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WALJAT COLLEGES OF APPLIED SCIENCES
In academic partnership with
BIRLA INSTITUTE OF TECHNOLOGY
Question Bank
Course: EC
Session: 2005-2006
Subject: EE 4301 Principles of Electrical Machines
Semester: IV
______________________________________________________________________________
DC Machines
1. a) What are the two basic components in a rotating electrical machine? Name the various
elements forming i) the magnetic field circuit. ii) the armature circuit.
2.
Draw a neat cross sectional view of a 4-pole d.c. machine showing the essential
components and prepare a list of the materials used for them.
3.
4.
Mention the prime functions of the following:
i)
Main magnetic poles and pole shoes
ii)
Armature core and armature winding
iii)
Commutator
iv)
Brushes and brush gears.
On what basis armature winding are classified as LAP and WAVE? In what respects the
two types differ from each other. Illustrate with a heat sketch the two schemes of winding.
5.
What rule is followed for marking the polarity and location of the br4ushes? Which type of
winding (Lap/Wave) do you recommend for (i) High voltage. Low current d.c. machine. (ii)
Low voltage, high current d.c. machine and why?
6.
Define the following terms in context to armature winding. (i) Pole pitch. (ii) Back
pitch.(iii) Front pitch. (iv) Progressive and Retrogressive windings. Illustrate each with
suitable sketch/ diagram.
7.
Mention the characteristic features of the various types of pitches.
8.
Test for the feasible type of armature winding with the following arrangements:
9.
i) 28 conductors, 4 poles;
ii) 38 conductors, 4 poles;
iii) 26 conductors, 4 poles;
iv) 24 conductors, 4 poles with 2 conductors per slot.
Why do you consider commutator as a mechanical rectifier? Explain with necessary
schematic diagrams. How unidirectional voltage is obtained from the armature coil in a
simple d.c. generator?
Prepared By Mohammad Mohatram
10.
Give the winding table and draw the developed armature-winding diagram for the possible
winding pattern (Lap/ wave) and possible pitches (vF & vB).
i)
46 conductors with 4 poles.
ii)
36 conductors with 4 poles.
iii)
26 conductors with 4 poles.
iv)
30 conductors with 4 poles.
Show the polarity and arrangement of the brushes in each case.
11.
What is commutation? Explain the process of commutation with the help of neat diagrams.
12.
What is reactance voltage? How does it affect the operation of a d.c. machine? Develop an
expression for the reactance voltage.
13.
Explain the difficulties of commutation in a d.c. generator. What methods are used to
overcome these difficulties?
14.
What is Armature reaction? Explain with the aid of neat diagram armature reaction in a d.c.
machine.
15.
Discuss the various effects of armature reaction on working of a d.c. machine.
16.
Derive expressions for demagnetizing and cross magnetizing ampere-turns.
17.
(a) What for interlopes and compensating winding are provided?
(b) How is the polarity of an interlope marked in case of a d.c. generator and a d.c. motor.
(c) Where are the interlope located and how is the interlope winding connected?
(d) How is the compensating winding connected?
18.
Derive the expression for the e.m.f. induced in a d.c. machine. State the compensations
made and the symbol is used.
19.
A 6 pole 2 circuit wave connected armature has 250 conductor and 1200rom. The
electromotive force generated on open circuit is 600V. Find the useful flux per pole.
(0.04wb).
20.
An 8 pole Lab connected armature has 960 conductors, a flux of 4 mega lines (40mwb) per
pole, and a speed of 400 rpm .Calculate the e.m.f. generated on open circuit. (256V).
22.
A 6 pole armature winding has 48- conductors. Average e.m.f generated in each conductor
is 2.2V and each conductor can carry 100A at full load. Calculate (a) the voltage generated
(b) output full load current (c) Total output power developed when the armature is (i) lap
connected, (ii) wave connected. (d) 176V, 600A, 105.6KW. (ii) 528V, 200A, 105.6KW.
Prepared By Mohammad Mohatram
23.
A shunt machine, connected to 250V mains, has an armature resistance (including brushes)
of 0.12 ohm, and the resistance of the field circuit is 100 ohms. Find the ratio of the speed
as a generator to the speed as a motor the line current in each being 80A. (1.08).
24.
