controlled using rotor position. The rotor position can be sensed using Hall
sensors, resolvers, or optical encoders [1-137]. These position sensors increase cost, size and complexity of
control thereby reducing the reliability and acceptability of these drives. Due to the high cost of the motor and
controller, very few commercial applications of PMBLDC motors have been reported in the literature [113-137].
Recently some additional applications of PMBLDC motors have been reported in electric vehicles (EVs) and
hybrid electric vehicles (HEVs) due to environmental concerns of vehicular emissions. PMBLDC motors have
been found more suitable for EVs/HEVs and other low power applications, due to high power density, reduced
volume, high torque, high efficiency, easy to control, simple hardware and software and low maintenance [36-
137]
.
The cost of a PMBLDCM drive has two main components; one is the motor and other is the controller.
Extensive research attempts [13-29] have been made to reduce the cost and to increase the efficiency of these
motors. However, the cost of controllers and the power quality aspects of the drives are still under
consideration. Due to ease of control in PMBLDC motors, they are preferred for numerous applications in low
power and variable speed drives.

3. Classification of PMBL Motors

Permanent magnet brushless motors can be divided into two subcategories. The first category uses
continuous rotor-position feedback for supplying sinusoidal voltages and currents to the motor. The ideal
motional EMF is sinusoidal, so that the interaction with sinusoidal currents produces constant torque with
very low torque ripple. This called a Permanent Magnet Synchronous Motor (PMSM) drives, and is also called
a PM AC drive, brushless AC drive, PM sinusoidal fed drive, sinusoidal brushless DC drive, etc.

The second category of PMBL motor drives is known as the brushless DC (BLDC) motor drive and it is also
called a trapezoidal brushless DC drive, or rectangular fed drive. It is supplied by three-phase rectangular
current blocks of 120° duration, in which the ideal motional EMF is trapezoidal, with the constant part of the
waveform timed to coincide with the intervals of constant phase current. These machines need rotor-position
information only at the commutation points, e.g., every 60°electrical in three-phase motors [1-29].
The PMBLDC motor has its losses mainly in the stator due to its construction; hence the heat can easily be
dissipated into the atmosphere. As the back EMF is directly proportional to the motor speed and the
developed torque is almost directly proportional to the phase current, the torque can be maintained constant
by a stable stator current in a PMBLDC motor. The average torque produced is high with fewer ripples in
PMBLDC motors as compared to PMSM [1-15]. Amongst two types of PMBL motors, PMSM is, therefore,
preferred for applications where accuracy is desired e.g. robotics, numerical controlled machines, solar
tracking etc. However, the PMBLDCM can be used in general and low cost applications. These motors are
preferred for numerous applications, due to their features of high efficiency, silent operation, compact in size
and low maintenance.

4. Construction of PMBLDC Motors

The stator of a PMBLDC motor usually has three phase concentrated windings; however, the rotor
construction varies according to desired requirements. Various geometries for PM rotors have been reported
in the literature [21], for improved power density and efficiency by adopting flux enhancement, armature
reaction reduction or high-speed operation. Two main configurations of PM rotors are surface mounted
magnet type where magnets are mounted on the outer surface of the rotor, and the buried magnet type
where the magnets are mounted inside the magnetic structure of the rotor.

Another type of PMBL motor is the axial field machine where the direction of the magnetic field is axial instead
of radial. The configurations of axial field PMBL motors include a single stator and single rotor, a single stator
sandwiched between two rotors (double air gaps), a single rotor sandwiched between two stators (double air
gaps) and a variety of multiple stators and rotors (multiple air gaps) [21].
Any of these PMBLM rotor configurations can be selected on the basis of application and power rating.
Various other configurations of PMBL machines include Axial Flux Permanent Magnet (AFPM) machines, PM
alternators and torous alternators with different rotor geometries like surface type, interior type, radial and
axial field machines in two, three and multi phase PMBLDC machines.

