UNIVERSITY OF NAIROBI

DESIGN OF AN INVERTER DRIVE FOR A DC MOTOR

Project Number: PRJ 077

By OWITI EMMANUEL ONDIEGE

F17/8281/2004

Supervisor: MR. S.L OGABA

Examiner: MR. V.M DHARMADHIKARY

Project report submitted in partial fulfillment of the requirement for the award of the degree of

Bachelor of Science in Electrical and Electronic Engineering of the University of Nairobi

20th MAY 2009

Department of Electrical and Information Engineering

ii

ABSTRACT

Today in industries the world over, a large part of electric energy consumption is by electric

motor drives. This brings out the need for the design of electric motor drive control systems with

the main aims being to increase the efficiency, economize power consumption and achieve a

fully controllable variable speed drive system.

The project entailed the study of the conventional methods of dc motor control hence noting their

inefficiencies, then the design and implementation of an inverter drive that can be used to control

the speed of a dc motor.

iii

Table of Contents ABSTRACT .................................................................................................................................... ii

ACKNOWLEDEGMENT ............................................................................................................... v

CHAPTER 1: INTRODUCTION .................................................................................................. 1

1.1 Advantages of Electronic Control ......................................................................................... 1

1.2 Objectives ............................................................................................................................. 2

CHAPTER 2: ELECTRIC MOTORS ........................................................................................... 4

2.1 How a motor works ............................................................................................................... 4

2.2 Direct Current (DC) motors .................................................................................................. 6

2.2.1 Separately excited DC motor ......................................................................................... 8

2.2.2 Self excited DC motor: series motor .............................................................................. 8

2.2.3 Self excited DC motor: shunt motor .............................................................................. 9

2.2.4 DC Compound Motor .................................................................................................. 10

2.3 AC Motors .......................................................................................................................... 11

2.3.1 Synchronous motor ...................................................................................................... 12

2.3.2 Induction motor ............................................................................................................ 13

CHAPTER 3: SPEED CONTROL OF DC MOTORS ............................................................... 15

3.1 SPEED CONTROL OF DC SHUNT MOTORS. ............................................................... 16

3.1.1 Flux control method. .................................................................................................... 16

3.1.2 Armature control method. ............................................................................................ 17

3.1.3 Voltage control method................................................................................................ 18

3.2 SPEED CONTROL OF DC SERIES MOTORS ................................................................ 19

3.2.1 Flux control method. .................................................................................................... 20

3.2.2 Armature resistance control. ........................................................................................ 21

3.2.3 Series-Parallel control method. .................................................................................... 22

iv

CHAPTER 4: FINAL DESIGN .................................................................................................. 24

4.1 THEORY OF THE CIRCUIT ............................................................................................ 24

4.2 COMPONENT DESIGN .................................................................................................... 27

4.2.1 Input rectifier ............................................................................................................... 27

4.2.2 Control circuit .............................................................................................................. 28

4.2.3 DC – AC Converter ..................................................................................................... 30

4.2.4 Rectifier and filter ........................................................................................................ 32

4.3 RESULTS AND ANALYSIS ............................................................................................. 34

CHAPTER 5: CONCLUSION ..................................................................................................... 36

5.1 RECOMMENDATIONS AND FURTHER WORK .......................................................... 36

REFERENCES ............................................................................................................................ 37

APPENDIX : DATA SHEETS .................................................................................................... 38

v

ACKNOWLEDEGMENT

I would like to thank Mr. Ogaba for spending his valuable time in supervising my project and

assisting me in all ways possible in order for me to meet the project requirements, and to Mr.

Dharmadhikary for taking his time to examine my project.

I would also like to thank the technical staff; Mr. Muraba and Mr. Wanyoike for their technical

assistance.

Lastly I would like to sincerely thank my classmates who, not just throughout the project but

throughout the five years, have provided immense support and advice.

1

CHAPTER 1: INTRODUCTION

It is essential to vary the speed of electric drives in different field of applications. Normally in all

process industries, it is desired that the system be set at slow speed in the beginning and then

gradually increased to meet the maximum production rate. An example of a newspaper printing

press may be taken here. Various pages of the newspaper are printed by different stands and then

combined, cut and folded. The entire process is first set at a slow pace so that, the wastage of

newsprint is minimum. Once the process is set, the speed of the entire system is increased to

quicken the production. There can be many processes where variable speed drives are required.