A 6 pole armature is wound with 498 conductors. The flux and the speed are such that the
average e.m.f generated in each conductor is 2V. The current in each conductor is 120 A,
Find the total current and the generated e.m.f of the armature if the winding is connected (a)
wave (b) Lap. Also find the total power generated in each case.
{Ans.= 240A, 498V, 166V, 199.5KW}.
25.
A 4 pole armature is wound with 564 conductors and driven at 800 rpm, the flux pole being
20mwb. The current in each conductor is 60A. Calculate the total current and the electrical
power generated in the armature if the conductors are connected (a) wave, (b) Lap,
{(a) 120A, 36.096kw), (b) 240A, 36.096kw,}.
26.
An eight pole Lap-connected armature has 96 slots with 6 conductors per slot and is driven at
500rpm. The useful flux per pole is 0.09wb. Calculate the generated e.m.f. {Ans: 432V}.
27.
A 4 pole armature has 624 lap connected conductors and is driven at 1200 rpm. Calculate
the useful flux per pole required to generate an emf of 2500V. [ans: 0.02wb].
28 A 6 pole armature has 410 wave connected conductors. The useful flux per pole is 0.025 wb.
Find the speed at which the armature must be driven if the generated emf is to be 485V. [ans:
946rpm].
29. The wave connected armature of a 4 pole d.c. generator is required to generate an emf of 250V
when driven at 660 rpm. Calculate the flux per pole required if the armature has 144 slote with
2 coil sides per slot, each coil consisting of 3 turns. [Ans: 0.0274wb].
30. A 6 pole dc generator runs at 850 rpm and each pole has a flux of 0.2  10-2 wb. If there are 150
conductors in series between each pair of brushes. What is the value of the generated emf ?
[Ans: 153V].
31. A 4 pole wave wound d.c. machine running at 1500 rpm has a commutator of 30.5 cm
diameter. If armature current is 150 amperes, thickness of brush 1.27 cm and self inductance of
each armature coil is 0.07mH, Calculate the average emf induced in each coil during
commutation. Assume linear communication. [19.8V App.]
Prepared By Mohammad Mohatram
32. A 300 KW, 500V, 8-pole d.c. generated has 768 armature conductors, Lap connected.
Calculate the number of demagnetizing and cores amp-turn per pole when the brushes are given
a lead of 5 electrical degrees from the geometrical neutral. [ Ans: 200 At 3400At].
33. A 4 pole motor has a wave connected armature with 888 conductors. The brushes are displaced
backwards through 5(mech.) degree from the geometrical neutral. If the total armature current
is 90A, Calculate: (a) The cross and the back ampere turns per pole; and (b) the additional
field current to neutralize this demagnetization. If the field winding has 1200 turns per pole. [
Ans: 4440At. 555At, 0.4625A].
34. Calculate the number of turns/pole required for commutating poles of the d.c. generator
referred to in Q.(32) assuming the compole ampere turns/pole to be about 1.33 times the
armature ampere-turns / pole and the brushes to be in the geometrical neutral. [Ans: 8].
35. An eight pole generator has a lap connected armature with 640 conductors. The ratio of pole
arc/pole pitch is 0.7. Calculate the ampere-turns/pole of a compensating winding to give
uniform air gap density when the total armature current is 900A. [Ans: 3150At].
36. What are the different methods of excitation used in a d.c. machine? Illustrate them with
suitable schematic diagrams.
37. What is meant by (I) magnetization characteristic, (ii) internal characteristics, and (iii) external
characteristics of a d.c. generator? Mention their distinguishing features. Sketch the
characteristics of series, shunt, compound, and separately excited d.c. generators.
38. What are conditions of building up of voltage in a d.c. shunt and series generator? What is
understood by critical resistance and critical speed of a d.c. generator?.
39. Discuss the performance characteristics of d.c. series, shunt and compound generators and
motors. Mention their important applications.
40. Enumerate the various losses and derive expression for maximum efficiency of a d.c. shunt
machine.
41. Why starter is needed for a d.c. motor? Give a neat illustrative schematic diagram showing the
essential parts of a three point and four point d.c. motor starter. Explain their working with
particular attention to the function of no load and over load protection schemes.
