Post on 26-Nov-2014
AKSHAYA BHARATHI INSTITUTE OF TECHNOLOGY R.S. NAGAR, SIDDAVATAM, KADAPA (Dist).
DEPARTMENT: ELECTRICAL & ELECTRONICS
PAPER PRESENTATION ON “DYNAMIC SIMULATION OF BRUSHLESS DC MOTOR”
Submitted By:
V. Venkata Bhaskar C.Chaitanya 3rd EEE-1-sem 3rd EEE-1-sem
ABIT ABIT Ph: (0)9000114733 Ph: (0)9441637285 E-mail:
Under the Guidance of: J.Sivavara Prasad M.Tech Associate Professor ABITUNIVERSITY: JAWAHERLAL NEHRU TECHNOLOGICAL UNIVERSITY, ANANTAPUR.
Abstract:
Due to the brushes, DC motors suffer from a lower
reliability, since the brushed wear down by operation and need
time to time maintenance or replacement. This drawback can be
eliminated by using a BLDC motor. Several simulation models
have been proposed for the analysis of the BLDC motor drives.
These models are based on state space equations, Fourier series
and the d-q axis model. Even though the previous works made a
great contribution to simulate the BLDC motor drives, no models
are appropriate for development and analysis of control
algorithm of BLDC motor drives. BLDC motor drive is widely
used because of its particularly high mechanical power density,
simplicity, and cost effectiveness.
The main purpose of this project is to develop a model
of brushless dc motor drive considering behavior of the motor
during phase commutation and considering the waveform of
back EMF, for the dynamic simulation of BLDC motor drive
under MATLAB/Simulink environment. The torque of the
BLDC motor is mainly influenced by the waveform of back
EMF. Ideally, BLDC motors have trapezoidal back EMF
waveform and are fed with rectangular stator currents, which
give a theoretically constant torque. However, in practice, torque
ripple exist, mainly due to emf waveform imperfections, current
ripple and phase current commutation. The current ripple result
from PWM or hysteresis control. The emf waveform
imperfections result from variations in the shapes of slot, skew
and magnet of BLDC motor. First a simple mathematical model
of the BLDC drive is developed. The motor model is then
simulated using MATLAB/Simulink, with sinusoidal and
trapezoidal waveforms of back EMF. The model proposed in this
project can be an easy to design tool for the design and control
algorithms. The speed, torque, current of the BLDC motor drive
considering phase commutation has been analyzed through the
simulation. A comparison study of results of both the cases is
done.
I. BRUSHLESS DC MOTORINTRODUCTION
A Brushless DC (BLDC) motor can be considered as
a dc motor turned inside out, so that the field is on the rotor
and the armature is on the stator. The BLDC motor is actually
a permanent magnet ac motor whose torque-current
characteristics mimic the dc motor. Instead of commutating
the armature current using brushes, electronic commutation is
used. This eliminates the problems associated with the brush
and the commutator arrangement, for example, sparking and
wearing out of the commutator-brush arrangement, thereby,
making a BLDC more rugged as compared to a dc motor.
Having the armature on the stator makes it easy to conduct
heat away from the windings, and if desired, having cooling
arrangement for the armature windings is much easier as
compared to a dc motor.
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 [8].
Dynamic Simulation of Brushless DC Motor
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.
Generally an increase in the electric loading is possible,
providing a greater specific torque. The efficiency is likely to
be higher than of a commutator motor of equal size, and the
absence of brush friction helps further in this regard.
The absence of commutator and brush gear reduces
the motor length. This is useful not only as a simple space
saving, but also as a reduction in the length between bearings,
so that for a given stack length the lateral stiffness of the rotor
is greater. Permitting higher speeds or a longer active
length/diameter (L/D) ratio is required. The removal of the
commutator reduces the inertia still further.
The brushless configuration does not come without
some disadvantages. The main disadvantages relative to the
commutator motor are
The need for shaft position sensing
Increased complexity in the electronic controller and
The brushless motor is not necessarily less expensive
to manufacture that the commutator motor.
The power electronic converter is required with the
brushless motor as similar to the P.W.M inverter based
Induction motor drives. The device ratting may be lower,
especially if only a ‘constant torque’ characteristics is
required. Of course, the induction motor can be inexpensively
controlled with triacs or series SCRs, but the performance so,
obtained is inferior to that of the brushless D.C system in
efficiency, stability, response, and controlled speed range.
II. CONSTRUCTIONAL DETAILS OF BLDC MOTORBLDC motors are a type of synchronous motor. This
means the magnetic field generated by the stator and the
magnetic field generated by the rotor rotates 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. Out of these, 3-phase motors are the
most popular and widely used. The disassembled view of a
brushless dc motor is as shown in Fig. 2.1. it consists of
stator
rotor
position sensors
Fig. 2.1 Disassembled view of a brushless dc motor
A. Stator
The stator of a BLDC motor is as shown in Fig. 2.2.
It consists of stacked steel laminations with windings placed
in the slots that are axially cut along the inner periphery.
