Speed control of DC motor by using PWM technique project report

Speed control of DC motor by using PWM technique

INTRODUCTION

Today’s industries are increasingly demanding process automation in all sectors. Automation results into better quality, increased production an reduced costs. The variable speed drives, which can control the speed of A.C/D.C motors, are indispensable controlling elements in automation systems. Depending on the applications, some of them are fixed speed and some of the variable speed drives. The variable speed drives, till a couple of decades back, had various limitations, such as poor efficiencies, larger space, lower speeds, etc., However, the advent power electronic devices such as power MOSFETs, IGBTs etc., and today we have variable speed drive systems which are not only in the smaller in size but also very efficient, highly reliable and meeting all the stringent demands of various industries of modern era. Direct currents (DC) motors have been used in variable speed drives for a long time. The versatile characteristics of dc motors can provide high starting torques which is required for traction drives. Control over a wide speed range, both below and above the rated speed can be very easily achieved. The methods of speed control are simpler and less expensive than those of alternating current motors. There are different techniques available for the speed control of DC motors. The phase control method is widely adopted in which ac to dc converters are used to supply the dc motors, but has certain limitations mainly it generates harmonics on the power line and it also has poor p.f. when operated at lower speeds. The second method is pwm technique, which has got better advantages over the phase control. In our proposed project, a 5 H.P DC shunt motor circuitry is designed, and developed using pulse with modulation (PWM).The pulse width modulation can be achieved in several ways. In the present project, the PWM generation is done using timer IC.

Veermata Jijabai Technological Institute

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Speed control of DC motor by using PWM technique In order to have better open loop speed control as demand varies frequently like in traction system and many operations in industry must be control manually, PWM is most efficient and cheap speed control method for dc drives. By varying resistor pot only we can control the speed of motor states that simple and easy method. The project proposed is a real time working project, and this can be further improvised by using more no. of IGBT provides two or four quadrant chopper which will vary the motor in bidirectional mode.


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1. DC MOTOR


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1. DC MOTOR 1.1 INTRODUCTION TO SPEED CONTROL: Speed control means intentional change of drive speed to a value required for performing the specific work process. This concept of speed control or adjustment should not be taken to include the natural change in speed which occurs due to change in the load on the shaft. Any given piece of industrial equipment may have its speed change or Adjusted mechanically by means of stepped pulleys, sets of change gears, variable speed friction clutch mechanism and other mechanical devices. Historically it is proved to be the first step in transition from non adjustable speed to adjustable speed drive. The electrical speed control has many economical as well as engineering advantages over mechanical speed control The nature of the speed control requirement for an industrial drive depends upon its type. Some drives may require continues variation of speed for the whole of the range from zero to full speed or over a portion of this range , while the others may require two or more fixed speeds

1.2 CLASSIFICATION OF DC MOTORS: DC motors are classified into three types depending upon the way their field windings are excited. Field windings connections for the three types Of DC motors have been shown in fig.1.1

.


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V

Saturating field M SHUNT MOTOR

V

Series field M

SERIES MOTOR

V

Shunt field Series field

M

COMPOUND MOTOR Fig.1.1 Classification of DC Motor


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1.3 SPEED CONTROL OF DC MOTORS: The DC motors are in general much more adaptable speed drives than AC motors which are associated with a constant speed rotating field. Indeed one of the primary reasons for the strong competitive position of DC motors in modern industrial drives is the wide range of specified afforded we know the equation

N= K (

ϕ)

=K (V-Ia Ra / ϕ) Where V=supply voltage (volts) Ia = armature current (amps) Ra=armature resistance (ohms) Φ=flux per pole (Weber)

This equation gives two methods of effective speed changes i.e. a)

The variation of field excitation, if this causes in the flux per pole Φ and is known as the field control.

b)

The variation of terminal voltage (V).this method is known as armature control.


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1.4 SPEED CONTROL METHODS 1.4.1 FLUX CONTROL METHOD: It is known that N α 1/ Φ by decreasing the flux, thus speed can be increased and vice versa. Hence, name flux or field control method. The flux of DC motor can be changed by changing field rheostat. Since

with help of a shunt

in relatively small, shunt field rheostat has to carry only a small,

so that rheostat is small in size. This method therefore very efficient in non-interpolar machines the speed can be increased by this method in the ratio 2:1 any further weakening of flux Φ adversely affect the communication And hence puts a limit to the maximum speed obtainable with this method in machines fitted with interlopes in ratio of maximum to minimum speeds of 6:1 is fairly common. The connection diagram for this type of speed control is shown in fig1.2.

Field rheostat

V

Fig.1.2 Flux Control Method


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1.4.2 ARMATURE OR RHEOSTAT CONTROL METHOD:

Controller Resistance Field

V Ia

Armature in

Ristence in

Speed,N

armature

Armature current, Ia

Fig 1.3

Rheostat Control Method and Characteristics


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Speed control of DC motor by using PWM technique This method is used when speeds below the no load speed are required. As the supply voltage is normally constant, the voltage across the armature is varied by inserting a variable rheostat or controller resistance in series with the armature circuit as shown in fig1.3 as controller resistance is increased, potential difference across the armature is decreased, thereby decreasing the armature speed. For a load of constant torque, speed is approximately proportional to the potential difference. Across the armature current characteristics in fig. in seen that greater the resistance In the armature circuit, greater is the fall in speed Let Ia1

= Armature current in the first case

Ia2

=

N1, N2 = V

=

Armature current in the second case corresponding speeds Supply voltage

Then N1 (v-Ia1Ra )αEb1 Let some controller resistance of value R be added to the armature circuit resistance so that its value becomes (R+Ra) = Rt Then,

