MAHARANA PRATAP ENGINEERING COLLEGE

AFFILIATED TO

APJAKTU LUCKNOW( UP)

SEMINAR REPORT

ON

“BRUSHLESS DC MOTOR”

Submitted to: Submitted by:

DEPORTMENT OF VINAY SINGH

ELECTRICAL ENGINEERING ROLL NO- 1404620904

B.Tech 3rd YEAR

Session 2015-2016

ACKNOWLEDGEMENT

In the accomplishment of this project successfully, many people have best owned

upon me their blessings and the heart pledged support, this time I am utilizing to

thank all the people who have been concerned with seminar.

Primarily I would thank god for being able to complete this project with success.

Then I would like to thank my project guide Mr. KULDEEP BHARADWAJ and Mr.

DINESH TIWARI, whose valuable guidance has been the ones that helped me patch

this seminar and make it full proof success his suggestions and his instructions has

served as the major contributor towards the completion of the seminar.

Then I would like to thank my parents and friends who have helped me with their

valuable suggestions and guidance has been helpful in various phases of the

completion of the project.

SIGNATURE;

HOD OF ELECTRICAL ENGG.

Mrs. Lovi kaushal

INDEX ;

1. ABSTRACT

2. INTRODUCTION

3. STRUCTURE OF PMBLDC MOTOR

3.1. BASIC STRUCTURE

3.2. STATOR

3.3 ROTOR

4. SIMPLIFIED MODEL OF A BLDC MOTOR

5. HALL SENSOR

6. PHASE COMMUTATION

7. ELECTRONIC COMMUTATIOM

8. PULSE WIDTH MODULATION

9. CLASSIFICATION OF BLDC MOTOR

10. BRUSHLESS VERSUS BLDC MOTOR

11. CONTROLLERS FOR PMBLDC MOTOR

12. COMPRESION B/W BLDC & CDC MOTOR

13. ADVANTAGES OF BLDC MOTOR

14. DISADVANTAGES OF BLDC MOTOR

15. APPLICATION OF BLDC MOTOR

16. CONCLUSION

17. REFERENCES

ABSTRACTThis seminar is a general overview of BLDC motors, including their advantages against other commonly-used motors, structure, electromagnetic principles, and mode of operation. This seminar also examines control principles using Hall sensors for both single-phase and three-phase BLDC motors, and a brief introduction to sensor less control methods using BEMF for a three-phase BLDC motor.

2. INTRODUCTION

A Brushless DC motor is similar to that brushless DC motor in that it has an internal shaft position feedback which tells which winding to switch on and what exact moment. This internal feedback gives both the brushed DC motor and brushless DC motor with their unique characteristics. Linear speed- torque curves which are well suited for speed and position control and high starting torque . The internal feedback is accomplished in a brush type DC motor with the mechanical commutator (a series of copper brush which are insulated from each other) and the mechanical brushes through which the current is fed into commutator bars and switched sequentially into the appropriate winding in the armature.

In a BLDC motor, internal feedback is accomplished by shaft position sensor some type which gives the required shaft position information to the drive electronics. The drive electronics in turn switched on the appropriate windings exactly at the right moment. this internal shaft position feedback also gives the BLDC motor characteristics. Which are the similar to DC motor characteristics. Linear torque-speed characteristics and high starting torque .The power supplied to a BLDC motor can be DC power but it can also be AC if drive electronics has necessary circuitry to convert the AC power to DC.

The brushless DC motor is actually a permanent magnet AC motor whose torque speed characteristics mimic the DC motor. instead of commutating the armature current using brushes ,electronic commutation is used. 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 winding is much easier as compared to a DC motor. A BLDC motor is modified PMSM with the modification being that the back-emf is trapezoidal instead of being sinusoidal as in the case of PMSM. The position of the rotor can be sensed by Hall Effect position sensors, namely Hall A, Hall B and Hall C, each having a phase lag of 120 degree with respect to the earlier one. Three Hall position sensors are use to determine the position of the rotor field. BLDC motor model is explained as, the electromagnetic torque, Tem is linearly proportional to the armature current Ia i.e. Tem = Kr Ia, where KT is torque constant.

Brushless DC motors (BLDC) are more reliable than standard DC (mechanically commutated) motors. DC motors start turning when a supply voltage is applied, but BLDC motors require electronics for commutation. Speed control or remote control of a BLDC motor requires electronics as for mechanically commutated DC motors. BLDC motor are more suitable for control and regulation. Operating a BLDC motor in sensorless beyond its typical speed limit makes it similar to a BLDC motor equipped with Hall sensors.