One of the major achievements of power electronic technology in the field of control is the

control of dc motor drives. Because of their tremendous control capabilities, solid state motor

control schemes have almost replaced the conventional electrical control methods.

Because of their manifold advantages such as compactness, fast response, higher efficiency,

more control capabilities, more reliability and less cost, power electronic controlled schemes

have totally dominated the field of control of dc motors.

There are several advantages of electronic control systems as compared to the conventional

methods. They are as follows.

1.1 Advantages of Electronic Control

1. Electronic control systems are very compact and small in size, and thus require

comparatively less space.

2. They require less power.

3. They are fast in response.

4. They are much more accurate and efficient than the conventional methods.

5. System reliability is much more than the usual conventional methods.

6. An electronic control system is economical as compared to other systems as it involves

minimum maintenance cost.

7. An electronic system is highly protective and the devices under control are much safe as

compared to other systems.

2

1.2 Objectives

The objectives of the project were to:

• Do a study of dc motor control by conventional methods.

• Design an inverter drive to control a dc motor.

• Test the designed inverter drive.

A study of the control of dc motors was carried out and was dealt with in chapter 3 of the project

report. The three main methods that were discussed were:

a. Varying the flux per pole. This is known as flux control method.

b. Varying the resistance in the armature circuit. This is known as armature control

method.

c. Varying the applied voltage. This is known as voltage control.

It is seen that these conventional methods, though they formed the background by which the

power electronic control devices are used in control, they are found to be wasteful in terms of the

amount of power consumed.

3

Figure 1.1 Block diagram of the inverter drive

In chapter 4, the final design schematic is shown (figure 4.1). Here an in depth explanation of the

circuit is given with the circuit being segmented to give the reader better understanding. The

circuit could also be represented as shown in the block diagram in figure 1.1.

Control Unit - Rectifier - 555 timer - Voltage Comparator - Optical Coupler

Semiconductor Switching Device (MOSFETs)

Dc output Dc motor

AC MAINS

4

CHAPTER 2: ELECTRIC MOTORS

An electric motor is an electromechanical device that converts electrical energy to mechanical

energy. This mechanical energy is used, for example, in rotating a pump impeller, fan or blower,

driving a compressor, lifting materials etc. Electric motors are used at home (mixer, drill, fan)

and in industry. Electric motors are sometimes called the “work horses” of industry because it is

estimated that motors use about 70% of the total electrical load in industry.

2.1 How a motor works

The general working mechanism is the same for all motors. (Figure 2.1):

An electric current in a magnetic field will experience a force.

If the current carrying wire is bent into a loop, then the two sides of the loop, which are at right

angle to the magnetic field, will experience forces in opposite directions.

The pair of forces creates a turning torque to rotate the coil.

Practical motors have several loops on an armature to provide a more uniform torque and the

magnetic field is produced by electromagnet arrangement called the field coils.

5

Figure 2.1 Basic Principle of how Electric Motors Work

In understanding a motor it is important to understand what a motor load means. Load refers to

the torque output and corresponding speed required. Loads can generally be categorized into

three groups:

• Constant torque loads are those for which the output power requirement may vary with

the speed of operation but the torque does not vary. Conveyors, rotary kilns, and

constant-displacement pumps are typical examples of constant torque loads.

• Variable torque loads are those for which the torque required varies with the speed of

operation. Centrifugal pumps and fans are typical examples of variable torque loads

(torque varies as the square of the speed).

• Constant power loads are those for which the torque requirements typically change

inversely with speed. Machine tools are a typical example of a constant power load.

Components of electric motors vary between different types of motors and are therefore

described for each motor separately.

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7

• Armature. When current goes through the armature, it becomes an electromagnet. The

armature, cylindrical in shape, is linked to a drive shaft in order to drive the load. For the

case of a small DC motor, the armature rotates in the magnetic field established by the

poles, until the north and south poles of the magnets change location with respect to the

armature. Once this happens, the current is reversed to switch the south and north poles

of the armature.