N2 α (V-Ia2 Rt) α Eb2 N2/N1=Eb2/Eb1

Considering no load speed, we have N/N0 (I-(Ia Rt)/ (V-Ia0 Ra) Neglecting Iao Ra w.r.t.toV, we get N=No (I-(Ia Rt)/ V Veermata Jijabai Technological Institute

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No

Speed, N

Ia

Im

FIG.1.4 SPEED vs ARMATURE CURRENT CHARACTERISTICS It is seen that for a given resistance Rt the speed is a linear function of armature current Ia as shown in fig.1.4 The load current for which the speed would be zero is found by putting N=0 in above relation 0 = N0 ((I-Ia Rt)/V) Or Ia = V/Rt This maximum current and is known as stalling current. This method is very wasteful, expensive and unsuitable for rapidly changing loads because for a given value of Rt, speed will change with load. A more stable operation can be obtained by using a diverter across the armature in addition to armature control resistance. Now, the changes in armature current will not be so effective in changing the potential difference across the armature. The connection diagram for this type of speed control arrangement is shown in fig.


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1.4.3 VOLTAGE CONTROL METHOD: A) MULTIPLE CONTROL VOLTAGE : In this method, the shunt field of the motor is connected permanently to a fixed exciting voltage but the armature is supplied with different voltages by connecting it across one at the several different voltages by means of suitable switchgear. The armature will be approximately proportional to these different voltages. The intermediate speeds can be obtained by adjusting the shunt field regulator.

B) WARD-LEONARD SYSTEM: This system is used where an unusually wide (upto 10:1) and very sensitive speed control is required as for colliery winders , electric excavators and the main drives in steel mills and blooming in paper mills. The field of the motor (M1) is permanently connected across the DC supply lines whose speed control can be done. The other motor M2 is directly connected to Generator G.

Fig. STRUCTURAL ARRANGEMENT OF WARD LEONARD SYSTEM The output voltage of G is directly is fed to the main motor M1. The voltage of generator can be varied from zero to upto its maximum value by means of field regulator.


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Speed control of DC motor by using PWM technique By reversing the direction of the field current of G by means of the reversing switch which RS, generated voltage can be reversed and hence the direction of rotation of M1. It should be remembered that motor set always runs in the same direction. The addition of a flywheel whose function is to reduce fluctuations in the Power demand from the supply circuit . The chief advantage of system is its overall efficiency especially at right loads. It has the outstanding merit of giving wide speed Control from maximum in one direction through zero to the maximum in the opposite direction and of giving a smooth acceleration.

1.5 MOTOR APPLICATIONS: DC motor possesses excellent torque speed characteristics and offer a wide range of speed control. Though efforts are being made to obtain wide range speed control with ac motors, yet the versatility and flexibility of a dc motors can’t be matched by a ac motors. In view of this, the demand for dc motors would continue undiminished even in figure. A brief discussion regarding the dc motor applications is given below.

1.5.1 SHUNT MOTORS: 

It is the type generally used in commercial practice and is usually recommended where starting conditions are not usually severs. Speed of the shunt motors may be regulated in two ways: first, by inserting resistance in series with the armature, thus decreasing speed: and second, by inserting resistance in the field circuit, the speed will vary with each change in load: in the latter, the speed is practically constant for any setting of the controller. This latter is the most generally used for between synchronous motors and dc shunt motors. It is because the construction of high performance poly phase induction motor with large number of poles is difficult. However, for adjustable speed service at low operating speed, dc shunt motor is a preferred choice


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Speed control of DC motor by using PWM technique 

When the driven load requires a wide range of speed control (both below base speed and above base speed), a dc shunt motor is employed, e.g. .in latches etc.

1.5.2 SERIES MOTORS The main feature of series motor is the automatic decrease in speed as soon as increased load torque is required. The decreasing speed with increase in load torque or vice versa has only a marginal effect on the power taken by the series motor. 

Since a series motor can withstand severe starting duties and can furnish high starting torques , it is best suited for driving hoists, trains , excavators ,cranes, etc. wound motor induction motors compete favorably with series motor’s ,but the choice is governed by the economics . However for traction purposes , series motor is the only choice. Therefore series motors are widely used in all types of electric vehicles, eletrictrains, streetcars, battery powered tools, automotive starter motors etc.



Speed regulation in the series motor is quite poor. With the increase in the load speed of the machine decreases. (DC shunt motor maintains almost constant speed from no load to full load)..

1.5.3 COMPOUND MOTORS A compound motor with a strong series field has its characteristics approaching that of a series motor. Therefore such type of compound motors are used for loads requiring heavy starting torque which are likely to be reduced to zero A compound motor with weak series field has its characteristics approaching that of a shunt motor. Weak series field causes more drooping speed torque characteristics than with an ordinary shunt motors. Such compound motors with steeper characteristics, are used where load fluctuates between wide limits intermittently.


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2. SWITCHING DEVICES AND PWM TECHNIQUES


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2. SWITCHING DEVICES AND PWM TECHNIQUE

2.1 POWER SEMICONDUCTOR DEVICES CLASSIFICATION:

Power semiconductor devices

2 Terminal devices

PN Diode

3 Terminal devices

Schotkey diode

Power MOSFET

JFET

Thyristor

IGBT

BJT

Fig.2.1. Classification of Switching Devices

Today’s power semiconductor devices are almost exclusively based on silicon material and can be classified as follows: 

Diode



Thyristor or silicon-controlled rectifier (SCR)



Bipolar Junction Transistor(BJT)



Power MOSFET



IGBT


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2.2 DIODE Power diodes provide uncontrolled rectification of power and are used in applications such as electroplating, anodizing, battery charging, welding, power supplies (dc and ac), and variable frequency drives. They are also used in feedback and the freewheeling functions of converters and snubbers. Fig 2.2 shows the diode symbol and its volt-ampere characteristics. In the forward biased condition, the diode can be represented by a junction offset drop and a series-equivalent resistance that gives a positive slope in the V-I characteristics. The typical forward conduction drop is 1.0 V. This drop will cause conduction loss, and the device must be cooled by the appropriate heat sink to limit the junction temperature. In the reverse-biased condition, a small leakage current flows due to minority carriers, which gradually increase with voltage. If the reverse voltage exceeds a threshold value, called the breakdown voltage, the device goes through avalanche breakdown, which is when reverse current becomes large and the diode is destroyed by heating due to large power dissipation in the junction.

