MEGHNAD SAHA INSTITUTE OF TECHNOLOGY
Department of Electrical Engineering
Assignment
Subject: ELECTRICAL MACHINE [EE 401]
DC machine & DC machine – motoring and generation: [MODULE 3 & 4]
1. What do you mean by electrical degree and mechanical degree? Derive a relation between electrical degree and
mechanical degree.
2. Explain how voltage builds up in a dc shunt generator. Explain the terms critical field resistance and critical speed.
3. What do you understand by armature reaction? Explain the concept of demagnetizing and cross-magnetizing
armature ampere turns.
4. Derive an expression for total no of demagnetizing ampere-turns and total no of cross-magnetizing turns.
5. What do you mean by commutation in a DC machine? How commutation can be improved?
6. Derive an expression for torque developed in armature of a DC motor. Also derive an expression of back emf of
DC motor
7. Write short notes on any three of the following:
a. Commutation in DC machine
b. Interpole/Compole
c. Compensation winding
d. Open circuit characteristics of DC generator
e. Full pitch and Chorded coil
f. Emf polygon
8. Discuss in brief different types of commutation---resistance commutation, linear commutation, delayed
commutation and voltage commutation.
9. Explain what do you mean by external and internal characteristics of separately excited DC generator with proper
diagrams?
10. ‘A DC series motor should never be run on no-load’— explain.
11. How does armature reaction affect commutation? What steps are taken to have effective commutation?
12. “DC series motor is suitable for traction purpose”--------explain
13. Explain with neat circuit diagram, the Ward-leonard method of speed control of DC series motor. What are the
advantages and disadvantages?
14. Explain why we get speed below rated speed by armature control method and above rated speed by field control
method.
15. What are the advantages of distributed winding in rotating electrical machines?
16. Draw the external characteristics of a DC separately excited generator and explain how it is different from a series
and a shunt generator.
17. Explain with neat circuit diagram, the Ward-leonard method of speed control of DC motor. What are the advantages
and disadvantages?
18. Draw and explain the method of speed control of a DC motor by flux control method. Discuss the ranges of speed
control by the flux control method.
19. Answer the following questions:
a) What type of generator is used for general power supply?
b) A DC generator can run to generate the same no-load voltage with different modes of excitation. For which
mode of excitation the short-circuit current will be minimum?
c) For what purpose differential compound wound DC generator can be used?
20. An 8-pole generator has 500 armature conductors and has a useful flux per pole of 0.065wb. What will be the emf
generated if it is lap connected and runs at 1000rpm? What must be the speed at which it is to be driven to produce
the same emf if it is wave wound?
21. A short shunt compound generator supplies a current of 100A at a voltage of 220V. The resistance of shunt field,
series field and armature are 50, 0.025 and 0.05 respectively. Total brush drop is 2V and the iron & friction loss
amounts to 1KW. Find---i) the generated emf ii) the copper loss iii) the output of the prime mover driving the
generator iv) the generator efficiency
22. A 4-pole DC shunt generator with wave wound armature has 41 slots each having 12 conductors. Armature
resistance Ra=0.5ohm and shunt field resistance Rsh=200ohm, the flux per pole is 25mwb. If the load of 10ohm is
connected across the armature terminal, calculate the voltage across the load when the generator is driven at
1000rpm.
23. A 230V DC shunt machine has armature circuit resistance (including brushes) of 0.5ohm and field circuit resistance
of 115ohm. If this machine is connected to 230V supply mains, find the ratio of speed as a generator to the speed
as a motor. The line current in each case is 40A.
24. A shunt generator delivers 50KW at 250V and 400rpm. The armature and field resistance are 0.02ohm and 50ohm.
Find the speed when the machine works as a motor, taking an input of 50KW at 250V. Allow 1V per brush as
contact drop.
25. A 4-pole DC series motor has wave-connected winding with 600 conductors. The total resistance of motor is
0.8ohm. When fed from 250V DC source, the motor supplies a load of 10KW and takes 50A with a flux per pole
of 3mwb. For these operating conditions, calculate the developed torque and shaft torque.
