Department of Electrical and Electronics Engineering. II year - III semester ECE Electrical Engineering. 2-Marks Question Bank & University Qn/Ans

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1 Department of Electrical and Electronics Engineering II year - III semester ECE Electrical Engineering 2-Marks Question Bank & University Qn/Ans Prepared by Thenmozhi.N, AP/EEE

2 EC 2201 ELECTRICAL ENGINEERING L T P C AIM To expose the students to the concepts of various types of electrical machines and transmission and distribution of electrical power. OBJECTIVES To impart knowledge on Constructional details, principle of operation, performance, starters and testing of D.C. machines. Constructional details, principle of operation and performance of transformers. Constructional details, principle of operation and performance of induction motors. Constructional details and principle of operation of alternators and special machines. Power System transmission and distribution. UNIT I D.C. MACHINES 9 Constructional details emf equation Methods of excitation Self and separately excited generators Characteristics of series, shunt and compound generators Principle of operation of D.C. motor Back emf and torque equation Characteristics of series, shunt and compound motors - Starting of D.C. motors Types of starters - Testing, brake test and Swinburne s test Speed control of D.C. shunt motors. UNIT II TRANSFORMERS 9 Constructional details Principle of operation emf equation Transformation ratio Transformer on no load Parameters referred to HV/LV windings Equivalent circuit Transformer on load Regulation - Testing Load test, open circuit and short circuit tests. UNIT III INDUCTION MOTORS 9 Construction Types Principle of operation of three-phase induction motors Equivalent circuit Performance calculation Starting and speed control Single-phase induction motors (only qualitative treatment). UNIT IV SYNCHRONOUS AND SPECIAL MACHINES 9 Construction of synchronous machines-types Induced emf Voltage regulation; emf and mmf methods Brushless alternators Reluctance motor Hysteresis motor Stepper motor. UNIT V TRANSMISSION AND DISTRIBUTION 9 Structure of electric power systems Generation, transmission and distribution systems - EHVAC and EHVDC transmission systems Substation layout Insulators cables. TOTAL = 45 PERIODS TEXT BOOKS 1. D.P.Kothari and I.J.Nagrath, Basic Electrical Engineering, Tata McGraw Hill publishing company ltd, second edition, 2007 (Reprint). 2. C.L. Wadhwa, Electrical Power Systems, New Age International, fourth edition, REFERENCES 1. S.K.Bhattacharya, Electrical Machines, Tata McGraw Hill Publishing company ltd, second edition, V.K.Mehta and Rohit Mehta, Principles of Power System, S.Chand and Company Ltd, second edition,

3 UNIT I - D.C. MACHINES Constructional details emf equation Methods of excitation Self and separately excited generators Characteristics of series, shunt and compound generators Principle of operation of D.C. motor Back emf and torque equation Characteristics of series, shunt and compound motors - Starting of D.C. motors Types of starters - Testing, brake test and Swinburne s test Speed control of D.C. shunt motors. Two mark questions and answers 1. What is an electrical generator? An electrical generator is a machine which converts mechanical energy into electrical energy. 2. What is the basic principle of a dc generator? The basic principle of a dc generator is Faraday s law of electromagnetic induction. According to faraday s law, whenever the flux linking with a conductor changes, an electromotive force is set up in that conductor. Such an emf is called dynamically induced emf. 3. What are the essential parts of a d.c generator? Magnetic frame or yoke Poles shoes and pole cores Armature core Commutator Brushes and bearings 4. What is the purpose of yoke in d.c machine? It acts as a protecting cover for the whole machine and provides mechanical support to the poles. It carries the magnetic flux produced by the poles. 5. State the functions of poles in a d.c machine Poles carries a field winding It directs flux produced through air gap to armature core, to the next pole. Pole shoe enlarges the area of armature core to come across the flux 6. The core of armature is laminated in DC machines. Justify? The purpose of armature core lamination is to reduce the eddy current losses. 7. List the different types of armature windings. Lap winding (A = P) Wave winding (A = 2) where A- number of parallel paths p-number of poles 2

4 8. What is the purpose of Commutator in d.c generator? To facilitate the collection of current from the armature conductors. To convert alternating emf into uni-directional emf. 9. Write the emf equation of d.c generator. φzn E g = 60 P volts A Where φ - flux/pole in webers. Z total number of armature conductors N speed of the armature in rpm. P number of poles A number of parallel paths Lap winding (A = P) Wave winding (A = 2) 10. What are the different types of generator? 1) Separately excited d.c generator 2) Self excited d.c generator a. Series generator b. Shunt generator c. Compound generator Long shunt compound Short shunt compound 11. Define back emf and write its significance. There is an induced emf in the rotating armature conductors of DC motor, according to Faraday s Law of electromagnetic induction. According to Lenz s law, the direction of induced emf is to oppose the cause producing it and hence the induced emf opposes the supply voltage. So, this induced emf is called the back emf. Significance: The presence of back emf makes the dc motor as a self regulating machine. This means that the motor adjusts itself to draw the armature current just enough to satisfy the load demand. The basic principle of this fact is that the back emf is proportional to speed. 12. Classify methods of excitation and explain? 1) Separately excitation- The field winding is excited by separate d.c source 2) Self excitation- The field winding is excited by using emf induced in the generator itself. Initially at start, the emf is due to the presence of residual flux in the poles. 13. Why d.c series motor cannot be started without any load? In d.c series motor, φ I a where φ - flux/pole I a - Armature current On no load, as I a is small, flux produced is also small. Also, N α 1 as back emf E b is constant. φ 3

