Electrical Insulators

Proper operation of a transmission or distribution line is highly dependent upon the proper working of insulators. A good insulator should have a good mechanical strength to withstand the mechanical load and stresses. It should have a high dielectric strength to withstand operating and flashover voltages. Also, an insulator must be free from pores or voids, which may damage it. Therefore, to ensure desired performance of insulators, each insulator has to undergo various tests.


Testing of insulators

Following are the different types of tests that are carried out on overhead line insulators.
1. Flashover tests
2. Performance tests
3. Routine tests

Flashover tests of insulators

Three types of flashover tests are conducted before the insulator is said to have passed the flashover test.
1. Power frequency dry flashover test
2. Power frequency wet flashover test
3. Impulse frequency flashover test

Power frequency dry flashover test

The insulator to be tested is mounted in the same manner in which it is to be used. Then, a variable voltage source of power frequency is connected between the electrodes of the insulator. The voltage is gradually increased up to the specified voltage. This specified voltage is less than the minimum flashover voltage. The voltage at which surrounding air of the insulator breaks down and become conductive is known as flashover voltage. The insulator must be capable of withstanding the specified voltage for one minute without flashover.

Power frequency wet flashover test (Rain test)

In this test also, the insulator to be tested is mounted in the same manner in which it is to be used. Similar to the above test, a variable voltage source of power frequency is connected between the electrodes. Additionally, in this test, the insulator is sprayed with water at an angle of 45° in such a manner that its precipitation should not be more than 5.08 mm/min. The voltage is then gradually increased up to the specified voltage. The voltage is maintained at the specified value for 30 seconds or one minute and the insulator is observed for puncture or breakdown. If the voltage is maintained for one minute, this test is also called as one-minute rain test.

Impulse frequency flashover test

This test is to ensure that the insulator is capable of sustaining high voltage surges caused by lightning. The insulator under test is mounted in the same manner as in above tests. An impulse voltage generator which generates a very high voltage at a frequency of several hundred kilohertz is connected to the insulator. This voltage is applied to the insulator and spark-over voltage is noted. The ratio of impulse spark-over voltage to spark-over voltage at power frequency is called as the impulse ratio. This ratio should be approximately 1.4 for pin type insulators and 1.3 for suspension type insulators.

Performance tests of insulators

1. Temperature cycle test
2. Puncture voltage test
3. Mechanical strength test
4. Electro-mechanical test
5. Porosity test

Temperature cycle test
In this test, the insulator under test is first heated in water at 70° for one hour. Then the insulator is immediately cooled at 7° for another hour. This cycle is repeated three times. Then the insulator is dried and its glazing is thoroughly observed for any damages or deterioration.

Puncture voltage test
The purpose of this test is to determine the puncture voltage. The insulator to be tested is suspended in insulating oil. A voltage is applied and increased gradually until the puncture takes place. The voltage at which insulator starts to puncture is called as puncture voltage. This voltage is usually 30% higher than that of the dry flash-over voltage for a suspension type insulators.


Mechanical strength test
In this test, the insulator under test is applied by 250% of the maximum working load for one minute. This test is conducted to determine the ultimate mechanical strength of the insulator.

Electro-mechanical test
This test is conducted only for suspension type insulators. In this test, a tensile stress of 250% of maximum working tensile stress is applied to the insulator. After this, the insulator is tested for 75% of dry spark-over voltage.

Porosity test
In this test, a freshly manufactured insulator sample is broken into pieces. These pieces are then immersed into a 0.5% to 1% alcohol solution fuchsine dye under pressure of 150 kg/cm2 for several hours (say 24 hours). After that, the pieces are removed from the solution and examined for the penetration of the dye into it. This test indicates the degree of porosity.

Routine tests of insulators
1. High voltage test
2. Proof load test
3. Corrosion test

High voltage test
This test is usually carried out for pin insulators. In this test, the insulator is inverted and placed into the water up to the neck. The spindle hole is also filled with water and a high voltage is applied for 5 minutes. The insulator should remain undamaged after this test.

Proof load test
In this test, each insulator is applied with 20% in excess of working mechanical load (say tensile load) for one minute. The insulator should remain undamaged after this test.

Corrosion test

In this test, the insulator with its metal fitting is suspended into a copper sulfate solution for one minute. Then the insulator is removed from the solution and wiped and cleaned. This procedure is repeated for four times. Then the insulator is examined for any metal deposits on it. There should be zero metal deposits on the insulator.

NETWORK THEORY VIDEO CLASSES

CLASSIFICATION OF ELEMENTS

CLASSIFICATION OF SOURCES

KCL,KVL,SERIES ,PARALLELCKTS ETC

NODE,MESH ANALYSIS TELLEGENS  THEOREM

SUPERPOSITION THEOREM ETC

THEVINENS THEOREM

MAXIMUM POWER TRANSFER

RECIPROCITY THEOREM

TWO PORT NW

POWER SYSTEMS NOTES PDF

http://www.vssut.ac.in/lecture_notes/lecture1424265031.pdf


ANOTHER IMPORTANT LINKS
https://www.smartzworld.com/notes/power-system-ii-ps-ii/

Ward Leonard speen control method

Ward Leonard control system is introduced by Henry Ward Leonard in 1891. Ward Leonard method of speed control is used for controlling the speed of a DC motor. It is a basic armature control method. This control system is consisting of a dc motor M₁ and powered by a DC generator G. In this method the speed of the dc motor (M_1) is controlled by applying variable voltage across its armature. This variable voltage is obtained using a motor-generator set which consists of a motor M₂ (either AC or DC motor) directly coupled with the generator G. It is a very widely used method of speed control of DC motor.