26. A 250V shunt motor has an armature resistance of 0.6ohm and field resistance of 250ohm. When driving at 650rpm
a constant torque load, the armature takes 20A. If it is required to raise the speed from 650rpm to 850rpm, what
resistance must be inserted in the shunt field circuit assuming linear magnetization characteristics?
27. Determine the demagnetizing and cross-magnetizing ampere-turns per pole of a DC machine having 420 conductors
lap connected in a 6-pole machine. The load current is 100amps when brushes are shifted by 10 mechanical degrees
in the direction of rotation.
28. A 220V DC series motor has an armature and series field resistance of 0.4 and 0.6ohm respectively. The motor
while working with an unsaturated field takes 100amp at 1000rpm speed. Calculate the speed of motor when it
develops 50% of the torque as compared to first case mentioned above.
29. A 6-pole lap wound dc generator has 240 coils of 2 turns each. Resistance of one turn is 0.03ohm. The armature is
50cm long and 40cm diameter. Air gap flux density of 0.6T is uniform over pole-shoe. Each pole subtends an angle
of 40 mechanical. For armature speed of 1200rpm, find---- (a) generated emf at no-load and (b) the terminal voltage
at full load armature current of 40A.
30. A 15KW, 230V, 80A, 1000rpm dc series motor has the following full load losses expressed in percentage of motor
input.
Armature circuit ohmic loss (including brush loss) =2.8%
Field ohmic loss=2.6% and Rotational loss=2.2%
Neglect the armature reaction and magnetic saturation and assume the rotational loss to remain constant. If the
motor draws half the rated current at rated voltage, determine----
(a) Speed in rpm and (b) Shaft power output.
31. A 230V, 250rpm, 100A separately-excited dc motor has an armature resistance of 0.5ohm. The motor is connected
to 230V dc supply and rated dc voltage applied to the field winding. It is driving a load whose torque-speed
characteristic is given by TL=500-10w, where ‘w’ is the rotational speed expressed in rad/sec and T L is the load
torque in Nm. Find the steady state speed at which the motor will drive the load and the armature current drawn by
it from the source. Neglect rotational losses of the machine.
32. A 250V dc shunt motor has an armature resistance of 0.5ohm and a field resistance of 250ohms. When driving a
constant load torque at 600rpm, the motor draws 21A.What will be the new speed of the motor if an additional
250ohms resistance is inserted in the field circuit.
33. A 10KW, 240V dc shunt motor draws a line current of 5.2A while running at no-load speed of 1200rpm from a
240V dc supply. It has an armature resistance of 0.25ohm and a field resistance of 160ohms. Estimate the efficiency
and speed of the motor when it delivers rated load.
34. An 8-pole compensated generator has 96slots, each containing 2000amp conductors. The ratio (pole arc/pole pitch)
is 0.7. The length of interpole air-gap is 1.25cm and flux-density in the interpole air-gap is 0.3wb/m2. Find the amp-
turns/pole for the compensating and interpole winding.
35. A 5KW, 230V, shunt motor has an armature resistance of 0.5ohm and a field resistance of 230ohms. At no load,
the motor runs at 1000rpm and draws a current of 3amp. At full-load and rated voltage, current drawn is 30A. If the
armature reaction weakens the field by 2% and total brush voltage drop is 2V, determine---- (i) full-load speed (ii)
full-load torque.
36. Two identical dc machines when tested by Hopkinson’s method gave the following results-----Line voltage: 220V,
Field Currents: 2.5A and 2A, Line current=10A (including field currents), Motor armature current=73A. Armature
resistance of each machine is 0.05ohm. Calculate the efficiency of both the machine.
37. A 250V shunt motor has an armature resistance of 0.6ohms and a field resistance of 250ohms. When driving at
650rpm, a constant torque load, the armature takes 20A. If it is required to raise the speed from 650rpm to 850rpm,
what resistance must be inserted in the shunt field circuit assuming linear magnetization characteristics?