• Commutator. Its purpose is to overturn the direction of the electric current in the

armature. The commutator also aids in the transmission of current between the armature

and the power source.

The main advantage of DC motors is speed control, which does not affect the quality of power

supply. It can be controlled by adjusting:

The armature voltage – increasing the armature voltage will increase the speed

The field current – reducing the field current will increase the speed.

DC motors are available in a wide range of sizes, but their use is generally restricted to a few low

speed, low-to-medium power applications like machine tools and rolling mills because of

problems with mechanical commutation at large sizes. Also, they are restricted for use only in

clean, non-hazardous areas because of the risk of sparking at the brushes. DC motors are also

expensive relative to AC motors.

The relationship between speed, field flux and armature voltage is shown in the following

equation:

Back electromagnetic force: E = KvΦN (2.1)

Torque: T = KtΦIa (2.2)

Where:

E = electromagnetic force developed at armature terminal (volt)

Φ = field flux which is directly proportional to field current

N = speed in RPM (revolutions per minute)

8

T = electromagnetic torque

Ia = armature current

Kv = voltage constant

Kt = torque constant

2.2.1 Separately excited DC motor

In many traction applications where both armature voltage and stator current are needed to

control the speed and torque of the motor from “no load” to “full load”, the separately excited

DC motor is used for it’s high torque capability at low speed achieved by separately generating a

high stator field current and enough armature voltage to produce the required rotor torque

current. As torque decreases and speed increases, the stator field current requirement decreases

and the armature voltage increases. Without any load the speed of the separately excited motor is

limited by the armature voltage and stator field current.

2.2.2 Self excited DC motor: series motor

In the series motor, the field windings, consisting of a relatively few turns of heavy wire, are

connected in series with the armature winding. The same current flows through the field winding

and the armature winding. Any increase in current, therefore, strengthens the magnetism of both

the field and the armature.

Because of the low resistance in the winding, the series motor is able to draw a large current

during starting. This high starting current is what produces a high starting torque, which is the

series motor’s principal advantage.

The speed of a series motor is dependent upon the load. Any change in load is accompanied by a

substantial change in speed. A series motor will run at high speed when it has light load and at

low speed with a heavy load. If the load is removed, the motor may operate at such high speed

that the armature will fly apart. Series motors are suited for applications requiring a high starting

torque, such as cranes and hoists.

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12

2.3.1 Synchronous motor

A synchronous motor is an AC motor, which runs at constant speed fixed by frequency of the

system. It requires direct current (DC) for excitation and has low starting torque, and

synchronous motors are therefore suited for applications that start with a low load, such as air

compressors, frequency changes and motor generators. Synchronous motors are able to improve

the power factor of a system, which is why they are often used in systems that use a lot of

electricity.

The main components of a synchronous motor are:

Rotor. The main difference between the synchronous motor and the induction motor is that the

rotor of the synchronous motor travels at the same speed as the rotating magnetic field. This is

possible because the magnetic field of the rotor is no longer induced. The rotor either has

permanent magnets or DC-excited currents, which are forced to lock into a certain position when

confronted with another magnetic field.

Stator. The stator produces a rotating magnetic field that is proportional to the frequency

supplied.

This motor rotates at a synchronous speed (Ns), which is given by equation (2.3):

Ns = 120 f / P (2.3) Where:

f = frequency of the supply

P= number of poles

13

2.3.2 Induction motor

Induction motors are the most common motors used for various equipments in industry. Their

popularity is due to their simple design, they are inexpensive and easy to maintain, and can be

directly connected to an AC power source.

An induction motor has two main electrical components (Figure 2.6):

1. Rotor : Induction motors use two types of rotors:

A squirrel-cage rotor consists of thick conducting bars embedded in parallel slots. These bars

are short-circuited at both ends by means of short-circuiting rings.

A wound rotor has a three-phase, double-layer, distributed winding. It is wound for as many

poles as the stator. The three phases are wired internally and the other ends are connected to slip-

rings mounted on a shaft with brushes resting on them.

2. Stator. The stator is made up of a number of stampings with slots to carry three-phase

windings. It is wound for a definite number of poles. The windings are geometrically spaced 120

degrees apart.