Principle of Ward Leonard Method

Basic connection diagram of the Ward Leonard speed control system is shown in the figure below.

The speed of motor M₁ is to be controlled which is powered by the generator G. The shunt field of the motor M₁ is connected across the DC supply lines. Now, generator G is driven by the motor M₂. The speed of the motor M₂ is constant.

When the output voltage of the generator is fed to the motor M₁ then the motor starts to rotate. When the output voltage of the generator varies then the speed of the motor also varies. Now controlling the output voltage of the generator the speed of motor can also be controlled. For this purpose of controlling the output voltage, a field regulator is connected across the generator with the dc supply lines to control the field excitation. The direction of rotation of the motor M₁ can be reversed by excitation current of the generator and it can be done with the help of the reversing switch R.S. But the motor-generator set must run in the same direction.

Advantages of Ward Leonard System

1. It is a very smooth speed control system over a very wide range.
(from zero to normal speed of the motor).
2. The speed can be controlled in both the direction of rotation of
the motor easily.
3. The motor can run with a uniform acceleration.
4. Speed regulation of DC motor in this ward Leonard
system is very good.
5. It has inherent regenerative braking property.

Disadvantages of Ward Leonard System

1. The system is very costly because two extra machines
(motor-generator set) are required.
2. Overall efficiency of the system is not sufficient especially
if it is lightly loaded.
3. Larger size and weight. Requires more floor area.
4. Frequent maintenance.
5. The drive produces more noise.

Application of Ward Leonard System

This Ward Leonard method of speed control system is used where a very wide and very sensitive speed control is of a DC motor in both the direction of rotation is required. This speed control system is mainly used in colliery winders, cranes, electric excavators, mine hoists, elevators, steel rolling mills, paper machines,diesel-locomotives, etc.

COMPLETE ELECTRICAL ENGG MATERIAL BASICS WITH DRAWINGS AND PICTURES

http://www.textbooksonline.tn.nic.in/books/11/stdxi-voc-ema-em-1.pdf

Basics

How Does Solar Power Work?

As the electricity flows into your solar power electrical system, it is either used in your building or “sold” back to your electric utility service for later use. Your electric meter literally runs backwards when you are producing net energy and forward when you use it.

Your solar system produces more energy in the summer, when energy rates are highest, and you can bank excess energy credits to use when you need them within a one-year cycle. You only pay for your yearly net power usage.

CONVERTING SUN LIGHT TO ELECTRICAL POWER

Solar power cells convert sunlight into electricity, using the energy of speeding photons to create an electrical current within a solar panel.

Photons are created in the center of the sun by the fusion of atoms. It takes a photon about a million years to work its way to the surface of the sun, but once free it is hurled through space so fast that it reaches earth in just eight minutes - after traveling 93 million miles.

This tremendous energy from the sun is abundant, and has been powering the earth for billions of years - feeding plants, redistributing and refreshing water supplies and ultimately creating other forms of energy (such as fossil fuels) that largely power our civilization today.

Over the past several decades, scientists have been learning to harness this ancient energy source with more efficiency to do the work of non-renewable fuels - without pollution, noise or radiation, and not subject to economic whims that drive costs higher each year.

An interesting side note: Photons are also called quanta. They are literally "packets" of sunlight" . Albert Einstein got his Nobel Prize for his study of quantum mechanics.

Advantages  of Solar Panels

As solar energy becomes increasingly accepted as many people start to take note of the rapidly increasing global warming problems, it is important to weigh both the advantages and disadvantages of this renewable energy source. The main aspect of solar energy that differentiates it from other sources of energy is that it is free. The only cost is usually linked to purchasing the solar panels that are then used to capture solar energy. Discussed below are several benefits and demerits of solar panels.

Decentralizes energy production

Another vital benefit that solar panels provide is that it helps to decentralize production of energy, which leads to a more self reliant society. Gas, oil and coal used to generate traditional electricity is usually transported internationally or cross-country. This transportation is associated with lots of other additional costs, such as air pollution and monetary expenses. All these costs are easily avoided with the use of solar energy.

Benefit is appreciated by people living in remote locations of the world with limited access to the main power lines. On grid normally means that a home stays connected to state power grid lines. However, off grid does not require a connection to state power grid lines, so the home or business being powered will be solely relying on solar power. This will benefit those in remote regions since power costs, particularly installation, are usually very high.

Disadvantages of solar panels

Inefficiency in production
A common criticism of solar energy is that it is comparatively inefficient. Solar efficiency refers to the amount of radiation that solar panels can convert to electrical energy, which is roughly 20%. Hence, you require a large surface area so as to produce lots of electricity. Nevertheless, solar efficiency continues to improve steadily and it is expected to rise over the coming years.

Inadequate storage
Storage of solar energy is yet to reach its full potential. Even though there are numerous solar batteries available on the market, these are bulky and costly. Furthermore, these batteries are more suitable for small scale solar panels and not huge solar farms.

High installation cost

Capital costs associated with solar energy installation are very high. Particularly unhelpful is the high upfront costs of buying solar panels. Imagine paying upfront for roughly 25 years of power. That is an exceedingly disadvantageous aspect of solar power, especially during this period of recession where people are struggling to meet their daily needs.

Solar power and other green technologies help to get people thinking with a view to be more environmentally friendly. Many governments continue to encourage solar use by providing rebates, subsidies and tariffs. A good example is INDIA, which is a success story of how these measures can really work to benefit the society as a whole.