Traditionally, the stator resembles that of an induction motor;
however, the windings are distributed in a different manner.
Most BLDC motors have three stator windings connected in
star fashion. Each of these windings 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.
Depending upon the control power supply capability,
the motor with the correct voltage rating of the stator can be
chosen.
Fig. 2.2 Stator of a BLDC motor
B. RotorThe 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.
Fig.2.3 shows cross sections of different
arrangements of magnets in a rotor [8].
Fig..2.3 Rotor magnet cross sections
C. Position 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.
The position of the rotor can be sensed by using an
optical position sensor and its associated logic. Optical
position sensors consist of phototransistors (sensitive to light),
revolving shutters, and a light source. The output of an optical
position sensor is usually a logic signal. This is especially
useful when unipolar switching is used to drive the BLDC
motor.
Another option is using Hall effect position sensors
which are 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.
Other types of position sensors are pulse encoders
and Variable Differential Transformers [4][8][11].
III. OPERATING PRINCIPLE
In BLDC motor, torque is produced by the
interaction between the magnetic fields 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. A six-step commutation defines the sequence of
energizing the windings [8] is as shown in Fig. 2.4.
The conducting interval for each phase is 120o
electrical angle. The commutation phase sequence is AB-AC-
BC-BA-CA-CB, where A, B, C are the three phases. Each
conducting stage is called one step. Therefore, only two
phases conduct current at any time, leaving the third phase
floating. In order to produce maximum torque, the inverter
should be commutated every 60o so that current is in phase
with the back EMF. The commutation timing is determined by
the rotor position, which can be detected by position sensors
[5].
IV. BACK EMFWhen 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
Back EMF,
...(2.1)
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,
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 [8].
Generally, the torque of the BLDC motor is mainly
influenced by the waveform of back EMF and hence it is
needed to predict its precise back EMF. The model of back
EMF approximated to ideal trapezoidal/sinusoidal models in
BLDC motor system has been generally used in computer
simulation tools. For brushless dc motors with a trapezoidal
back EMF and fed with rectangular stator currents, the torque
produced is theoretically constant. However, in practice,
torque ripple may exist, due to the machine itself and also due
to the feeding system. The causes of ripple coming from the
machine are cogging torque and emf waveform imperfections,
and those coming from the supply are current ripple (resulting
from the PWM or hysteresis control) and phase current
commutation.
Fig. 2.4 A six step commutation sequence
V. BRUSHLESS DC MOTOR CHARACTERISTICS: BLDC motors have identical speed and torque
characteristics to brushed DC motors.
The torque produce by BLDC motors is given by
...(2.2)
From the Eqn (2.2), it is clear that the motor torque is directly
proportional to motor Current
The speed, ω, of a BLDC motor is given by
...(2.3)
Where Kspeed is the motor speed constant in radians per volt.
It is clear that the motor speed is directly
proportional to the applied voltage (V). Fig.2.5 summarizes
the torque-current characteristics of a BLDC motor. The
actual torque drops slightly due to core saturation at higher
currents. It should also be noted that BLDC motors produce
maximum torque from standstill. This is one of the main
reasons they are preferred over induction motors. Induction
motors draw up to twice their rated current on starting but
product only typically 30 percent of their operating torque
from standstill. Synchronous motors product zero starting
torque, as a result they are usually started and brought to
synchronous speed using a DC motor.
Fig. 2.5 BLDC motor Torque - Current characteristics
Fig. 2.6 shows torque/speed characteristics of BLDC
motor. There are two torque parameters used to define BLDC
motor, peak torque (TP) and rated torque (TR).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.
Fig. 2.6 BLDC motor Torque - speed curve
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.
VI. MODELING OF BLDC MOTOR
INTRODUCTION
Most generic and easiest mode of operation of the BLDC
motor is sensor mode. In sensor mode Hall sensors or optical
encoders or revolvers or shaft position sensors are used which
give information for commutation by means of rotor position
sensing. The sensor controlled mode of operation is generally
called as self controlled mode. The block diagram of a
brushless dc motor in sensor mode is as shown in Fig. 3.1. It
consists a PM ac motor, position sensor with logic circuit.
This drive can be divided into several blocks to
develop the dynamic model as discussed in Section.3.2. A
dynamic model is one, which takes into account time-varying
characteristics and interactions.
Fig. 3.1 Block diagram of a brushless dc motor
A. BLDC Motor Block Diagram ModelThe block diagram of the BLDC motor drive is as
shown in Fig. 3.2. The system mainly consists of four
components:
PWM signal generator with 120 degree
conduction
Voltage source inverter
Electrical part of BLDC motor and
Mechanical part of BLDC motor [14] [16].
Fig. 3.2 Block diagram of the BLDC motor drive system
Here, the control signal is given to the PWM signal
generator. It generates perfect square wave switching patterns,
which are given to the inverter as input. Inverter converts dc
into ac then it is given to the BLDC motor. This can be
mathematically modeled based on their performance
equations. The Complete BLDC Motor Drive model is as
shown in Fig. 3.3.
