UNIT III
ELECTROMECHANICAL ENERGY
CONVERSION
PART-A
1. What is an electromechanical
system?
.2. Describe multiply excited magnetic
field system.
3. Define co energy.
4. How energy is stored?
5. Define field energy.
6. Write the expression for the
principle of energy conversion.
7. What is the significance of co
energy?
8. How the energy stored in magnetic
field?
9. Give any four examples if single
excited magnetic system.
10. Write the applications of singly
excited and doubly excited magnetic
system .
11. State the necessary conditions for
the production of steady torque the
interaction of stator and rotor fields in
an electric machine.
PART-B
1. Derive the expression for field
energy produced in a doubly excited
magnetic field system? (16)
2. The magnetic flux density on the
surface of an iron face is 1.6 T which
is a typical saturation level
value for ferromagnetic material. Find
the force density on the iron face. (16)
3. What are the special applications
where the electric field is used as a
coupling medium for
electromechanical energy conversion?
Also explain why electric field coupling
is preferred in such applications? (16)
4. Find an expression for the force
per unit area between the plates of a
parallel plate condenser in terms of
the electric field intensity. Use both
the energy and co energy methods.
Find the value of the force per unit
area when E = 3 x 106 V/m, the
breakdown strength of air. (16)
5. Explain with neat diagram and
sufficient expressions, the multiply
excited magnetic field systems. (16)
6. Explain i - characteristics of a
magnetic system. Also derive the
expression for co energy density.
Assume i - relationship of the
magnetic circuit is linear. (16)
7. Explain the concept of singly –
excited machines and derive the
expression for the electromagnetic
torque. (16)
U NIT IV
BASIC CONCEPTS IN ROTATING
MACHINES
PART-A
1. Define the term pole pitch
2. Define pitch factor
3. Define the term breadth factor
4. Write down the advantages of short
pitched coil.
5. What is distributed winding?
6. Explain the following terms with
respect to rotating electrical machines.
7. Write the expressions for the
synchronous speed.
8. Write the mmf equation of dc
machine.
9. What is meant by electromagnetic
torque?
10. State the torque equation for
round rotor machine.
11. Define rotating magnetic field.
PART-B
1. Derive the expression for the r.m.s
value of emf induced in a.c. machines.
(16)
2. Prove that mmf wave of a single
phase ac winding is pulsating or
standing. (16)
3. Prove that the resultant mmf wave
of three phase ac winding is rotating
in space with
speed but its magnitude is constant.
(16)
4. Derive the torque equation for
round rotor machine. (16)
5. Explain the various concepts of
magnetic fields in rotating machines.
(16)
6. Explain with neat diagram the
concept of mmf space wave of a
single coil. (16)
7. Write in detail about mmf space
wave of three phase distributed
winding. (16)
UNIT V
DC MACHINES
PART – A
1. What is prime mover?
2. Give the materials used in machine
manufacturing
3. How will you change the direction
of rotation of a d.c motor?
4. What is back emf in d.c motors?
5. Under what condition the
mechanical power developed in a dc
motor will be maximum?.
6. What is the function of a no-voltage
release coil provided in a dc motor
starter?
7. Name the two types of automatic
starters used for dc motors.
8. Enumerate the factors on which the
speed of a dc motor depends.
9. List the different methods of speed
control employed for dc series motor
10. Name the different methods of
electrical breaking of dc motors.
11. Under what circumstances does a
dc shunt generator fail to build up?
12. To what polarity the interpoles
excited in dc motors?
13. What is back emf in d.c motors?
14. Name any four applications of DC
series motor.
15. Why DC motors are not operated
to develop maximum power in
practice?
16. Name the starters used for series
motors.
17. Name Different types of starters.
18. Name the Protective devices in a
starter.
20. What are the modification in ward
Leonard linger system?
21. What type of DC motors are
suitable for various torque
operations?
22. Define speed regulation.
23. What are the performance curves?
24. To what polarity are the interpoles
excited in dc generators?
25. Why are carbon brushes preferred
for dc machines?
26. What are the various types of
commutation?
27. Name the two methods of
improving commutation.
28. What is reactance emf in dc
machine?
29. Define the term commutation in
dc machines.
30. How and why the compensating
winding in dc machine excited?