Figure 2.6 An Induction Motor

14

Induction motors can be classified into two main groups:

Single-phase induction motors. These only have one stator winding, operate with a single-phase

power supply, have a squirrel cage rotor, and require a device to get the motor started. This is by

far the most common type of motor used in household appliances, such as fans, washing

machines and clothes dryers, and for applications for up to 3 to 4 horsepower.

Three-phase induction motors. The rotating magnetic field is produced by the balanced three-

phase supply. These motors have high power capabilities, can have squirrel cage or wound rotors

(although most have a squirrel cage rotor), and are self-starting. It is estimated that about 70% of

motors in industry are of this type, are used in, for example, pumps, compressors, conveyor belts,

heavy-duty electrical networks, and grinders. They are available in 1/3 to hundreds of

horsepower ratings.

Induction motors work as follows. Electricity is supplied to the stator, which generates a

magnetic field. This magnetic field moves at synchronous speed around the rotor, which in turn

induces a current in the rotor. The rotor current produces a second magnetic field, which tries to

oppose the stator magnetic field, and this causes the rotor to rotate.

In practice however, the motor never runs at synchronous speed but at a lower “base speed”. The

difference between these two speeds is the “slip”, which increases with higher loads. Slip only

occurs in all induction motors. To avoid slip, a slip ring can be installed, and these motors are

called “slip ring motors”. The following equation can be used to calculate the percentage slip:

% Slip = 100% (2.4)

Where:

Ns = synchronous speed in RPM

Nb = base speed in RPM

15

CHAPTER 3: SPEED CONTROL OF DC MOTORS

Although a far greater percentage of electric motors in service are ac motors, the dc motor is of

considerable industrial importance. The principal advantage of a dc motor is that its speed can be

changed over a wide range by a variety of simple methods. Such fine speed control is not

possible with ac motors.

In fact fine speed control is one of the reasons for the strong competitive position of dc motors in

modern industrial applications. In this chapter the various methods of speed control of dc motors

are discussed.

The speed (N) of a dc motor is given by:

N α

N = rpm (3.1)

Where R = Ra shunt motor

R = Ra + Rse series motor

Ia – armature current

Eb – back emf

From equation (3.1), it is clear that there are three main methods of controlling the speed of a dc

motor, namely:

i. By varying the flux per pole ( . This is known as flux control method.

ii. By varying the resistance in the armature (Ra) circuit. This is known as armature control

method.

iii. By varying the applied voltage, V. This is known as voltage control method.

16

3.1 SPEED CONTROL OF DC SHUNT MOTORS.

The speed of a shunt motor can be changed by:

i. Flux control method.

ii. Armature control method.

iii. Voltage control method.

The first method i.e. flux control, is frequently used because it is simple and inexpensive.

3.1.1 Flux control method.

It is based on the fact that by varying the flux , the motor speed (N α 1 ) can be changed and

hence flux control method. In this method, a variable resistance (known as shunt field rheostat) is

placed in series with shunt field winding Rsh.

Figure 3.1 Flux control method

The shunt field rheostat reduces the shunt field current Ish and hence the flux . Therefore, we

can only raise the speed of the motor above the normal speed.

17

Advantages

i. This is an easy and convenient method.

ii. It is an inexpensive method since very little power is wasted in the shunt field resistor due

to relatively small value of shunt field current Ish.

Disadvantages

i. Only speeds higher than normal can be obtained since the total field circuit resistance

cannot be reduced below the shunt field winding Rsh.

ii. There is a limit to the maximum speed obtainable by this method. It is because if the flux

is too much weakened, the commutation becomes poorer.

3.1.2 Armature control method.

This is based on the fact that by varying the voltage available across the armature, the back emf

and hence the speed of the motor can be changed. This is done by inserting a variable resistance

i.e. controller resistance Rc,in series with the armature.

Figure 3.2 Armature control method

Due to the voltage drop in the controller resistance, the back emf Eb is decreased. Since N α Eb, the speed of the motor is reduced. The highest speed obtainable is that corresponding to Rc = 0

i.e. normal speed. Hence this method can only provide speeds below the normal speed.

18

Disadvantages.

i. A large amount of power is wasted in the controller resistance since it carries full

armature current Ia.

ii. The speed varies with load since the speed depends upon the voltage drop in the

controller resistance and hence on the armature current demanded by the load.

iii. The output and efficiency of the motor are reduced.

iv. This method results in poor speed regulation.