38. A 4-pole dc generator has a simplex lap winding with 144 coils each of 5turns and runs at 300rpm. The resistance
of each turn is 0.01ohm. The pole-pitch is 25cm, the armature length is 50cm and pole arc/pole-pitch ratio is 0.7.
If the air-gap flux-density is 0.5wb/m2, find----- a) Average value of induced emf , (b) Armature Resistance.
Single phase transformer: [MODULE 5]
1. Provide basic definition of a transformer. Is it a machine?-explain.
2. Is it possible to operate a transformer with DC supply?
3. Explain proper sequence to place HV winding, LV winding, CORE.
4. What is the approximate value of flux density inside transformer core-MCQ
5. Explain the operation of breather
6. What is the utility of conservator tank?
7. Explain the principle of operation of a single phase two winding transformer.
8. What type of material is used to make transformer core?
9. Write a short note on Bushing.
10. Explain transformer ratio.
11. Draw no load phasor diagram.
12. Explain operation of transformer under loaded condition with proper phasor diagram (lagging, leading, unity power
factor)
13. What is the physical interpretation of power factor?
14. Explain the equivalent circuit of two winding transformer under loaded and no-load condition.
15. Why silicon lamination is provided in case of transformer core?
16. Explain different types of losses.
17. Explain different types of core loss.
18. How do we minimise hysteresis loss, eddy current loss.
19. How do hysteresis loss and eddy current loss are separated?
20. What is the basic definition of efficiency for 2 winding transformer?
21. How do we maximise efficiency of 2 winding transformer?
22. What do you mean by voltage regulation? Explain its significance.
23. Is it possible to operate a 50 Hz 2 winding transformer (single phase) with 60 Hz. Explain its significance.
24. Derive the expression for voltage regulation of a single phase 2 winding transformer under lagging power factor
under no load condition with phasor.
25. Explain at which condition voltage regulation will be maximum with phasor diagram.
26. Derive the expression for zero voltage regulation/ condition for zero voltage regulation with phasor.
27. Is it possible to have zero voltage regulation under lagging power factor- explain (with phasor diagram).
28. Write down the necessary and desired conditions for parallel operation of single phase 2 winding transformer.
29. Explain parallel operation of transformer under equal voltage ratio.
30. Explain parallel operation of transformer under unequal voltage ratio.
31. What do you mean by magnetostriction?
32. What are the basic properties of transformer oil?
33. What will be the effect if outer moisture (atmospheric) will directly come in contact with oil.
34. Explain different types of cooling in single phase transformer.
35. Explain operation of no load tap changer.
36. Explain the operation of on load tap changer.
37. Explain polarity test.
38. Write a short note on Sumpner’s test/back to back test/ hit- run test.
39. Why core loss is treated as constant loss?
40. Why no load current is considered negligible in comparison to load component of current under on load operation?
41. Is it possible to provide larger air gap in between the transformer winding and core?
42. Why is it not possible to place the exciting current branch in case of equivalent circuit of 2 winding transformer in
series?
43. Explain the term - all day efficiency
44. Write the working principle of auto transformer.
45. What are the advantages of auto transformer over 2 winding single phase transformer if both have same rating?
46. The emf per turn for a single phase, 2310/220 V, 50 Hz transformer is approximately 13 volts. Calculate (a) the no.
of primary and secondary turns and (b) the net cross-sectional area of the core, fore a maximum flux density of 1.4
T.
47. A 33 kVA, 2200/220 V, 50 Hz single phase transformer has the following parameters:
Primary winding: r1 = 2.4 Ω, x1 = 6.0 Ω.
Secondary winding: r2 = 0.03 Ω, x2 = 0.07 Ω
(a) Find the primary resistance and leakage reactance referred to secondary
(b) Find the secondary resistance and leakage reactance referred to primary
(c) Find the equivalent resistance and leakage reactance referred to (i) primary and (ii) secondary.
(d) Calculate the total ohmic loss at full load.
(e) Calculate the voltage to be applied to the h.v side in order to obtain a short circuit current of 160 A in the l.v.
winding. Under these conditions, find the power input also.