SOURCE COPIED FROM SOLAR TECH.COM

POWER ELECTRONICS COMPLETE COURSE ---SOURCE NPTEL

PART I -- INTRODUCTION

silicon control rectifier

Power Systems- generation,transmission,distribution & protection video classes

http://nptel.iitm.ac.in/video.php?subjectId=108102047

Click here

1 - Electric Energy Systems A Perspective (54:14)

2 - Structure of Power Systems (59:38)

3 - Conventional Sources of Electric Energy (51:36)

4 - Hydroelectric Power Generation (53:24)

5 - Non Conventional Energy Sources (53:18)

6 - Renewable Energy (Contd.) (51:07)

7 - Energy Storage (49:41)

8 - Deregulation (48:35)

9 - Air Pollutants (48:20)

10 - Transmission Line Parameters (44:16)

11 - Capacitance of Transmission Lines (53:35)

12 - Characteristics and Performance of Transmission Lines (47:13)

13 - Voltage Regulation (VR) (52:21)

14 - Power Flow through a Line (52:19)

15 - Methods of Voltage Control (46:17)

16 - Compensation of Transmission Lines (48:25)

17 - Compensation of Transmission Lines (Contd.) (26:26)

18 - Underground Cables (34:59)

19 - Cables (Contd.) (24:29)

20 - Insulators for Overhead Lines (55:38)

21 - HVDC (50:06)

22 - HVDC (Contd.) (31:12)

23 - Distribution Systems (45:19)

24 - Automatic Generation Control (48:36)

25 - Automatic Generation Control (Contd.) (48:52)

26 - Load Flow Studies (51:10)

27 - Load Flow Problem (52:06)

28 - Load Flow Analysis (Contd.), Gauss Siedel Method (43:40)

29 - Newton Raphson (NR), Load Flow Method (42:06)

30 - Fast Decoupled Load Flow (40:11)

31 - Control of Voltage Profile (48:17)

32 - Optimal System Operation (Economic Operation) (41:04)

33 - Optimal Unit Commitment (44:55)

34 - Optimal Generation Scheduling (50:37)

35 - Optimal Load Flow (Contd.) and Hydro Thermal Scheduling (52:40)

VIDEO CLASSES-CONTROL SYSTEMS FOR GATE,IES- ELECTRICAL,ELECTRONICS&INSTRUMENTATION

CONTENTS

1- The Control Problem (01:03:24)
2 - Some More Examples (01:08:59)
3 - Different Kinds of Control Systems (01:05:16)
4 - History of Feedback (01:04:11)
5 - Modern Control Problems (47:25)
6 - DC Motor Speed Control (01:08:11)
7 - System Modelling, Analogy (01:13:09)
8 - Causes of System Error (01:02:50)
9 - Calculation of Error (01:03:21)
10 - Control System Sensitivity (01:05:00)
11 - Automatic Control of DC Motor (01:15:03)
12 - Proportional Control (01:13:12)
13 - Non-Unity Feedback (01:06:41)
14 - Signal-Flow Graph (01:04:57)
15 - Mason's Gain Formula (01:06:52)
16 - Signal-Flow Graph for DC Motor Control (01:04:25)
17 - Steady-State Calculations (57:23)
18 - Differential Equation Model and Laplace Transformation Model (01:00:12)
19 - D-Operator Method (57:29)
20 - Second-Order System Response (01:01:15)
21 - Frequency Response (52:07)
22 - Laplace Transformation Theorems (50:20)
23 - Final Value Theorem (55:30)
24 - Transfer Function and Pole-Zero Diagram (56:10)
25 - 'Good' Poles and 'Bad' Poles (57:31)
26 - Signal Flow Graph with Transfer Functions (49:33)
27 - s-Domain and t-Domain (54:55)
28 - Second-Order System Response in s-Domain (55:32)
29 - Integral Feedback (45:34)
30 - Root-Locus Method (1:00:28)
31 - Root-Locus Rules (49:16)
32 - Asymptotes of Root Locus (44:53)
33 - Routh Array (54:07)
34 - Singular Cases (54:40)
35 - Closed Loop Poles (36:22)
36 - Controller in the Forwarded Path (56:30)
37 - Mapping of Control in the Complex-Plane (51:09)
38 - Encirclement by a Curve (50:32)
39 - Nyquist Criterion (52:49)
40 - Application of the Nyquist Criterion (54:23)
41 - Polar Plot and Bode Plots (52:40)
42 - Logarithmic Scale for Frequency (53:04)
43 - 'Asymptotic' DB Gain (55:07)
44 - Compensating Network (50:51)
45 - Nichols' Chart (50:06)
46 - Time Domain Methods of Analysis and Design (51:21)
47 - State-Variable Equations (52:22)

PATTERN OF GATE-EE

BLUE PRINT OF GATE AS PER OBSERVATION 
 
DESCENDING ORDER OF THE SUBJECTS


1)CONTROL SYS(HIGHEST MARKS )
2)NETWORKS
3)MACHINES
4)ANALOG ELECTRONICS
5) ELECTRICAL INSTRUMENTS
6)POWER SYS.
7)DIGITAL ELECTRONICS
8)ELEC. FIELDS 
9)POWER ELECTRONICS(LOWEST MARKS)

ELECTRICAL ENGINEERING MPORTANT QUESTION AND ANSWERS FOR XAMS N INTERVIEWS

Question: What will happen when power factor is leading in distribution of power?
Ans: If there is high power factor, i.e., if the power factor is close to one:
1. Losses in form of heat will be reduced
2. Cable becomes less bulky and easy to carry, and very cheap to afford, &
3. It also reduces over heating of transformer

Question: What is the one main difference between UPS & inverter and electrical engineering & electronics engineering?
Ans: Uninterrupted power supply is mainly used for short time. Means according to ups VA it gives backup. UPS is also two types: online and offline. An online ups having high volt and amp for long time backup with high dc voltage. But ups start with 12v dc with 7 amps. But inverter is start with 12v, 24, dc to 36v dc and 120amp to 180amp battery with long time backup.

