dc drive with over current protection.pdf

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DC DRIVE WITH OVER CURRENT, OVER VOLTAGE

PROTECTION AND SOFT START

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RRAAJJUU.. KK ((0088224411AA00228877))

SSUURRYYAAKKAANNTTHH.. MM ((0088224411AA0022AA88))

Under the Guidance of

Sri. K.SATISH KUMAR

ASSISTANT PROFESSOR IN DEPARTMENT OF

ELECTRICAL & ELECTRONICS ENGINEERING

JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY

HYDERABAD – 500 085

2012

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CCEERRTTIIFFIICCAATTEE

This is to certify that this project report entitled ―DC DRIVE WITH OVER

CURRENT, OVER VOLTAGE PROTECTION AND SOFT START” that is

being submitted by RAJU. K, MAYUR KARTHIK. K, SURYAKANTH. M in partial

fulfillment for the award of the degree of Bachelor of Technology in Electrical and Electronics

Engineering to the Gokaraju Rangaraju Institute of Engineering and Technology is a record

of bonefied work carried out by him under my guidance and supervision. The results embodied

in this project report have not been submitted to any other University or Institute for the award of

any degree or diploma.

GUIDE:

Sri. SATISH KUMAR Prof P. M. SHARMA Associate Professor Head of Department

Department of Electrical & Electronics Department of Electrical & Electronics

GRIET, Bachupally GRIET, Bachupally

Hyderabad – 500 090. Hyderabad – 500 090.

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ACKNOWLEDGEMENT

I express my deep sense of gratitude to Sri. SATHISH KUMAR, Assistant Professor

Department of Electrical and Electronics Engineering, Gokaraju Rangaraju College of

Engineering and Technology, Hyderabad, under who’s esteemed Guidance this project work was

carried out. His generous help and valuable suggestions at each stage of this work are

acknowledged.

I have immense pleasure in expressing my thanks and deep sense of Gratitude to my

External Project Guide Pro. P.M. SHARMA, HOD of E.E.E Department, G.R.I.E.T, Hyderabad

for his everlasting encouragement and immense cooperation offered in amicable and pleasant

manner throughout the project work.

I take this opportunity to express my deep sense of gratitude to Professors, Asst.

Professors, Lecturers and staff of EEE Dept, G.R.I.E.T, Hyderabad for their help throughout this

course.

Mayur karthik. K (08241A0276)

Raju. K (08241A0287)

Suryakanth. M (08241A02A8)

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(ii)

Contents:

Certificate ………………… (i)

Acknowledgment ………………… (ii)

1. Introduction (7-14) 1.1 Motor control

1.2 Triggering of thyristor

1.3 5HP motor

1.4 Types of motor controllers

1.5 Applications

1.6 Triggering circuit diagram

1.7 Multisim software

1.8 Circuit simulation in Multisim

2. Power supply (15-20) 2.1 Power supply circuit

2.2 Cosine wave generation

3. Inverting amplifier circuit (21-26) 3.1 Inverting amplifier

3.2 Circuit using LM741 IC

3.3 Simulation result

3.4 Practical output

4. Comparator circuit (27-35)

4.1 Comparator circuit

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4.2 Comparator operation

4.3 Comparator circuit using LM339 IC


4.5 Practical output

4.6 Differentiator circuit


5. 555 Timer Circuit (36-40) 5.1 About 555 Timer

5.2 555 Timer circuit for generation of pulses


5.4 Practical result

6. Pulse transformer (41-45)

6.1 Pulse transformer operating principle

6.2 Pulse transformer circuit

7. Fully controlled thyristor bridge circuit (46-49)

7.1 Thyristor

7.2 Fully controlled thyristor bridge circuit


8. Snubber circuit (50)

8.1 Snubber circuit

9. Over current protection (51-52) 9.1 current protection

10. Soft start (53) 10.1 Potentiometer

11. PCB Design (54-57) 11.1 Eagle software

11.2 Triggering circuit

11.3 Eagle schematic

11.4 PCB Design

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Conclusion 57

List of figures 58

References

Appendix 59

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CHAPTER 1

INTRODUCTION

1.1 MOTOR CONTROLLER: A motor controller is a device or group of devices that serves to govern in some predetermined

manner the performance of an electric motor. A motor controller might include a manual or

automatic means for starting and stopping the motor, selecting forward or reverse rotation,

selecting and regulating the speed, regulating or limiting the torque, and protecting against

overloads and faults.

1.2 TRIGGERING OF THYRISTOR:

Turning on the thyristor by giving a gating pulse to it is known as triggering. With anode positive

with respect to cathode, a thyristor can be turned on by any one of the following techniques :

(a) Forward voltage triggering

(b) Gate triggering

(c) dv/dt triggering

(d) Temperature triggering

(e) Light triggering.

These methods of turning-on a thyristor are now discussed one after the other.

(a) Forward Voltage Triggering:

When anode to cathode forward voltage is increased with gate circuit open, the reverse

biased junction J2 will break. This is known as avalanche breakdown and the voltage at

which avalanche occurs is called forward break over voltage VB0. At this voltage,

thyristor changes from off-state (high voltage with low leakage current) to on-state

characterized by low voltage across thyristor with large forward current. As other

junctions J1, J3 are already forward biased, breakdown of junction J2 allows free

movement of carriers across three junctions and as a result, large forward anode-current

flows. As stated before, this forward current is limited by the load impedance. In practice,

the transition from off-state to on-state obtained by exceeding VB0 is never employed as

it may destroy the device. The magnitudes of forward and reverse break over voltages are

nearly the same and both are temperature dependent. In practice, it is found that VBR is

slightly more than VB0. Therefore, forward break over voltage is taken as the final

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voltage rating of the device during the design of SCR applications. After the avalanche

breakdown, junction J2 loses its reverse blocking capability. Therefore, if the anode

voltage is reduced below VB0 SCR will continue conduction of the current. The SCR can

now be turned off only by reducing the anode current below a certain value called

holding current (defined later).