3.1.3 Voltage control method.

In this method, the voltage source supplying the field current is different from that which

supplies the armature. This method avoids the disadvantages of poor speed regulation and low

efficiency as in armature control method. However, it is quite expensive. Therefore this method

of speed control is employed for large size motors where efficiency is of great importance.

Multiple voltage control.

In this method, the shunt field of the motor is connected permanently across a fixed voltage

source. The armature can be connected across several different voltages through a suitable

switchgear. In this way, voltage applied across the armature can be changed. The speed will be

approximately proportional to the voltage applied across the armature.

Ward-Leonard System.

In this method, the adjustable voltage for the armature is obtained from an adjustable voltage

generator while the field circuit is supplied from a separate source. The armature of the shunt

motor (whose speed is to be controlled) is connected directly to a dc generator driven by a

constant speed ac motor. The field of the shunt motor is supplied from a constant-voltage exciter.

The field of the generator is also supplied from the exciter. The voltage of the generator can be

varied by means of its field regulator.

19

By reversing the field current of the generator by a controller, the voltage applied to the motor is

reversed. Sometimes a field regulator is included in the field circuit of the shunt motor for

additional speed adjustment.

With this method, the motor may be operated at any speed up to its maximum speed.

Advantages.

i. The speed of the motor can be adjusted through a wide range without resistance losses

which results in high efficiency.

ii. The motor can be brought to a standstill quickly, simply by rapidly reducing the voltage

of generator. When the generator voltage is reduced below the back emf of the motor,

this back emf sends current through the generator armature, establishing dynamic

braking. While this takes place, the generator operates as a motor driving the ac motor

which returns power to the line.

iii. This method is used for the speed control of large motors when a dc supply is not

available.

The disadvantage of this method is that a special motor-generator set is required for each motor

and the losses in this set are high if the motor is operating under light loads for long periods.

3.2 SPEED CONTROL OF DC SERIES MOTORS

The speed of dc series motors can be obtained by:

i. Flux control method.

ii. Armature resistance control method.

iii. Series-Parallel control method

20

3.2.1 Flux control method.

In this method, the flux produced by the series motor is varied and hence the speed. The

variation of flux is achieved in the following ways:

Field diverter.

In this method, a variable resistance (field diverter) is connected in parallel with series field

winding. Its effect is to shunt some portion of the line current from the series field winding, thus

weakening the field and increasing the speed. (N α 1 ).

The lowest speed obtainable is that corresponding to zero current in the diverter (i.e. diverter is

open). The lowest speed is hence the normal speed of the motor. Consequently, this method can

only provide speeds above the normal speed. The series field diverter method is often employed

in traction work.

Figure 3.3 Field diverter

Armature diverter.

In order to obtain speeds below the normal speed available, a resistance (called armature

diverter) is connected in parallel with the armature. The diverter shunts some of the line current,

thus reducing the armature current. Now for a given load, if Ia is decreased, the flux Φ must

increase. Since N α1 , the motor speed is decreased. By adjusting the armature diverter, any

speed lower than the normal speed can be obtained.

21

Figure 3.4 Armature diverter

Tapped field control.

In this method, the flux is reduced (and hence speed is increased) by decreasing the number of

turns of the series field winding. The switch S can short circuit any part of the field winding, thus

decreasing the flux and raising the speed. With full turns of the field winding, the motor runs at

normal speed and as the field turns are cut out; speeds higher than normal speed are achieved.

Figure 3.5 Tapped field control

3.2.2 Armature resistance control.

In this method, a variable resistance is directly connected in series with the supply to the

complete motor. This reduces the voltage available across the armature and hence speed falls. By

changing the value of variable resistance, any speed below the normal speed can be obtained.

This is the most common method employed to control the speed of dc series motors.

22

Figure 3.6 Armature resistance control

3.2.3 Series­Parallel control method.

Another method used in the speed control of DC series motors is the series- parallel method. In

this system which is widely used in traction system, two (or more) similar DC series motors are

mechanically coupled to the same load.

