Anna University, Chennai
UNIT – I
D.C. MACHINES
PART – A
1. What is prime mover?
The basic source of mechanical power which drives the armature of the generator is called prime mover.
2. Give the materials used in machine manufacturing?
There are three main materials used in m/c manufacturing they are steel to conduct magnetic flux copper to conduct electric current insulation.
3. What are factors on which hysteresis loss?
It depends on magnetic flux density, frequency & volume of the material.
4. What is core loss? What is its significance in electric machines?
| |
5. What is eddy current loss?
When a magnetic core carries a time varying flux, voltages are induced in all possible path enclosing flux. Resulting is the production of circulating flux in
core. These circulating current do no useful work are known as eddy current and have power loss known as eddy current loss.
6. How hysteresis and eddy current losses are minimized?
Hysteresis loss can be minimized by selecting materials for core such as silicon steel & steel alloys with low hysteresis co-efficient and electrical
resistivity. Eddy current losses are minimized by laminating the core.
7. How will you find the direction of emf using Fleming‟s right hand rule?
The thumb, forefinger & middle finger of right hand are held so that these fingers are mutually perpendicular to each other, then forefinger gives the direction of the lines of flux, thumb gives the direction of the relative motion of conductor and middle finger gives the direction of the emf induced.
8. How will you find the direction of force produced using Fleming‟s left hand rule?
The thumb, forefinger & middle finger of left hand are held so that these fingers are mutually perpendicular to each other, then forefinger gives the direction of magnetic field, middle finger gives the direction of the current and thumb gives the direction of the force experienced by the conductor.
9. What is the purpose of yoke in d.c machine?
1. It acts as a protecting cover for the whole machine and provides mechanical support for the poles.
2. It carries magnetic flux produced by the poles
10. What are the types of armature winding?
1. Lap winding, A=P,
| |
11. How are armature windings are classified based on placement of coil inside the armature slots?
Single and double layer winding.
12. Write down the emf equation for d.c.generator?
E=(ФNZ/60)(P/A)V.
p---------no of poles
Z---------Total no of conductor
Ф---------flux per pole
N---------speed in rpm.
13. Why the armature core in d.c machines is constructed with laminated steel sheets
instead of solid steel sheets?
Lamination highly reduces the eddy current loss and steel sheets provide low reluctance path to magnetic field.
14. Why commutator is employed in d.c.machines?
Conduct electricity between rotating armature and fixed brushes, convert alternating emf into unidirectional emf(mechanical rectifier).
15. Distinguish between shunt and series field coil construction?
Shunt field coils are wound with wires of small section and have more no of turns. Series field coils are wound with wires of larger cross section and have less no of turns.
16. How does d.c. motor differ from d.c. generator in construction? Generators are normally placed in closed room and accessed by skilled operators only. Therefore on ventilation point of view they may be constructed with large opening in the frame. Motors have to be installed right in the place of use which may have dust, dampness, inflammable gases, chemical etc. to protect the motors against these elements the motor frames are used partially closed or totally closed or flame proof.
17. How will you change the direction of rotation of d.c.motor?
Either the field direction or direction of current through armature conductor is reversed.
| |
As the motor armature rotates, the system of conductor come across alternate north and south pole magnetic fields causing an emf induced in the
conductors. The direction of the emf induced in the conductor is in opposite
to current. As this emf always opposes the flow of current in motor operation it is called as back emf.
19. What is the function of no-voltage release coil in d.c. motor starter?
As long as the supply voltage is on healthy condition the current through the
NVR coil produce enough magnetic force of attraction and retain the starter handle in ON position against spring force. When the supply voltage fails or becomes lower than a prescribed value then electromagnet may not have enough force to retain so handle will come back to OFF position due to spring force automatically.
20. Enumerate the factors on which speed of a d.c.motor depends?
N= (V-IaRa)/Ф so speed depends on voltage applied to armature, flux per
pole, resistance of armature.