Fig.2.2.Symbol & Characteristics of Diode Veermata Jijabai Technological Institute

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2.3 THYRISTORS: Thyristors or silicon-controlled rectifiers (SCRs) have been the traditional workhorses for bulk power conversion and control in industry. The modern era of solidstate power electronics started due to the introduction of this device in the late 1950s. Basically, it is a trigger into conduction device that can be turned on by positive gate current pulse but once the device is on, a negative gate pulse cannot turn it off. The device turn on process is very fast and turn off process is slow because the minority carriers are to be cleared from the inner junctions by “recovery and recombination” processes Commercial thyristors can be classified as phase control and inverter types. The thyristors have been widely used in dc and ac drives, lighting, heating and welding control.

Fig.2.3. Thyristor symbol and V-I characteristics


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2.4 BIPOLAR POWER OR JUNCTION TRANSISTORS (BPTS OR BJTS) A bipolar junction transistor (BJT), unlike a thyristor-like device, is a twojunction, self-controlled device where the collector current is under the control of the base drive current. Bipolar junction transistors have recently been ousted by IGBTs (Insulated Gate Bipolar Transistors) in the higher end and by power MOSFETs in the lower end. The dc current gain of a power transistor is low and varies widely with collector current and temperature. The gain is increased to a high value in the Darlington connection, as shown in Fig2.4 However, the disadvantages are higher leakage current, higher conduction drop, and reduced switching frequency. The shunt resistances and diode in the base-emitter circuit help to reduce collector leakage current and establish base bias voltages. A transistor can block voltage in the forward direction only (asymmetric blocking). The feedback diode, as shown, is an essential element for chopper and voltage-fed converter applications. Double or triple Darlington transistors are available in module form with matched parallel devices for higher power rating. Power transistors have an important property known as the second breakdown effect. This is in contrast to the avalanche breakdown effect of a junction, which is also known as first breakdown effect. When the collector current is switched on by the base drive, it tends to crowd on the base-emitter junction periphery, thus constricting the collector current in a narrow area of the reverse-biased collector junction. This tends to create a hot spot and the junction fails by thermal runaway, which is known as second breakdown. The rise in junction temperature at the hot spot accentuates the current concentration owing to the negative temperature coefficient of the drop, and this regeneration effect causes collapse of the collector voltage, thus destroying the device.


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Fig.2.4. Two stage Darlington transistor with bypass diode

2.5 POWER MOSFETS: Unlike the devices discussed so far, a power MOSFET (metal-oxide semiconductor field effect transistor) is a unipolar, majority carrier, “zero junctions,” voltage-controlled device. Fig 2.5 shows the symbol of an N-type MOSFET and Fig.2.6 shows its volt-ampere characteristics. If the gate voltage is positive and beyond a threshold value, an N-type conducting channel will be induced that will permit current flow by majority carrier (electrons) between the drain and the source. Although the gate impedance is extremely high at steady state, the effective gate-source capacitance will demand a pulse current during turn-on and turn-off. The device has asymmetric voltageblocking capability, and has an integral body diode, as shown, which can carry full current in the reverse direction. The diode is characterized by slow recovery and is often bypassed by an external fast-recovery diode in high-frequency applications.


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Fig.2.5.Power MOSFET Symbol

Fig.2.6. V-I characteristics of power MOSFET


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2.6 IGBT: The Insulated Gate Bipolar Transistor (IGBT) is a minority-carrier device with high input impedance and large bipolar current-carrying capability. Many designers view IGBT as a device with MOS input characteristics and bipolar output characteristic that is a voltage-controlled bipolar device. To make use of the advantages of both Power MOSFET and BJT, the IGBT has been introduced. It’s a functional integration of Power MOSFET and BJT devices in monolithic form. It combines the best attributes of both to achieve optimal device characteristics. The IGBT is suitable for many applications in power electronics, especially in Pulse Width Modulated (PWM) servo and three-phase drives requiring high dynamic range control and low noise. It also can be used in Uninterruptible Power Supplies (UPS), Switched-Mode Power Supplies (SMPS), and other power circuits requiring high switch repetition rates. IGBT improves dynamic performance and efficiency and reduced the level of audible noise. It is equally suitable in resonant-mode converter circuits. Optimized IGBT is available for both low conduction loss and low switching loss. The main advantages of IGBT over a Power MOSFET and a BJT are:

1. It has a very low on-state voltage drop due to conductivity modulation and has superior on-state current density. So smaller chip size is possible and the cost can be reduced.

2. Low driving power and a simple drive circuit due to the input MOS gate structure. It can be easily controlled as compared to current controlled devices (thyristor, BJT) in high voltage and high current applications.

3. Wide SOA. It has superior current conduction capability compared with the bipolar transistor. It also has excellent forward and reverse blocking capabilities.


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Speed control of DC motor by using PWM technique The main drawbacks are: 1. Switching speed is inferior to that of a Power MOSFET and superior to that of a BJT. The collector current tailing due to the minority carrier causes the turnoff speed to be slow. 2. There is a possibility of latchup due to the internal PNPN thyristor structure. The IGBT is suitable for scaling up the blocking voltage capability. In case of Power MOSFET, the on-resistance increases sharply with the breakdown voltage due to an increase in the resistively and thickness of the drift region required to support the high operating voltage. For this reason, the development of high current Power MOSFET with high-blocking voltage rating is normally avoided. In contrast, for the IGBT, the drift region resistance is drastically reduced by the high concentration of injected minority carriers during on-state current conduction. The forward drop from the drift region becomes dependent upon its thickness and independent of its original resistivity.