5. Controllers for PMBLDC Motors

The control of PMBLDC motors can be accomplished by various control techniques using conventional six
pulse inverters which can be classified in two broad categories as voltage source inverter (VSI) and current
source inverter (CSI) based topologies. The controllers can further be divided on the basis of solid state
switches and control strategies. The PMBLDCM needs rotor-position sensing only at the commutation points,
e.g., every 60°electrical in the three-phases; therefore, a comparatively simple controller is required for
commutation and current control.

The commutation sequence is generated by the controller according to the rotor position which is sensed
using Hall sensors, resolvers or optical encoders. These sensors increase the cost and the size of the motor and
a special mechanical arrangement is required for mounting the sensors. The system reliability also reduces
due to the additional components and wiring. Therefore, the control complexity and high cost of the drive
hold back the widespread use of PMBLDC motors.

Reduced cost controllers for PMBLDC motors are more in demand and many schemes and algorithms for
reduced cost controllers have been reported in the literature [82-102]. The cost reduction of controllers for
PMBLDCM drives can be accomplished by two approaches, namely topological approach and control
approach. In the topological approach, the number of switches, sensors and associated circuitry used to
compose the power converter is minimized, whereas, new algorithms are designed and implemented in
conjunction with the converter to produce the desired characteristics, in the control approach.
To begin with the topological approach, topologies with more than one switch per phase, but less than
conventional two switches per phase can be considered for low cost applications. However, there are some
conventional topologies (i.e. six switch topology) for low cost

applications also reported in the literature [92, 95]. As the majority of applications of these motors are at low
power levels, therefore, single phase AC mains fed PMBLDCM drives are considered in this work.

Fig. 1 Load commutated converter topology [92]

A single phase AC mains input based thyristorised load commutated converter topology as shown in Fig. 1,
has been reported [92] based on a current source inverter. Four-quadrant operation, current sensorless
control and wide operating speed range are good features of the proposed topology. However, the
requirement of a big inductor for high capacity applications has been a major disadvantage of this topology.

Fig. 2 Buck converter-CSI based topology [95]

Fig. 3 Ćuk converter-CSI based topology [95]

Some modifications in this thyristorised drive based on the buck and Ćuk topology (schematic shown in Figs. 2
and 3) have been proposed in the literature [95] for reduction of harmonics and cost as well.

The topologies with switches less than one per phase

reported in the literature [82-83, 85, 89-91, 94, 99-102] are modified from the basic VSI topology [1-16] (conventional six
switch configuration as shown in Fig. 4). One such reported topology is a three phase four switch topology
shown in Fig. 5, which has been tested with different schemes like PWM and hysteresis current control
methods [90-91]. It has been modified for power factor correction [91, 94] resulting in a topology with a total of six
switches as shown in Fig. 6. This topology has single phase to three phase conversion with sinusoidal input
current close to unity power factor. This topology enables regenerative braking due to bidirectional power
flow between ac input and PMBLDC motor via the DC link. This topology requires a symmetric PWM scheme
for switching control, which can be generated using a digital signal processor (DSP) or a field programmable
gate array (FPGA) [102].

Fig. 4

Conventional VSI based topology [1-16]

Fig. 5 Three phase four switch topology [90, 91]

Fig. 8 Split supply converter topology [83]
A half bridge power converter topology known as a split supply converter topology (shown in Fig.8) has also
been used for PMBLDCM [83] having one switch per phase and only two diodes for rectification. This topology
can be used with bifilar winding after incorporating some modifications; however it reduces motor utilization
[2, 13]
.
Fig. 10 Variable DC link converter topology [85]

Fig. 11 SEPIC converter based topology [99]

[99, 132]
Some of researchers have proposed unipolar excitation for PMBLDC motors, which need less electronic
components and use a simple circuit as compared to conventional bipolar excitation of PMBLDC motors. This
leads to converter cost minimization and opens up scope for substantial applications where cost matters more
than the accuracy of control.