Fig. 3.3 BLDC motor drive model.
For the development of mathematical model of BLDC
motor dive the following assumptions were made
The motor is not saturated.
Iron losses are negligible.
Stator resistances of all the windings are
equal and self and mutual inductances are
constant.
Power semiconductor devices in the inverter
are ideal.
B. Dynamic Simulation of BLDC Motor Drive1. Simulation results:
The development of mathematical modeling and simulink
implementation of the BLDC motor drive with sinusoidal and
trapezoidal back emfs has been discussed in the previous
chapter. The Simulation is carried with sinusoidal and
trapezoidal back emfs.
For the both the cases it considered that the motor is operating
with 80V DC bus voltage with load torque of 0.5 Nm.
Fig. 4.1 shows simulation results of the three phase currents
with sinusoidal back emf. Which shows that the variation of
currents in each sector. In each sector any two phases are in
conducting, one is under commutation interval. For example
in sector 1 phase C is positively conducting, phase B is
negatively conducting and phase A is in commutation interval.
Sector positions: 1 2 3 4 5 6
Fig. 4.1 Simulation results of three phase currentswith sinusoidal back emf and sector locations
Fig. 4.2 shows simulation results of the three phase currents with trapezoidal back emf.
Fig. 4.2 Simulation results of three phase currentswith trapezoidal back emf and sector locations
Three phase inverter output voltages for each phase
and for three phases with the sinusoidal and trapezoidal back
emfs are as shown in Figs. 4.3(a), 4.3(b), 4.4(a), and 4.4(b)
respectively. The inverter is operated with PWM control in
accordance with the sector identification. The PWM gate
patterns for the both excitations are shown in Fig 4.5.
Fig. 4.3(a) Simulation results of Inverter output for each phase with the sinusoidal back EMF.
Fig. 4.3(b) Simulation results of three phase Inverter output (Uabc)
with the sinusoidal model.
Fig. 4.4(a) Simulation results of Inverter output for each phase with the trapezoidal backEMF.
Fig. 4.4(b) Simulation results of three phase Inverter output (Uabc) with the trapezoidal model.
Fig. 4.5 (a) Simulation results of comparison of square modulation wave
and triangular carrier,
4.5(b) Resulting modulating wave (normalized w.r.t V).
VII. FUTURE SCOPE
What has been presented in this project is only the
first step in the development of the model of BLDC
Motor Drive System. It is suitable to analyze
tendency of dynamic behaviours of BLDC Motor
Drive System. In the future, it will be trying to
increase accuracy of the model for quantity analysis
through the analyzing the primary factors of error.
The bldc motor model will integrated with the other
models of the automotive application and a
dynamic simulation of the automotive applications
will be implemented. One of The automotive
applications is an Electro-Mechanical Actuator
(EMA). An EMA translates electrical signals to
mechanical action of surfaces, for example, flaps
and ailerons in aircrafts or windows and sunroofs in
vehicles. The EMA has wide range of applications
in automotive and aerospace industry.
CONCLUSIONS
A model for dynamic simulation of Brushless DC
motor drive, considering phase commutation and the wave
shape of back EMF, is implemented under
MATLAB/Simulink environment. The feasibility and
performance of the model is examined by simulation.
Comprehensive analysis of the BLDC motor drives is allowed
and dynamic characteristic can be effectively monitored and
predicted. This model was implemented under Simulink
environment with modular manner. Thus the proposed model
can be used very effectively in analysis and design of control
algorithms of the BLDC motor drive system.
The model was simulated using a trapezoidal back
EMF waveform as well as a sinusoidal back EMF waveform.
Speed, Torque, Current of a BLDC Motor Drive for both the
cases are observed. Speed obtained with the sinusoidal back
EMF (i.e.2650rpm) is nearer to the rated speed of the BLDC
motor drive (i.e.2800rpm) than the speed obtained with the
trapezoidal back EMF (i.e.2566rpm). A comparison of both
the cases shows that the sinusoidal model gives a better
performance compared to the trapezoidal model.
Hence, it was concluded that for better simulation
results, i.e., to get simulation results more similar to practical
cases, a sinusoidal back EMF waveform should be considered,
instead of trapezoidal back EMF waveform.
5.2 Future Scope
What has been presented in this project is only the
first step in the development of the model of BLDC
Motor Drive System. It is suitable to analyze
tendency of dynamic behaviours of BLDC Motor
Drive System. In the future, it will be trying to
increase accuracy of the model for quantity analysis
through the analyzing the primary factors of error.
The bldc motor model will integrated with the other
models of the automotive application and a
dynamic simulation of the automotive applications
will be implemented. One of The automotive
applications is an Electro-Mechanical Actuator
(EMA). An EMA translates electrical signals to
mechanical action of surfaces, for example, flaps
and ailerons in aircrafts or windows and sunroofs in
vehicles. The EMA has wide range of applications
in automotive and aerospace industry.