5 So on no load or very light load, as flux is very small, the motor tries to run at dangerously high speed and this will damage the motor. So a d.c series motor should never be started without any load. 14. Write down torque equation of d.c motor. 1 PZ Ta = φi a N-m 2π A PZ Ta = 0.159φI a N-m A Where, - Torque in N-m - Flux per pole in Wb - Armature current in Amps P- Number of poles Z- Total number of conductors A-Number of parallel paths 15. What are the applications of d.c motors? D.C Shunt motor - Constant speed motor with medium starting torque Blowers and fans Centrifugal and reciprocating pumps Lathe machines Machine tools Milling machines Drilling machines D.C Series motor - High starting torque, variable speed, no load condition dangerous. Cranes Hoists and Elevators Trolleys Conveyors Electric locomotives D.C Compound motor(cumulative) - High starting torque, No load condition allowed. Rolling mills Punching machines Driving a heavy machine tools for intermittent loads shears Heavy planers Elevators. D.C Compound motor(differential) - Speed increases as load increases Not very suitable for practical applications. 16. When is a four point starter required in D.C motors? A four point starters required for dc motor under field control. 17. If the speed is decreased in a DC motor, what happens to the back emf and the armature current? 4

6 As speed decreases, back emf also decreases since E b N and the net voltage across the armature (V - E b ) increases and motor draws more armature current. So, as speed decreases, armature current increases. 18. What is the necessity of starter? When a dc motor directly switched on, at the time of starting, motor back emf is zero, as speed is zero (E b N). So, the voltage equation, in general, V = E b + I a R a reduces to V = I a R a. this implies that the armature current is very high, and the motor winding gets damaged. So, starter is used to reduce starting current of the motor. 19. List the important parts of D.C motor starter. Starting resistance Over Load Release No Voltage Coil Spring Starter handle 20. What are the functions of NVC and OLR? No voltage release coil (NVR) - Under low voltage condition, the coil is deenergized and the motor is disconnected from the supply. Over load release coil (OLR) - Under over load condition, the coil is energized and attracts the arm upwards. This shorts the NVC and motor is disconnected from supply. 21. What is the disadvantage of a three point starter/ What is the need for a four point starter in place of three point starter? In a three point starter the NVC and field winding are in series. So, while controlling the speed of the motor above rated speed, field current is reduced due to the extra resistance in series with the field winding. So, the current through NVC also reduces. This reduces the magnetism produced by the NVC, which releases the handle to the OFF position. To rectify his problem, a four point starter is used, where NVC and field winding are connected in parallel. 22. Where are three point and four point starters recommended? Three point starter- armature speed control (below rated speed) Four point starter- field speed control (above rated speed) 23. Why brake test is performed in d.c motors? (Nov 2005) It is performed to obtain actual efficiency of the motor under working conditions. With the help of this direct test, the motor characteristics can also be obtained. 24.What are the drawbacks of brake test on DC machine? The brake test can be used in small rating motors only. Due to friction, heat is generated and there is a large dissipation of energy. Cooling arrangement is necessary. This method cannot be used for determining internal losses The power developed gets wasted. 5

7 25. What is the use of swinburne s test? It is also no-load or lossless method. The efficiency can be determined for any desired load conditions, without actually loading the machine. 26. What are the advantages and disadvantages of Swinburne s test? Advantages: Economical operation. The efficiency can be determined for any load condition. Losses can be determined without applying actual load. Disadvantages: It is not applicable for d.c series motor. The change in iron losses from no-load to full load is not considered. The test is conducted for no-load condition. When applying full load, no idea about satisfactory commutation. 27. Write the expression for the speed of a d.c motor and therefrom suggest the parameter to be varied for speed control. (Nov 2002) The speed is given by Eb V I a Ra Nα α φ φ But I a R a is very small compared to V. Hence, N α V φ Thus to control the speed, the following parameters can be varied. The applied voltage The flux produced by the field winding The armature voltage control called rheostatic control 28. What are the different types of speed control of a d.c motor? Voltage control Field control Armature control Detailed Answer Questions 1.Draw and explain about the constructional details of D.C Machine Diagram of the cross-sectional view Explanation of each parts Yoke Poles Field winding Armature Commmutator Brushes 2. Explain the EMF Equation of a dc machine. Assumptions for notations 6

8 Derivation EMF equation for lap and wave winding φzn P E g = volts 60 A 3. Describe the principle of operation of D.C motor. Faradays law of Electromagnetic Induction. Diagram showing change in flux Explanation of the working 4. Explain the different types of dc generators with respect to excitation Separately excited dc generator Self-excited dc generator Series Generator: Rising voltage characteristics-used as booster in feeders Shunt Generator: Constant voltage- Battery charging,ordinary lighting Compound Generator: Differential compound -Drooping characteristics-welding power supply Flat compound-constant voltage-ordinary lighting overcompound-voltage increases as load increases-booster in traction lines 5. Explain characteristics of series, shunt and compound motors Graph and explanation for the following characteristics Torque Vs Armature current Speed Vs Armature current Speed Vs Torque 6. Explain about starting and types of starters used in D.C motors. Need for starter Types of starters Two point starter Three Point Starter Four point Starter For each starter, Draw diagram Explain about working Write Advantages and Disadvantages, if any 7. Explain the testing of D.C machines. Brake test Swinburne s test For each test, Draw experimental set up diagram Tabulation Formulae, if any Method of calculation to find efficiency. 7