(b) Gate Triggering :

Turning on of thyristors by gate triggering is simple, reliable and efficient; it is therefore

the most usual method of firing the forward biased SCRs. A thyristor with forward break

over voltage (say 800 V) higher than the normal working voltage (say 400 V) is chosen.

This means that thyristor will remain in forward blocking state with normal working

voltage across anode and cathode and with gate open. However, when turn-on of a

thyristor is required, a positive gate voltage between gate and cathode is applied. With

gate current thus established, charges are injected into the inner p layer and voltage at

which forward break over occurs is reduced. The forward voltage at which the device

switches to on-state depends upon the magnitude of gate current. Higher the gate current,

lower is the forward break over voltage.

Figure 1.1- Gate current versus break over voltage

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When positive gate current is applied, gate P layer is flooded with electrons from the cathode.

This is because cathode N layer is heavily doped as compared to gate P layer. As the thyristor is

forward biased, some of these electrons reach junction J2. As a result, width of depletion layer

around junction J2 is reduced. This causes the junction J2 to breakdown at an applied voltage

lower than forward break over voltage VB0. If magnitude of gate current is increased, more

electrons will reach junction J2, As a consequence thyristor will get turned on at a much lower

forward applied voltage. Figure shows that for gate current Ig = 0, forward break over voltage is

VB0. For Igl , forward break over voltage, or turn-on voltage is less than VB0 For Ig2 > Ig1 ,

forward break over voltage is still further reduced. The effect of gate current on the forward

break over voltage of a thyristor can also be illustrated by means of a curve as shown in Fig. 1.1.

For Ig < oa, forward break over voltage remains almost constant at VB0. For gate currents Ig1,

Ig2 and Ig3 the values of forward break over voltages are ox, oy and oz, respectively as shown.

In Figure the curve marked Ig = 0 is actually for gate current less than oa. In practice, the

magnitude of gate current is more than the minimum gate current required to turn on the SCR.

Typical gate current magnitudes are of the order of 20 to 200 mA.

Once the SCR is conducting a forward current, reverse biased junction J2 no longer exists. As

such, no gate current is required for the device to remain in on-state. Therefore, if the gate

current is removed, the conduction of current from anode to cathode remains unaffected.

However, if gate current is reduced to zero before the rising anode current attains a value, called

the latching current, the thyristor will turn-off again. The gate pulse width should therefore be

judiciously chosen to ensure that anode current rises above the latching current. Thus latching

current may be defined as the minimum value of anode current which it must attain during turn-

on process to maintain conduction when gate signal is removed. Once the thyristor is conducting,

gate loses control. The thyristor can be turned-off (or the thyristor can be returned to forward

blocking state) only if the forward current falls below a low-level current called the holding

current. Thus holding current may be defined as the minimum value of anode current below

which it must fall for turning-off the thyristor. The latching current is higher than the holding

current. Note that latching current is associated with turn-on process and holding current with

turn-off process. It is usual to take latching current as two to three times the holding current. In

industrial applications, holding current (typically 10 mA) is almost taken as zero.

(c) dv/dt Triggering :

With forward voltage across the anode and cathode of a thyristor, the two outer junctions

are forward biased but the inner junction is reverse biased. This reverse biased junction J2

has the characteristics of a capacitor due to charges existing across the junction. In other

words, space-charges exist in the depletion region around junction J2 and therefore

junction J2 behaves like a capacitance. If the entire anode to cathode forward voltage Va

appears across J2 junction and the charge is denoted by Q, then a charging current i given

by

i = dQ/dt =d (Cj Va)/dt

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=Cj (d Va /dt) + Va (d Cj /dt)

As Cj the capacitance of junction J2 is almost constant, the current is given by

i = Cj (d Va /dt)

If the rate of rise of forward voltage dVa/dt is high, the charging current i will be more. This

charging current plays the role of gate current and turns on the SCR even when gate signal is

zero. Such phenomena of turning-on a thyristor, called dv/dt turn-on must be avoided as it leads

to false operation of the thyristor circuit. For controllable operation of the thyristor, the rate of

rise of forward anode to cathode voltage dVa/dt must be kept below the specified rated limit.

Typical values of dv/dt are 20 – 500 V/µsec. False turn-on of a thyristor by large dv/dt can be

prevented by using a snubber circuit in parallel with the device

This method is discussed further in.

(d) Temperature Triggering :

During forward blocking, most of the applied voltage appears across reverse biased

junction J2. This voltage across junction J2 associated with leakage current may raise the

temperature of this junction. With increase in temperature, leakage current through

junction J2 further increases. This cumulative process may turn on the SCR at some high

temperature.

(e) Light Triggering:

For light-triggered SCRs, a recess (or niche) is made in the inner p-layer as shown in Fig.

4.5 (a). When this recess is irradiated, free charge carriers (holes and electrons) are

generated just like when gate signal is applied between gate and cathode. The pulse of

light of appropriate wavelength is guided by optical fibers for irradiation. If the intensity

of this light thrown on the recess exceeds a certain value, forward-biased SCR is turned

on. Such a thyristor is known as light-activated SCR (LASCR).

LASCR may be triggered with a light source or with a gate signal. Sometimes a

combination of both light source and gate signal is used to trigger an SCR. For this, the gate is

biased with voltage or current slightly less than that required to turn it on, now a beam of light

directed at the inner p-layer junction turns on the SCR. The light intensity required to turn-on the

SCR depends upon the voltage bias given to the gate. Higher the voltage (or current) bias, lower

the light intensity required. Light-triggered thyristors have now been used in high-voltage direct

current (HVDC) transmission systems. In these several SCRs are connected in series-parallel

combination and their light-triggering has the advantage of electrical isolation between power

and control circuits.

1.3 5HP MOTOR: 5 HP DC Drive has thyristor controlled full converter output with single phase

or two phase input, up to 5HP motor applications.

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1.3.1 Types:

1. Side plate type:

These types of drives are simply covered with powder coated plates. It can be easily mounted on

the wall or panel.

2. Box type with voltmeter and Ammeter:

These types of drives are covered with powder coated box having feather touch ON/OFF switch,

variable pot, power ON indication, volt meter and ammeter.