3. STRUCTURE OF PERMANENT MAGNET BRUSHLESS DC

MOTOR

3.1. BASIC STRUCTURES

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. Here we focus on 3-phase motors.

The construction of modern brushless motors is very similar to the ac motor, known as the permanent magnet synchronous motor. Illustrates the structure of a typical three-phase brushless dc motor. The stator windings are similar to those in a polyphase ac motor, and the rotor is composed of one or more permanent magnets. Brushless dc motors are different from ac synchronous motors in that the former incorporates some means to detect the rotor position (or magnetic poles) to produce signals to control the electronic switches as shown in fig. The most common position/pole sensor is the Hall element, but some motors use optical sensors.

Fig-3.1

3.2. 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. 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 interconnected coils, with one or more coils are placed in the stator slots. Each of these windings are distributed over the stator periphery to form an even numbers of poles. As their names indicate, the trapezoidal motor gives a back trapezoidal EMF.

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. Depending upon the 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.

Fig:3.2

3.3. 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 were traditionally used to make the permanent magnet pole pieces. For new design rare earth alloy magnets are almost universal. 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 using a smaller rotor and stator for the same torque. Accordingly, 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. Figure shows cross sections of different arrangements of magnets in a rotor.

Fig:3.3

4. SIMPLIFIED MODEL OF A BLDC MOTOR

A simplified model of a BLDC motor consists of three coils arranged in three direction A, B and C as shown in figure. A permanent magnet forms the rotor. Here the rotor is outlined as a bar magnet with its rotary axis at the intersection of the three axis A,B,C perpendicular to the plane of these axis. The orientation position of the permanent magnet can be controlled by driving a configuration of currents through the three coils. The bar magnet comes the position 1 when a current is driven from C through B and it comes to the opposite orientation when a current is driven from B to C. For a BLDC motor that is equipped with hall sensors these given the actual rotor position.

Fig:4.1

The motion of the rotor induces alternating voltages called Back Electromotive Force (BEMF) within the coils. The amplitude of the BEMF is proportional to the angular velocity of the rotor. Hall sensors are mounted in such a way that the zero crossing of the BEMF occurs as close as possible to the zero crossing of the hall sensor signal associated with the corresponding coil. H1 is associated with A, H2 is associated with B, and H3 IS associated with C. Alternatively, the connections A,B & C of BLDC motors also labelled as U, V, W respectively. The BEMF can be modelled as a voltage source in series with each coil that has voltage amplitude proportional to the speed of the rotor. The BEMF voltage varies with the angle between the coil axis and the angle of the rotor. Following, the shape of the BEMF is assumed to be sine wave. Alternatively, the shape can be triangular or trapezoidal or somewhat between these shape.

5. HALL SENSORS

The Hall effect is basic to solid-state physics and an important diagnostic tool for the characterization of materials – particularly semi-conductors. It provides a direct determination of both the sign of the charge carriers, e.g. electron or holes (appendix A), and their density in a given sample. The basic setup is shown in Fig. 1: A thin strip (thickness δ) of the material to be studied is placed in a magnetic field B oriented at right angles to the strip.

Fig:5.1

A current I is arranged to flow through the strip from left to right, and the voltage difference between the top and bottom is measured. Assuming the voltmeter probes are vertically aligned, the voltage difference is zero when B = 0.

The current I flows in response to an applied electric field, with its direction established by convention. However, on the microscopic scale I is the result of either positive charges moving in the direction of I, or negative charges moving backwards. In either case, the magnetic Lorentz force qv ×B causes the carriers to curve upwards. Since charge cannot leave the top or bottom of the strip, a vertical charge imbalance builds up in the strip. This charge imbalance produces a vertical electric field which counteracts the magnetic force, and a steady-state situation is reached. The vertical electric field can be measured as a transverse potential difference on the voltmeter.

Fig:5.2

Suppose now that the charge carriers where electrons ( q = −e ). In this case negative charge accumulates on the strip’s top so the voltmeter would read a negative potential difference. Alternately, should the carriers be holes ( q = +e ) we measure a positive voltage.

The above argument provides a simple picture in which to think about the Hall effect — and in fact leads to the correct answer if pursued. However, it presupposes a steady current of charge carriers flowing in the conductor all in a single direction with constant speed. Why, for instance, don’t the carriers accelerate? The true nature of macroscopic conduction is a bit more complicated, relying on a statistical average over individual carrier’s motions. In the next section we will briefly look at this issue.