9 UNIT II. TRANSFORMERS Constructional details Principle of operation emf equation Transformation ratio Transformer on no load Parameters referred to HV/LV windings Equivalent circuit Transformer on load Regulation - Testing Load test, open circuit and short circuit tests. Two mark questions and answers 1. What is a transformer? A transformer is a static piece of device which transforms electrical power from one alternating current circuit to another with desired change in voltage and current, without any change in frequency. It accomplishes this by electromagnetic induction, where the two electric circuits are in mutual inductive influence of each other. 2. What is the purpose of laminating the core in a transformers? To reduce eddy current loss. 3. Mention the difference between core and shell type transformers. Or How transformers are classified according to their construction? S. Core Type Shell type No 1. The winding encircles the core. The core encircles most part of the winding. 2. It has a single magnetic circuit It has a double magnetic circuit 3. The core has two limbs The core has three limbs 4. Cylindrical coils are used Multilayer disc or sandwich type coils are used 5. The coils can be easily removed for The coils cannot be removed easily maintenance 6. As the windings are uniformly distributed on two limbs, natural cooling Natural cooling does not exist as the windings are surrounded by the core. is effective. 7. Preferred for low voltage transformers Preferred for high voltage transformers 4. Give the emf equation of a transformer and define each term. Emf induced in primary coil E 1 = 4.44 f Ф m N 1 volt Emf induced in secondary coil E 2 = 4.44fФ m N 2 volt Where f - frequency of AC input Ф m - the maximum value of flux in the core N 1, N 2 - are the number of primary and secondary turns. 8

10 5. Does the transformer draw any current when secondary is open? Why? Yes, it (primary) will draw the current from the main supply in order to magnetize the core and to supply iron and copper losses on no load. There will not be any current in the secondary since secondary is open. 6. Define voltage regulation of a transformer The change in secondary terminal voltage from no-load to full load expressed as a percentage of no- load or full load voltage is termed as regulation. % regulation down = (V 2o -V 2 ) /V 2o x 100 % regulation up = (V 2o -V 2 ) /V 2 x 100 where V 20 - secondary voltage on no-load V 2 - secondary voltage no full load 7. Define all day efficiency of a transformer. It is the computed on the basis of energy consumed during a certain period, usually a day of 24 hrs. η all day = output in kwh /input in kwh, for 24 hrs. 8. Explain on the material used for core construction. The core is constructed of transformer sheet steel laminations assembled to provide a continuous magnetic path with a minimum of air gap included. The steel used is of high silicon content sometimes heat treated to produce a high permeability and a low hysteresis loss, at the usual operating flux densities. The eddy current loss is minimized by laminating the core, the laminations being insulated from each other by light coat of coreplate vanish or by an oxide layer on the surface.the thickness of laminations varies from 0.35 mm for a frequency of 50 Hz and 0.5 mm for a frequency of 25 Hz. 9. When will a Bucholz relay operate in a transformer? Bucholz rely is a protective device in a transformer. If the temperature of the coil exceeds its limit, Bucholz relay operates and gives an alarm. 10. How does change in frequency affect the operation of a given transformer? With a change in frequency, iron loss, copper loss, regulation, efficiency and heating varies and thereby the normal operation of the transformer is affected. 11. What is the angle by which no-load current will lag the applied voltage in an ideal transformer? In an ideal transformer, there are no copper loss and no core loss, (i.e. loss free core). The no load current is only magnetizing current. Therefore the no-load current lags behind the voltage by an angle of 90. However, in a practical transformer, the windings possess resistance and leakage reactance and therefore the no-load current lags the applied voltage by an angle slightly less than List the advantages of stepped core arrangement in a transformer. (i) To reduce the space effectively. (ii) To obtain reduced length of mean turn of the windings. 9

11 (iii) To reduce I 2 R loss. 13. Why are breathers used in transformers? Breathers are used to entrap the atmospheric moisture and thereby not allowing it to pass on to the transformer oil. to permit the oil inside the tank to expand and contract as its temperature increases and decreases. to avoid sledging of oil i.e. decomposition of oil. Addition of 8 parts of water in reduces the insulations quantity of oil. Normally silica gel is filled in the breather having pink colour. This colour will be changed to white due to continuous use, which is an indication of bad silica gel. It is normally heated and reused. 14. What is a transformer oil? What is the function of transformer oil in a transformer? Nowadays instead of natural mineral oil, synthetic oils known as ASKRELS (trade name ) are used. They are non-inflammable & under an electric arc do not decompose to produce inflammable gases. PYROCOLOR oil possess high dielectric strength. The function of the transformer oil is to provide, (i)good insulation and (ii)cooling. 15. A 1100/400 V, 50 Hz single phase transformer has 100 turns on the secondary winding. Calculate the number of turns on its primary. V2 N 2 We know, transformation ratio is = = K V1 N Substituting = 1100 N N1 = 100/400 x 1100 = 275 turns What are the functions of no-load current in a transformer? No-load current produces flux and supplies iron loss and copper loss on no-load. 17. How will you transfer the quantities from secondary to primary in a transformer? Where K = N2/N1 = transformation ratio 18. Can the voltage regulation of a transformer go to negative? If so under what condition? 10