1.3.2 Features:

Suitable for up to 5 HP DC motor.

Constant RPM.

Constant torque.

Armature feedback.

Taco feedback.

Current trip adjustment.

Acceleration / deceleration time adjustable.

Inching with adjustable voltage.

Min. & maximum speed limits.

Synchronized drives for multi motor application.

Indication of overload.

Safe, Secure, Serviceable and Simple to use.

1.4 TYPES OF MOTOR CONTROLLERS

Motor controllers can be manually, remotely or automatically operated. They may include only

the means for starting and stopping the motor or they may include other functions.

An electric motor controller can be classified by the type of motor it is to drive such as

permanent magnet, servo, series, separately excited, and alternating current. A motor controller

is connected to a power source such as a battery pack or power supply, and control circuitry in

the form of analog or digital input signals.

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1.5 APPLICATIONS

Every electric motor has to have some sort of controller. The motor controller will have differing

features and complexity depending on the task that the motor will be performing.

The simplest case is a switch to connect a motor to a power source, such as in small appliances

or power tools. The switch may be manually operated or may be a relay or contactor connected

to some form of sensor to automatically start and stop the motor. The switch may have several

positions to select different connections of the motor. This may allow reduced-voltage starting of

the motor, reversing control or selection of multiple speeds. Overload and overcorrect protection

may be omitted in very small motor controllers, which rely on the supplying circuit to have over

current protection. Small motors may have built-in overload devices to automatically open the

circuit on overload. Larger motors have a protective overload relay or temperature sensing relay

included in the controller and fuses or circuit breakers for over currect protection.

An automatic motor controller may also include limit switches or other devices to protect the

driven machinery.

More complex motor controllers may be used to accurately control the speed and torque of the

connected motor (or motors) and may be part of closed loop control systems for precise

positioning of a driven machine. For example, a numerically controlled lathe will accurately

position the cutting tool according to a preprogrammed profile and compensate for varying load

conditions and perturbing forces to maintain tool position.

1.6 TRIGGERING CIRCUIT DIAGRAM USED FOR THIS

PROJECT:

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Figure 1.2- Triggering circuit for thyristors

The figure1.2 shows the circuit used for triggering the thyristors of fully controlled bridge ,used

for controlling speed of the motor. The circuit consists of amplifier circuit, comparator 8 circuit,

555imer circuit for pulse generation and finally the pulse transformer circuit. The circuit is

simulated using MULTISIM software.

1.7 MULTISIM SOFTWARE:

Multisim is s an electronic schematic capture and simulation program which is part of a suite of

circuit design programs. Simulating circuits with Multisim catches errors early in the design

flow, saving time and money. Multisim includes all the tools necessary to take a design from

inception to finished project.

1.7.1 Features:

Changing settings while the simulation is running.

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Re-wiring terminals while simulation is running.

Using multiple instances of the same instrument in one circuit.

Saving instrument settings and displaying data with the circuit file.

Populating data displayed in the measurement instruments in the Graphic View also.

Resizing the measurement instrument panel to account for screen resolution or

presentation mode.

1.8 CIRCUIT SIMULATION IN MULTISIM:

Figure 1.3- Triggering circuit for thyristors in MULTISIM

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CHAPTER 2

POWER SUPPLY

2.1 POWER SUPPLY CIRCUIT:

The circuit given here is of a regulated dual power supply that provides +12V and -12V from

the AC mains. A power supply like this is a very essential tool on the work bench of an

Electronic hobbyist. The transformer T1 steps down the AC mains voltage and diodes D1,

D2, D3 and D4 does the job of rectification. Capacitors C1 and C2 does the job of

filtering.C3, C4, C7and C8 are decoupling capacitors. IC 7812 and 7912 are used for the

purpose of voltage regulation in which the former is a positive 12V regulator and later is a

negative 12V regulator. The output of 7812 will be +12V and that of 7912 will be -12V.

Assemble the circuit on a good quality PCB.

Transformer T1 can be a 230V primary; 12-0-12 V, 1A secondary step-down

transformer.

Fuse F1 can be a 500mA fuse.

Capacitor C1, C2, C5 and C6 must be rated at least 50V.

Figure 2.1-Dual power supply circuit

The ICs used for voltage regulation are 7812 and 7912 for +12v and -12v supply respectively.

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Figure 2.2- 7812 Figure 2.3- General pin description

Figure 2.4- 7912

These ICs have three terminals – input, output, and ground. 7812 and 7912 are voltage regulators

integrated circuit. It is a member of 78xx series of fixed linear voltage regulator ICs. The voltage

source in a circuit may have fluctuations and would not give the fixed voltage output. The

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voltage regulator IC maintains the output voltage at a constant value. The xx in 78xx indicates

the fixed output voltage it is designed to provide. 7812 provides +12V regulated power supply.

Capacitors of suitable values can be connected at input and output pins depending upon the

respective voltage levels.

2.1.1 Power supply practical output:

The output waveform obtained at the positive terminal of dual power supply circuit, when

connected to CRO is show in following figure.

Figure 2.5- CRO output for +12V with 5V/div

The output waveform obtained at the negative terminal of dual power supply circuit, when connected

to CRO is show in following figure.

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Figure 2.6- CRO output for -12V with 5V/div

2.2 COSINE WAVEFORM GENERATION:

Input sine wave:

Circuit:

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For inverse cosine control scheme, which is usually adopted in line commutated thyristor control

circuits, a cosine waveform is to be generated from the supply sine wave. This circuit is used to

convert the input sine wave into cosine wave. The circuit uses a suitable combination of resistor

and three capacitors to phase shift the input sine wave so as to obtain a cosine wave.