21. Under what circumstances does a dc shunt generator fails to generate? Absence of residual flux, initial flux setup by field may be opposite in direction to residual flux, shunt field circuit resistance may be higher than its critical field resistance, load circuit resistance may be less than its critical load resistance.
22. Define critical field resistance of dc shunt generator?
Critical field resistance is defined as the resistance of the field circuit which will cause the shunt generator just to build up its emf at a specified field.
23. Why is the emf not zero when the field current is reduced to zero in dc generator?
Even after the field current is reduced to zero, the machine is left out with some flux as residue so emf is available due to residual flux.
24. On what occasion dc generator may not have residual flux?
The generator may be put for its operation after its construction, in previous operation, the generator would have been fully demagnetized.
25. What are the conditions to be fulfilled by for a dc shunt generator to build back emf?
The generator should have residual flux, the field winding should be connected in such a manner that the flux setup by field in same direction as
| |
26. Define armature reaction in dc machines?
The interaction between the main flux and armature flux cause disturbance called as armature reaction.
27. What are two unwanted effects of armature reactions? Cross magnetizing effect & demagnetizing effect.
28. What is the function of carbon brush used in dc generators?
The function of the carbon brush is to collect current from commutator and supply to external load circuit and to load.
29. What is the principle of generator?
When the armature conductor cuts the magnetic flux emf is induced in the conductor.
30. What is the principle of motor?
When a current carrying conductor is placed in a magnetic field it experiences a force tending to move it.
31. What are different methods of speed control in D.C shunt motor?
1. Armature control
2. Flux or field control
3. Applied voltage control
32. When is a four point DC starter required in DC motors?
A four point DC starter is required for dc motor under field control
33. If speed is decreased in a dc motor, what happens to the back emf decreases and armature current?
If speed is decreased in a dc motor, the back emf decreases and armature current increases.
34. How does a series motor develop high starting torque?
A dc series motor is always started with some load. Therefore the motor armature current increases. Due to this, series motor develops high starting
torque.
35. What is the necessity of starter in dc motors?
When a dc motor is directly switched on, at the time of starting, the motor back emf is zero. Due to this, the armature current is very high. Due to the
very high current, the motor gets damaged. To reduce the starting current of the motor a starter is used.
36. Mention the types of braking of dc motor?
1. Regenerative braking
2. Dynamic braking
3. Plugging
37. What are the losses in dc motor?
1. Copper losses
2. Iron losses
3. Mechanical losses
38. Name any 2 non-loading method of testing dc machines?
1. Swinburne‟s test
2. Hopkinson test
PART – B
1. Explain in detail the constructional details of DC machine.
2. Derive the e.m.f. equation (8)
3. Draw and explain the characteristics of DC generator.
4. Derive the torque equation/
5. Write down the significance of back e.m.f.
6. Write down the principle of operation of DC motor.
7. Draw and explain the characteristics of DC motor.
8. Draw and explain 3 point starter.
9. What is the draw back in 3 point starter and how it is overcome in 4 point starter?
10.Explains the procedure of swin burne's test.
11.Draw and explain the speed control methods of DC shunt motor.
12.Explain in detail about the ward-leonard system of speed control of DC
motor.
UNIT – II TRANSFORMERS PART – A
1. What are the 2 types of 3phase induction motor? Squirrel cage and slip ring induction motor.
2. Write two extra features of slip ring induction motor?
Rotor has 3-phase winding. Extra resistance can be added in rotor circuit for improving PF with the help of three slip rings.
3. Why an induction motor is called as rotating transformer?
The rotor receives same electrical power in exactly the same way as the secondary of a two winding transformer receiving its power from primary.
That is why induction motor is called as rotating transformer.
4. Why an induction motor never runs at its synchronous speed?
| |
torque to maintain rotation.
5. What are slip rings?
The slip rings are made of copper alloys and are fixed around the shaft insulating it. Through these slip rings and brushes rotor winding can be
connected to external circuit.
6. What are the advantages of cage motor?
Since the rotor have low resistance, the copper loss is low and efficiency is very high. On account of simple construction of rotor it is mechanically
robust, initial cost is less, maintenance cost is less, simple starting
arrangement.