Figure 3.7a Motors connected in series Figure 3.7b Motors Connected in parallel

23

When the motors are connected in series (figure 3.7a), each motor armature will receive half the

normal voltage. Therefore the speed will be low. When the motors are connected in parallel

(figure 3.7b), each motor armature receives the normal voltage and the speed is high. Thus two

speeds can be obtained. It can be noted that for the same load on the pair of motors, the system

would run approximately four times the speed when the machines are in parallel as when they

are in series. This is shown in equation (3.2) and (3.3).

When in parallel:

N α α ⁄ α (3.2)

When in series:

N α α α ⁄

(3.3)

24

CHAPTER 4: FINAL DESIGN

The circuit was finally designed to meet the required objectives and specifications.

4.1 THEORY OF THE CIRCUIT

The purpose of the design was to come up with an inverter drive to control a dc motor by

variation of armature voltage. The motor to be controlled having a voltage rating of 110V DC yet

the supply available from the mains being 240V AC.

The inverter drive consists of an astable multi-vibrator (555 timer), that produces a 2 KHz square

wave pulse, a triac optical coupler that allows the pulse from the 555 timer to reach the switching

devices (MOSFETs) only when required. The configuration of the MOSFETs is such that they

switch in an anti-phase design such that when Q4 is ON, Q5 is OFF and vice-versa (see figure

4.1). The MOSFETs are connected to a high frequency centre-tapped transformer that has its

centre tap supplied by a dc voltage that has been rectified from the input ac mains. This produces

an ac voltage at the secondary of the transformer (Tx2). The output is then rectified and supplied

to the dc motor.

This output voltage is sampled and fed-back to the positive terminal of the voltage comparator

that also is connected to resistors R5, R4 and variable resistor R6 (figure 4.1). The negative

terminal of the voltage comparator is kept at a reference voltage (Vref) of 12V that is provided for

by the zener diode (D5) that is rated 12V. The voltage required at the output (Vout) is set by the

variable resistor R6.

This is shown in the equation (4.1), since R4 and R5 are fixed then the variable resistor R6 sets the

required voltage.

Vout = 12V 4 5 6 5 6

(4.1)

Should the voltage at the positive terminal of the comparator be less than the voltage at the

negative terminal (Vref) then the output of the comparator is taken to negative hence reverse

biasing the diode that is within the triac optical coupler IC. This turns ON the triac which allows

the pulse from the 555 timer to pass through.

25

On the positive pulse period, the signal passes through resistor R10 through to the N-P-N

transistor Q1. This then turns ON the P-N-P transistor Q3 allowing the MOSFET Q5 to be turned

ON. The rectified voltage from rectifier consisting of diodes D6, D7, D8 and D9 to the centre tap

of transformer (Tx2) hence now passes through the MOSFET Q5.

On the negative pulse period, the P-N-P transistor Q2 is turned ON this provides a gate signal to

the MOSFET Q4 which turns it ON. This action of switching between Q4 and Q5 produces an ac

voltage at the secondary of the high frequency transformer.

Should there be a change in load or some cause of voltage change such that the voltage at the

positive terminal of the comparator is higher than that at the negative terminal (Vref), then the

comparator swings to +Vcc hence forward biasing the diode that is within the triac optical

coupler. This turns OFF the triac, hence blocks the pulse from the 555 timer. The dc-ac

configuration that consists of the BJTs and MOSFETs is then turned OFF. This causes the

voltage at the output to decrease to the required voltage. Hence once the required voltage has

been set from the variable resistor R6 then the circuit is able to control itself due to the feedback

path provided.

26

Figure 4.1 Final Circuit Design

27

4.2 COMPONENT DESIGN

4.2.1 Input rectifier

The input rectifier consists of diodes D1, D2, D3, D4 and shunt capacitor C1 which converts the ac

voltage from the transformer, Tx1 (240V-15V) to dc voltage.

Figure 4.2 Input rectifier

The ICs (555 timer and comparator) supplied by Tx1 are rated 100mA each so the total current

required = 2 100mA = 200mA.

Tx1 power = Output Voltage * Output Current (4.2)

= 15V 200mA

= 3VA

Diodes D1, D2, D3, and D4 form a full-wave bridge rectifier. The diodes used are determined by

the maximum voltage across each diode.