2.3 COSINE WAVES GENERATION PRACTICAL OUTPUT:

Figure 2.8- Cosine waveform in CRO

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CHAPTER 3

INVERTING AMPLIFIER CIRCUIT

3.1 INVERTING AMPLIFIER:

Figure 3.1 – Inverting amplifier using Op-Amp

In this Inverting Amplifier circuit the operational amplifier is connected with feedback to

produce a closed loop operation. For ideal op-amps there are two very important rules to

remember about inverting amplifiers, these are: "no current flows into the input terminal" and

that "V1 equals V2", (in real op-amps both these rules are broken). This is because the junction

of the input and feedback signal (X) is at the same potential as the positive (+) input which is at

zero volts or ground then, the junction is a "Virtual Earth". Because of this virtual earth node the

input resistance of the amplifier is equal to the value of the input resistor, Rin and the closed loop

gain of the inverting amplifier can be set by the ratio of the two external resistors. We said above

that there are two very important rules to remember about Inverting Amplifiers or any

operational amplifier for that matter and

these are.

1. No Current Flows into the Input Terminals

2. The Differential Input Voltage is Zero as V1 = V2= 0 (Virtual Earth).

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Then by using these two rules we can derive the equation for calculating the closed-loop gain of

an inverting amplifier, using first principles.

Current ( i ) flows through the resistor network as shown.

Figure 3.2 – current division through resistance network

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3.2 CIRCUIT USING LM741 IC FOR INVERTING AMPLIFIER:

Figure 3.3 – Inverting amplifier using LM741 IC

The IC used her in the circuit as inverting amplifier is LM741.

The LM741 series are general purpose operational amplifiers which feature improved

performance

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Figure 3.4 – Pin description of LM741 IC

Pin 1 (Offset Null): Since the op-amp is the differential type, input offset voltage must be

controlled so as to minimize offset. Offset voltage is nulled by application of a voltage of

opposite polarity to the offset. An offset null-adjustment potentiometer may be used to

compensate for offset voltage. The null-offset potentiometer also compensates for irregularities

in the operational amplifier manufacturing process which may cause an offset. Consequently, the

null potentiometer is recommended for critical applications. See 'Offset Null Adjustment' for

method.

Pin 2 (Inverted Input): All input signals at this pin will be inverted at output pin 6. Pins 2 and 3

are very important (obviously) to get the correct input signals or the op amp cannot do its work.

Pin 3 (Non-Inverted Input): All input signals at this pin will be processed normally without

inversion. The rest is the same as pin 2.

Pin 4 (-V): The V- pin (also referred to as Vss) is the negative supply voltage terminal. Supply-

voltage operating range for the 741 is -4.5 volts (minimum) to -18 volts (max), and it is specified

for operation between -5 and -15 Vdc. The device will operate essentially the same over this

range of voltages without change in timing period. Sensitivity of time interval to supply voltage

change is low, typically 0.1% per volt. (Note: Do not confuse the -V with ground).

Pin 5(Offset Null): Seepin1

Pin 6 (Output): Output signal's polarity will be the opposite of the input's when this signal is

applied to the op-amp's inverting input. For example, a sine-wave at the inverting input will

output a square-wave in the case of an inverting comparator circuit.

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Pin 7 (+V): The V+ pin (also referred to as Vcc) is the positive supply voltage terminal of the

741 Op-Amp IC. Supply-voltage operating range for the 741 is +4.5 volts (minimum) to +18

volts (maximum), and it is specified for operation between +5 and +15 Vdc. The device will

operate essentially the same over this range of voltages without change in timing period.

Actually, the most significant operational difference is the output drive capability, which

increases for both current and voltage range as the supply voltage is increased. Sensitivity of

time interval to supply voltage change is low, typically 0.1% per volt.

Pin 8 (N/C): The 'N/C' stands for 'Not Connected'. There is no other explanation. There is

nothing connected to this pin, it is just there to make it a standard 8-pin package. The LM741 IC

is therefore used for inverting amplification purpose. The cosine wave generated previously is

given as input to this op-amp. As the op-amp circuit functions as inverting amplifier, the cosine

wave will be inverted and amplified. Thus, the output of the inverting amplifier circuit wills an

inverted and amplified cosine wave. The output from the inverting amplifier circuit will be fed to

comparator circuit.

3.3 SIMULATION RESULTS:

Figure 3.5 – Output of inverting amplifier circuit

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3.4 PRACTICAL OUTPUT AT INVERTING AMPLIFIERS:

FIGURE 3.6(b) Practical output at LM741

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CHAPTER 4

COMPARATOR CIRCUIT

4.1 COMPARATOR CIRCUIT:

Figure 4.1 – Voltage comparator circuit

Comparator circuits find a number of applications in electronics. As the name implies they are

used to compare two voltages. When one is higher than the other the comparator circuit output is

in one state, and when the input conditions are reversed, then the comparator output switches.

These circuits find many uses as detectors. They are often used to sense voltages. For example

they could have a reference voltage on one input, and a voltage that is being detected on another.

While the detected voltage is above the reference the output of the comparator will be in one

state. If the detected voltage falls below the reference then it will change the state of the

comparator, and this could be used to flag the condition. This is but one example of many for

which comparators can be used. In operation the op amp goes into positive or negative saturation

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dependent upon the input voltages. As the gain of the operational amplifier will generally exceed

100 000 the output will run into saturation when the inputs are only fractions of a millivolt apart.

A typical comparator circuit will have one of the inputs held at a given voltage. This may often

be a potential divider from a supply or reference source. The other input is taken to the point to

be sensed.

4.2 COMPARATOR OPERATION:

Figure 4.2 – Comparator operation

The above drawings show the two simplest configurations for voltage comparators. The

diagrams below the circuits give the output results in a graphical form. For these circuits the

REFERENCE voltage is fixed at one-half of the supply voltage while the INPUT voltage is

variable from zero to the supply voltage.

In theory the REFERENCE and INPUT voltages can be anywhere between zero and the supply

voltage but there are practical limitations on the actual range depending on the particular device

used.

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4.3 COMPARATOR CIRCUIT USING LM339 IC:

Figure 4.3 – Comparator circuit using LM339

The above figure shows the comparator circuit using LM339 IC for comparing the voltage levels

and generating the required pulses. The circuit generates the pulse width modulated (PWM)

pulses that are given to thyristors for triggering.