7. Give the condition for maximum torque for 3phase induction motor, when it is
running?
The rotor resistance and reactance should be same for max.torque i.e. R2=Sx2
8. List out the method for speed control of 3phase cage type induction motor?
By changing supply frequency
By changing no of poles
By operating the two motors in cascade
9. Name the two winding of single phase induction motor? Running and starting winding.
10. What are methods available for making single phase induction motor a self starting?
By slitting the single phase, by providing shading coil in the poles.
11. What is the function of capacitor in single phase induction motor?
To make phase difference between starting and running winding, to improve
PF and to get more torque.
12. State any 4 use of single phase induction motor?
Fans, wet grinders, vacuum cleaner, small pumps, compressors, drills.
13. What kind of motors used in ceiling fan and wet grinders?
| |
14. What is the application of shaded pole induction motor?
Because of its small starting torque, it is generally used for small toys, instruments, hair driers, ventilators.etc.
15. In which direction a shaded pole motor runs?
The rotor starts rotation in the direction from unshaded part to the shaded part.
16. Why single phase induction motors have low PF?
The current through the running winding lags behind the supply voltage by large angle so only single phase induction motor have low PF.
17. Differentiate between “capacitor start” & “Capacitor start capacitor run”
single
Phase induction motor (IM)?
Capacitor start – capacitor run is connected series with starting winding, but it will be disconnected from supply when motor pick up its speed. Capacitor
start capacitor run- starting winding and capacitor will not be disconnected
from supply even though motor pickup its speed.
18. Explain why single phase induction motor is not a self starting one?
When motor fed supply from single phase, its stator winding produces an alternating flux, which doesn‟t develops any torque.
19. Define slip in an IM?
The slip of an IM is defined as the ratio of difference between sync. speed
(Ns) and rotor speed (N) to the sync. speed. s= (Ns – N)/ Ns
20. Define slip speed in an IM?
The slip speed is defined as the difference in speed between the rotating magnetic field produced by stator (Ns) and rotor speed (N).
21. What is the speed of the rotor field in space?
The speed of the rotor field in space is speed of rotating field.
86. What is sync. speed in 3-phase IM? Ns = 120f/p
Where f- supply frequency
P- No of poles on the stator.
22. List the various methods of speed control of 3 phase IM? Types of stator side control
1. Stator voltage control
2. Stator frequency control
3. v/f control
| |
23. In which type of motor can resistance be introduced in the rotor circuit? What is the effect of it?
Slip ring IM. Effects:
1. starting torque increased
2. starting current decreased
3. motor speed can be controlled
24. Why the slots on the IM are usually skewed?
In order to obtain a uniform torque, reduce the magnetic locking of the stator and rotor and reduce the magnetic humming noise while running.
90. What are the types of poly phase IM?
1. Squirrel cage IM
2. Slip ring IM
25. What will be the effect when stator voltage and freq of a IM are reduced proportionally?
1. Motor speed increases
2. Maximum torque is constant
26. What is slip power recovery scheme?
Slip power can be returned to the supply source and can be used to supply an additionalmotor which is mechanically coupled to the main rotor. This type
of drive is known as slip power recovery system and improves overall
efficiency of the system.
PART - B
1. Derive the e.m.f. equation of transformer.
2. Explain in detail about the transformer on no load.
3. Draw and explain ideal transformer. Draw its vector diagram.
4. Obtain the equivalent circuit of the transformer by conducting suitable tests.
5. Draw and explain in detail about the transformer an load.
6. Derive the regulation formula for the transformer.
7. Explain in detail about the testing procedures for the transformer.
UNIT – III INDUCTION MOTORS PART – A
1. Define a transformer?
A transformer is a static device which changes the alternating voltage from one level to another.
2. What is the turns ratio and transformer ratio of transformer? Turns ratio = N2/ N1
Transformer = E2/E1 = I1/ I2 =K
3. Mention the difference between core and shell type transformers?
| |
4. What is the purpose of laminating the core in a transformer? In order to minimise eddy current loss.
5. Give the emf equation of a transformer and define each term?
Emf induced in primary coil E1= 4.44fФmN1 volt emf induced in secondary
Coil E2 =4.44 fФmN2.
f--------freq of AC input
Ф-----------maximum value of flux in the core
N1, N2---------Number of primary & secondary turns.