Question: What is ACSR cable and where we use it?
Ans: ACSR means Aluminium conductor steel reinforced, this conductor is used in transmission & distribution.

Question: What is the principle of motor?
Ans: Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movement is called as torque

Question: What is meant by armature reaction?
Ans: The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.

Question: Why Human bodies feel Electric shock?? In an Electric train during running, we don't feel any Shock. Why?
Ans: Unfortunately our body is a pretty good conductor of electricity, The golden rule is Current takes the lowest resistant path if you have insulation to our feet as the circuit is not complete (wearing rubber footwear which doing some repairs is advisable as our footwear is a high resistance path not much current flows through our body).The electric train is well insulated from its electrical system.

Question: Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?
Ans: DC Series motor has high starting torque. We can not start the Induction motor and Synchronous motors on load, but can not start the DC series motor without load

Question: What is vacuum circuit breaker? Define with cause and where be use it Device?
Ans: A breaker is normally used to break a circuit. While breaking the circuit, the contact terminals will be separated. At the time of separation an air gap is formed in between the terminals. Due to existing current flow the air in the gap is ionized and results in the arc. Various mediums are used to quench this arc in respective CB's. But in VCB the medium is vacuum gas. Since the air in the CB is having vacuum pressure the arc formation is interrupted. VCB's can be used up to 11kv.

Question: Why when birds sit on transmission lines or current wires don't get shock?
Ans: It?s true that if birds touch the single one line (phase or neutral) they don't get electrical shock... if birds touch 2 lines than the circuit is closed and they get electrical shock. so if a human touch single one line (phase) then he doesn't get shock if he is in the air (not touching - standing on the ground if he is standing on the ground then touching the line (phase) he will get a shock because the ground on what we standing is like line (ground bed - like neutral) and in the most of electric lines the neutral is grounded. so that means that human who touch the line closes the circuit between phase and neutral.

Question: What happens if we give 220 volts dc supply to a bulb or tube light?
Ans: Bulbs [devices] for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element.

Question: What are the advantages of speed control using thyristor?
Ans: Advantages:
1. Fast Switching Characteristics than Mosfet, BJT, IGBT
2. Low cost
3. Higher Accurate.

Question: How tube light circuit is connected and how it works?
Ans: A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is provided, the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will generate around 1000volts. This volt will capable of to break the electrons inside the tube to make electron flow. Once the current passes through the tube the starter circuit will be out of part. Now there is no change of supply causes choke voltage normalized and act as minimize the current.

Question: Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?
Ans: Link is provided at a Neutral common point in the circuit from which various connections are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e. load) only. So if any malfunction happens the fuse connected in the particular control circuit alone will blow off.

Question: What is MARX CIRCUIT?
Ans: It is used with generators for charging a number of capacitor in parallel and discharging them in series. It is used when voltage required for testing is higher than the available.

Question: What is "pu" in electrical engineering?
Ans: ?pu? stands for per unit and this will be used in power system single line diagram there it is like a huge electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the component is considered as base values, then all other component ratings will get back into this basis. Those values are called as pu values. (pu = actual value/base value).

Question: What is electric traction?
Ans: Traction means using the electric power for traction system i.e. for railways, trams; trolleys etc. electric traction means use of the electricity for all these. Now a day, magnetic traction is also used for bullet trains. Basically dc motors are used for electric traction systems.

Question: Describe the operation carried out in thermal power station.
Ans: The water is obtained in the boiler and the coal is burnt so that steam is obtained this steam is allowed to hit the turbine; the turbine which is coupled with the generator generates the electricity

Electrical Engineering Interview Questions Question: What are the advantages of speed control using thyristor?
Ans: Advantages:
1. Fast Switching Characteristics than Mosfet, BJT, IGBT
2. Low cost
3. Higher Accurate. TOP Question: How tube light circuit is connected and how it works?
Ans: A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is provided, the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will generate around 1000volts. This volt will capable of to break the electrons inside the tube to make electron flow. Once the current passes through the tube the starter circuit will be out of part. Now there is no change of supply causes choke voltage normalized and act as minimize the current.

Question: Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?
Ans: Link is provided at a Neutral common point in the circuit from which various connections are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e. load) only. So if any malfunction happens the fuse connected in the particular control circuit alone will blow off.


Question: How can you start-up the 40w tube light with 230v AC/DC without using any choke/Coil?
Ans: It's possible by means of Electronic choke. Otherwise it's not possible to ionize the particles in tube.

Question: Which type of A.C motor is used in the fans (ceiling fan, exhaust fan, padestal fan, bracket fan etc) which are found in the houses?
Ans: It is Single Phase Induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.

Question: Why syn. generators r used for the production of electricity?
Ans: Synchronous machines have capability to work on different power factor (or say different imaginary power varying the field emf. Hence, synchronous generators are used for the production of electricity.