12 Yes. If the load has leading power factor, the voltage regulation of a transformer goes negative. 19. Distinguish between power transformer and distribution transformer. Power transformers have very high power ratings in the order of MVA. They are used in generating and receiving stations. Sophisticated controls are required. Voltage ranges will be very high. Distribution transformers are used in consumer side. Voltage levels will be medium. Power ranging will be small in order of kva. Complicated controls are not needed. 20. What is the purpose of providing taps in transformer and where these are provided? In order to attain the required voltage, taps are provided. They taps are used to vary the number of turns in a winding, so as to vary the voltage. Normally it is provided at low voltage side. 21. What are the factors on which hysteresis loss depends? The hysteresis loss depends on the magnetic flux density B, frequency f and the volume of the material V. 22. What is core loss? What is its significance in electric machines? When a magnetic material undergoes cyclic magnetization, two kinds of power losses occur on it hysteresis and eddy current loss which together are known as core loss. It is important in determining heating, temperature rise, rating and efficiency of transformers, machines and other a.c run magnetic devices. 23. What is eddy current loss? When a magnetic core carries a time varying flux, voltages are induced in all possible paths enclosing flux. Result is the production of circulating current in the core. These induced currents do no useful work and are known as eddy currents. The corresponding power loss is known as eddy current loss. 24.How are hysteresis and eddy current losses minimized? Hysteresis loss can be minimized by selecting materials for core such as silicon steel & steel alloys with low hysteresis coefficient and electrical resistivity. Eddy current losses are minimized by laminating the core. 25. What is an ideal transformer? An ideal transformer is one which has the following properties. It has no losses Its windings have zero resistance There is no magnetic leakage Permeability of the core is so high that negligible current is required to establish the flux in it. 26. What is the purpose of the magnetizing current in transformer? The magnetizing current is the component of the primary current which is responsible for the production of flux in the core. 11

13 27. What are the losses occurring in a transformer. The losses occurring in a transformer are a) Core loss loss occurring in the core of the transformer. This has two components. Hysteresis loss The loss occurring due to the magnetization characteristics of the core. Eddy current loss The loss occurring due to the eddy current produced in the core. a) Copper loss (I 2 R loss) loss occurring in the windings of the transformer. This has two components. Primary copper loss The loss occurring due to the current flowing through the primary winding. Secondary copper loss The loss occurring due to the current flowing through the secondary winding. 28. What are the applications of a step-up and step-down transformers? Step-up transformers are used in generating stations. Normally the generated voltage will be 11 kv.this voltage(11 KV) is stepped up to 33 kv or 110 kv or 220 kv or 400 kv and transmitted through transmission lines. (In short it may Be called as sending end). Step-down transformers are used in receiving stations. The voltage are again stepped down to 11 kv or 22 kv and transmitted through feeders.(in short it may be called as receiving end).further these 11 kv or 22kV are stepped down to 3 phase 400 V by means of a distribution transformer and made available at consumer premises. The transformers used at generating stations and receiving stations are called power transformers. 29. What are the typical uses of auto transformer? To give small boost to a distribution cable to correct for the voltage drop. As induction motor starters. Detailed Answer Questions 1. Explain the construction of various types of transformers. Draw the diagrams and explain for the types mentioned below o Core type transformer In the core-type, the windings surround a considerable part of steel core o Shell type transformer In the shell-type, the steel core surrounds a considerable part of the windings 12

14 o Berry Type Transformer Diagram and Explanation 2. Explain the principle of operation of a transformer Diagram Explanation (write the following in a detailed manner) The primary winding is connected to an alternating voltage source, an alternating current I 1 starts flowing through N 1 turns. The alternating mmf N 1 I 1 sets up alternating flux Ø which is confined to the high permeability iron path as shown above. The alternating flux induces voltage E 1 in the primary winding and E 2 in the secondary winding. If the load is connected across the secondary, a load current starts flowing. In addition to the secondary winding, there may be a third winding on the same iron core. The transformer action requires the existence of alternating mutual flux linking the various windings on a common magnetic core. 3. Derive the emf equation of a single phase transformer. Mention the various notations used Derive the expressions for emf induced in both primary and secondary winding E = 4.44 f φ N volts 1 max 1 E = 4.44 f φ N volts 2 max 2 4. Derive the equivalent circuit of a transformer. Mention about all the paarmeters Derive the following final circuit step by step 5. i)draw and explain the phasor diagram of transformer when it is operating under no load. ii) Modify the above phasor diagram to take into account the loading effect of the transformer. When the transformer is operating at no load, the current in the secondary winding is zero, while the primary winding carries a small current I o (no load current ) consisting of two components i) a reactive or magnetizing component I m 13

15 V 1 ii) an active or power component, I w From the phasor diagram, we have I = I cosφ w o o I w Ø o I o I = I sinφ m o o I m Ø I o = I w I m I cosφ o= I w o E 1, Phasor diagram of transformer on no load 6. Discuss the various the calculation of efficiency in a transformer. outputpower Efficiency of transformer, η = inputpower where input power = output power + total losses total losses = iron losses + copper losses = W i + (I 2 1 R 1 + I 2 2 R 2 ) output power = V 2 I 2 cos Ф Condition for maximum efficiency is, Total copper losses = iron losses (constant losses) Let x be the percentage of load at which total copper losses are equal to iron losses. Hence, for maximum efficiency, x 2 W c = W i x W W i = where W c is the full load copper loss c 7. Discuss about the importance of open and short circuit tests in a transformer. Open Circuit Test/No Load Test V o volts I o amps W o watts (Rated) Thus the watt meter reading, W = V I cosφ = Iron losses o o o o 14