6. Does 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 for iron and copper losses on no load. There will not be any current in the secondary since secondary is open.
7. Define voltage regulation of a transformer?
When a transformer is loaded with a constant primary voltage, the secondary voltage decreases for lagging PF load, and increases for leading PF load because of its internal resistance and leakage reactance. 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=(V2noload- V2full load)*100/ V2noload,
%regulation up=(V2noload-V2F.L)*100/V2F.L
8. Define all day efficiency of a transformer?
It is computed on the basis of energy consumed during a certain period, usually a day of 24 hrs. all day efficiency=output in kWh/input in kWh for 24 hrs.
9. Why transformers are rated in kVA?
| |
10. What determines the thickness of the lamination or stampings?
1. Frequency
2. Iron loss
11. What are the typical uses of auto transformer?
1. To give small boost to a distribution cable to correct for the voltage drop.
2. As induction motor starter.
12. What are the applications of step-up & step-down transformer?
Step-up transformers are used in generating stations. Normally the generated voltage will be either 11kV. This voltage (11kV) is stepped up to
110kV or 220kV or 400kV and transmitted through transmission lines (simply
called as sending end voltage).
Step-down transformers are used in receiving stations. The voltage are stepped down to 11kV or 22kV are stepped down to 3phase 400V by means
of a distribution transformer and made available at consumer premises. The transformers used at generating stations are called power transformers.
13. How transformers are classified according to their construction?
1. Core type 2.shell type. In core type, the winding (primary and secondary)
surround the core and in shell type, the core surround the winding.
14. Explain on the material used for core construction?
The core is constructed by sheet steel laminations assembled to provide a continuous magnetic path with 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 used from each other by light coat of core-plate vanish or by oxide
layer on the surface. The thickness of lamination varies from 0.35mm for a frequency of 50Hz and 0.5mm for a frequency of 25Hz.
15. How does change in frequency affect the operation of a given transformer?
With a change in frequency, iron and copper loss, regulation, efficiency &
heating varies so the operation of transformer is highly affected.
16. What is the angle by which no-load current will lag the ideal applied voltage?
In an ideal transformer, there are no copper & core loss i.e. loss free core.
| |
17. List the arrangement of stepped core arrangement in a transformer?
1. To reduce the space effectively
2. To obtain reduce length of mean turn of the winding
3. To reduce I2R loss.
18. 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. Also to permit the oil inside the
tank to expand and contract as its temperature increases and decreases.
19. What is the function of transformer oil in a transformer?
1. It provides good insulation
2. Cooling.
20. Can the voltage regulation go –ive? If so under what condition? Yes, if the load has leading PF.
21. Distinguish power transformers & distribution transformers?
Power transformers have very high rating 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 receiving side. Voltage levels will be medium. Power ranging will be small in
order of kVA. Complicated controls are not needed.
22. Name the factors on which hysteresis loss depends?
1. Frequency 2. Volume of the core 3. Maximum flux density
23. Why the open circuit test on a transformer is conducted at rated voltage?
The open circuit on a transformer is conducted at a rated voltage because core loss depends upon the voltage. This open circuit test gives only core loss or iron loss of the transformer.
24. What is the purpose of providing Taps in transformer and where these are provided?
In order to attain the required voltage, taps are provided, normally at high voltages side(low current).
25. What are the necessary tests to determine the equivalent circuit of the transformer?
1. Open circuit test
2. Short circuit test
26. Define regulation and efficiency of the transformer?
The regulation of the transformer is defined as the reduction in magnitude of the terminal voltage due to load, with respect to the no-load terminal
voltage.