For proper operation of a PMBLDC motor, the flow of current in the stator windings must be synchronized to
the instantaneous position of the rotor and therefore, the current controller must receive information about
the position of the rotor. However, the presence of the position sensor is undesirable in many applications;
therefore, position sensorless schemes may be employed in which rotor position information is deduced from
the voltages and currents in the motor windings.

6. Position Sensorless Control Methods

The basic idea of position sensorless control methods is to eliminate the position sensors (usually three Hall
sensors). To accomplish this task, additional circuitry and computational efforts are required to estimate the
commutation instances of the PMBLDC motor from the voltage and current signals which can easily be sensed.
Therefore, sensorless techniques demand high performance processors with large memory and program
codes for computation and estimation, as compared to sensor-based drive systems.

PMBLDC motors can be modeled by the same equivalent circuit for each phase winding, where the source
voltage ‘v’ supplies current ‘i’ to the phase circuit consisting of series-connected resistance ‘R’, inductance ‘L’,
and back EMF ‘e’. The back EMF is a result of the movement of the PM rotor, thereby, dependent on rotor
position and proportional to rotor velocity. The machine voltage and current waveforms reflect the rotor-
position dependence of the inductance and back EMF. Therefore, the voltage and current waveforms can be
analyzed to extract the back EMF or inductance (or a combination of the two), from which the rotor position
can be estimated in the position sensorless schemes [17-20, 49-81, 131-136]. The position sensorless approach has
many advantages, e.g. minimum installation cost, minimum space requirement, no environmental restrictions
(e.g. high pressure and temperature environment in HVAC compressors), EMI free position information,
reduced controller cost etc.

These sensorless techniques may be broadly categorized as: back electromotive force (BEMF) sensing,
inductance or flux-linkage variation sensing [17-20, 49-81]. Closed-loop observer based methods to address position
sensing in PM machines and sensorless schemes for permanent magnet synchronous motors have also been
reported in the literature [17-20, 30, 127, 131], which can be extended to brushless dc motors in the same fashion or
with some modifications.

6.1 Back EMF Sensing

In PM brushless DC machines, the magnitude of the back EMF is a function of the instantaneous rotor position
and has trapezoidal variation with 120º flat span. However, in practice, it is difficult to measure the back EMF,
because of the rapidly changing currents in machine windings and induced voltages due to phase switching.
The back EMF is not sufficient enough at starting until the rotor attains some speed. Therefore, it is a usual
practice to make the initial acceleration under open-loop control using a ramped frequency signal so that the
back-EMF is measurable for the controller to lock in.

One of the popular starting methods is “align and go”

[17-20, 49, 131-136]
, in which the rotor is aligned to the specified position by energizing any two phases of the stator
and then the rotor is accelerated to the desired speed according to the given commutation sequences. The
“align and go” method suffers demagnetization of permanent magnets due to large instantaneous peak
currents at starting. The zero-crossing points of the back EMF in each phase may be an attractive feature to
use 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 before they can be used for commutation [49, 100].

The detection of the third harmonic component in back EMF [59, 72], direct current control algorithm [100] and
phase locked loops [66] have been proposed to overcome the phase-shifting problem. However, the direct
current control algorithm suffers filtering problem of sensed voltage signals which limits the operation range
above 200 rpm. The third-harmonic approach assumes equal inductance in all three phases, which is only valid
for surface-mounted magnet motors; however, in the case of rotors with saliency, errors in position
estimation arise due to rapidly changing phase currents. To measure the back EMF across the terminals of a
star-connected machine, it is necessary to have the machine’s star neutral terminal.

The back EMF method has been applied in special-purpose low-cost applications for fans and pumps
[74, 124]
while ignoring these problems.