42. Explain why the terminal voltage of a d.c. shunt generator falls as the current supplied by the
machine is increased. The resistance of the field circuit of a d.c. shunt generator is 200 ohms
when the output of the generator is 100kw, the terminal voltage is 500V and the generator emf
Prepared By Mohammad Mohatram
is 525V. Calculate (a) the armature resistance and (b) the value of the generated emf when the
output 60kw, if the terminal voltage is then 250V. [Ans: 0.123ohm, 534.5V]. {279.67}.
43. A short shunt compound generator has armature, shunt field, and series field resistance of 0.8
ohm, and 0.6 ohm respectively and supplies a load of 5 kw at 250V. Calculate the wmf
generated in the armature {Ans: 282.66V]
44. The following table gives the open circuit voltage for different field currents of a shunt
generator driven at a constant speed.
Terminal voltage(V) 120
240
334
400
444
470
Field Current ( C )
1.0
1.5
2.0
2.5
3.0
0.5
Plot a graph showing the variation of generated emf with exciting current and from this graph
determine the value of the generated emf when the shunt circuit has a resistance of (a) 160
ohms (b) 210 ohms © 300 ohms. Also find the value of the critical resistance of the shunt
circuit. [Ans: 467V, 373V, 0.240ohms]
45.
The following table relates to the open circuit curve of a shunt generator running at 750 rpm.
Generated emf (v)
10
Field Current (A)
172
300
360
385
395
0
1.0
2.0
3.0
4.0
5.0
Determine the no load terminal voltage if the field circuit resistance is 125 ohms. If the speed is
now halved, what is the resultant terminal voltage? At the reduced speed, what value of field
circuit resistance will give a no load terminal voltage of 175V? [Ans: 345V, 5V, 64.5ohms].
46.
The open circuit characteristics of a shunt generator when separately excited and running at
1000rpm is given by:
Open circuit voltage(V)
56
112
150
180
200
216
230
Field current (A)
0.5
1.0
1.5
2.0
2.5
3.0
3.5
If the generator is shunt connected and runs at 1100 rpm with a total field resistance of 80
ohms, determine : (a) the no load emf (b) the output current when the terminal voltage is 200V
if the armature resistance is 0.1 ohm © the terminal voltage of the generator when giving the
maximum output current. Neglect the effect of armature reaction and of brush contact drop.
[Ans: 236V 100A……………]
47.
A shunt machine has armature and field resistance of 0.04 ohm and 100 ohm respectively.
When connected to a 460V d.c. supply and driven as a generated at 600rpm, it delivers 50 kw
Prepared By Mohammad Mohatram
from the same supply. Show that the direction of rotation of the machine as a generator and as a
motor under these conditions remains unchanged. [Ans: 589 rpm].
48.
A 100 kw , 500V, 750rpm, d.c. shunt generator, connected to constant voltage bus, has field
and armature resistances of 100 ohms and the machine continues to run taking 50 A from the
bus bars, Calculate its speed. Neglect brush drop and armature reaction effects. [Ans: 714 rpm].
49.
A 4-pole d.c. motor is connected to a 500V d.c. supply and takes an armature current of 80A.
The resistance of the armature circuit is 0.4 ohm. The armature is wave wound with 522
conductors and the useful flux per pole is 0.025 wB. Calculate (a) the back emf of the motor (b)
the speed of the motor © The torque in Nw-m developed by he motor. [ans: 468V, 1075rpm,
333Nw-m].
50.
A d.c. shunt generator delivers 5 kw at 250 V when driven at 1500 rpm. The shunt circuit
resistance is 250 ohms and the armature circuit resistance is 0.4 ohm. The iron, friction and
windage losses are 250W. Determine the torque in Nw-m required to drive the machine at the
above load. [Ans: 36.2Nw-m]
51.
A shunt wound motor has a field resistance of 350 ohms and an armature resistance of 0.2 ohm
and runs of a 250V supply. The armature current is 55A and the motor speed is 1000 rpm.
Assuming a straight line magnetization curve, calculate (a) an additional resistance required in
the field circuit to increase the speed of 1100 rpm for the same armature current, and (b) the
speed with the original field current and an armature current of 100A. [Ans; 35 ohms, 962
rpm].
52.
Why a d.c. series motor should never be started at no load? A d.c. series motor, connected to a
440V supply, runs at 600 rpm when taking current 0f 50A. Calculate the value of a resistor
which, when inserted in series with the motor, will reduce the speed to 400 rpm, the gross
torque being then half its previous value. Resistance of motor = 0.2 ohm. Assume the flux to be
proportional to the field current [Ans: 6.53 ohms].