The LM339 series consists of four independent precision voltage comparators with an offset

voltage specification as low as 2 mV max for all four comparators. These were designed

specifically to operate from a single power supply over a wide range of voltages. Operation from

split power supplies is also possible and the low power supply current drain is independent of the

magnitude of the power supply voltage. These comparators also have a unique characteristic in

that the input common-mode voltage range includes ground, even though operated from a single

power supply voltage. Application areas include limit comparators, simple analog to digital

converters; pulse, square wave and time delay generators; wide range VCO; MOS clock timers;

multivibrators and high voltage digital logic gates. When operated from both plus and minus

power supplies, they will directly interface with MOS logic- where the low power drain of the

LM339 is a distinct advantage over standard comparators.

29

Figure 4.4 – Pin diagram Figure 4.5 – Package diagram

4.4 SIMULATION RESULTS:

Figure 4.6 – Output at the comparator

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4.5 PRACTICAL OUTPUT:

FIGURE 4.7(a) Practical output at comparator LM339

FIGURE 4.7(b) Practical output at comparator LM339

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4.5 DIFFERENTIATOR CIRCUIT:

A Differentiator is a circuit that is designed such that the output of the circuit is proportional to

the time derivative of the input. There are two types of differentiator circuits, active and passive.

Figure 4.8 – Differentiator circuit

A differentiator circuit consists of an operational amplifier, resistors and capacitors. The circuit

is based on the capacitors current to voltage relationship:

where I is the current through the capacitor, C is the capacitance of the capacitor, and V is the

voltage across the capacitor. The current flowing through the capacitor is then proportional to the

derivative of the voltage across the capacitor. This current can then be connected to a resistor,

which has the current to voltage relationship:

Where R is the resistance of the resistor. If Vout is the voltage across the resistor and Vin is the

voltage across the capacitor, we can rearrange these two equations to obtain the following

equation:

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Thus, it can be shown that in an ideal situation the voltage across the resistor will be proportional

to the derivative of the voltage across the capacitor with a gain of RC.

4.6 DIFFERENTIATOR CIRCUIT:

Figure 4.9 – Differentiator circuit used in triggering circuit

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4.7 SIMULATION RESULT:

Figure 4.10 – Output at the Differentiator

4.7.1 2222A Transistor:

Figure 4.11 – Package diagram Figure 4.12 – Schematic

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The 2N2222, is a small, common NPN BJT transistor used for general purpose low-power

amplifying or switching applications. It is designed for low to medium current, low power,

medium voltage, and can operate at moderately high speeds. It was originally made in the TO-18

metal can as shown in the picture, but is more commonly available now in the cheaper TO-92

packaging, where it is known as the PN2222 or P2N2222.

4.7.2 Diode IN4148:

Figure 4.13 – IN4007 Diodes

In electronics, a diode is a two-terminal electronic component that conducts electric current in

only one direction. The term usually refers to a semiconductor diode, the most common type

today. This is a crystalline piece of semiconductor material connected to two electrical terminals.

A vacuum tube diode (now little used except in some high-power technologies) is a vacuum tube

with two electrodes: a plate and a cathode.

The most common function of a diode is to allow an electric current to pass in one direction (

called the diode's forward direction) while blocking current in the opposite direction (the reverse

direction). Thus, the diode can be thought of as an electronic version of a check valve. This

unidirectional behavior is called rectification, and is used to convert alternating current to direct

current. Here in this circuit the diode used is IN4007.

35

CHAPTER 5

555 TIMER CIRCUIT

5.1 555 TIMER:

The 555 timer is an integrated circuit (chip) implementing a variety of timer and multivibrator

applications. It is one of the most popular and versatile integrated circuits which can be used to

build lots of different circuits. It includes 23 transistors, 2 diodes and 16 resistors on a silicon

chip installed in an 8-pin mini dual-in-line package (DIP-8). The 555 Timer is a monolithic

timing circuit that can produce accurate and highly stable time delays or oscillations. The timer

basically operates in one of the two modes—monostable (one-shot) multivibrator or as an astable

(free-running) multivibrator. In the monostable mode, it can produce accurate time delays from

microseconds to hours. In the astable mode, it can produce rectangular waves with a variable

duty cycle. Frequently, the 555 is used in astable mode to generate a continuous series of pulses,

but you can also use the 555 to make a one-shot or monostable circuit. The 555 can source or

sink 200 mA of output current, and is capable of driving wide range of output devices. The

output can drive TTL (Transistor-Transistor Logic) and has a temperature stability of 50 parts

per million (ppm) per degree Celsius change in temperature, or equivalently 0.005 %/°C.

Applications of 555 timer in monostable mode include timers, missing pulse detection, bounce

free switches, touch switches, frequency divider, capacitance measurement, pulse width

modulation (PWM) etc. In astable or free running mode, the 555 can operate as an oscillator. The

uses include LED and lamp flashers, logic clocks, security alarms, pulse generation, tone

generation, pulse position modulation, etc. In the bistable mode, the 555 can operate as a flip-

flop and is used to make bounce-free latched switches, etc. The 555 can be used with a supply

voltage (VCC) in the range 4.5 to 15V (18V absolute maximum).

5.1.1 Pin description

Figure 5.1: Pin out diagram of 555 Timer Figure 5.2 555 Timer package

36

Figure 5.3 Functional Block Diagram of 555 Timer

1) Ground: Connect this to ground. Remember to connect all grounds in a circuit together.

2) Trigger: A short low (less than 1/3 Vcc) pulse on the trigger starts the timer. By connecting

this to ground we "turn on" the 555 timer.

3) Output: During timing interval, the output stays at +VCC Can source up to 200mA.

4) Reset: Forces pin 3 low if pulled to ground.

5) Control: Can be used to adjust threshold trigger voltage. Not used in our applications.

Connect to ground with a .01uF cap to eliminate supply noise from Vcc.

6) Threshold: When threshold crosses above 2/3 Vcc timing interval ends.

7) Discharge: Connects to ground when output goes low, Controls timing.

8) Vcc. Power supply. Typical range is 4.5v to 16v.