Fig:5.3

For the estimation of the rotor position, the motor is equipped with three hall sensors. These hall sensors are placed every 120 degree . With these sensors, 6 different commutation are possible. Phase commutation depends on hall sensor values. Power supply to the coils changes. With right synchronized commutations, the torque remains nearly constant and high. Figure shows the hall sensor signals for the clockwise rotation.

Fig:5.4

Fig:5.5

Figure shows the Hall sensor image. The Hall sensor is this little component under the right electromagnet. When it senses the south pole, it keeps the coils turned off. When the sensor no magnetic field (or could be also the south pole), then it turns on the coils. The coils have both the same magnetic polarity which is north. So they pull the opposite pole and torque is then created. If you put a probe to the hall sensor and which the signal, then you will discover that during a full rotation of the rotor, the Hall sensor is tow times HIGH and two times LOW. Fig shows the Back Emf, Current Waveforms and Hall position sensors for a BLDC motor drive. Motor commutation is usually related to hall effect sensor output.

All of the electrical motors that do not require an electrical connection (made with brushes) between stationary and rotating parts can be considered as brushless permanent magnet (PM) machines, which can be categorized based on the PMs mounting and the back-EMF shape. The PMs can be surface mounted on the rotor (SMPM) or installed inside the rotor (IPM), and the back-EMF shape, PMAC synchronous motor (PMAC or PMSM) have trapezoidal back-EMF. A PMAC motor is typically excited by a three-phase sinusoidal current and a BLDC motor is usually powered by a set of current having a quasi-squre waveform.

Fig:5.6

In recent years PWM techniques wave effectively introduced to improve the performance of non linear systems. The application of PWM is very promising in system identification and control due to learning ability, massive parallelism, fast adaptation, inherent approximation capability, and high degree of tolerance.

6. PHASE COMMUTATIONS

To simplify the explanation of how to operate a three phase . BLDC motor, a typical BLDC motor with only three coils is considered. As previously shown, phases commutation depends on the hall sensors values. When motor coils are correctly supplied, a magnetic field is created and the rotor moves. The most elementary commutation driving method used for BLDC motor is an ON & OFF scheme: a coil is either conducting or not conducting. Only two windings are supplied at the same time and the third winding is floating. Connecting the coils to the power and neutral bus induces the current flow. This is referred to as trapezoidal commutation or block commutation. Figure shows the power circuit diagram for BLDC motor.

HALL SENSOR VALUE

H1 H2 H3

PHASE SWITCHES

1 0 1 U-V Q1:Q4

0 0 1 U-W Q1: Q6

0 1 1 V-W Q3: Q6

0 1 0 V-U Q3: Q2

1 1 0 W-U Q5: Q2

1 0 0 W-V Q5: Q4

Fig:6.1

Far motors with multiple poles the electrical rotation does not correspond to a mechanical rotation. A four pole BLDC motor uses four electrical rotation cycles to have one mechanical rotation. The strength of the magnetic field determines the force and speed of the motor. By

varying the current flow through the coils, the speed and torque of the motor can be adjusted. The most common way to control the current flow is to control the average current flow throw the coil. PWM (pulse width modulation) is used to adjust the average. Voltage and thereby the average current, inducing the speed.

7. ELECTRONIC COMMUTATION

The polarities of two coil currents with one coil left unconnected define six different position for rotor. Switching the currents in a way the currents pull the rotor to the position next to the current position lift the rotor turn. Each position of the rotor is associated with a configuration of coil currents by a successive switching scheme configuration that pulls the rotor to its next position. The coil currents are driven by three voltage sources.

Fig:7.1

The voltage source are realized with fast switches (power MOSFETs) that are PWM controlled for adjustment of effective voltage. The block commutation scheme is outlined by figure for each commutation step there is one terminal connected to ground (ground symbol), one terminal is connected to a power supply (circle), and one terminal is left open (terminal A, B&C). Permanent connection to ground to power supply drives the maximum current through the coil of the motor and will turn it with maximum speed that is possible for a given motor with a given supply voltage.

For the block commutation, each sector of the rotor is mapped to the successive sector concerning current switching. So, the commutation via interrupts become simple if each change of a Hall sensor signal forces an interrupt. Then, the actual triple of Hall sensor signals defines the commutation sector. In other words, the block commutation can be described as a periodic sequence of 0Z11Z0 where 0 is connection to ground, Z represents an open terminal, and 1 is the connection to the supply voltage source. The coils of the motor can be connected in star (Y connection) or triangle (Delta connection). Whatever is the type of connection, the idea is to get an access to the null point to be able to measure the BEMF and some motors allows this access via an additional wire. Direct access to the null point N enables direct measurement of the BEMF. The voltage of null point N is affected by the supply voltage together with a given PWM scheme.