53.
A series motor runs at 900 rpm when taking 30 A at 230V. The total resistance of the armature
and field circuits is 0.8 ohm. Calculate the values of the additional resistance required in series
with the machine to reduce the speed to 500 rpm if the gross torque is (a) constant; (b)
proportional to speed; © proportional to the square of the speed. Assume the magnetic circuit to
be unsaturated. [Ans: 3.05ohms, 5.67ohms, 9.2 ohms].
Prepared By Mohammad Mohatram
54.
A d.c. shunt motor takes an armature current 20A from a 230V supply. Resistance of the
armature is 0.5 ohm. Calculate the resistance required in series with the armature to have the
speed if (a) the load torque is constant (b) the load torque is proportional to the square of the
speed. [Ans: 5.5 ohms, 23.5 ohms].
55.
A d.c. shunt motor runs at 900 rpm from a 480V supply when taking an armature current of
25A. Calculate the speed at which it will run from a 240 V supply when taking an armature
current of 15A. Assume the flux per pole at 240V to have decreased to 75 percent of its value at
4800V. [Ans: 595 rpm].
56.
A shunt wound generator has a full load output of 10 KW at a terminal voltage of 240V. The
armature and shunt circuit resistances are 0.6 ohm and 160 ohms respectively. The mechanical
and iron losses total 500W. Calculate the power in KW required at the driving shaft at full load
and the efficiency. What will be the approximate output power of the generator at maximum
efficiency and what will be the value of that efficiency? [Ans: 11.98KW, 0.8345 p.u; 8.72KW
0.8355 p.u.]
57.
If the d.c. machine referred to in Q.56 is run as a motor taking 40A from a 240V supply, what
are the values of (a) the output power in KW, and (b) the efficiency? Calculate also (c) the
approximate value of the input power when the efficiency is a maximum, and (d) the value of
that efficiency? [Ans: 7.85KW, 0.8177 p.u.; 9.44 KW, 0.818 p.u.].
58.
A d.c. shunt motor has an output of 8 KW when running at 750 rpm of a 480V supply. The
resistance of the armature circuit is 1.2 ohms and that of shunt circuit is 800 ohms. The
efficiency at that load is 83 percent. Determine (a) the no load armature current; (b) the speed
when the motor takes 12A, and (c) the armature current when the gross torque is 60 Nw-m.
Assume the flux to remain constant [Ans: 1.87A, 766 rpm, 10.3A].
59.
The current taken by a 460V shunt motor when running light is 7A. The resistance of the
armature circuit is 0.15 ohm and that of the field circuit is 230 ohms. Calculate the output in
KW and the efficiency when the current taken is 130A. Calculate also the armature current at
which the efficiency is a maximum. [Ans: 54.1KW, 0.905 p.u., 146.5A]
60.
A 230 V shunt motor, running on no load and at normal speed, takes an armature current of
2.5A from 230V mains. The field circuit resistance is 230 ohms and the armature circuit
resistance is 0.3 ohm. Calculate the motor output (in Kilowatts), and the efficiency when the
total current taken from the mains is 35A. If the motor is used as a 230V shunt generator. Find
Prepared By Mohammad Mohatram
the efficiency and the input power for an output current of 35 A. [Ans: 6.9KW, 0.857 p.u.;
0.871 p.u. 9.24KW]
Transformer
61.
Define a transformer. Distinguish between primary and secondary winding. Give different
classifications of transformer.
62.
Sketch the cross sections of different types of core used for transformer. What is the purpose of
laminating the core? Why is the L.V. winding place near the core?
63.
What is the principle of action of a transformer?. How the primary current adjusts itself as the
load on the transformer changes? Show that a constant frequency, constant voltage transformer
is also a constant flux transformer under steady state.
64.
Derive the emf equation of a two winding transformer. Explain the concept of an ideal
transformer stating the underlying assumptions.
65.
Explain the terms:
(i)
Transformation ratio
(ii)
Magnetizing current, and
(iii)
Leakage flux.
How do the leakage fluxes affect the operation of a transformer? How are they minimized?
66.