6.2 Inductance Variation Sensing

The fundamental concept behind the inductance variation is the rate of current change in the motor which
depends on the inductance of the winding. The inductance variation can be sensed after injection of a current
pulse in the armature windings [17-20, 55, 60, 75]. This scheme is particularly useful at zero speed when there is no
back EMF. This method is suitable for the IPM (Interior Permanent Magnet) BLDC motor with high
performance material such as the NdFeB magnet. In order to get various inductance profiles, a large current
pulse is required. Thus, these methods are not suitable for a SPM-type BLDC motor with ferrite magnets [135].
Therefore, the application of inductance variation sensing methods may be useful to address the problem of
starting, including identification of the rotor position before full excitation of the machine. Initial rotor position
identification is particularly important in applications such as traction, where any reverse motion is not
acceptable. Some authors [17-20, 77] have also reported the detection of initial rotor position of a salient pole PM
motor by high-frequency injection methods using voltage pulses.

Despite implementation difficulties, several methods of position sensing from inductance variation have been
applied for sensorless operation. Low frequency excitation pulse results in large current amplitudes which
facilitate easy detection, but can cause audible noise from the motor. Whereas high frequency avoids audible
noise, but reduces current amplitudes. Therefore, choice of an appropriate modulation frequency and
modification in the machine rotor can further improve rotor position sensing using this method.

6.3 Flux Linkage Variation Sensing

Another method reported in the literature [17-20] is flux-linkage variation sensing, which is based on the phase
voltage equation of the motor. Since the phase flux linkages are a function of current and rotor position,
therefore, phase flux linkage can be estimated continuously by integrating the voltage after subtracting the
resistive voltage drop from the phase voltage [17-20]. The open-loop integration is prone to errors caused by
drift, which can be reduced if the pure integrator is replaced by a low pass filter or an alternative integrator
structure. In most electrical machines, it is not practical to measure the phase voltages directly, because of
isolation related issues; therefore, applied phase voltage is estimated from DC supply voltage of the solid-state
converter [17-20].

7. Application Potential of PMBLDC Motors

Classic electric motors are mostly preferred for motion control, in general and household appliances, in
particular. The most common motors for household appliances are single phase AC induction motors,
including split phase, capacitor start, capacitor run types and universal motors. These motors operate at
constant speed directly from AC mains irrespective of efficiency; however, consumers now demand appliances
with low energy consumption, improved performance, reduced acoustic noise, and many more convenience
features. Therefore, household appliances are expected to be one of largest end product market for PMBLDC
motors over the next few years. The major household appliances include fans, blowers, washing machines,
room air-conditioners, refrigerators, vacuum cleaners, food processors, etc.

The possibilities of cost reduction have to be explored to commercialize PMBLDCM drives, apart from
technological advancements. The cost of a PMBLDCM drive has two main components; one is motor and other
is the controller. Extensive research attempts [13-29] have been made to reduce the cost and increase the
efficiency of the motor. Comparative analysis has also been presented in the literature [36, 117] for the choice of
the motor to suit a particular application.