Question: Give two basic speed control scheme of DC shunt motor?
Ans: 1. By using flux control method: in this method a rheostat is connected across the field winding to control the field current. So by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled
2. Armature control method: in this method a rheostat is connected across armature wedge. By varying the resistance the value of resistive drop (IaRa)can be varied, and since speed is directly proportional to Eb-IaRa the speed can be controlled

Question: What is the significance of vector grouping in Power Transformers?
Ans: Every power transformer has a vector group listed by its manufacturer. Fundamentally it tells you the information about how the windings are connected (delta or wye) and the phase difference between the current and voltage. E.g. DYN11 means Delta primary, Wye Secondary and the current is at 11 o clock referred to the voltage.

Question: Give two basic speed control scheme of DC shunt motor?
Ans: 1. By using flux control method: in this method a rheostat is connected across the field winding to control the field current. So by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled.
2. Armature control method: in this method a rheostat is connected across armature winding by varying the resistance the value of resistive drop (IaRa) can be varied, and since speed is directly proportional to Eb-IaRa the speed can be controlled.

Question: What is meant by armature reaction?
Ans: The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.

Question: What is 2 phase motor?
Ans: A two phase motor is a motor with the starting winding and the running winding have a phase split e.g.; ac servo motor where the auxiliary winding and the control winding have a phase split of 90 degree.

Question: What is the Polarization index value? (pi value)and simple definition of polarization index?
Ans: It?s ratio between insulation resistance (IR) i.e. meggar value for 10min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5.

Question: Advantages of vvvf drive over non-vvvf drives for EOT cranes.
Ans: 1. Smooth start and stop.
2. No jerking of load.
3. Exact positioning.
4. Better protection for motor.
5. high/low speed selection.
6. Reliability of break shoe.
7. Programmable break control.
8. Easy circuitry.
9. Reduction in controls.
10. Increases motor life.

Notes for an Introductory Course On Electrical Machines and Drives by E.G.Strangas

http://www.egr.msu.edu/~fzpeng/ECE320/ECE320-Notes-Part1.pdf
copy the link and paste on your adress bar or click on the title - notes for introductory course on elec. mach.

Contents


1 Three Phase Circuits and Power 1

1.1 Electric Power with steady state sinusoidal quantities 1
1.2 Solving 1phase problems 5
1.3 Threephase Balanced Systems 6
1.4 Calculations in threephase systems 9

2 Magnetics 15

2.1 Introduction 15
2.2 The Governing Equations 15
2.3 Saturation and Hysteresis 19
2.4 Permanent Magnets 21
2.5 Faraday’s Law 22
2.6 Eddy Currents and Eddy Current Losses 25
2.7 Torque and Force 27

3 Transformers 29

3.1 Description 29
3.2 The Ideal Transformer 30
3.3 Equivalent Circuit 32
3.4 Losses and Ratings 36
3.5 Perunit System 37
3.6 Transformer tests 40
3.6.1 Open Circuit Test 41
3.6.2 Short Circuit Test 41
3.7 Threephase Transformers 43
3.8 Autotransformers 44

4 Concepts of Electrical Machines; DC motors 47

4.1 Geometry, Fields, Voltages, and Currents 47

5 Threephase Windings 53

5.1 Current Space Vectors 53
5.2 Stator Windings and Resulting Flux Density 55
5.2.1 Balanced, Symmetric Threephase Currents 58
5.3 Phasors and space vectors 58
5.4 Magnetizing current, Flux and Voltage 60

6 Induction Machines 63

6.1 Description 63
6.2 Concept of Operation 64
6.3 Torque Development 66
6.4 Operation of the Induction Machine near Synchronous Speed 67
6.5 Leakage Inductances and their Effects 71
6.6 Operating characteristics 72
6.7 Starting of Induction Motors 75
6.8 Multiple pole pairs 76

7 Synchronous Machines and Drives 81

7.1 Design and Principle of Operation 81
7.1.1 Wound Rotor Carrying DC 81
7.1.2 Permanent Magnet Rotor 82
7.2 Equivalent Circuit 82
7.3 Operation of the Machine Connected to a Bus of Constant Voltage and Frequency 84
7.4 Operation from a Source of Variable Frequency and Voltage 88
7.5 Controllers for PMAC Machines 94
7.6 Brushless DC Machines 95

8 Line Controlled Rectifiers 99

8.1 1and 3Phase circuits with diodes 99
8.2 One Phase Full Wave Rectifier 100
8.3 Threephase Diode Rectifiers 102
8.4 Controlled rectifiers with Thyristors 103
8.5 One phase Controlled Rectifiers 104
8.5.1 Inverter Mode 104
8.6 ThreePhase Controlled Converters 106
8.7 *Notes 107


9 Inverters 109

9.1 1phase Inverter 109
9.2 Threephase Inverters 111

10 DC to DC Conversion 117

10.1 StepDown or Buck Converters 117
10.2 Stepup or Boost Converter 119
10.3 Buckboost Converter 122

Electronics and communications subject most important n frequently asked multiple choice questions of BEL,AAI,DRDO,JTO paper 2