16 Wo cosφ o= V I o Active component of no load current, I = I cosφ Magnetising current, I = I sinφ o m o o V Magnetising reactance, X = m o I m w o o Resistance equivalent to core loss, Vo Ro = I w Short Circuit Test V sc volts I sc amps W sc watts (Rated) W = V I cosφ = Full load copper losses sc sc sc sc Wsc cosφ sc = = short circuit power factor V I W = I R R sc 2 sc sc 1e W = sc 1e 2 Isc sc V Z = = R + X sc 2 2 1e 1e 1e Isc 15

17 UNIT III - INDUCTION MOTORS Construction Types Principle of operation of three-phase induction motors Equivalent circuit Performance calculation Starting and speed control Single-phase induction motors (only qualitative treatment). Two mark questions and answers 1. What are types of 3- phase induction motor? 1. Squirrel cage induction motor 2.Slip ring induction motor 2. Why the rotor slots of a 3-phase induction motor are skewed? The rotor slots of a three -phase induction motor are skewed 1. to make the motor run quietly by reducing the magnetic hum 2.to reduce the locking tendency of the rotor 3. Why the induction motor is called asynchronous motor? Since the induction motor runs always at a speed lesser than synchronous speed, it is called asynchronous motor. 4. What are slip rings? The slip rings are used to connect the external stationary circuit to the internal rotating circuit. The slip rings are made of copper alloys and are fixed around the shaft. Through these slip rings and brushes, the rotor winding can be connected to external resistance for improving starting torque and speed control. 5. State the difference between slip ring rotor and cage rotor of an induction motor? S. No Wound or Slip ring rotor Squirrel Cage rotor 1. Rotor consists of a three phase winding similar to the stator winding Rotor consists of copper or aluminium bars which are shorted at the ends with the help of end rings. 2. Resistance can be added externally As permanently shorted, external resistance cannot be added. 3. Slip rings and brushes are present. Slip rings and brushes are absent. 4. Frequent maintenance is necessary Construction is robust and maintenance is less 5. High starting torque can be obtained Moderate starting torque which cannot be controlled. 6. Rotor must be wound for the same number of poles as that of the stator. Rotor adjusts itself for the same number of poles as that of the stator. 7. Speed control by rotor resistance is possible Speed control by rotor resistance is not possible 8. Applications - lifts, hoists, cranes, elevators, compressors Applications Lathes, Drilling machines, fans, blowers, water pumps, grinders, printing machines 16

18 6. Write an expression for the slip of an induction motor. N s N Percentage slip = 100 N s where N s - Synchronous speed of the rotating magnetic field N - Speed of the rotor. 7. Explain why the no load current of an induction motor is much higher than that of an equivalent transformer. In induction motor, due to the presence of the air gap, the magnetizing current that is required to set up the flux is much higher. The working component of the current has to meet the hysteresis loss, eddy current loss, friction and windage losses. Hence the no load current of induction motor is higher. 8. State the effect of rotor resistance on starting torque? Starting torque increases with increase in value of rotor resistance. 9. Give the conditions for maximum torque for 3-phase induction motor? The condition for maximum torque for 3-phase induction motor is R2 = s X2 i.e where s is the slip under running conditions. At start, the condition reduces to R2 = X2 (as slip = 1) 10. What is reason for inserting additional resistance in rotor circuit of a slip ring induction motor? Introduction of additional resistance in the rotor circuit will increase the starting torque as well as running torque. Also it limits the starting current, thus acting as a starter and improves the power factor. 11. Why the reactance of three phase induction motor greatly varies between starting and running condiction? At start, the reactance of the induction motor is high. In running condition, the frequency of the rotor becomes sf which is very small as slip s is very small. The reactance is directly proportional to the frequency. Thus the reactance also reduces according to the frequency. Hence there is considerable variation between reactance of induction motor at start and in running condition. 12. What are the advantages of 3-phase squirrel cage induction motor? a) It is very simple and extremely rugged, almost unbreakable construction b) Its cost is very low and it is very reliable c) It has high efficiency.no brushes are needed and hence frictional losses are reduced d) It requires minimum of maintenance. 13. What are the types of starters? a) Stator resistance starter b) Autotransformer starter c) Star Delta starter d) Rotor resistance starter. 17