(a) Explain with the aid of phasor diagram the functions of the no load current (b) Explain why
the no load current may be non-sinusoidal when the primary voltage is purely sinusoidal. (c)
Draw the phase diagram of a single phasor transformer on load showing the phasor relations
between primary and secondary voltage and currents. Assume ideal transformer.
67.
Define the terms “ equivalent resistance” and “equivalent reactance” as applied to transformers
and deduce an expression for these quantities in terms of the actual values of the primary and
secondary winding.
68.
What are the various losses occurring in a transformer? Mention the factors governing these
losses. Shoe that the maximum efficiency of a transformer occurs when the Cu loss is equal to
the iron loss.
69.
Distinguish between power efficiency and all – day efficiency. Why is all-day efficiency
considered more reasonable basis for comparison than power (ordinary) efficiency?
Prepared By Mohammad Mohatram
70.
Describe the open circuit test and the short circuit test on a transformer. Explain, on which side
(H.V/ L.V.) these tests are carried on and Draw the associated phasor diagrams and why?
71.
Describe how O.C. and S.C. tests help in obtaining equivalent circuit diagram of a transformer.
Proceed step- by – step to obtain an approximate equivalent circuit diagram from the exact
equivalent circuit diagram stating the simplifying assumptions.
72.
Draw relevant phasor diagrams of a two winding transformer for lagging and leading load
conditions.
73.
Define regulation of a transformer. How can the efficiency and regulation of a transformer be
calculated by the aid of the data obtained from the O.C. and S.C. tests. Draw the appropriate
phasor diagrams and the expression for determining regulation of a transformer at
a) Lagging p.f.
b) Leading p.f., and
c) unity p.f. load.
74.
Discuss the concept of per unit and percentage resistance and reactance drops in a transformer.
Mention its advantages over the ohmic representation.
75.
(a) Give various arrangements of primary and secondary winding for three-phase transformer.
Enumerate the advantages of three phase transformer over Single-phase transformer.
(b) Explain why in a power transmission system normally star side of a star/delta transformer is
the HV side while in a distribution system it is the LV Side.
76.
A 200 KVA, 3300V/240V, 50Hz, Single phase transformer has 80 turns on the secondary
winding. Assuming an ideal transformer, Calculate:
(a) The primary and secondary currents on full load,
(b) The maximum value of the flux;
© The number of primary turns
77.
[Ans: 60.6A, 833A, 0.0135wb< 1100 turns].
The following data apply to a Single phase transformer; output 100KVA; secondary volts 400;
primary turns 200; secondary turns 40; Calculate , neglecting losses, (a) the primary applied
voltage (b) the normal primary and secondary currents (c) Secondary current when loaded to 50
KW at 0.8 p.f. [Ans: 2000, 50A, 250A, 156.3A]
78.
A 125 KVA transformer having a primary voltage of 2000V at 50 Hz has 182 primary turns
and 40 secondary turns. Neglecting losses, Calculate (a) the full load primary and secondary
Prepared By Mohammad Mohatram
current, (b) the no load secondary induced emf (c) the maximum flux in the core. [Ans: 62.5A,
284A, 440V, 4.9510-2wb].
79.
A 3300/240, single phase transformer, on no load takes 2A at p.f. of 0.25, Determine
graphically , or otherwise, the primary current and p.f. when the transformer is supplying a load
of 60A at a p.f. of 0.9 leading {Ans: 4.43A, 1.0}
80.
If three-single phase transformers, each with a turns ratio of 2:1, are connected star/Delta and
the primary line voltage is 6600V, What is the value of secondary no load voltage? If the
transformer s are connected delta/star with the same primary voltage. What is the value of
secondary line voltage. {Ans: 317.5V, 952.5V].
81.
The no load current of a transformer is 0.5A at 0.3 p.f. when supplied at 230V, 50Hz. The
number of turns on the primary winding is 200. Calculate
(a) The maximum value of the flux in core
(b) The core loss
(c) The magnetizing current
82.
[Ans: 5.18 mwb, 345W 0.477A rms]
The ratio of turns of a single-phase transformer is 8, The resistance on the primary and
secondary winding are 0.85 ohms and 0.012ohms respectively, and leakage reactance of these
winding are 4.8 ohms and 0.07 ohms respectively. Determine the voltage to be applied to the
primary to obtain a current of 150A in the secondary when the secondary terminals are shortcircuited, ignore magnetizing current. [Ans: 176.5V]
83.