VP = Vrms√2 (4.3)

= 15√2

= 21.2V

The diodes used should be rated more than 200mA and 21.2V, diodes IN4001 rated 50V and 1A

were used.

28

Capacitor C1 is a shunt capacitor filter. The minimum capacitor that could be used was

determined from the equation (4.4):

Vdc = (4.4)

With VP = 21.2, Idc = 200mA, f = 50Hz. The value of C can be found by substitution which is

114µf. The value used was 470µf with a voltage rating of 35V.

4.2.2 Control circuit

Figure 4.3 Control circuit

The control circuit comprises of an astable multi-vibrator (555 timer) which generates a high

frequency (2 KHz) pulsating signal, a voltage comparator (LM393N) and a triac optical coupler

(MOC3020). The 555 timer is rated 15V maximum.

The values of R1 and R2 were found from the equation (4.5a)

f = .

(4.5a)

with f = 2 KHz

C2 (R1 +2R2) =

29

Setting C2 to be 10nf, then

R1+ 2R2 = 72.5KΩ (4.5b)

To obtain a square wave signal, R2 should be greater than R1. Hence setting R1 to 6.8 KΩ,

substitution in equation (4.5b) gives R2 = 33K Ω.

The data sheet recommends that voltage control terminal be connected to ground via a 10nf

decoupling capacitor C3 = 10nf.

R3 and D5 set a reference voltage. A reference voltage of 12V is set by the zener diode D5. D5 is

BZX79C12 rated Vz = 12V and Iz = 5mA.

R3 = = = 600Ω

Hence R3 design is 680Ω.

R4, R5 and R6 sample the output voltage to the motor. The voltage drop across R6 and R5 at 110V

(this is the rated voltage of the dc motor), should be equal to the reference voltage (12V).

110V 12V (4.6a)

0.11 (4.6b)

To get the maximum voltage output the variable resistor (R6) is set at a minimum (zero) hence

equation (4.6b) becomes:

0.11

R5= 0.11(R4 + R5)

Setting R4 = 470KΩ then R5= 56 KΩ.

To attain a lower voltage, the value of the variable resistor (R6) is increased. At a lower scale, to

obtain a voltage of 50V:

30

50V = 12V (4.7a)

= 0.24 (4.7b)

By substitution R6 = 87KΩ, Potentiometer available = 100KΩ

The triac optical coupler is used to allow the pulse from the 555 timer to the inverter when the

voltage drop across resistors R5 and R6 is less than the reference voltage. The optical coupler

MOC3020 rated If 15mA. Therefore the resistor R7 has a value of 1KΩ.

R7 = (4.8)

= = 1 KΩ

4.2.3 DC – AC Converter

This employs a push-pull configuration having a high frequency coupling transformer Tx2.

Figure 4.4 DC- AC Converter

31

The transformer which is centre tapped at the primary is fed from the input rectifier made up of

diodes D6, D7, D8 and D9. The diodes used were determined by the maximum voltage across

each diode.

VP = 240√2 =339V.

The output current = 9A.

Output power = Output Voltage Output Current

= 110V 9A

= 990VA

Supply current =

= = 4.125A

C4 is the shunt capacitor filter. Its value was determined by substitution in equation (4.4). A

capacitor of 330µf and voltage rating of 470V was used.

The MOSFETs Q4 and Q5 are IRF740 rated 10A and 400V VDS max.

Transistors Q2 and Q3 should be rated more than 100mA. Transistor 2N2907 rated 600mA was

used. Resistors R12 and R16 were used to ground all leakage currents flowing through the

transistors.

R12, R16 = = = 150Ω

R9, R14 =

IB = = = 1mA

Therefore;

R9, R14 = = 15KΩ

32

The current flowing through the base of Q3 is the collector current flowing through Q1. The

transistor C1815 rated 100mA and hfe 100 was used for Q1.

R13 = = = 15KΩ

R10 =

IB = = = 10µA

R10 = = 1.5MΩ

4.2.4 Rectifier and filter

This consists of full-wave bridge rectifier and L-C filter.