Recently, there has been growing research interest for

use of PMBLDC motors in electric vehicles (EVs) and hybrid electric vehicles (HEVs) due to environmental
concerns of vehicular emissions. An electric drive is one of the main parts of an EV/HEV and requires
multidisciplinary power electronic technologies, including motors, converter topologies, switching devices,
microprocessors/DSPs, and control strategies. The PMBLDC motor is more suitable for EVs/HEVs and low
power applications, due to high power density, less volume, high torque, high efficiency, easy to control,
simple hardware and software, and low maintenance. It is very difficult to identify a unique drive solution for
all kinds of electric vehicles (i.e. bikes, cars, vans, trucks, etc.). For many applications, the motor should have
shape flexibility, compactness, robustness, high efficiency and high torque [113-137].
Axial flux motors can be an interesting option for EVs, because it can be directly coupled, to or inside, the
vehicle wheels. With this kind of solution known as a “wheel motor,” the mechanical differential and reduction
gear can both be avoided. Among the available types of axial flux motors, the axial flux interior permanent
magnet (AFIPM) motor in which the stator windings are allocated in slots, has some very attractive
characteristics for application as wheel motors e.g. robust construction and ability to deliver the desired
torque in the flux-weakening region, too. It has higher output torque as compared to other motors due to the
cumulative effects of field and reluctance [36-48].
PMBLDC motors have been used in various high-speed applications such as the hard disk drive (HDD) of
computers which run at very high speed to reduce the access time of the data written on the surface of a
rotating disk. In order to run the motor at high speed, back electromotive force (EMF) constant is designed to
be small to reduce the voltage drop due to back EMF. But, it results in small starting torque, thereby a long
transient period. It is one of the drawbacks of a PMBLDC motor in high-speed applications. Therefore a
combination of unipolar and bipolar drives has been used which utilizes the advantage of the large starting
torque of a bipolar drive and the high operating speed of a unipolar drive. A DSP/FPGA based controller can be
used to drive the PMBLDC motor with the bipolar or unipolar method and to switch from one method to the
other at any speed [132] .
Many other applications of PMBLDC motors have been reported which include, tread mills [124], washers [127],
dexterous robotic hands [128], wheelchairs [130], compressors of household air conditioners [107, 120], automotive
HVACs [113, 133] and commercial freezers [136], fans [123, 129] and pumps [44, 116, 118, 125]. The use of an application
specific integrated circuit (ASIC) such as ML-4425 [22, 23] for generating the commutation pulses based on back-
EMF sensing, has the flexibility of adding desired features through software modifications rather than with
additional hardware.

8. Power Quality Considerations

In recent years, the power quality considerations for various drives have been reported reasonably due to
increased use of electronic equipment and AC motor drives in all walks of society i.e. household, commercial
and industrial applications. A diode rectifier with a smoothing dc capacitor behaves as a harmonic voltage
source, however, thyristor converters are a common and typical source of harmonic currents. Therefore, any
of these kinds of drives which behave as a nonlinear load are not a good option for power utilities.
In view of these problems, some suitable measures are required for the compensation of these current
harmonics. One very popular method is the use of filters i.e. passive or active wave shaping (series or parallel).
The current source nonlinear loads and voltage source nonlinear loads have dual relations to each other in
circuits and properties and can be used with parallel and series filters, respectively, for harmonic
compensation [103-112].

Fig. 12 Boost converter based PFC topology [94,99]

Despite increased awareness about power quality improvements, in general, the topologies for power quality
improvement in PMBLDC motors have been reported less

frequently in the literature [91, 94-95, 99, 104-106, 112]. Fig. 6 shows a three phase four switch topology voltage source
inverter (VSI) having total six switches including rectifier for PFC and Fig. 12 shows a conventional six switch
VSI topology with single phase PFC at input mains of PMBLDCM drive. Conventional six switch converters have
been reported [94-95, 99, 106] with various PFC converters. A six switch single phase to three phase converter has
been reported [91, 94, 102] which draws sinusoidal input current at close to unity power factor. Some of these
topologies are designed and modeled for a PMBLDC Motor of 1.5 kW (data is given in Appendix) in the
MATLAB/Simulink environment.

(a)
 (b)
Fig. 13 The source current (is) waveform and its THD during steady state at rated torque for (a) conventional VSI based topology fed
PMBLDCM drive (Fig.4) and (b) conventional VSI based boost PFC topology fed

PMBLDCM drive (Fig.12)

Fig. 13 shows the supply currents and harmonic spectra of the conventional PWM VSI fed PMBLDC motor
drive with and without PFC converter. The harmonic spectra shows 81.19% THD in the AC mains current at
rated torque with a crest factor (CF) of 2.95 for conventional VSI. The THD of AC mains current is reduced to
2.13% with the boost PFC topology with a crest factor of 1.45 at same load on the motor.
Fig. 14 shows the variation of source voltage (vs), source current (is), DC link voltage (vdc), speed (N), motor
phase current (ia) and torque (Te) for conventional VSI based topology fed PMBLDCM drives with and without
PFC during starting and load perturbation (i.e. load application and load removal). The performance of the
PMBLDCM drive is improved with boost PFC topology in terms of low torque ripples, smooth speed variation
and unity power factor at AC mains. Other attempts [107-111] have been reported on various wave shaping
techniques, which can be used with PMBLDC motor drives after careful analysis and evaluation.