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5.1.2 555 TIMER OPERATING MODES:

The 555 has three operating modes:

Monostable mode: in this mode, the 555 functions as a "one-shot". Applications include

timers, missing pulse detection, bounce free switches, touch switches, frequency divider,

capacitance measurement, pulse-width modulation (PWM) etc

Astable - free running mode: the 555 can operate as an oscillator. Uses include LED and

lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse

position modulation, etc.

Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not

connected and no capacitor is used. Uses include bounce free latched switches, etc.

5.2 555 TIMER CIRCUIT FOR GENERATION OF PULSES:

Figure 5.4 555 TIMER CIRCUIT

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The output of the transistor is given as input of the 555 timer and output pulses are obtained. The

width of the pulses are varied by varying the values of Capacitance and Resistance connected to

the timer.

5.3 SIMULATION RESULT FOR THE CIRCUIT:

Figure 5.5 Simulation result at 555 timer output

39

5.4 PRACTICAL RESULT FOR THE CIRCUIT:

Figure 5.6(a) Practical output at 555 Timer output

Figure 5.6(b) Practical output at 555 Timer output

40

CHAPTER 6

PULSE TRANSFORMER

6.1 PULSE TRANSFORMER OPERATING PRINCIPLE:

The magnetic flux in a typical A.C. transformer core alternates between positive and negative

values. The magnetic flux in the typical pulse transformer does no. The typical pulse transformer

operates in an unipolar mode (flux density may meet but does not cross zero.)

Figure 6.1(b) Pulse transformer

A fixed D.C. current could be used to create a biasing D.C. magnetic field in the transformer

core, thereby forcing the field to cross over the zero line. Pulse transformers usually (not always)

operate at high frequency necessitating use of low loss cores (usually ferrites). Figure shows the

electrical schematic for a pulse transformer and equivalent high frequency circuit representation

for a transformer which is applicable to pulse transformers. The circuit treats parasitic elements,

leakage inductances and winding capacitance, as lumped circuit elements, but they are actually

distributed elements. Pulse transformers can be divided into two major types, power and signal.

An example of a power pulse transformer application would be precise control of a heating

element from a fixed D.C. voltage source. The voltage may be stepped up or down as needed by

the pulse transformer’s turns ratio. The power to the pulse transformer is turned on and off using

a switch (or switching device) at an operating frequency and pulse duration that delivers the

required amount of power. Consequently, the temperature is also controlled. The transformer

provides electrical isolation between the input and output. The transformers used in forward

converter power supplies are essentially power type pulse transformers. There exist high-power

pulse transformer designs that have exceeded 500 kilowatts of power capacity.

41

The design of ―signal‖ type of pulse transformer focuses on the delivery of a signal at the

output. The transformer delivers a ―pulse-like‖ signal or a series of pulses. The turns ratio of

the pulse transformer can be used to adjust signal amplitude and provide impedance matching

between the source and load. Pulse transformers are often used in the transmittal of digital data

and in the gate drive circuitry of transistors, F.E.T.s, S.C.R.s, and etc. In the latter application,

the pulse transformers may be referred to as ―gate transformers‖ or ―gate drive transformers‖.

Signal type of pulse transformers handle relatively low levels of power. For digital data

transmission, transformers are designed to minimized signal distortion. The transformers might

be operated with a D.C. bias current. Many signal type pulse transformers are also categorized as

wideband transformers. Signal type pulse transformers are frequently used in communication

systems and digital networks.

Figure 6.2 Electrical schematic for a pulse transformer and its equivalent high frequency circuit

representation

Pulse transformer designers usually seek to minimize voltage droop, rise time, and pulse

distortion. Droop is the decline of the output pulse voltage over the duration of one pulse. It is

42

cause by the magnetizing current increasing during the time duration of the pulse. To understand

how voltage droop and pulse distortion occurs, one needs to understand the magnetizing (

exciting, or no-load ) current effects, load current effects, and the effects of leakage inductance

and winding capacitance. The designer also needs to avoid core saturation and therefore needs to

understand the voltage-time product.

Magnetizing (No-Load) Current, its Effects, and Its Relation to Saturation

Consider the simple pulse transformer circuit and its equivalent circuit.

Figure 6.3 Pulse transformer circuit and its equivalent circuit

There is no source impedance, winding capacitances, or secondary leakage inductance to worry

about. With both switches open, there cannot be any primary or secondary currents flowing.

Now close the primary switch. Since the secondary load is not connected, the pulse transformer’s

primary winding acts like an inductor placed across a voltage source. Primary current begins to

flow. This is the magnetizing current ( no secondary current ) and is governed by the differential

equation V(t) = L x d(I)/dt + Rp x I(t), with units of volts, henries, amps, and seconds. If the

power supply has constant voltage, Rp = zero, & L = Lkp+Lm is constant, the differential

43

equation can be solved for I(t), I(t) = Io + V x t / ( Lkp+Lm ), where Io = the initial current which

equals zero. Notice that the current increases at a linear rate over time and that the rate in

inversely proportional to the inductance. The current flows through Np turns creating Np x I(t)

amount of magnetizing force ( amp-turns ) which in turns creates a magnetic flux density in the

pulse transformer core. Eventually the increasing primary magnetizing current would exceed the

magnetic flux capacity of the pulse transformer core and will saturate the core. Once saturation

occurs the primary current rapidly increases towards infinity ( in theory ). In a real circuit the

primary winding resistance ( and source impedance ) would limit the current. See Figure below

for graphical illustration. For non-zero Rp, I(t) = Io + ( V/Rp ) x ( 1 – e to the ( -Rp x t / ( Lkp +

Lm )) power ). The effect of Rp is graphically illustrated in Figures. Rp extends the time it takes

for the unloaded transformer ( or an inductor ) to saturate. If Rp is sufficiently large, it prevents

the transformer (or inductor) from saturating altogether. Regardless of saturation, Rp places an

upper limit on the primary current value.

Figure 6.4: Effect of Rp by graphical illustration

44

6.2 PULSE TRANSFORMER CIRCUIT:

Figure 6.5 Pulse transformer used in triggering circuit

The single pulse produced by the 555 timer is converted into two pulses to trigger a pair of

thyristors in the fully controlled bridge rectifier.