SET - 2

1. Resistivity of silicon in ohms cm. is approx. equal to

a. 50 b. 1012 c. 230k d. 10-6

2. Rsistivity ofGermanium in ohms cm. is approx. equal to

a. 50 b. 10-12 c. 50k d. 10-6

3. The number of free electrons/cubic cm intrinsic Germanium at room temperature

is approx. equal to

a. 1.5*1010 b. 2.5*1013 c. 1000 d. 5*106

4. The number of free electrons/cubic cm of intrinsic silicon at room temperature is

approx. equal to

a. 1.5*1010 b. 2.5*1013 c. 10000 d. 5*106

5. The forbidden energy gap for silicon is

a. 1.1eV b. 067eV c. 0.97eV d. 1.7eV

6. The forbidden energy gap for Germanium is

a. 1.1eV b. 067eV c. 0.97eV d. 1.7eV

7. N type material is formed by the addition of the following (penta valent )atom in n

to semiconductor material

a. Antimony

b. Arsenic

c. Phosphorous

d. Any of the above

8. P type material is formed by the addition of the following [Trivalent] atom tn to

semiconductor material

a. Boron b. Gallium c. Indium d. Any of the above

9. Impurity atoms that produces N type material by its addition in semiconductor is

called

a. Donar b. Acceptor c. Conductor d. Insulator

10. Impurity atoms that produces P type material by its addition in semiconductor is

called

a. Donar b. Acceptor c. Conductor d. Insulator

11. Dynamic resistance of a diode Rd is if voltage changes is DVd and the current

change is D Id

a. D Vd / D Id

b. D Id / D Vd

c. 1 / DVd

d. 1 / D Id

12. Point contact diodes are preferred at very high frequency, because of its low

junction

a. Capacitance and inductance

b. Inductance

c. Capacitance

13. Identify the circuit given below

a. AND gate

b. OR gate

c. Rectifier

d. NOR gate

14. Identify the circuit given below

a. AND gate

b. OR gate

c. Rectifier

d. NOR gate

15. DC value of a Half wave rectifier with Em as the peak value of the input is

a. 0.318Em

b. 0.418Em

c. 0.518Em

d. 0.618Em

16. Change in Zener voltage of 10V at 100o C if temperature co-efficient is 0.072%o

C as

a. 0.54 V b. 0.74 V c. 0.64 V d. 0.14 V

17. If Tc is th e% temperature co of / oC and Vz as zener voltage and T as change in

temperature then the change in zener voltage is

c. 100. Vz. Tc DT

d. None of these of the above

18. PIV for a full wave rectifier, if Em is the peak voltage is

a. Em b. 1.5Em c. 0.636Em d. 2Em

19. Schottky Barrier diodes becomes important at

a. DC level operation

b. Low frequency operation

c. High frequency operation

d. None of these

20. Clamping network is the one that will clamp the signal to a

a. Different peak value

b. Different DC level

c. Different polarity level

d. Different RMS level

21. Clipping network is the one that will clip a portion of the

a. Input signal without distorting the remaining portion

b. Input signal with distorting the remaining portion

c. Any of the above

d. None of these

22. Transition capacitance Ct of a Varicap diode with Knee voltage Vt, reverse

voltage Vr and K, the constant based on semiconductor material and the construction

technique & N dependent on type of junction is given by

a. 1 / K (Vt + Vr)N/2

b. 1 / K (Vt + Vr)N

c. K / (Vt + Vr)N

d. K / (Vt + Vr)1/N

23. Ct = K / (Vt + Vr)N where Vt Knee voltage, Vr reverse voltage, K manufacturing

dependent constant and N dependent on type of junction, for alloy junction the value

of N is

a. 1/3 b. 2/3 c. 1/2 d. 1/4

24. Ct = K / (Vt + Vr)N where Vt Knee voltage, Vr reverse voltage, K manufacturing

dependent constant and N dependent on type of junction, for diffused junction the

value of N is

a. 1/3 b. 2/3 c. 1/2 d. 1/4

25. In JFET, the drain current Id is given by (Idss drain – source saturation current

Vgs – Gate – source voltage, Vp the pinch off voltage)

a. Idss[1 – Vp/Vgs]

b. Idss(1 – Vgs/Vp)2

c. Idss[1 – Vgs/Vp)

d. Idss(1 – Vgs/Vp)3/2

26. The shadow mask in colour tube is used to

a. Reduce X-Ray emission

b. Ensure each beam hits its own dots

c. Increase screen brightness

d. Provide degaussing for the screen

27. Indicate which of the following signal is not transmitted in colour TV

a. Y b. Q c. R d. I

28. Another name for horizontal retrace in TV receiver is the

a. Ringing b. Burst c. Damper d. Fly back

29. Another name for the colour sync in the colour TV system

a. Ringing b. Burst c. Damper d. Fly back

30. The HV anode supply for a picture tube of a TV receiver is generated in the

a. Mains transformer

b. Vertical output stage

c. Horizontal output stage

d. Horizontal oscillator

31. The output of vertical amplifier is

a. Direct current

b. Amplified vertical sync pulse

c. A saw tooth voltage

d. A saw tooth current

32. In a transistor if Alpha = 0.98, current gain is equal to

a. 29 b. 59 c. 69 d. 49

33. The active region in the common emitter configuration means

a. Both collector and emitter junction is reverse biased

b. The collector junction is forward biased and emitter junction

c. The collector junction is reverse biased and emitter junction is forwared biased

d. Both collector & emitter junction are forward biased

34. The saturation region in the common emitter configuration means that

a. Both collector & emitter junction are reverse biased

b. The collector junction is forward biased and emitter junction

c. The collector junction is reverse biased and emitter junction is forwared biased