A single-phase transformer operates from a 230V supply. It has an equivalent resistance of 0.1
ohm and an equivalent leakage reactance of 0.5 ohm referred to the primary. The secondary is
connected to a coil having a resistance of 200 ohms and a reactance of 100 ohms. Calculate the
secondary terminal voltage. The secondary winding has four times as many turns as the
primary. {Ans: 900]
84.
A 10 KVA single phase transformer, for 2000V/400V at no load, has resistance and leakages
reactance as follows: Primary winding: resistance 5.5 ohms, reactance 12Ohms; Secondary
winding: resistance 0.2 ohms, reactance 0.45 ohm. Determine the approximate value of the
secondary voltage at full load, 0.8 p.f. (Lagging), When the primary supply voltage is 2000V.
{Ans: 377.6V]
85.
A single – phase transformer 125 KVA, 2000 to 440 V has a primary resistance of 0.17 ohm,
and secondary resistance of 0.0083 ohm. The iron loss is 1.3KW. Calculate the efficiency for
Prepared By Mohammad Mohatram
¼, ½, ¾, 4/4 and 5/4 of full load (a) for unity p.f. (b) for 0.8 p.f. and plot graphs of efficiency
against load. [Ans: (a) 98% at full load, (b) 97.8% at full load].
86.
Calculate the total resistance referred to both primary and secondary for the transformer of Q
(85). [Ans: 0.34 ohm, 0.0166 ohm].
87.
Calculate the KVA loading at which the efficiency of the transformer in Q(85) is a maximum.
What is the value of the maximum efficiency at (a) unity p.f. (b) 0.8 p.f. [Ans: 123KVA, (a)
98% (b) 97.5%].
88.
A 100 KVA transformer has an ordinary efficiency at full load, unity p.f. 98%, and the copper
losses are then twice the iron losses. Calculate the KVA loading at which the efficiency is a
maximum. [Ans: 70.7 KVA].
89.
Draw the equivalent circuit of a loaded transformer and calculate the circuit constants of a
250/500 volt transformer from the following test results:
Open circuit test: 250V, 1 Amp, 80W as the low voltage side
Short circuit test: 20V, 12 Amp, 100W as the high voltage side
Calculate the efficiency and the primary voltage applied when the output is 10A, 500V at 0.8
p.f. lagging. [Ans: 96.4%, 257.4V].
90.
A 50 KVA , 6360V/240V transformer is tested on open and short circuit to obtain its
efficiency, the results of the test being as follows:
Open circuit: primary voltage 6360V, primary current 1A, power input 2 KW.
Short circuit: primary voltage 180V, primary current 175A, power input 2 KW.
Find the efficiency of the transformer when supplying full load at a p.f. of 0.8 lagging. Draw a
phasor diagram (neglecting impedance drop) for this condition. [Ans: -.892 p.u.]
91.
A 240/400 single phase transformer absorbs 35W when its primary winding is connected to a
240V 50Hz supply, the secondary being on open circuit. When the primary is short circuited
and a 10V; 50Hz supply is connected to the secondary winding the power absorbed is 48W
when has the full load value 15A.
Estimate the efficiency of the transformer at half load, 0.8 p.f. lagging [Ans: 0.981 p.u.]
92.
A transformer working at unity p.f. has an efficiency of 90 % at both half load and the full load
of 500W. Determine the efficiency at 75% of full load. [Ans; -.905].
Prepared By Mohammad Mohatram
93.
A 400 KVA transformer has an iron loss of 2KW and the maximum efficiency at 0.8 p.f. occurs
when the load is 240 kw. Calculate (a) the maximum efficiency at unity p.f. (b) the efficiency
on full load at 0.71 p.f. [Ans: (a) 0.9868 p.u. (b) 0.9808 p.u.].
94.
A 40 KVA transformer has core loss of 450w and full load copper loss of 850W, If the p.f. of
the load is 0.8, Calculate (A) the full load efficiency (b) the maximum efficiency, ansd (c) the
load at which maximum efficiency occurs. [Ans: (a) 0.961 p.u. (b) 0.9628 p.u. (c) 23.3 kw].
95.