19 e) Direct on line starter 14. List out the methods of speed control of cage type 3-phase induction motor? a) By changing supply frequency b) By changing the number of poles c) By operating two motors in cascade 15. Mention different types of speed control of slip ring induction motor? a) By changing supply frequency b) By changing the number of stator poles c) By rotor rheostat control d) By operating two motors in cascade 16. List out the methods of speed control of 3-phase induction motor from stator side. a) Supply voltage control to control N s, called V/f control b) Supply voltage control c) Controlling number of stator poles to control N s d) Adding rheostats in stator circuit 17. Write down the methods of speed control of 3-phase induction motor from rotor side. a) Adding external resistance in the rotor circuit b) Cascade control. 18. Why is the efficiency of a 3-phase induction motor less than of a transformer? In induction motor, there is a mechanical loss due to the rotation of the rotor. Hence the efficiency of an induction motor is less than that of the transformer. 17. Are single phase induction motors self-starting? Why? Or Explain why single phase induction motors are not self-starting using double field revolving theory. The single phase induction motors are not self-starting. According to double field revolving theory, an alternating quantity can be resolved into two components rotating in opposite directions and having equal magnitude. In single phase induction motors, alternating stator flux has two such rotating components. Each interacts with rotor flux to produce two torques, forward and backward. At start, these two torques are equal and opposite. Thus net torque experienced by rotor is zero at start. Hence, single phase induction motors are not self-starting. 18. Name the two windings of a single-phase induction motor. I. Running winding ii. Starting winding. 19. What are the various methods available for making a single-phase motor selfstarting? i) By splitting the single phase into 2 phases ii) By providing shading coil in the poles. 18

20 20. What is the function of capacitor in a single-phase induction motor? i. To make more phase difference between the starting and running winding. ii. To improve the power factor and to improve the torque. 21. Give the names of three different types of single-phase motor. a) Split phase motor b) Capacitor start induction motor c) Capacitor start capacitor run induction motor d) Shaded pole motor. 22. What is the use of shading ring in a pole motor? The shading coil causes the flux in the shaded portion to lag behind the flux in unshaded portion of pole. This gives in effect a rotation of flux across the pole face and under the influence of this moving flux, a stating torque is developed. 23. State any four use of single-phase induction motor. a) Fans b) Wet grinders c) Vacuum cleaners d) small pumps e) compressors f) small drilling machines 24.What does crawling of induction motor mean? Squirrel cage type, sometimes exhibit a tendency to run stably at speeds as low as 1/7 the of their synchronous speed, because of the harmonics. This phenomenon is known as crawling. 25. What is cogging of an induction motor? Sometimes, even when full voltage is applied to the stator winding, the rotor of squirrel cage 3-phase induction motor refuses to start and pick-up speed. This happens when the number of stator slots is equal to the number of rotor slots and is due to the magnetic locking between the stator and rotor teeth. This phenomenon is called as Cogging or teeth locking. When number of stator slots and rotor slots are equal, the reluctance for the magnetic path is minimum, due to which rotor slots align with stator slots and remain standstill. Detailed Answer Questions 1. Explain the construction & different types of rotors in induction motor. An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field An inside rotor attached to the output shaft that is given a torque by the rotating field. Type of rotors (explain in detail with diagrams) 1) Squirrel cage rotor 2) Wound rotor 19

21 2. Develop the equivalent circuit for 3-phase induction motor? Derive the equivalent circuit step by step I 1 = stator current/phase V 1 = applied voltage to the stator/phase R 1 = stator winding resistance/phase X 1 = stator winding reactance/phase R2 = rotor winding resistance/phase X 2 = rotor winding reactance/phase I 2 = rotor current I o = I c +I m (Im-magnetising component, Ic-core loss component) 3. Draw the slip-torque characteristics for a three-phase induction motor and Explain. For small values of slip s, the torque is directly proportional to s. For large values of slip s, the torque is inversely proportional to s. 4. Draw and explain the torque/slip curves of a three-phase induction motor for different values of rotor resistance Explain the torque-slip characteristics with the following graph 20

22 5. Explain the powerflow in an induction motor 6. Explain the different types of starters for an induction motor. Stator resistance starter Auto-transformer starter Star-Delta starter Rotor Resistance Starter Direct On-line starter 7. Explain the different speed control methods of squirrel cage induction motor. Frequency Control or V/f control Supply Voltage control Controlling by number of poles Adding rheostat in stator circuit Adding rheostat in rotor circuit 21

23 UNIT IV - SYNCHRONOUS AND SPECIAL MACHINES Construction of synchronous machines-types Induced emf Voltage regulation; emf and mmf methods Brushless alternators Reluctance motor Hysteresis motor Stepper motor. Two mark questions and answers 1. What is principle of an alternator? An alternator works on the principle of Faraday s law of electromagnetic induction. According to faraday s law, whenever the flux linking with a conductor changes, an electromotive force is set up in that conductor 2. What are the two types of alternators based on construction? 1) Salient pole type 2) Non-Salient type or Smooth cylindrical type 3. What are advantages of having rotating field system over rotating armature? (Rotating field means stationary armature) 1) For stationary armature, large space can be provided to accommodate large number of conductors and insulation 2) It is easy to collect larger currents at very high voltages from a stationary member than from slip ring and brush assembly. 3) The field requires only small amount of power, which can be supplied to the rotating field through slip rings and brushes. 4) Insulation to stationary system of conductors is not subjected to mechanical stresses due to centrifugal action. 5) It is easy to provide necessary cooling arrangement for a stationary system of conductors. 6) For a rotating field, lesser number of slip rings is required. 4. What are the causes for the variations in terminal voltage of an alternator? The voltage variation in the terminal voltage are due to the following causes. 1. Due to the resistance of the winding, R 2. Due to the leakage reactance of the winding, X l 3. Due to the armature reaction effect, X a 5. What is armature reaction? The effect of armature flux on the main flux is called as armature reaction. 6. Define the regulation of an alternator. (Apr/May2007) It is defined as the ratio of the change in the terminal voltage when full load is removed to the rated terminal voltage, keeping field excitation and speed constant. E ph V ph % Re gulation = 100 V where V ph rated terminal voltage E ph - No load induced emf ph 7. Why is EMF method called pessimistic method? (Apr/May2007) 22