2.7 PWM TECHNIQUE: Pulse-width modulation (PWM) or duty-cycle variation methods are commonly used in speed control of DC motors. The duty cycle is defined as the percentage of digital ‘high’ to digital ‘low’ plus digital ‘high’ pulse-width during a PWM period.

Fig.2.7 shows the 5V pulses with 0% through 100% duty cycle. The average DC Voltage value for 0% duty cycle is zero; with 20% duty cycle the average value is 1.2V (20% of 5V). With 50% duty cycle the average value is 2.5V, and if the duty cycle is 80%, the average voltage is 4V and so on. The maximum duty cycle can be 100%, which is equivalent to a DC waveform. Thus by varying the pulse-width, we can vary the average voltage across a DC motor and hence its speed.


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Fig.2.7. 5V Pulses With 0% Through 100% Duty Cycle The average voltage is given by the following equation: ý = D. Ymax + (1- D) Ymin But usually minimum equals zero so the average voltage will be: ý = D. Ymax The circuit of a simple speed controller for a mini DC motor, such as that used in tape recorders and toys, is shown in Fig2.8

Fig.2.8. DC motor speed control using PWM method Veermata Jijabai Technological Institute

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Speed control of DC motor by using PWM technique The major reason for using pulse width modulation in DC motor control is to avoid the excessive heat dissipation in linear power amplifiers. The heat dissipation problem often results in large heat sinks and sometimes forced cooling. PWM amplifiers greatly reduce this problem because of their much higher power conversion efficiency. Moreover the input signal to the PWM driver may be directly derived from any digital system without the need for any D/A converters. The PWM power amplifier is not without disadvantages. The desired signal is not translated to a voltage amplitude but rather the time duration (or duty cycle) of a pulse. This is obviously not a linear operation. But with a few assumptions, which are usually valid in motor control, the PWM may be approximated as being linear (i.e., a pure gain).The linear model of the PWM amplifier is based on the average voltage being equal to the integral of the voltage waveform. Thus VS * Ton = Veq * T Where VS = the supply voltage (+12 volts) Ton = Pulse duration Veq = the average or equivalent voltage seen by the motor T = Switching period (1/f) The recommended switching frequency is 300Hz. The switching frequency (1/T), is determined by the motor and amplifier characteristics. The control variable is the duty cycle which is Ton / T. The duty cycle must be recalculated at each sampling time. The voltage that the motor sees is thus Veq, which is equal to the duty cycle times the supply voltage


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2.8 Principle Pulse width modulation control works by switching the power supplied to the motor on and off very rapidly. The DC voltage is converted to a square wave signal, alternating between fully on (nearly 12v) and zero, giving the motor a series of power “kicks”. Pulse width modulation technique (PWM) is a technique for speed control which can overcome the problem of poor starting performance of a motor. PWM for motor speed control works in a very similar way. Instead of supplying a varying voltage to a motor, it is supplied with a fixed voltage value (such as 12v) which starts it spinning immediately. The voltage is then removed and the motor ‘coasts’. By continuing this voltage on/off cycle with a varying duty cycle, the motor speed can be controlled. The wave forms in the below figure to explain the way in which this method of control operates. In each case the signal has maximum and minimum voltages of 12v and 0v.

  

In wave form, the signal has a mark space ratio of 1:1.with the signal at 12v for 50% of the time, the average voltage is 6v, so the motor runs at half its maximum speed. In wave form, the signal has mark space ratio of 3:1.which means that the output is at 12v for 75% of the time. This clearly gives an average output voltage of 9v, so the motor runs at 3/ 4 of its maximum speed. In wave form, the signal has mark space ratio is 1:3, giving an output signal that is 12v for just 25% o the time. The average output voltage of this signal is just 3v, so the motor runs at 1/4 of its maximum speed.

By varying the mark space ratio of the signal over the full range, it is possible to obtain any desired average output voltage from 0v to12v .The motor will work perfectly well, provided that the frequency of the pulsed signal is set correctly, a suitable frequency being 30Hz.setting the frequency too low gives jerky operation. And setting it too high might increase the motor’s impedance.


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1:1 Mark space ratio (50% duty cycle)

3:1 Mark space ratio (75% duty cycle)

1:3Mark space ratio (25%dutycycle)

Fig.2.9. Pulse Width Modulation Waveforms

2.9 METHODS The pwm signals can be generated in a number of ways. there are several methods:   

analogue method digital method discrete IC


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Speed control of DC motor by using PWM technique Analogue method: A block diagram of an analogue PWM generator is

Triangle wave generator from radio comparator

control receiver

PWM

Receiver signal to demand signal converter

Fig.2.10. Block Diagram of an Analogue PWM Generator The simplest way to generate a PWM signal is the intersective method, which requires only a saw tooth or a triangle wave form (easily generated using a simple oscillator) and a comparator. When the value of the reference signal is more than the modulation wave form, the PWM signal is in the high state, otherwise it is in the low state. Digital Method: The digital method involves incrementing a counter, an comparing the counter value with a pre-loaded register value, or value set by an ADC. They normally use a counter that increments periodically and is reset at the end very period of the PWM. When the counter value is more than the reference value, the PWM output will change state from high to low. PWM generator chips: There are several IC’s available which converts a DC level into a PWM output. Many of these are designed for use in switch mo power supplies .unfortunately, the devices designed for switch mode power supplies not to allow the mark-space ratio to alter over the entire 0 – 100% range. Many limit the maximum to 90% which is effectively limiting the power you can send to the motors. Veermata Jijabai Technological Institute

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3. COMPONENTS DESCRIPTION


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3. COMPONENTS DESCRIPTION

3.1 INTRODUCTION: The main aim of the dc motor speed control using pwm is after power on the power supply generates +5v dc.The logic section works on +5v dc and the IGBT triggering sections are working on +30v dc. The Motor is driven on +220v dc.