% regulation = (V2 on no-load- V2 when loaded/ V2 on no-load)x 100
Transformer efficiency ƞ= (output power/input power)x 100
PART - B
1. Explain the constructional details of 3 f induction motor.
2. How Rmf is produced with the help of 3 f AC supply?
3. Explain in detail the principle of operations of 3 phase induction motor.
4. Explain the procedure to obtain the equivalent circuit of 3 phase induction motor.
5. Derive the torque equation of 3 phase induction motor.
6. Draw and explain the characteristics of 3 phase induction motor.
7. Explain in detail about (i) Star-delta starter (ii) Auto transformer starter (iii) DOL starter.
8. Write short notes on single phase transformer.
9. Why single phase induction motor is not self starting? Explain the methods available to start the motor.
10. Write short notes on speed control of 3 f induction motor.
UNIT IV
SYNCHRONOUS AND SPECIAL MACHINES
PART – A
1. What are the principal advantages of rotating field type construction? Relatively small amount of power required for field system can easily supplied to rotating system using slip rings and brushes, more space is available in the stator part of the machine to provide more insulation, it is easy to provide cooling system, stationary system of conductors can easily be braced to prevent deformation.
2. What are the advantages of salient type pole construction used in sync.machines?
They allow better ventilation, the pole faces are so shaped radial air gap length increases from pole center to pole tips so flux distortion in air gap is
sinusoidal so emf is also sinusoidal.
3. Which type of sync. generators are used in hydroelectric plants and why? As the speed of operation is low, for hydro turbines used in hydroelectric plants, salient pole type sync. generator is used because it allows better ventilation also better than smooth cylindrical type rotor
4. Why are alternators rated in KVA and not in KW?
As load increases I2R loss also increases, as the current is directly related to apparent power delivered by generator, the alternator has only their
apparent power in VA/KVA/MVA as their power rating.
5. Why the sync. impedance method of estimating voltage regulation is is considered as pessimistic method?
Compared to other method, the value of voltage regulation obtained by this method is always higher than the actual value so it is called as pessimistic method.
6. Why MMF method of estimating voltage regulation is considered as optimistic method?
Compared to EMF method, MMF method involves more no. of complex calculation steps. Further the OCC is referred twice and SCC is referred once
while predetermining the voltage regulation for each load condition. Reference of OCC takes core saturation effect. As
this method require more effort, final result is very close to actual value,
hence this method is called as optimistic method.
| |
It is defined as the increase in terminal voltage when full load is thrown off, assuming field current and speed remaining the same.
% reg = [(E0 – V)/V]x100
Where E0 = no terminal voltage V = full load rated terminal voltage
8. How is arm. winding in alternators is different from those used in dc machines?
The arm. winding of the alternator is placed in the stator, but in the case of dc machines the arm winding is placed in the rotor.
9. What is hunting how can it be prevented?
When a sync motor is used for driving a fluctuating load, the rotor starts oscillating about its new position of equilibrium corresponding to the new
load. This is called hunting or phase swinging. To prevent hunting dampers
are damping grids are employed.
10. what are different torques of a sync motor?
1. Starting torque
2. Running torque
3. Pull-in torque
4. Pull-out torque
11. define step angle?
It is defined as angle through which the stepper motor shaft rotates for each
command pulse. It is denoted as β, i)β=[(Ns-Nr)/ Ns.Nr]x360o
Where Ns = no. of stator poles or stator teeth
Nr = no. of rotor poles or rotor teeth
ii) β = 3600/mNr
Where m= no. of stator poles
12. What are different types of stepper motor?
1. Variable reluctance (VR) motor
2. Permanent magnet (PM) stepper motor
3. Hybrid stepper motor
13. What is the advantage in using stepper motor?
1. it can drive open loop without feedback
2. it requires little or no maintenance.
14. Give the applications of stepper motor?
1. Robotics
2. Computer peripherals
3. Facsimile machine
4. Aerospace
15. What are the adv. of reluctance m/c?
1. Motor speed is constant
2. Simple construction
PART - B
1. Derive the emf equation of alternator.
2. Write down the procedure of finding voltage regulation of alternator using synchronous impedance method.
3. Write down the procedure for finding voltage regulation using ampere-
turns method.