d. Both collector & emitter junction are forward biased

35. The % of Red, Green & Blue in 100% White Y is given by

a. 30%, 59%, 11%

b. 50%, 30%, 11%

c. 30%, 11%, 50%

d. 33.3%, 33.5%, 38.3%

36. Equalizing pulse width, if H is the Horizontal sync rate

a. 0.64 H b. 0.07 H c. 0.04 H d. 0.16 H

37. In a simple RC network the bandwidth is equal to

a. 1/2 p RC

b. RC / 2

c. 2 C / p R

d. 2 p / RC

38. The time constant of a RC network is given by

a. RC b. C/R c. R/C d. None of these

39. First zero crossing of pulse frequency spectrum occurs at if d is the pulse width,

T is the pulse repetition rate

a. 1/d b. d/T c. T/d d. T

40. The distortion less output characteristic of a network means

a. Constant amplitude and linear phase shift over frequency

b. Linear phase shift and amplitude need not be constant

c. Any amplitude and phase

d. None of these

41. Single sideband means suppressed

a. Carrier

b. Carrier and one side band

c. One side band

d. None of these

42. In an amplitude modulated signal, lower side band frequency is equal to (if the

carrier frequency is fc and modulation frequency is fm)

a. fm + fc b. fc – fm c. fm r fc d. fc / fm

43. Modulation index of the frequency modulation depends on

a. Amplitude & frequency of the modulation signal

b. Frequency and amplitude of carrier signal

c. Carrier frequency

d. None of these

44. The BW of the narrow band FM if modulating frequency is fm

a. 3 r fm b. 2 r fm c. 2.5 r fm d. 10 r fm

45. Reactance tube modulator is known for

a. FM b. AM c. PPM d. PAM

46. Bandwidth and rise time product is

a. 0.35 b. 0.45 c. 0.30 d. 0.49

47. Energy gap, Lg, for Germanium at room temp [300o K] is

a. 0.72eV b. 1.1eV c. 1.53eV d. 0.2eV

48. Volt equivalent of temperature VT, at 116o K is

a. 0.11V b. 0.01V c. 1.16V d. 0.1V

49. Reverse saturation current of a Ge.diode is in the range of

a. mA b. uA c. nA d. pA

50. Cut-in voltage V for silicon is approximately

a. 0.2V b. 0.6V c. 0.9V d. 1.1V

51. Every 10o C rise in temp. the reverse saturation current

a. Doubles

b. Halves

c. Triples

d. No change

52. Hall effect with reference to Metal or Semiconductor carrying a current I is placed

in a transverse magnetic field B, an electric field E is induced in

a. Parallel to B

b. Perpendicular to I

c. Perpendicular to both B & I

d. Perpendicular to B

53. 1 eV (electron volt) is equal to:

a. 1.9 r 10-20 J

b. 1.6 r 10-19 J

c. 1.6 r 10-20 J

d. 1.16 r 10-19 J

54. Donar impurity is having a valency of:

a. 2 b. 3 c. 4 d. 5

55. Acceptor impurity is having a valency of

a. 2 b. 3 c. 4 d. 5

56. Electron volt arises from the fact that if any electron falls through a potential of 1

volt, its kinetic energy will

a. Decrease, & potential energy will increase

b. Increase & potential energy decrease

c. Be unaltered & potential energy decreases

d. Increase & potential energy increase

57. Hole is created in a semiconductor material if one of following impurities are

added

a. Antimony

b. Arsenic

c. Indium

d. Phosphorus

58. Excess electron is created by

a. Boran

b. Gallium

c. Indium

d. Arsenic

59. A snubber circuit is used across the SCR to protect against

a. The di/dt of the anode current

b. The dv/dt turn on

c. L.di/dt of load inductance

d. None of these

60. Germanium has the valency of

a. 2 b. 3 c. 4 d. 5

61. Silicon has the valency of

a. 2 b. 3 c. 4 d. 5

62. Hole acts as a free charge carrier of polarity

a. Negative

b. Positive

c. Neutral

d. None of these

63. Burst signal in NTSC system is 8 cycles of the frequency of

a. Colour sub carrier

b. Picture carrier

c. Sound carrier

d. None of these

64. Colour sub carrier reference burst is superimposed on the

a. Back porch of the each horizontal sync pulse

b. Front porch of the each horizontal sync pulse

c. Front porch of the each vertical sync pulse

d. Back porch of the each vertical sync pulse

65. The law of mass action with reference to semiconductor technology states that

the product of free negative & positive concentration is a constant and

a. Independent of amount of donor and acceptor doping

b. Dependent on amount of donor and independent of the amount acceptor impurity

doping

c. Depend on amount of both donor & acceptor impurity doping

d. None of these

66. The snubber circuit used across SCR is a simple

a. R-L network

b. RLC network

c. LC network

d. RC network

67. To limit the rate of rise of SCR anode current a small

a. Inductor is inserted in cathode circuit

b. Inductor is inserted in anode circuit

c. Capacitor is inserted in anode circuit

d. Capacitor is inserted in cathode circuit

68. Torque developed by a DC servo motor is proportional to the

a. Product of power and time

b. Product of armature current and back emf

c. Armature voltage and armature current

d. Field voltage and field current

69. Proportional Integral control

a. Reduces steady state error but reduces the forward gain

b. Increases the forward gain and reduces the steady state error

c. Increases the steady state error and increases the forward gain.

d. None of these

70. Increasing the servo bandwidth:

a. Improves signal to noise ratio

b. Improves speed response and lowers signal to noise ratio

c. Improves power output

d. None of these

71. Notch filter is

a. Low pass filter

b. High pass filter

c. Narrow stop band filter

d. Narrow pass band filter

72. In TV Receivers the Electron beam deflection method used is

a. Electro static

b. Electro magnetic

c. Magnetic

d. All the above

73. In a line of sight communication the maximum range R in miles between the

receiver antenna and transmitter antenna of height H in feet is approximately

a. R = 1.93 ÖH

b. R = 1.23 ÖH

c. R = 1.53 ÖH

d. R = 2.03 ÖH

74. In wavelength of the 60 MHz carrier frequency is

a. 10 metres.