3.2 POWER SUPPLY:

The Power Supply is a Primary requirement for the project work. The required DC power supply is 0-30 volt & 0-5 volt can be available from the DC Power supply else can be rectify by means of rectifier circuit through diode or thyristors.


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Components

Ratings

Quantity

Use

Resistors

1K

1

It

0.5K

1

charging

0.01mf

1

discharging time

0.1mf

2

of

1mf

1

and hence can

10mf

1

change

the

100mf

1

frequency

by

PIV 75V

2

changing

the

Capacitors

Diodes(1N4148)

decides

a

and

capacitor

capacitor Power diode

10A

1

Used to flywheel the

inductive

load seen across the IGBT Tachometer

3000rpm

1

To measure the speed of motor

Voltmeter

0-500V(DC)

1

To

measure

Armature voltage Ammeter

0-5A(DC)

2

To

measure

winding current

Table:3.1 Circuit components


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3.3) 555 timer ic The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation, and oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-flop element. Derivatives provide up to four timing circuits in one package.

Fig 3.1 Pin diagram of 555 IC


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Pins

Name

Purpose

1

GND

Ground reference voltage, low level (0 V)

2

TRIG

The OUT pin goes high and a timing interval starts when this input falls below 1/2 of CTRL voltage (which is typically 1/3 VCC, CTRL being 2/3 VCC by default if CTRL is left open)

3

OUT

This output is driven to approximately 1.7 V below +VCC, or to GND.

4

RESET

A timing interval may be reset by driving this input to GND, but the timing does not begin again until RESET rises above approximately 0.7 volts. Overrides TRIG which overrides THR

5

CNTL

Provides "control" access to the internal voltage divider (by default, 2/3 VCC)

6

THR

The timing (OUT high) interval ends when the voltage at THR ("threshold") is greater than that at CTRL (2/3 VCC if CTRL is open)

7

DIS

Open collector output which may discharge a capacitor between intervals. In phase with output.

8

OUT

Positive supply voltage, which is usually between 3 and 15 V depending on the variation

Table:3.2 Pin description


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Modes The IC 555 has three operating modes

• Bistable

Fig:3.1 Schematic diagram of a 555 in bistable mode In bistable (also called Schmitt trigger) mode, the 555 timer acts as a basic flipflop. The trigger and reset inputs (pins 2 and 4 respectively on a 555) are held high via pull-up resistors while the threshold input (pin 6) is simply floating. Thus configured, pulling the trigger momentarily to ground acts as a 'set' and transitions the output pin (pin 3) to Vcc (high state). Pulling the reset input to ground acts as a 'reset' and transitions the output pin to ground (low state). No timing capacitors are required in a bistable configuration. Pin 5 (control voltage) is connected to ground via a small-value capacitor (usually 0.01 to 0.1 μF). Pin 7 (discharge) is left floating.


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2) Monostable

Fig:3.2 Schematic of a 555 in monostable mode and operation waveforms The output pulse ends when the voltage on the capacitor equals 2/3 of the supply voltage. The output pulse width can be lengthened or shortened to the need of the specific application by adjusting the values of R and C. The output pulse width of time t, which is the time it takes to charge C to 2/3 of the supply voltage, is given by


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Speed control of DC motor by using PWM technique Where t is in seconds, R is in ohms (resistance) and C is in farads (capacitance). While using the timer IC in monostable mode, the main disadvantage is that the time span between any two triggering pulses must be greater than the RC time constant. Conversely, ignoring closely spaced pulses is done by setting the RC time constant to be larger than the span between spurious triggers.

• Astable

Fig:3.4 Schematic of a 555 in astable mode In astable mode, the 555 timer puts out a continuous stream of rectangular pulses having a specified frequency. Resistor R1 is connected between VCC and the discharge pin (pin 7) and another resistor (R2) is connected between the discharge pin (pin 7), and the trigger (pin 2) and threshold (pin 6) pins that share a common node. Hence the capacitor is charged through R1 and R2, and discharged only through R2, since pin 7 has low impedance to ground during output low intervals of the cycle, therefore discharging the capacitor. In the astable mode, the frequency of the pulse stream depends on the values of R 1, R2 and C:


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The high time from each pulse is given by:

and the low time from each pulse is given by:

where R1 and R2 are the values of the resistors in ohms and C is the value of the capacitor in farads.

The power capability of R1 must be greater than Particularly with bipolar 555s, low values of

. must be avoided so that the output stays

saturated near zero volts during discharge, as assumed by the above equation. Otherwise the output low time will be greater than calculated above. The first cycle will take appreciably longer than the calculated time, as the capacitor must charge from 0V to 2/3 of VCC from power-up, but only from 1/3 of VCC to 2/3 of VCC on subsequent cycles. To have an output high time shorter than the low time (i.e., a duty cycle less than 50%) a small diode (that is fast enough for the application) can be placed in parallel with R2, with the cathode on the capacitor side. This bypasses R2 during the high part of the cycle so that the high interval depends only on R1 and C, with an adjustment based the voltage drop across the diode. The voltage drop across the diode slows charging on the capacitor so that the high time is a longer than the expected and often-cited ln(2)*R1C = 0.693 R1C. The low time will be the same as above, 0.693 R1C. With the bypass diode, the high time is