A 100 KVA lighting transformer has a core loss of 3 KW. The losses being equally divided
between the iron and copper. During one day the transformer operates on full load for 3 hours,
on half load for 4 hours, the output being negligible for the reminder of the day. Calculate the
all day efficiency. [0.9225 p.u.]
96.
A 10 KVA, single phase transformer has a full load efficiency at unity power factor leading of
97%, the copper and iron losses then being equal. Calculate its all day efficiency if it is loaded
throughout the 24 hours as follows: no load for 10 hours, quarter load for 6 hours, half load for
five hour, full load for three hours. [94%]
97.
Calculate the voltage regulation at 0.8 p.f. lagging for a transformer, which has an equivalent
resistance of 2 % and an equivalent leakage reactance of 4%. [4%]
98.
A 75 KVA transformer rated at 6600V/230V on no load, require 310 volts across the primary
to circulate full current on short circuit, the power being absorbed being 1.6 KW. Determine (a)
the % voltage regulation, and (b) the full load secondary terminal voltage for power factors of
(i) unity, (ii) 0.8 lagging and (iii) 0.8 leading. If the input power to the transformer on load is
0.9 KW, Calculate per unit efficiency at full load and at half load for power factor 0.8 and find
the KVA at which the efficiency is a maximum. [2.13 %, 225.1 Volt; 4.22%, 220.3 Volts;
-0.81%, 231.86 V; 0.960 p.u., 0.9585 p.u., 56.25 KVA ]
99.
The primary and secondary of a winding of a 30 KVA, 6000 V/230V transformer have
resistance of 10 ohms and 0.016 ohms respectively. The total resistance of the transformer
referred to the primary is 23 ohms. Calculate the % regulation of the transformer when
supplying full load current at a power factor of 0.8 lagging. [2.54 %]
100.
Prove that with the secondary on open circuit if the primary voltage is E and the frequency f
are raised so that E/f remains constants, then the maximum value of the flux in the core will
also remain constant, if the primary resistance is neglected.
Prepared By Mohammad Mohatram
Three phase induction motor
101.
How is the synchronous speed of an a.c. motor defined? What is an asynchronous motor?
102.
How a rotating magnetic field in three-phase induction is motor obtained?
103.
Name the conditions that must be satisfied in order that the revolving magnetic field of a threephase induction motor is of constant amplitude and of constant peripheral velocity.
104.
Why is it that the rotor speed of a three phase singly excited induction motor can never attain
synchronous speed exactly?
105.
Describe, what is meant by the slip of an induction motor.
106.
What is slip frequency?
107.
How does the magnetizing current of the induction motor compare with that of the transformer,
which is larger, explain?
108.
Show how the power that is transferred across the air gap of the three phase induction is
represented. Explain the terms. What portion of this is useful power?
109.
Draw the complete equivalent circuit of the three phase induction motor and explain the
meaning of each parameter and electrical variable appearing in the circuit.
110.
What constitute the rotational losses of the induction motor? How are these supplied?
111.
By means of a power flow diagram show the flow of power in a three phase induction motor
from the electrical source to the mechanical load at the motor shaft.
112.
Explain the torque-speed curve of the induction motor.
113.
What is meant by the break down torque of an induction motor?
Single phase induction motor
114.
Explain why a single phase induction motor has no starting torque?
115.
What is a resistance split-phase induction motor? What is the function of auxiliary winding?
Why is a cutout switch placed in series with the auxiliary winding?
116.
The capacitor-start induction-run motor has a much higher starting torque than the resistance
split-phase induction motor. Explain.
117.
What is a permanent-split capacitor motor? Its starting torque is lower than that of the
resistance split-phase induction motor. Explain.
Prepared By Mohammad Mohatram
118.
Describe the construction features of the shaded-pole induction motor and explain how a
starting torque is produced?
119.
When quietness of operation is essential in a fractional horsepower operation, which single
phase induction motor type is preferred? and why?
120.
Explain double field revolving theory.
Synchronous Machines
121.
Explain the difference between induction machine and synchronous machine.
122.
What are the two construction of a synchronous machine?
123.
What are V-curves? Explain
124.
Explain the working principle of a three phase synchronous motor.
125.
Derive the EMF equation of a synchronous machine.
126.
What are the advantages of rotating field and stationary armature?
Prepared By Mohammad Mohatram