4. Explain in detail the construction of synchronous machine.
5. Write short notes on
(i) Brushless alternator (ii) Reluctance motor (iii) Hysteresis motor (iv) Stepper motor.
UNIT – V TRANSMISSION AND DISTRIBUTION PART – A
1. What are the principle divisions of an electric motor?
1. Generating station
2. Transmission systems
3. Distribution systems
2. List out the types of power generating systems?
1. Steam thermal power station
2. Nuclear power station
3. Hydro electric power station
4. Diesel power station
3. What is the adv. of EHVDC system?
1. Requires less space compared to ac for same voltage rating and size
3. cheaper for long distance transmission.
4. There is no inductance, capacitance, phase displacement and surge problems.
5. There is no skin effect. Therefore entire cross section of the conductor is used.
4. What are the desirable properties of insulators?
1. High mechanical strength to withstand conductor load
2. High electrical strength to prevent leakage current.
3. It should not have any pores in air spaces.
| |
1. Pin type
2. Suspension type
3. Strain type
4. Shackle
6. What are the adv and dis adv of under ground cables? Adv:
1. Low vtg drop
2. Low faults
3. Good appearance
4. Less damage through lightening and storms.
Dis adv:
1. High installation cost
2. Insulation problems at high voltage
7. How cables are classified according to the voltage? Low tension cable – upto 1000V
High tension cable – upto 11kV
Super tension cable – from 22kV to 33kV Extra high tension cable – from 33kV to 66kV
Extra super tension cable – above 132kV
8. What is the voltage level of sub-transmission system?
33kV or 66kV
9. What are the major electrical equipments used in the power system? Generator, power transformer, bus bars, circuit breakers, switching circuits
10. What is the function of circuit breaker in power system ?
It is one of the protective equipments in substation. The main function of the
CB is to open or close a circuit under normal as well as fault conditions.
11. What is the sub-transmission network?
The primary transmission line goes upto the receiving station. The voltage is stepped down to 33kV by step-down transformer i.e. 132/33kV. This 33kV is transmitted by 3-phase, 3-wire, overhead system. It is a secondary
| |
12. Name few insulating materials used in cables.
Rubber, vulcanised India rubber, what are the major impregnated paper, PVC, varnished cambric.
13. what are the major sources of energy used in the power system?
1. Coal
2. Water
3. Nuclear
14. What are the different types of HVDC links?
1. Monopolar link
2. Bipolar link
3. Homopolar link
15. What is meant by distribution system?
The part of power system which distributes electric power for local use is known as distribution system.
16. What are the parts of distribution system?
1. Feeders
2. Distributors
3. Service mains
17. What are the different types of distributions? A)AC distribution:
i) primary distribution system ii) secondary distribution system b) DC distribution
i)2-wire dc system ii) 3-wire dc system
18. Define sub station
There are number of transformers and switching stations built between generating stations and the ultimate consumers. These are generally known as substations.
19. What are the classifications of substation based on?
1. Service requirement
2. Physical features
20. Compare EHVAC with EHVDC systems?
1. Corona loss and radio interference will be more in EHVAC whereas less in
EHVDC
2. EHVAC requires three conductors whereas EHVDC requires only two conductors
3. Skin effect will occur in EHVAC whereas skin effect is absent in EHVDC
4. Installation cost of EHVAC is less where cost is more in EHVDC
5. It has inductance, capacitance, phase displacement and surge problems whereas EHVDC does not has these problems.
PART - B
1. Explain in detail, the single line diagram of electrical power system.
2. Explain in detail the various types of transmission and distribution systems.
3. Distinguish EHVAC and EHVDC systems.
4. Explain in detail the substation lay out.
5. Write short notes as (i) Insulators (ii) cables.
6. Explain the various types of insulators used in overhead transmission system
7. Sketch and explain the suspension type insulators
8. Discuss about the types of cables used in the power systems
9. Describe the types of DC links
10.Describe the construction of thermal power system
11.Explain the different types of distribution sub systems
No comments:
Post a Comment