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Speed control of DC motor by using PWM technique where Vdiode is when the diode's "on" current is 1/2 of Vcc/R1 which can be determined from its datasheet or by testing. As an extreme example, when Vcc= 5 and Vdiode= 0.7, high time = 1.00 R1C which is 45% longer than the "expected" 0.693 R1C. At the other extreme, when Vcc= 15 and Vdiode= 0.3, the high time = 0.725 R1C which is closer to the expected 0.693 R1C. The equation reduces to the expected 0.693 R1C if Vdiode= 0. The operation of RESET in this mode is not well-defined. Some manufacturers' parts will hold the output state to what it was when RESET is taken low, others will send the output either high or low. The astable configuration, with two resistors, cannot produce a 50% duty cycle. To produce a 50% duty cycle, eliminate R1, disconnect pin 7 and connect the supply end of R2 to pin 3, the output pin. This circuit is similar to using an inverter gate as an oscillator, but with fewer components than the astable configuration, and a much higher power output than a TTL or CMOS gate. The duty cycle for either the 555 or inverter-gate timer will not be precisely 50% due to the fact the timing network is supplied from the devices output pin, which has different internal resistances depending on whether it is in the high or low state (high side drivers tend to be more resistive).

Specifications Supply voltage (VCC)

4.5 to 15 V

Supply current (VCC = +5 V)

3 to 6 mA

Supply current (VCC = +15 V)

10 to 15 mA

Output current (maximum)

200 mA

Maximum Power dissipation

600 mW

Power

consumption

(minimum 30 mW@5V, 225 mW@15V

operating) Operating temperature


0 to 75 °C

37


HCNW 3120 (Optocoupler IC)

Description: The HCNW3120 contain an AlGaAs LED. The LED is optically coupled to an integrated circuit with a power output stage. These optocouplers are ideally suited for driving power IGBTs and MOSFETs used in motor control inverter applications. The high operating voltage range of the output stage provides the drive voltages required by gate controlled devices. The voltage and current supplied by these optocouplers make them ideally suited for directly driving IGBTs with ratings up to 1200V/100A. For IGBTs with higher ratings, the HCNW3120 series can be used to drive a discrete power stage which drives the IGBT gate. The HCNW3120 has the highest insulation voltage of VIORM=1414Vpeak



Features 

2.5 A maximum peak output current



2.0 A minimum peak output current



25 kV/μs minimum Common Mode Rejection (CMR) at VCM= 1500 V



0.5 V maximum low level output voltage (VOL) Eliminates need for negative gate drive



ICC = 5 mA maximum supply current



Under Voltage Lock-Out protection (UVLO) with hysteresis



Wide operating VCC range: 15 to 30 Volts



500 ns maximum switching speeds



Industrial temperature range: –40 °C to 100 °C



Safety Approval: UL Recognized — 5000 Vrms for 1 min. for HCNW3120


38


Applications



IGBT/MOSFET gate drive



AC/Brushless DC motor drives



Industrial inverters



Switch mode power supplies

Functional Diagram:

Fig:3.5 Pin diagram of HCNW3120

Recommended Operating Conditions: Parameter`

Symbol

Min.

Max.

Units

Power supply voltage Input current (ON) Input Voltage(Off) Operating temperature

(VCC – VEE)

15

30

Volts

IF(ON)

10

16

mA

VF(OFF)

-3.6

0.8

Volts

TA

-40

100

°C

Table 3.3


39


Electrical specifications: Parameters

Symbol

Min.

High level output current

IOH

0.5

A

Low level output current

IOL

0.5

A

High level output voltage

VOH

0.5

V

Low level output voltage

VOL

0.5

V

High level supply current

ICCH

5.0

A

Low level supply current

ICCL

5.0

A

Input Forward Voltage

VF

1.95

V

Temperature Coefficient

V

of Forward voltage Input Reverse Breakdown

1.2

Max.

Units

mV/C

F/TA

BVR

5

V

Voltage Input Capacitance

CIN

pF

Table 3.4


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3.5 IGBT HGTG12N60A4D This is the IGBT made by FAIRCHILD SEMICONDUCTOR INDUSTRIES has rating as specified below: Features • Greater than 100kHz Operation for 390V, 12A • 200kHz Operation for 390V, 9A • 600V Switching SOA(Switching Operating Area) Capability • Typical Fall Time. For 70ns at TJ(Junction Temperature) = 1250C • Low Conduction Loss

600V, SMPS Series N-Channel IGBT with Anti-Parallel Hyperfast Diode The HGTG12N60A4D is MOS gated high voltage switching device combining the best features of MOSFETs and bipolar transistors. These devices have the high input impedance of a MOSFET and the low on-state conduction loss of a bipolar transistor. The much lower on-state voltage drop varies only moderately between 25oC and 150oC. The IGBT used is the development type TA49335. The diode used in anti-parallel is the development type TA49371. This IGBT is ideal for many high voltage switching applications operating at high frequencies where low conduction losses are essential. This device has been optimized for high frequency switch mode power supplies.


41


Table no.3.5 Electrical specifications of IGBT HGTG12N60A4D.