9. Future Trends

In spite of being a most promising machine, PMBLDC motors have faced many hurdles in order to come to
their present stage in terms of cost, torque ripple, noise, vibration, reliability, operational constraints such as
temperature rise etc. Numerous applications of PMBLDC motors have been discussed with emphasis on low
cost topologies and sensorless techniques. The applications of PMBLDC motors, as reported in the literature [1-
16, 101-123]
, are mostly in EVs/HEVs, some household and commercial appliances and very few industrial drives.
However these motors may be employed in a number of such applications, if cost reduction with sensorless
operation is possible. The power quality improvement at AC mains adds to the benefits of the drive in many
applications. Investigations are being made in the direction of controller cost reduction using various
topologies with and without position/speed sensors.

Many additional applications of PMBLDCMs may be explored in ceiling and pedestal fans, domestic juicers,
mixers and grinders, industrial and domestic pumps, dryers and many more small appliances.
AN885

Conventional VSI topology (Fig. 4)

Conventional VSI based boost PFC topology (Fig.12)

Fig. 14 Variation of source voltage (vs), source current (is), dc link voltage (vdc), speed (N), motor phase current (ia) and torque (Te) for
PMBLDCM drive during starting (0.15 sec.), load application (0.35 sec) and load removal (1.25 sec.)
Moreover, PMBLDCM drives can be employed for automobiles, hand tools, and small-process drives with
people carriers in airport lobbies, golf carts, freezers, precise control for packaging, bottling, food processing
and other similar applications. However, in most of these applications, the role of controller and
operating conditions are different, therefore, the controller design for a particular application plays a
major role in the performance and efficiency of the drive. The cost of the controller and complexity of
control become the key factor for the commercialization of these drives. Hence the acceptability of
PMBLDC motors in a variety of applications solely depends upon the research in the area of simplified
and low cost controller design. Therefore, general or application specific controller topologies have to
be designed for PMBLDC motors with prime consideration of simple and low cost controllers having
improved power quality at the input mains of the drive.

The future research in PMBLDCM drives is expected to focus on sensorless starting, reduction of motor
cost, controller cost reduction, comprehensive sensorless control, application specific controller design,
improved PQ controllers, reduced cost controllers with PFC features. With the above objectives of
PMBLDCM drives, the economic viability and performance of PMBLDC motors in a wide range of
applications is expected to grow in the future.

10. Conclusions

An exhaustive overview of PMBLDCM drives has been presented to provide a clear perspective on
various aspects of these drives to the researchers and engineers working in this field. The PMBLDCM
drives are suitable for many applications; however, the choice of the motor (i.e. rotor configuration),
control scheme (i.e. sensorless or with sensors) and controller topology depends on the accuracy, cost,
complexity and reliability of the system. ASICs are one step in the direction of low cost controllers and
many more such ICs with cost effective solutions will be developed in the near future. A customer can
select a PMBLDCM drive with their desired features, however, there is a tradeoff between the number
of parameters (e.g. sensorless or with sensors, accuracy, complexity, reliability and cost of controller). It
is hoped that this investigation on PMBLDCM drives will be a useful reference for users and
manufacturers.

Appendix

Motor data:

Rated Power: 1.5 kW, Rated Voltage: 400 V, Rated Speed: 1500 rpm, Rated Current: 4.0 A, Rated torque:
10 Nm, No of poles: 4, Resistance: 2.8 ohm/ph., Inductance (L s+M): 0.00521 H/ph., back EMF constant :
1.23 Vsec/rad, Moment of Inertia = 0.013 Kg-m2. The Circuit Parameters used for simulations: Source
impedance: 0.03 pu, DC link capacitance: 1100 µF, dc link inductance for PFC: 5 mH, The switching
frequency of boost switch = 20 kHz.