24 The value of voltage regulation obtained by EMF method is always larger than the actual value, therefore it is called as pessimistic method. 8. How is armature winding in alternators different from those used in DC machines? (Apr/May2007) The armature winding of the alternator is placed in the stator, but in the case of the DC machines, armature winding is placed in the rotor. 9. What are the squirrel cage windings of synchronous motors? How and why are they used? (Nov/Dec2006) Damper windings are the squirrel cage windings of the alternator. This winding is placed in rotor pole shoes. They are used for starting the synchronous motor They are used for damping out the oscillations, whenever the motor is subjected to change in load condition. 10. What is a synchronous capacitor? (Apr/May2007) An over excited synchronous motor, running without any mechanical load, used specifically for power factor correction is known as synchronous capacitor. 11. What are inherent disadvantages of Synchronous motor? (Apr/May2006) Low-starting torque Sensitivity to system disturbances Necessity of a DC excitation High maintenance cost 12. Mention few applications of synchronous motor?(apr/may2006) Power factor correction To improve voltage regulation of transmission lines Direct current generator drives Rubber mills Pulp grinders Ball mills Ship propulsions 13. What are the different torques of a synchronous motor? 1. Starting torque 2. Running torque 3. Pull in torque 4. Pull out torque 14. Define pull-out torque in synchronous motor? Pull-out torque is the maximum sustained torque the motor develops at synchronous speed for one minute with rated frequency and normal excitation. Normal pull-out torque is usually 150% of full-load torque for unity-power-factor motors, and 175 to 200% for 0.8-leading-power-factor motors 15. What is meant by load angle of an alternator?(nov/dec2008) 23

25 The load angle or torque anlge of an alternator is defined as the angle between the induced emf and the terminal voltage. 16. Why does the synchronous motor always runs at synchronous speed? A synchronous motor always runs at synchronous speed because of the magnetic locking between the stator and the rotor poles. 17. What do you mean by hunting in synchronous motor? How is it overcome? The oscillations of the rotor about its new equilibrium position, due to sudden application or removal of load is called swinging or hunting in synchronous motor. The hunting can be overcome by the use of damper windings. 18. List any two methods of starting of a synchronous motor? By an extra small 3-phase cage induction motor By providing cage/damper winding in pole faces By operating the pilot exciter as a DC motor 19. What is a stepper motor? It is an electromechanical device which converts a train of input pulses into a precisely defined increment in the shaft position. 20. What is meant by step angle? The angle through which the motor turns or shaft moves for each pulse is called step angle, expressed in degrees. 21. Define resolution. It is defined as the no of steps needed to complete one revolution of the rotor shaft. 22. What are the different types of stepper motor? (Apr/May 2007) 1. Variable reluctance stepper motor 2. Permanent magnet stepper motor 3. Hybrid stepper motor. 23. Give the applications of stepper motor. 1) Serial printers 2) Tape drives 3) Floppy disc drives 4) Numerical control of machine tools 5) Robotics 6) Process control systems 7) X-Y recorders 8) Plotters 9) Space crafts 24.Give the advantages of reluctance motor. No d.c supply is needed for rotor Constant speed characteristics Robust construction 24

26 Less Maintenance 25. Mention some applications of Hysteresis motor Sound recording instruments High quality record players Tape recorders Electric clocks Teleprinters Timing devices Detailed Answer Questions 1. Explain in detail the EMF method of obtaining the regulation of an alternator Conducting Open circuit test and Short circuit test Graph for OCC and SCC Determination of synchronous impedance Calculation of Voltage regulation 2. Discuss the two types of synchronous generator based on construction. Explain the following types with diagram Two types of synchronous generators based on construction are, Salient pole type rotor Smooth cylindrical type or Non-salient pole type rotor. 3. How the synchronous motors are started? Explain the following methods of starting a synchronous motor, with diagrams. By an extra small 3-phase cage induction motor By providing cage/damper winding in pole faces By operating the pilot exciter as a DC motor 4. Explain the construction and principle of operation of synchronous motor. Principle of operation Diagram Synchronous motor is not self-starting. Justify. Features 5. Write notes on (a) Reluctance motor (6) (b) Stepper motor (6) For both Reluctance motor and Stepper motor, write the following Diagram Working principle Mathematical analysis Torque-speed curve Advantage and disadvantages Applications 25

27 6. Write notes on Stepper motor types. For each type, explain with diagram, working principle and characteristics. 1) Variable reluctance stepper motor 2) Permanent magnet stepper motor 3) Hybrid stepper motor. 26