42



43


4. BLOCK DIAGRAM AND CIRCUIT DIAGRAM


44


4.1 BLOCK DIAGRAM:

Fig.4.1.Block Diagram of D.C Motor Speed Control Using PWM


45


4.2 CIRCUIT DIAGRAM:

Fig.4.2 Circuit Diagram of D.C Motor Speed Control Using PWM


46


4.3 EXPLANATION: For the operation of very first component of the project kit i.e. 555 IC we need to have a dc supply of 4.5-15V. We used 5V in our operation. And then to get astable operation though 555 timer IC we will setup 555 IC and combination of resistors and capacitors as shown in fig. the connections are already shown in fig4.2. for astable mode. We change the resistor R2 by Variable pot of 10KΩ so we can get variable duty cycle. In real operation we have got 9-99% duty cycle variation by moving position of pot. One more change of capacitor jumper is there we can select the frequency of range 1Hz10Hz.To change the frequency select the appropriate capacitor and short it through the jumper. The connection of jumper is at one of the fixed terminal of pot so we can introduce this frequency in the input. At output we will get perfect square wave with variable duty cycle. We can check this through CRO. And the average voltage depends upon the variation of duty cycle like if given that for 5V input DC supply the duty cycle of 20% then average voltage will be 20% of 5V means 1V. HCWN3120 is 8 pin optocoupler IC which has input fed bay 555 IC through a current limiting resistor. The main function of this optocoupler IC is to increase the value of generated PWM wave which is suitable to trigger the IGBT base. This IC has to be supplied externally by another DC voltage regulator. Its operating range of input voltage is 15-30 V DC. Its characteristics already mention in part 3.5 and output is given by pin 7 The most important component of the circuit is IGBT(Insulated Gate Bipolar Transistor ) has three pins –Gate, Collector, Emitter and its look like voltage regulator IC. The IGBT HGTG12N60A4D which is used in the circuit has gate normally trigger to 15V DC which will be given by HCWN IC. 220V dc supply will be seen across collector and emitter terminal. The connections are shown in fig.4.2 and here the field winding of Dc shunt motor connected to fixed DC supply voltage while armature is placed in series with the IGBT. We also used flywheel diode of rating 10A to eliminate the effect of inductive load of motor which may damage to the IGBT.


47

Speed control of DC motor by using PWM technique Now when the positive voltage of PWM wave is passed through this IGBT base it conducts and for that duration of time collector and emitter get shorted which means complete the circuit of DC motor to the supply for that duration which we called t on time. Once waveform goes to zero base will not trigger hence collector and emitter remains open for toff time and motor has no supply for this duration. SO now we can change capacitor to change the frequency of PWM and hence fast switching can be available. If we look form start when we have done all the connections properly and given the rated supply to all components and motor, we will set pot initial position as zero and then gradually vary the speed by changing position of pot upto rated speed.


48


4.3 Calculations: • Losses in DC Machine As we know “Energy neither can be created nor it can be destroyed, it can only be transferred from one form to another”. In DC machine, mechanical energy is converted into the electrical energy. During this process, the total input power is not transformed into output power. Some part of input power gets wasted in various forms. The form of this loss may vary from machine to machine. These losses give in rise in temperature of machine and reduce the efficiency of the machine. In DC Machine, there are broadly four main categories of energy loss. • Copper Losses or Electrical Losses in DC Machine The copper losses are the winding losses taking place during the current flowing through the winding. These losses occur due to the resistance in the winding. In DC machine, there are only two winding, armature and field winding. Thus copper losses categories in three parts; armature loss, field winding loss, and brush contact resistance loss. The copper losses are proportional to square of the current flowing through the winding.  2.1Armature Copper Loss in DC Machine Armature copper loss = Ia2Ra Where, Ia is armature current and Ra is armature resistance. These losses are about 30% of the total losses.  Field Winding Copper Loss in DC Machine Field winding copper loss = If2Rf Where, If is field current and Rf is field resistance. These losses are about 25% theoretically, but practically it is constant. Veermata Jijabai Technological Institute

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• Core Losses or Iron Losses in DC Machine As iron core of the armature is rotating in magnetic field, some losses occurred in the core which is called core losses. Normally, machines are operated with constant speed, so these losses are almost constant. These losses are categorized in two form; Hysteresis loss and Eddy current loss.  Hysteresis Loss in DC Machine Hysteresis losses occur in the armature winding due to reversal of magnetization of the core. When the core of the armature exposed to magnetic field, it undergoes one complete rotation of magnetic reversal. The portion of armature which is under S-pole, after completing half electrical revolution, the same piece will be under the N-pole, and the magnetic lines are reversed in order to overturn the magnetism within the core. The constant process of magnetic reversal in the armature, consume some amount of energy which is called hysteresis loss. The percentage of loss depends upon the quality and volume of the iron. The Frequency of Magnetic Reversal

Where, P = Number of poles N = Speed in rpm

Steinmetz Formula The Steinmetz formula is for the calculation of hysteresis loss.


50


Where, η = Steinmetz hysteresis co-efficient Bmax = Maximum flux Density in armature winding F = Frequency of magnetic reversals V = Volume of armature in m3.  3.2 Eddy Current Loss in DC Machine According to Faraday’s law of electromagnetic induction, when an iron core rotates in the magnetic field, an emf is also induced in the core. Similarly, when armature rotates in magnetic field, small amount of emf induced in the core which allows flow of charge in the body due to conductivity of the core. This current is useless for the machine. This loss of current is called eddy current. This loss is almost constant for the DC machines. It could be minimized by selecting the laminated core. •

Mechanical Losses in DC Machine

The losses associated with mechanical friction of the machine are called mechanical losses. These losses occur due to friction in the moving parts of the machine like bearing, brushes etc, and windage losses occurs due to the air inside the rotating coil of the machine. These losses are usually very small about 15% of full load loss.


51


Comparison of usual Rheostatic method and PWM based speed control of DC shunt Motor at no load: Motor Ratings used: Name of the ratings Output power Max. Speed Armature resistance Field winding resistance Rated armature current Rated armature voltage Rated field current

1) rheostatic control method: Total power losses: PT1 = Pa1 + Pk1 Where, Pa1= armature losses= Ia12Ra1 Pin1= input power= V(Ia1+ If1) Pi= iron losses=hysteresis losses+ eddy current losses (consider negligible compare to copper losses)


52


Constant losses (Pk1) =No load power input-armature losses =V (Ia1+ If1) - Ia12Ra1 We will be tested by performing three sets of reading: Position

Armature

Armature

Armature

resistance

voltage

current

Speed

Initial Intermediate Final

Input power (Pin) = Rated current *Rated Voltage (at rated speed) Efficiency= Pin1-PT1/Pin1 *100

2) PWM method: Pa2= armature losses= Ia22Ra2 Pin2= input power= V(Ia2+ If2) Constant losses (Pk2) =No load power input-armature losses =V (Ia2+ If2) - Ia22Ra2


53


Position

Armature

Armature

Armature

resistance

voltage

current

Speed

Initial Intermediate Final

Efficiency= Pin2-PT2/Pin2 *100


54


Future scope: We have used single quadrant chopper operation, but if we use two or four quadrant operation we can have bidirectional speed control features in the circuits. Four Quadrant Chopper A chopper is a static device that converts fixed DC input to a variable DC output voltage directly. A chopper may be thought of as an AC transformer since they behave in an identical manner. It’s also known as DC-to-DC converter. It’s widely used for motor control. It’s also used in regenerative braking. Essentially, a chopper is an electronic switch that is used to interrupt one signal under the control of another.