45

CHAPTER 7

FULLY CONTROLLED THYRISTOR BRIDGE

7.1 THYRISTOR

A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type

material. They act as bistable switches, conducting when their gate receives a current pulse, and

continue to conduct while they are forward biased (that is, while the voltage across the device is

not reversed).

The thyristor is a four-layer, three terminal semiconducting device, with each layer consisting of

alternately N-type or P-type material, for example P-N-P-N. The main terminals, labelled anode

and cathode, are across the full four layers, and the control terminal, called the gate, is attached

to p-type material near to the cathode. (A variant called an SCS—Silicon Controlled Switch—

brings all four layers out to terminals.) The operation of a thyristor can be understood in terms of

a pair of tightly coupled bipolar junction transistors, arranged to cause the self-latching action:

Figure 7.1 : Thyristor representation

Thyristors have three states:

1. Reverse blocking mode — Voltage is applied in the direction that would be blocked by a

diode

46

2. Forward blocking mode — Voltage is applied in the direction that would cause a diode to

conduct, but the thyristor has not yet been triggered into conduction

3. Forward conducting mode — The thyristor has been triggered into conduction and will remain

conducting until the forward current drops below a threshold value known as the "holding current"

7.1.1 Function of the gate terminal

The thyristor has three p-n junctions (serially named J1, J2, J3 from the anode).

Figure 7.2 Layer diagram of thyristor.

When the anode is at a positive potential VAK with respect to the cathode with no voltage

applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As

J2 is reverse biased, no conduction takes place (Off state). Now if VAK is increased beyond the

breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and the thyristor

starts conducting (On state).

If a positive potential VG is applied at the gate terminal with respect to the cathode, the

breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value

of VG, the thyristor can be switched into the on state suddenly.

Once avalanche breakdown has occurred, the thyristor continues to conduct, irrespective of the

gate voltage, until: (a) the potential VAK is removed or (b) the current through the device

(anode−cathode) is less than the holding current specified by the manufacturer. Hence VG can be

a voltage pulse, such as the voltage output from a UJT relaxation oscillator.

These gate pulses are characterized in terms of gate trigger voltage (VGT) and gate trigger

current (IGT). Gate trigger current varies inversely with gate pulse width in such a way that it is

evident that there is a minimum gate charge required to trigger the thyristor.

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7.2 FULLY CONTROLLED THYRISTOR BRIDGE CIRCUIT:

Figure 7.3 Thyristor bridge circuit used to control speed of dc motor

The circuit of a single-phase fully-controlled bridge rectifier circuit is shown in the figure above.

The circuit has four SCRs. It is preferable to state that the circuit has two pairs of SCRs, with

THY1 and THY2 forming one pair and, THY3 and THY4 the other pair. The firing pulses

obtained from pulse transformers are given to the gates of the thyristors to trigger them. Pulses

from the first pulse transformer are given to the thyristors THY1 and THY2 to make them

operate in the positive cycle of the input wave and pulses from second pulse transformer are

given to thyristors THY3 The main purpose of this circuit is to provide a variable dc output

voltage, which is brought about by varying the firing angle. And THY4 to make them operate in

the negative cycle of the input wave and dc is obtained at the output of the bridge.

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7.3 CIRCUIT SIMULATION IN PSIM:

Figure 7.4 Simulation of fully controlled thyristor rectifier circuit (firing angle α=30deg)

49

CHAPTER 8

SNUBBER CIRCUIT

A Snubber circuit consists of a series combination of resistance Rs and capacitance Cs in parallel

with the thyristor as shown in Fig.8.1. Strictly speaking, a capacitor Cs in parallel with the device

is sufficient to prevent unwanted dv/dt triggering of the SCR. When switch S is closed, a sudden

voltage appears across the circuit. Capacitor Cs behaves like a short circuit, therefore voltage

across SCR is zero. With the passage of time, voltage across Cs builds up at a slow rate such that

dv/dt across Cs and therefore across SCR is less than the specified maximum dv/dt rating of the

device. Here the question arises that if Cs is enough to prevent accidental turn-on of the device

by dv/dt, what is the need of putting Rs in series with Cs.

Before SCR is fired by gate pulse, Cs charges to full voltage Vs. When the SCR is turned on,

capacitor discharges through the SCR and sends a current equal to Vs / (resistance of local path

formed by Cs and SCR). As this resistance is quite low, the turn-on di/dt will tend to be excessive

and as a result, SCR may be destroyed. In order to limit the magnitude of discharge current, a

resistance Rs is inserted in series with Cs as shown in Fig. 8.1. Now when SCR is turned on,

initial discharge current Vs/Rs is relatively small and turn-on di/dt is reduced.

Figure: 8.1 Snubber circuit

In actual practice; Rs, Cs and the load circuit parameters should be such that dv/dt across Cs

during its charging is less than the specified dv/dt rating of the SCR and discharge current at the

turn-on of SCR is within reasonable limits. Normally, Rs Cs and load circuit parameters form an

underdamped circuit so that dv/dt is limited to acceptable values.

The design of snubber circuit parameters is quite complex. In practice, designed snubber

parameters are adjusted up or down in the final assembled power circuit so as to obtain a

satisfactory performance of the power electronics system.

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CHAPTER 9

OVER CURRENT PROTECTION

In a dc motor there are mainly two types of currents will flow. They are і) field current іі)

armature current. For the protection we use two separate circuits. These two circuits leads to

avoid the damaging the motor in short circuit time and in over voltage. In the circuit we are using

LM339 comparator, 4N28 optocupler and 741 op amp etc.

9.1 OVER CURRENT PROTECTION CIRUIT

Figure 9.1: over current protection

51

A resistor of value .125Ω, 60W is connected in series with armature. The voltage across this

shunt is taken as 2.5V. From a 1kΩ pot taken a 3V, by putting point at 25% of pot. Both these

voltages are given to the LM339 comparator. The output value of comparator is -11.97V. The

output of the comparator is given to the inverting amplifier. LM741 is used as inverting

amplifier. The output of the amplifier is between +3V to +4V which is enough to input of the

optocoupler. As before said when the sufficient value of voltage and current is given to the

optocoupler, the output terminal goes to short. So across the terminal ‘6’ we get the minimum

output voltage. For ideal case it is zero. But in practical it is near to (10 to 20)mV. When the

armature current is bellow the rated value i.e.20amps, the output of optocoupler is zero.