28 UNIT V - TRANSMISSION AND DISTRIBUTION Structure of electric power systems Generation, transmission and distribution systems - EHVAC and EHVDC transmission systems Substation layout Insulators cables. Two mark questions and answers 1. List the important components of transmission and distribution. The flow of electrical power from the generating station to the consumer is called an electrical power system or electrical supply system. It consists of the following important components: Generating station Transmission network Distribution network 2. List the various voltage levels involved in typical transmission and distribution. Generating station 6.6 kv, 11 kv, 22 kv or 33 kv Primary transmission 66 kv, 132 kv, 220 kv up to 400 kv Secondary transmission 11 kv,22 kv,33 kv Primary distribution 6.6 kv or 11 kv Secondary distribution 400V or 230V 3. What is primary and secondary transmission? It is basically with the help of overhead transmission lines. For the economic aspects, the voltage level is increased to 132KV, 220KV or more, with the help of step up transformer. Hence this transmission is also called high voltage transmission. The primary transmission uses 3-phase 3-wire system. The primary transmission line continues via transmission towers till the receiving station. At the receiving station voltage level is reduced to 22KV or 33KV using the step down transformer. There can be more than one receiving stations. Then at reduced voltage level of 22KV or 33KV, the power is then transmitted to various substations using overhead 3 phase 3 wire system. This is the secondary transmission. 4. List the various components of distribution. Substation Local distribution system Feeders Distributors Service mains 5. State the advantages of a.c. system. 1) It is possible to build up high a.c. voltage levels, using high-speed a.c. generators of large capacities. 2) The cost of such a.c. generators is very low. 3) The high voltage levels can be lowered to safe potentials using step down transformers for the distribution purpose. 4) The a.c. supply can be converted to obtain d.c. whenever required. 5) The transforming a.c. substations are much efficient than d.c. system using rectifiers. 27

29 6. State the disadvantages of a.c. system. 1) The construction a.c. transmission line is more complicated than d.c. line. 2) The resistance of a.c. line is higher due to the skin effect causing more voltage drop. 3) The drop is also due to the inductance of a.c. line causing loss of power. 4) The copper requirement for a.c. line is more than a d.c. line. 5) The problem of synchronization of alternators exists in case of a.c. system. 6) The speed of alternators in a.c. system is not economical and required to be controlled within very low limits. 7. State the advantages of d.c. System 1) Due to reduced voltage drops, the voltage regulation is better. 2) Absence of skin effect makes use of entire cross-section of conductor. 3) As the stress on cable insulation is less, the economical use of underground cables is possible. 4) The stability problems as well as synchronizing difficulties are absent in the d.c. System. 5) The d.c. line has reduced interference with radio and communication circuits. 6) Copper requirement is less as only two lines are sufficient for a dc s.c. system. 8. State the disadvantages of d.c. system. 1) The power generation is not possible at high d.c. voltage levels due to commutation problems. 2) The transformer works on a.c. and not d.c. supply. So the d.c. voltage levels cannot be stepped up or stepped down is not possible. 3) Obtaining a.c. from d.c. is not easy in practice. 4) The d.c. generators and motors need a lot of maintenance and their construction cost is also more than a.c. machines of same capacity. 5) The limitation of d.c. switches as well as circuit breakers cause the problems in d.c. system. 9. Compare overhead and underground transmission. S.No Overhead transmission Underground transmission 1 In this system, the transmission of electric power is by using overhead transmission lines over long distance. The cables are generally preferred in underground transmission system 2 The spacing between conductors All the conductors must be provides insulation. insulated from each other. 3 Voltage level can be as high as 400 kv Voltage level is below 66 kv due to insulation difficulties 4 Overhead lines can be easily repaired Underground cables can t be easily and the faults can be easily located. repaired 5 Maintenance cost is high. Maintenance cost is less 6 Maximum stress exists between conductor and earth. Maximum stress exists on the insulation between the conductors. 28

30 10. What are the advantages of high voltage transmission system? 1) The line losses are inversely proportional to the square of voltage and power factor. So line losses are less. 2) For constant losses, the volume of copper required is inversely proportional to the square of the voltage and power factor. Hence, the copper required is much less for high voltage transmission. 3) For constant current density, the line efficiency is very high for high voltage transmission. 4) The percentage line drop is very small for the high voltage transmission. 11. Discuss the limitations of high voltage transmission. 1) Higher transmission voltage, higher is the insulation required which causes problems in connection with conductor supports and clearance between the conductors. 2) Higher insulation means higher cost. 3) The cost of transformer, switchgear and other equipments is also high for high voltage. 4) Higher the voltage, severe is the corona effect. 12. Explain the necessity of EHV transmission 1) With the increase in transmission voltage, for same amount of power to be transmitted, current in the line decreases which reduces I 2 R losses. This will lead to increase in transmission efficiency. 2) With decrease in transmission current, size of conductor required reduces which decreases the volume of conductor. 3) With increase in level of transmission voltage, the insulation cost of the transmission line per km decreases. 4) Its economical with EHV to interconnect the power systems on a large scale. 13. State the advantages of EHVAC transmission. 1) Reduction in the current 2) Reduction in the losses 3) Reduction in volume of conductors material required 4) Decrease I voltage drop and improvement of voltage regulation 5) Increase in transmission efficiency 6) Increased power handling capacity 14. State the disadvantages of EHVAC transmission. 1) Increases the corona loss and radio interference 2) With increase in transmission voltage, insulation required for line conductors also increases which increases its cost. 3) The cost of transformers, switchgear equipments and protective equipments increases with increase in transmission line voltage. 4) The EHV line generates electrostatic effects, which are harmful to human being and animals. 5) There are lots of problems that arise during the erection of EHV lines. 6) In order to protect the transmission line during storms and cyclones, extra amount of metal is required in the tower which may increase the cost. 29