Fig.4.3. Four Quadrant Chopper Circuit 

The output voltage swings in both directions i.e. from +Vdc to -Vdc. This mode of switching is referred to as PWM with bipolar voltage switching.



The output voltage swings either from zero to +Vdc or zero to -Vdc. This mode of switching is referred to as PWM with unipolar voltage switching.

The four quadrant chopper operates in the four quadrants in the following ways: 

Quadrant I: In the first quadrant, the voltage and current are positive making the power is positive. In this case, the power flows from source to load. In this operation T1is ON, T4 is OFF, T2 is continuously ON and T3 is continuously OFF. T1 and T2 are conducting in this mode.


55


Quadrant 2 : In the second quadrant, the voltage is still positive but the current is negative. Therefore, the power is negative. In this case, the power flows from load to source and this can happen if the load is inductive or back emf source such as a dc motor .Here T1 is OFF, T4 is ON,T2 is continuously ON and T3 is continuously OFF. As the inductor current cannot be changed instantaneously, D4 and T2 will be freewheeling the current.



Quadrant 3 : In the third quadrant both the voltage and current are negative but the power is positive. In this case, the power flows from source to load. In this operation T3is ON, T2 is OFF, T4 is continuously ON and T1 is continuously OFF. T3 and T4 are conducting in this mode.



Quadrant 4 : In the fourth quadrant voltage is negative but current is positive. The power is therefore negative. HereT3 is OFF, T2 is ON,T4 is continuously ON and T1 is continuously OFF . As the inductor current cannot be changed instantaneously, D2 and T4 will be freewheeling the current.

4.5.1 Applications: At the present DC choppers are used in 1) Tramcars Geneva, Switzerland: Vossloh Kiepe supplied 92 chopper modules using modern IGBT technology for the modernization of the 46 six/eight axle tram motorcars belonging to Transports Public Genovis (TPG).

2)Underground Rail Vehicles Philadelphia, PA, USA The Southeastern Pennsylvania Transportation Authority (SEPTA) are restoring their B-IV vehicle fl eet which is now over 25 years old. SEPTA ordered 127 new IGBT chopper traction systems in order to equip the vehicles (that were still using cam shaft control assemblies) with modern drive technology.


56

Speed control of DC motor by using PWM technique 3)Light-rail Vehicles (B Wagon) Bonn The Stadtwerke Bonn GmbH ordered 50 modern IGBT choppers from Vossloh Kiepe for the modernization of the drive train of 25 B100S series light-rail vehicles. This modernization included replacing each of the cam shaft control assemblies with two IGBT choppers. In addition Vossloh Kiepe supplied the corresponding brake resistors and drive controls.

4)Tramcars Mülheim an der Ruhr Verkehrsbetriebe Mühlheim ordered 22 modern choppers for the modernization of the drive equipment for eleven M6 S and/or M8 S series light-rail articulated railcars.

5)EL2 Electric Locomotives Cottbus Deutsche Bahn AG Fahrzeuginstandhaltung Werk Cottbus (DB AG Cottbus) and Vossloh Kiepe worked together in order to modernize 53 EL2 four axle mining electrolocomotives operated by Lausitzer Braunkohle AG (LAUBAG) located in Senftenberg. The IGBT DC choppers as well as all other electrical components were also delivered by Vossloh Kiepe.


57


4. RESULT AND CONCLUSION


58


5. RESULT AND CONCLUSION 5.1 RESULTS: By varying the ohmic pot we have done the speed control DC shunt motor by means of PWM method for triggering the base of controlled device called IGBT. We found out that this is very cheap and efficient speed control method where all components give reliable operation and we have checked it experimentally where the efficiency of rheostatic method is better than the PWM control method.

5.2 CONCLUSION: The dc motor speed is controlled by using power electronic device and the PWM is used which to control the speed of dc motor. The speed pulse train will be based on required input speed. This circuit is useful to operate the dc motors at required speed with very low losses and low cost. The circuit response time is fast. Hence high reliability can be achieved. The designed circuit was tested for various speed inputs satisfactorily. The method already employed in traction system and has a good scope ahead.


59


REFERENCES: 1.Gopal K Dubey “Fundamentals of Electric Drives” Narosa Publishing

House

New Delhi, 1989. 2.Muhammad H. Rashid, ‘‘Power Electronics Circuits, Devices, and Applications,” Prentice Hall, 3rd edition, 2003. 3. Kumara MKSC, Dayananda PRD, Gunatillaka MDPR, Jayawickrama SS, “PC based speed controlling of a dc motor”, A fmal year report University of Moratuwa Illiniaus USA, 2001102. 4. J Nicolai and T Castagnet , “A Flexible Micro controller Based Chopper Driving a Permanent Magnet DC Motor”, The European Power Electronics Application. 1993 5. A Khoei Kh. Hadidi, “MicroProcessor Based Closed- Loop Speed Control System for DC Motor Using Power MOSFET”, 3rd IEEE international conference on Electronics, Circuits and Systems( 1996) vol.2, pp.1247-1250.


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Speed control of DC motor by using PWM technique project report

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