8. PULSE WIDTH MODULATION

Pulse width modulation (PWM) is very popular method for controlling the speed of electric motors. The block diagram for a three-phase BLDC drive, which consists of a three-phase inverter and a BLDC motor, was shown in Figure. It can be controlled by the PWM technique to give proper commutations so that two of the three phases are with on states and the remaining one is with floating state. Moreover, the sequence of commutations is retained in proper order such that the inverter performs the functions of brush and commutator in a conventional DC motor, to generate a rotational stator flux. Figure shows the PWM waveforms for this conventional approach, which has low switching losses in the inverter side at the cost of significantly high harmonic contents. This results in increase of loss in the motor side.

Fig:8.1

In a typical inverter configuration, as Figure illustrates, two phases are always conducting current and one phase is only available to measure back-EMF. To measure the back-EMF across a phase, the conventional method requires monitoring the phase terminal

and the motor neutral point, as shown in Figure. The zero crossing of the back-EMF can be obtained by comparing the terminal voltage to the neutral point. In most cases, the motor neutral point is not available. The most commonly used method is to build a virtual neutral point that will be theoretically at the same potential as the neutral point of the wye-wound motor.

The conventional detection scheme is quite simple and when a PWM signal is used to regulate motor speed or torque/current, the virtual neutral point fluctuates at the PWM frequency. As a result, there is a very high common-mode voltage and high-frequency noise. Voltage dividers and low-pass filters, as shown in Figure, are required to reduce the common-mode voltage and minimize the high-frequency noise.

Fig:8.2

PWM applied to high side switches of a typical inverter for BLDC motors

9. CLASSIFICATION OF BLDC 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 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. The second category of permanent magnet motor drive is known as the brushless DC drive, or rectangular fed drive. It is supplied by three-phase rectangular current blocks of 120 degree 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 degree electrical in three-phase motors.

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 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. Amongst two types of permanent magnet brushless motors, PMSM is, therefore, preferred for applications where accuracy is desired e.g. robotics, numerical controlled machines etc. however, the PMBLDC motor can be used in general and low cost automotive and industrial applications. These motors are preferred for numerous applications, due to their features of high efficiency, silent operation, compact in size and low maintenance.

10. BRUSHLESS VERSUS BRUSHED DC MOTOR

Brushed DC motors have been in commercial use since 1886. Brushless DC motor on the other hand did not become commercially viable until 1962. Brushless motors developed a maximum torque when stationary, linearly decreasing as velocity increases. Some limitations of brushed motors can be overcome by brushless motors; they include higher efficiency and a lower susceptibility of the commutator assembly to mechanical wear. These benefits come at the cost of potentially less rugged, more complex, and more expensive control electronics.

A typical brushless DC motor has permanent magnets which rotate and a fix armature, eliminating problems associated with connecting with connecting current to the moving armature. An electronic controller replaces the brush/commutator assembly of the brushed DC motor, which continually switches the phase to the windings to keep the motor turning. The controller performs similar timed power distribution by using solid-state circuit rather than the brush/commutator system.

The maximum power that can be applied to a brushless motor is limited almost exclusively by heat; too much of which weakens the magnets, and may damage the winding’s insulation. A brushless motor’s main disadvantage is higher cost, which arises from two issues. First, brushless motors require complex electronic speed controllers (ESCs) to run. Brushed DC motor can be regulated by a comparatively simple controller, such as a rheostat (variable resistor). However, this reduces efficiency because power is wasted in the rheostat. Second, some practical uses have not been well developed in the commercial sector.

Brushless motors are more efficient at converting electricity into mechanical power than brushed motors. This improvement is largely due to motor’s velocity being determined by the frequency at which the electricity is switched, not the voltage. Additional gains are due to the absence of brushes, alleviating loss due to friction. The enhanced efficiency is greatest in the no-load and low-load region of the motor’s performance curve. Under high mechanical loads, brushless motors and high-quality brushed motors are comparable in efficiency.

Environments and requirements in which manufacturers use brushless-type DC motors include maintenance-free operation, high speed, and operation where sparking is hazardous (i.e. explosive environments), or could affect electronically sensitive equipment. Brushless motor commutation can be implemented in software using a microcontroller or computer, or may alternatively be implemented in analogue hardware or digital firmware using a PSIM.