b. 15 metres

c. 5 metres

d. 2.5 metres

75. In standard TV receiving antenna the dipole element is

a. 0.5 of the wave length

b. 0.25 of the wave length

c. 1.5 of the wave length

d. 1.0 of the wave length

76. The characteristics of FET are similar to:

a. Triode

b. Tertode

c. Pentode

d. Diode

77. Charge coupled device is an array of capacitors whose structure is similar to:

a. Shift register

b. Flip-flop

c. NAND gate

d. Amplifier

78. Operational amplifier characteristics are which of the following:

a. Infinite gain

b. Infinite input impedance

c. Output impedance is zero

d. All of the above.

79. The typical value of the open loop gain in dB of an amplifier at DC with no

feedback is:

a. 90 to 100

b. 80 to 90

c. 0 to 50

d. 50 to 70

80. The 3 dB band width means the frequency at which

a. The open loop voltage gain reduced to 0.707

b. The open loop gain reduced to unity

c. Maximum voltage of a signal is without distortion

d. It is a medium wave band width of radio receiver

81. Rise time of an amplifier is defined as time required

a. To change from 0 to 100 % of its final value

b. To change from 0 to 50 % of its final value

c. To change from 10 to 90 % of its final value

d. To change from 10 to 100 % of its final value

82. High speed amplifier design emphasized on

a. Extremely small bandwidth

b. Very slow response

c. Unity gain bandwidth after 10 MHz

d. None of these

83. Tuned amplifier having the frequency range between

a. 150 KHz – 50 MHz

b. 100 Hz – 100 KHz

c. 100 KHz – 120 KHz

d. 50 MHz – 100 MHz

84. The resonance frequency of a tuned circuit made up of R, L, C is given by

a. 1/2 pÖLC

b. 2 pÖLC

c. 2 p / ÖLC

d. ÖLC / 2

85. The voltage follower can be obtained using operational amplifier

a. Without any feedback

b. Series parallel feedback of unity

c. Parallel feedback

d. Series feedback

86. Fidelity of the amplifier is when

a. It is a linear amplifier

b. It does not add or subtract any spectral components

c. It amplifier each component by the same amount

d. All of the above

87. What would be the output when two input sine waves of frequency 50 KHz and

100 KHz passed through an amplifier in the medium signal

a. 50 KHz and 100 KHz

b. 100 KHz and 200 KHz

c. 50 KHz and 150 KHz

d. All of the above

88. The important application of Schmitt trigger is

a. To convert slowly varying input voltage to abrupt voltage change

b. To convert abruptly varying input voltage into slowly varying output

c. To change the frequency of the input

d. None of these

89. Meaning of decoding is

a. Binary addition

b. Data transmission

c. Demultiplexing

d. Storage of binary information

90. Approximately how many number of gates are incorporated in SSL chip

a. 12

b. 100

c. Excess of 100

d. Excess of 1000

91. The circuit diagram represents which one of the following

a. Half adder

b. Full adder

c. Exor gate

d. AND gate

92. Flip flop cannot be called as

a. Bistable multivibrator

b. 1 Bit memory unit

c. latch

d. combinational circuit.

93. The important use of low pass filter in power supply is

a. To get the regulation in the output voltage

b. To filter out the ripple frequency

c. To increase the current rating

d. To convert AC into DC

94. Binary equivalent of the decimal number 145 is

a. 10010001

b. 1001011

c. 1010001

d. 1100010

95. In which of the following gate the output will be high when all the maintained at

high level

a. NOR

b. AND

c. NAND

d. EXOR

96. Which of the following definition is true in the De Morgan’s theorem

a. Multiplication symbols are replaced by addition symbol

b. Addition symbols are replaced by Multiplication symbol

c. Each of the terms are expressed in the complementary form

d. All of the above

97. 8421/BCD code fro a decimal number 149 is

a. 0001 0100 1001

b. 10010101

c. 10101001

d. None of these

98. Combinational circuit are mainly characterized by

a. Output depends upon the previous state & presents state

b. Output depends upon the input at that particular instant

c. Output depends upon the presents state & the clock state

d. Output does not depends upon the input at all

99. A flip flop is defined as

a. A bistable device with two complementary outputs

b. It is memory element

c. It will respond to input and it is a basic memory element

d. All of the above

100. Four bit code is called

a. Nibble

b. Byte

c. Word

d. Register

1. c

2. a

3. a

4. b

5. a

6. b

7. d

8. d

9. a

10. b

11. a

12. c

13. b

14. a

15. a

16. a

17. a

18. d

19. c

20. b

21. a

22. c

23. c

24. a

25. b

26. b

27. c

28. d

29. b

30. c

31. d

32. d

33. c

34. d

35. a

36. c

37. a

38. d

39. a

40. a

41. b

42. b

43. a

44. b

45. b

46. a

47. a

48. b

49. b

50. b

51. a

52. c

53. b

54. b

55. b

56. b

57. c

58. d

59. b

60. c

61. c

62. b

63. a

64. a

65. a

66. d

67. b

68. b

69. a

70. b

71. c

72. c

73. b

74. c

75. a

76. c

77. a

78. d

79. d

80. a

81. c

82. c

83. a

84. a

85. b

86. d

87. a

88. a

89. c

90. a

91. a

92. d

93. b

94. a

95. b

96. d

97. a

98. b

99. d

100.a