PART-B
1. (a) Describe with sketches the
construction of a DC machine. (8) (b)
Derive the EMF equation of DC
generator. (8)
2. Draw and explain the no-load and
load characteristics of DC shunt, series
and compound generators. (16)
3. Explain the effect of armature
reaction in a DC shunt generator. How
is its demagnetizing and cross-
magnetizing ampere turns calculated?
(16)
4. Explain the process of
commutation in a DC machine. (16)
5. With a aid of a circuit diagram,
describe the procedure for paralleling
two DC shunt generators and for
transferring the load from one
machine to the other. (16)
6. A 4-pole, 50 kW, 250 V, wave
wound shunt generator has 400
armature conductors. Brushes are
given a lead of 4 commutator
segments. Calculate the
demagnetization ampere-turns per
pole if shunt field resistance is 50
ohm. Also calculate extra shunt field
turns per pole to neutralize the
demagnetization. (16)
7. A 4-pole, lap connected DC
machine has 540 armature
conductors. If the flux per pole is .03
Wb and runs at 1500 RPM, determine
the emf generated. If this machine is
driven as a shunt generator with same
field flux and speed, calculate the line
current if the terminal voltage is
400V.Given the RSH=450ohm and
RA=2phm. (16)
8. Two separately excited DC
generators are connected in parallel
and supply a load of 200A. The
machines have armature circuit
resistances of 0.05 ohm and 0.1 ohm
and induced emfs of 425V and 440V
respectively. Determine the terminal
voltage, current and power output of
each machine. The effect of armature
reaction is to be neglected. (16)
9. (a) Explain the principle of
operation of a DC motor. (8) (b) A
shunt machine, connected to 200V
mains has an armature resistance of
0.15 _ and field resistance is 100 _.
Find the ratio of its speed as a
generator to its speed as a motor, line
current in each case being 75 A. (8)
10. (a) Draw and explain the
mechanical characteristics of DC series
and shunt motor. (8)
(b) A 230V, DC shunt motor, takes an
armature current at 3.33A at rated
voltage and at a no load speed of
1000RPM. The resistances of the
armature circuit and field circuit are
0.3 _ and 160 _ respectively. The line
current at full load and rated voltage
is 40A. Calculate, at full load, the
speed and the developed torque in
case the armature reaction weakens
the no load flux by 4%. (8)
11. (a) Describe the working of 3 point
starter for DC shunt motor with neat
diagram. (8)
(b)Explain Ward-Leonard method of
speed control in DC motors. (8)
12. (a) Derive an expression for the
torque developed in a DC machine.
(8)
(b) A 220V, Dc shunt motor with an
armature resistance of 0.4 ohm and a
field resistance of 110ohm drives a
load, the torque of which remains
constant. The motor draws from the
supply, a line current of 32A when the
speed is 450 RPM. If the speed is to
be raised to 700RPM, what change
must be effected in the value of the
shunt field circuit resistance? Assume
that the magnetization characteristic
of the motor is a straight line. (8)
13. Explain the different methods
used for the speed control of D.C.
shunt motor. (16)
14. With the help of neat circuit
diagram, explain swinburne’s test and
derive the relations for efficiency
(both for generator and motor) also
state the merits and demerits of this
method. (16)
15. (a)Explain in detail about circuit
model of D.C. machine. (8)
(b) A 440 V D.C shunt motor takes 4A
at no load . its armature and field
resistances are 0.4 ohms
and 220 ohms respectively .estimate
the kW output and efficiency when the
motor takes 60A on full load. (8)
16. (a) Derive an expression for the
torque developed in the armature of a
D.C. motor. (8) (b) Determine
developed torque and shaft torque of
220V, 4 pole series motor with 800
conductors wave-connected supplying
a load of 8.2 kW by taking 45A from
the mains. The flux per pole is 25m/
Wb and its armature circuit resistance
is 0.6 ohm. (8)
17. With the help of neat circuit
diagram, explain Hopkinson’s test and
derive the relations for efficiency
(both for generator and motor) also
state the merits and demerits of this
method. (16)
18. (a) Explain in detail about different
methods of excitation. (8) (b)Derive
the expression for efficiency of D.C.
machines. (8)
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