48. A 200 kVA, 11000/400 V, delta/star distribution gave the following test results:
Open circuit test: 400 V, 9 A, 1.50 kW.
Short circuit test: 350 V, rated current, 2.1 kW.
Calculate the equivalent circuit parameters referred to h.v. side and its efficiency at half full load of unity power
factor.
49. A 10 kVA, 2000/200 V, single phase transformer has following parameters:
r1 = 3.6 Ω, x1 = 5.2 Ω, r2 = 0.04 Ω, x2 = 0.056 Ω. Find the per unit values of re1 , xe1 , re2 , xe2 .
50. For a 200 kVA, 4000/1000 V, single phase transformer, draw the equivalent circuit referred to l.v side and insert
all the values. It is given that the transformer efficiency at unity power factor is 97 both at full load and at 60
of full load. The no load pf is 0.25 and the full load regulation at a lagging pf of 0.8 is 5.
51. A short circuit test, when performed on the h.v side of a 10 kVA, 2000/400 V, single phase transformer gave the
following data: 60 V, 4 A, 100 watts.
If the l.v side is delivering full load (or rated) current at 0.8 p.f. lag and at 400 V, find the voltage applied to h.v
side.
52. A 220 V, 60 Hz, single phase transformer has hysteresis loss of 340 watts and eddy current loss of 120 watts. If the
transformer is operated from 230 V, 50 Hz supply mains, then compute its total core loss. Assume Steinmetz’s
constant equal to 1.6.
53. The efficiency of a 20 kVA, 2500/250 V. single phase transformer at unity p.f. is 98 at rated
load and also at half rated load. Determine
(i)The transformer core loss and ohmic losses, (ii) the per unit value of the equivalent resistance of the transformer.
54. The maximum efficiency of a 100 kVA, single phase transformer is 98 and occurs at 80 of full load at 0.8 p.f.
if the leakage impedance of the transformer is 5, find the voltage regulation at rated load of 0.8 p.f. lagging.
55. Two single phase transformers rated 100 kVA and 500 kVA have per unit leakage impedance of (0.02+j0.06) Ω
and (0.025+j0.08) Ω respectively. What is the largest kVA load that can be delivered by the parallel combination
of these two transformers without over loading any one?
Three phase Transformer: [MODULE 5]
56. Write a comparison analysis in choosing a three phase transformer and single phase transformer.
57. ‘Explain the basic principle of vector grouping of 3-phase transformers. How many vector groupings have been
developed? Explain the meaning of each vector grouping.
58. Draw the connection diagram and corresponding phasor diagram for the vector groups (i) Dy 11 (ii) Dd 6, (iii) Dd
0, (iv)Yy 6, (v) Yy 0, (vi) Dy 1, (vii) Yd1, (vii) Yd 11. (In addition to these connections zig-zag connections are
required).
59. Why zig-zag connection of transformer is used?
60. One of the windings of 3-phase transformer shall be delta connected. Explain why
61. How group-3 and group-4 transformers can be run in parallel?
62. What is open delta connection in transformers? Explain its utility.
63. In open-delta transformers, show that the secondary line voltages form a balanced 3-phase system of voltages, in
case the supply voltages are balanced.
64. Mention the conditions to be fulfilled for parallel operation of three-phase transformer.
65. What are the applications of Scott connection?
66. Explain Scott connection with proper connection diagram and vector diagram.
67. Write a short note on three phase to six phase conversion.
68. Explain the detailed operation of a three winding transformer.
69. What do you mean by exciting current & magnetizing current?
70. Explain the excitation phenomena in transformer.
71. What is the purpose of using earthing transformers in 3 phase circuits?
72. Write a short note on harmonics present in magnetization current.
73. Explain the various three phase transformer connections from magnetic circuit point of view.
74. Discuss the disadvantages of harmonics in transformers.
75. What are the methods to suppress the harmonics in transformers?
76. If one supply line feeding the primaries of three phase transformers gets disconnected, then what will happen?
77. Explain the effect of using tertiary winding in a Yy transformer?
78. The voltage applied to the primary winding of an unloaded single phase transformer is given by v = 400 coswt +100
sinwt. The primary has 500 turns and frequency of the fundamental component of the applied voltage is 50 Hz.