To maintain the constant field current and to protect from over currents a circuit is designed

which contains 1N4007 diodes, 4N28 optocoupler etc. Under normal condition the voltage

across the diodes is taken and given to the optocoupler. The voltage value is taken as 3.5V. It is

sufficient to run the optocoupler. So the output of ‘6’ terminal of optocoupler is minimum i.e.

zero voltage. The circuit for both the circuit is given in figure 9.2. The output of both

optocouplers is given to a NOR gate. According to principle of NOR gate, the output of gate

goes to HIGH when both inputs are LOW. So output of gate will be 5V. The obtained output

from the NOR gate is amplified by using non inverting amplifier because for getting the pulses

from 555 timer, its terminal ‘4’ should connect to +12V. So by amplifier we will get the required

12V.

Under normal operation we will get the required +12V to connect to terminal ‘4’ of 555

timer. Under abnormal condition mean either change in armature current or change in field

current corresponding circuit optocoupler output goes to high i.e. voltage across collector of

optocoupler goes to 12V. According to operation principle of NOR gate if any one input is high

output goes to LOW. So voltage across NOR gate is zero. This leads to stop the pulses from 555

timers. The output across the NOR gate under normal condition is shown in figure 9.2. Under

abnormal condition i.e. when any one of the currents either armature current or field current is

over than rated, then the output across the NOR gate is shown in figure 9.3.

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Figure 9.2 over current protection under normal condition

Figure 9.3 over current protection circuit under abnormal condition

53

CHAPTER 10

SOFT START OF MOTOR

Soft start of the motor is done by using cosine wave and a potentiometer. Initially potentiometer

arranged in such a manner, the output of the comparator will give the low value dc. From that the

output of the 555 timer gives a low amplitude and low Ton pulse. From that the trigger pulse to

the scr will be given through the pulse transformer.

The firing angle α is more than π/2 and less than π. The output voltage is very small in value.

Now varying the potentiometer the output of thyristor rectifier will change with increasing value

of average voltage. Finally the potentiometer is stop for our required value. The variance of the

output average voltage is shown in figure 10.1. Here we used a 10k potentiometer which is

shown in the figure 10.2.

Figure: 10.1 variance of rectifier output figure: 10.2 potentiometer

From this operation of the potentiometer and cosine wave the starting of the motor is done

smoother, and safely.

54

CHAPTER 11

PCB DESIGN USING EAGLE SOFTWARE

11.1 TRIGGERING CIRCUIT FOR THYRISTORS:

Figure 11.1 - Triggering circuit

55

11.2 EAGLE SCHEMATIC OF THE CIRCUIT:

Figure 11.2- EAGLE Schematic

56

11.3 PCB DESIGN:

Figure 11.3- Triggering circuit

57

CONCLUSION:

Thus by using the triggering circuit (which takes cosine wave as input ),the pwm pulses have

been generated and the pulse width being modulated by the 555 timers ( by RC combination) and

the resultant pulses after allowing through pulse transformer (that converts single pulse to two

pulses for firing of two thyristors at a time in a single cycle of input signal) are given to thyristor

bridge which controls the input voltage to the motor (by varying the 10 K potentiometer

connected at the reference voltage of the comparator IC IN 339) there by controlling the speed of

the motor thus finally using this project the speed control of dc motor is achieved.

By using the optocoupler(4N28), comparator(LM339), op amp(LM741), NOR

gate(DM7402) etc we controlled the triggering pulses. Under failure condition of the current

pulses stop the conducting of the thyristor. We have taken some reference values which useful to

easy operation of the circuit.

REFERENCE:

TEXT BOOKS:

1) Power Electronics P.S. BIMBRA

2) Op Amps and Linear ICs Ramakanth A Gayakwad

INTERNET:

1) EBooks 2) Google.com

58

List of figures:

1.1 Gate current vs breakover voltage

1.2 Triggering circuit for thyristors

1.3 Triggering circuit for thyristors in Multisim

2.1 Dual power supply circuit

2.2 7812 IC

2.3 General pin description

2.4 7912 IC

2.5 CRO output for +12V

2.6 CRO output for -12V

2.7 Cosine wave generation circuit

2.8 Cosine waveform in CRO

3.1 Inverting amplifier using Op-amp

3.2 Current division through resistor network

3.3 Inverting amplifier using LM741 IC

3.4 Pin description of LM741 IC

3.5 Simulation result of inverting amplifier

3.6 Practical output of LM741 IC

4.1 Voltage comparator circuit

4.2 Comparator operation

4.3 Comparator using LM339 IC

4.4 Pin diagram

4.5 Package diagram


4.7 CRO result

4.8 Differentiator circuit

4.9 Differentiator circuit used in triggering circuit

4.10 Simulation output

4.11 Package diagram

4.12 Schematic

4.13 IN4007 Diode

5.1 Pin diagram of 555 Timer

5.2 555 Timer Package

59

5.3 Functional block diagram of 555 Timer

5.4 555 Timer circuit

5.5 Simulation result at 555 Timer

5.6 Practical output at 555 Timer

6.1 Pulse transformer

6.2 Schematic for pulse transformer

6.3 Equivalent diagram of pulse transformer

6.4 Effect of Rp by graphical illustration

6.5 Pulse transformer used in triggering circuit

7.1 Thyristor representation

7.2 Layer diagram

7.3 Thyristor bridge circuit

7.4 Psim simulation result

8.1 Snubber circuit

9.1 Over current protection

9.2 Over current protection circuit under normal condition

9.3 Over current protection circuit under abnormal condition

10.1 Variance of rectifier output

10.2 Poteniometer

11.1 Eagle tool bar

11.2 Triggering circuit

11.3 Eagle schematic

11.4 PCB design

91

APPENDIX (F)

DM7402 NOR GATE

dc drive with over current protection.pdf

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