11. CONTROLLERS FOR PMBLDC MOTORS 11.1. STRUCTURE OF CONTROLLER

The structure of controller for the BLDC motor shown in figure . The BLDC motor controller consists of six power semiconductor devices connected across a DC supply. Feedback diodes are connected across the devices. Rotor position sensor (RPS) which is mounted on a shaft of the BLDC motor provides signal to the controller about the position of the rotor with respect to the reference axis. The PWM strategy is applied only to the lower phase lag transistors. This not only reduces the current ripple but also avoids the need for wide bandwidth in the level shifting circuit that feeds the upper leg transistor.

Fig:11.1

The upper transistor need not be of the same type s the lower ones and need only switch the commutation frequency. This controller employs an inner current control loop with in an outer speed loop. Consequently it is possible to implement current feedback and speed feedback in the same way as for the DC motor and generally this result in a well-behaved system although compensation may be necessary in either one or both loops to improve stability and transient response.

The purpose of a BLDC motor controller is to provide speed and/or torque control to the motor. Usually a controller will provide one of the two, either torque control or speed control. Speed control is achieved by monitoring motor speed and adjusting the applied phase voltage to maintain the desired speed. Torque control is achieved by monitoring motor current. The motor current can be controlled to hold a constant value thus providing constant torque on the motor shaft. A speed control is possible over a wide range of speed and torque using relatively simple techniques that are familiar with commutation of motors.

12. COMPRESION B/W BRUSHLESS DC MOTOR &

BRUSHED DC MOTOR

13. ADVANTAGES OF BLDC MOTOR

Performance- The dynamic accuracy of the brushless DC motor is very high. Dynamic accuracy means the machine performs consistently, with the same efficiency.

Size- The brushless DC motor is the smallest of the motors available with a given power rating. Thus the machine occupies lesser floor space, weighs lighter and hence it makes handling of the machine easier.

Efficiency: The brushless DC motor is the most efficient motor available in the present industry.

Bearing stress: In large AC motors, heat current flows from the rotor through the bearing, damaging the motor bearings. The rotor heating in the brushless DC motor is the least because there is no winding in the rotor since it has a permanent magnet.

Brushless motors offers several advantages over brushed DC motors, including more torque per weight, more torque per watt (increased efficiency), increased reliability, reduce noise, longer lifetime (no brush and commutator erosion), elimination of ionizing sparks from the commutator, and overall reduction of electromagnetic interference (EMI), with no windings on the rotor, they are not subjected to centrifugal forces, and because the winding are supported by the housing, they can be cooled by conduction, requiring no airflow inside the motor for cooling. This is turn means that the motor’s internals can be entirely enclosed and protected from dirt or other foreign matter. Brushless DC motors are available in wide range of torque, speed and power.

14. DISADVANTAGE 1. Requires Complex Drive Circuitry2. Requires additional Sensors3. Higher Cost4. Some designs require manual labour (Hand wound Stator Coils)

15. APPLICATION OF BLDC MOTORS

CONSUMER: Hard Drives, CD/DVD Drives, PC Cooling Fans, toys, RC airplanes, air conditioners.

MEDICAL: Artificial heart, Microscopes, centrifuges, Arthroscopic surgical tools, Dental surgical tools and Organ transport pump system.

VEHICLES: electronic power steering, personal electric vehicles.

AIRPLANES: an electric self launching sailplane, flies with a 42kW DC/DC brushless motor and Li-Ion batteries and can climb up to 3000m with fully charged cells.

16. CONCLUSION

In this seminar the preliminary knowledge related to the thesis was presented. This knowledge should allow the reader to understand the concepts presented in following parts of the thesis. The General Electric drive system, construction of BLDC motor, generation of hall sensor signal for the BLDC motor, phase commutations, classification of BLDC motors, Simplified model of BLDC motor, Electronic commutation, Pulse width modulation, controllers for PMBLDC motor, structure of controller, speed control, torque control, advantage of BLDC motor over DC motor and application potential of BLDC motor were presented in this chapter.

17. REFERENCES 1. Muhammad Mubeen, “Brushless DC Motor Primer,” Motion Tech Trends, July,

2008.2. Derek Liu, “Brushless DC Motors Made Easy,” Freescale, 2008.3. Hall, E. H., “On a new action of the magnet on electric currents”, American Journal of

Mathematics, 2, No. 3, pages 287–292 (1879). The original paper by Hall which describes the effect. An interesting historical read.

4. Wikipedia Website, The Free Encyclopedia, Brushless dc motor Article, http://www.wikipedia.org.