(a) Find the maximum value of the flux in mWb.
(b) If the no load current is found to be i0 = 0.43 cos (wt – 400) + 0.08 cos(3wt – 100)
Find the total core loss. Neglect primary winding resistance.
(c) If the primary voltage is free of third harmonic voltage, find the percentage change in eddy-current loss.
79. A 400 V, 3 phase factory load of 100 kW at 0.8 pf is to be supplied from 11 kV line through three three identical
single phase step down transformers. Determine the voltage, current and kVA ratings of each of the three
transformers if they are connected.
(a) Delta/delta, (b) Star/star, (c) Star/delta, (d) Delta/star, (v) open-delta.
80. It is desired to transform 2400 V, 5000 kVA three phase power to 2-phase power at 600 Vby Scott-connected
transformers. Determine the voltage and the current ratings of both primary and secondary of each transformer.
Neglect the transformer no-load currents.
Electromagnetic force and torque: [MODULE 2]
81. Explain B-H curve of magnetic materials.
82. What do you mean by linear and non-linear magnetic circuits?
83. Derive the expression of the energy stored in the magnetic circuit (or Electromagnet).
84. Show that force as a partial derivative of stored energy with respect to position of a moving element.
85. Show that torque as a partial derivative of stored energy with respect to angular position of a rotating element.
86. What do you mean by Co-energy?
87. Write a note on – (i) galvanometer coil, (ii) lifting magnet, (iii) rotating element with eccentricity or saliency.
88. An iron ring of mean length 30 cm has an air gap of 2mm and a coil with 200 turns on it. If relative permeability of
iron is 300 when a current of 1 A flows through the coil, find the flux density.
89. A toroidal iron ring having uniform cross-sectional area of 20 mm2, and mean magnetic path of 80 mm, has an air
gap of 1 mm. The excitation coil with 200 turns wound uniformly along its length carries a DC current of 2A. The
iron may be assumed to be perfect magnetic material. Fringing at the air gap increase the effective area offered to
magnetic flux by 5. Calculate (a) the exciting MMF of the coil, (b) the effective reluctance of the magnetic circuit,
(c) the magnetic flux in the air gap, (d) the inductance of the coil, (e) the energy stored in the magnetic field under
the above excitation.
90. Total mean length of the whole magnetic path is 200 cm and the cross sectional area of 25 cm2. Length of each air
gap is 2mm. Number of turns on the coil is 500. Calculate the value of the exciting current required to exert a force
of attraction of 2000 N on the armature. Magnetic field strength required to produce a flux density of 1Wb/m 2 in
steel is 12 AT/cm. (consider the magnetic path is U shaped).
Magnetic fields and magnetic circuits: [MODULE 1]
1. What do you mean by MMF?
2. Define flux. Classify different types of fluxes.
3. Explain fringing.
4. Explain Biot-Savart law.
5. Explain Ampere law.
6. Derive the expression for magnetic field produced by a bar magnet and a current carrying coil through air and
combination of iron and air.
7. Explain the influence of highly permeable material on the magnetic flux lines.
8. A series magnet circuit has an iron path of length 50 cm and an air gap of length 1 mm. The cross-sectional area of
the iron is 6 cm2 and the exciting coil has 400 turns. Determine the current required to produce a flux of 0.9mWb
in the circuit. The following points are taken from magnetization characteristic:
Flux density (Wb/m2): 1.2 1.35 1.45 1.55
Magnetizing force (AT/m): 500 1000 2000 4500
9. For the magnetic circuit shown in the figure below, all the dimensions are in cm and all the air gaps are of 0.5 mm
wide. Net thickness of the core is 3.75 cm throughout. The turns are arranged on the centre limb as shown. Calculate
the mmf required to produce a flux of 1.7 mWb in the centre limb. Neglect the leakage and fringing. The
magnetization data for the material is as follows:
H (AT/m): 400 440 500 600 800
B (Wb/m2): 0.8 0.9 1.0 1.1 1.2