Figure 4.5 Output rectifier

33

Diodes D10, D11, D12 and D13 form a full-wave bridge rectifier. The diodes used are determined

by the maximum voltage across each diode. The diodes used were rated 800Vmax. The inductor

L1 and capacitor C5 form the L-C filter. The filter should be such that the ripple factor is 1%.

r = 1.19 LC (4.9)

0.01 = 1.19 LC

LC = 0.0084

Selecting an inductance of 22µH for L,

C = .

= 382µf

Therefore a capacitance of 470µf with a voltage rating of 250V was chosen.

The figure 4.6 shows the constructional details of the inverter drive.

Figure 4.6 inverter drive designed

34

4.3 RESULTS AND ANALYSIS

The inverter drive designed was able to produce dc voltage at the output that could be varied, to

be supplied to the dc motor armature. Hence the control was by armature voltage control.

The different values of dc voltage were provided for by the variable resistor R6 (see figure 4.1).

On the final packaged device the resistor knob was evenly divided in five divisions, i.e. from dial

setting 0 to dial setting 5.

Dial setting 0 represented the minimum resistance i.e. zero hence highest value of voltage at the

output. Dial setting 5 represented the highest value of resistance i.e. 100KΩ hence the lowest

value of voltage available at the output.

The results expected (theoretical) and measured were tabulated in table 4.1.

Table 4.1 Output Voltages Theoretical and Measured

Dial Setting Output Voltage, Vdc

(Theoretical)

Output Voltage, Vdc

(Measured)

0 110.9 125

1 86.2 100

2 70.8 78

3 60.6 66

4 53.5 56

5 48.2 50

The measured voltages were slightly higher than the expected (theoretical) values. This could be

explained due to the fact that on design from calculation, the components such as resistors

calculated were not always the standard values of resistors available. The values of such

components implemented were chosen to the nearest value.

35

The results (table 4.1) show that the circuit implemented could be used as an inverter drive for a

dc motor. This could be used in situations where varying values of dc voltage are required but

what is available from the supply is 240 Vac mains supply. The tabulated results also show that

the inverter drive is capable of providing voltage of between 50Vdc to 125Vdc.

36

CHAPTER 5: CONCLUSION

From the results obtained (table 4.1), the circuit designed was capable of providing speed control

of a dc motor by producing varying dc voltage of between 50V and 125V. This dc voltage being

supplied to the armature winding of the dc motor.

A survey of the control of dc motors by conventional methods was carried out and the three main

methods of control i.e.

• Varying the flux per pole

• Varying the resistance of the armature circuit.

• Varying the applied voltage

were studied as required in the objectives of the project.

The implementation of the project provided valuable knowledge of dc motors in general and how

their control is useful in the industrial field. It also gave a good understanding of the

conventional control concepts used in the control of dc motors.

5.1 RECOMMENDATIONS AND FURTHER WORK

1. Diodes should be added to the existing diodes (D10, D11, D12 and D13) in parallel hence

enable the device handle more current and a motor with a bigger power rating.

2. The circuit could be further improved by implementing one that controls a specific dc

motor, such that the output voltage of the inverter drive and the corresponding speed of

the dc motor monitored. A digital display can hence be provided for showing the voltage

and exact speed of the dc motor.

37

REFERENCES

1. Rudolf F. Graf, Williams Sheets, The Encyclopedia of Electronic Circuits Vol.4, © 1992

McGraw-Hill Professional

2. V.K Mehta, Rohit Mehta, Principles of Electrical Machines, © 2002 S.Chand &

Company Ltd. 7361, Ram Nagar, New Delhi-110 055, Printed by Rajenda Ravindra

Printers (Pvt.) Ltd., 7361, Ram Nagar New Delhi-100 055

3. B. L. Theraja, A Textbook of Electrical Technology, © 2005 S. Chand & Company Ltd.

7361, Ram Nagar, New Delhi-110 055, Printed by Rajenda Ravindra Printers (Pvt.) Ltd.,

7361, Ram Nagar New Delhi-100 055

4. S K Bhattacharya, S Chatterjee, Industrial Electronics & Control, © 1995 Tata McGraw-

Hill

5. Hughes Austin, Electric Motors and Drives: Fundamentals, types and applications, ©

1990 Newnes, Printed by Biddles Ltd

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APPENDIX : DATA SHEETS

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40

41

42

43

44