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.