22523 Industrial AC Machines External Oral / Viva Practice Questions with Answers |
1) What is a single phase
Autotransformer?
A Single-phase autotransformer is
a single winding transformer in which a part of the winding is common to both
high-voltage and low-voltage sides.
2) What are the advantages of
Autotransformer?
The advantages of autotransformer
are as follows:
Less winding material is required
in case of the autotransformer.
These transformers are not costly.
These transformers are smaller in
size.
In Autotransformer ohmic losses
and core losses are less.
These transformers have high
efficiency.
3) What are the disadvantages of
Autotransformer?
The disadvantages of
autotransformer are as follows:
In case of Autotransformer
effective impedance is smaller.
In the case of open circuit
winding, full voltage is transferred from the primary side to the secondary,
and this can burn the equipment.
4) What are the applications of
Autotransformers?
The applications of
autotransformer are as follows:
These are used for the interconnection
of the power system at different voltage levels like 132KV and 230KV.
In the distribution system,
autotransformers are used to boost the supply voltage.
Autotransformers can be used for
starting induction motors and synchronous motors if it has many tappings.
Autotransformers can be used as
variac (variable a.c).
5) What is the general system
requirement of Alternator?
There is a requirement of two
types of system for the generation of e.m.f.
The Magnetic system is required
for the production of the magnetic field.
Armature system is required for
the houses of conductors on which the EMF is to be induced.
6) What are the advantages of
Stationary Armature and Rotating Field System?
The advantages of the stationary
armature and rotating field system are as follows:
The coils of the stationary
armature can be insulated easily.
The cooling of the winding is more
efficient.
In the motor high peripheral speed
can be achieved.
In this Slip rings and brushes are
not necessary, and we can easily supply the output current to the load circuit.
7) What is meant by Stator? What
is meant by Rotor?
In AC/DC motor or Generator, the
stationary part of the system is called the Stator, and the rotating components
of the system, i.e., either in generator or motor is called the Rotor.
8) What are the Advantages of the
three-phase motor over Single phase motor?
The advantages of three Phase
motor over single phase motor are as follows:
Starting Torques are higher.
Improved speed regulation is
there.
Less vibration is there.
The operation is quite.
9) What is the basic principle of
operation of Alternators/DC generators?
Alternators/DC generators work on
the fundamental principle of Faraday?s law of Electromagnetic Induction. This
law states that when we move any current carrying conductor in a magnetic
field, then an EMF is induced in it.
10) How to minimize the Eddy
Current Losses?
Eddy current losses can be
minimized by laminating the core and insulating the core from each other with
paper or varnish.
11) What is meant by Turbo
Alternators?
High-speed alternators are called
Turbo Alternators. Because of the high speed of rotation, the diameter of the
rotor is reduced, and the axial length is increased. In this two or four poles
machine are used, and steam turbines are used as prime movers.
12) What are the various types of
Rotor used in the Alternators?
The various types of a rotor are:
Salient pole Rotor.
Squirrel Cage Rotor.
Wound Rotor.
Cylindrical Rotor.
13) Define Pole Pitch?
Pole pitch is defined as the
distance between the Centres of two adjacent poles. In which one pole pitch is
equal to 180 electrical degrees. We can also describe it as the number of slots
per pole.
14) Define Coil Span?
Coil Span is defined as the
distance between the two coil sides of a coil. We can also express it in
electrical degrees or many slots.
15) What is meant by full pitched
winding?
Full pitched winding are those
winding in which the coil span is equal to the pole pitch.
16) What is meant by short-pitched
winding?
Short pitched winding are those in
which the coil span is less than the pole pitch. These winding are also called
as short-chorded winding.
17) What are the advantages and
disadvantages of Short-pitched winding?
The advantages of the
short-pitched winding are:
Copper is saved for end
connections.
The Waveform of the generated EMF
is improved.
The generated EMF in the winding
can be made approximate to a sine wave, and the distorting harmonics can also
be reduced in these winding.
The disadvantages of the
short-pitched winding are:
In these winding the total voltage
around the coil reduces. Because the voltage that induces in the two coils is
out of phase.
18) Why the efficiency of a
three-phase induction is the motor is less than that of a three-phase
transformer?
The efficiency of a three-phase
induction motor is less than three-phase transformer because three-phase
induction motor is like a rotating transformer and because of the rotating
nature of induction motor there is friction and winding losses in it. Three
phase transformer is a static device; therefore, there are no rotational
losses.
19) Why is the air gap between the
pole pieces and the armature kept very small?
The air gap between the pole
pieces and the armature is kept very small because of the following reasons:
To maintain the field strength at
a high value.
To generate the high voltage.
If the air gap will be large than
stronger air gap is needed to produce the required voltage; therefore, the air
gap is kept very small.
Because of the reason that flux
lines can pass through iron very quickly than the air gap.
20) Why does curving the pole
faces in a DC machine contributes to a smoother DC output voltage from it?
The commutation of the DC machine
must be as ideal as possible for having smoother output voltage. For good
commutation, the flux density in the air gap must decrease gradually from
maximum value under the center of the pole to zero.
21) The series field winding has
low resistance while the shunt field winding has high resistance. Why?
In the case of the generator, the
series winding takes the full load current so, it requires very few turns to
produce the necessary magnetic field, and therefore the series field winding
has low resistance. But in the case of shunt winding the shunt resistance has
full load voltage across it and requires a large number of turns to limit the
current in it; therefore, the shunt field windings has high resistance.
22) What are the conditions to be
satisfied for parallel operation of DC generators?
The conditions for parallel
operation are as follows:
The generator's polarity must be
the same, or the connections must be interchanged until the polarity becomes
the same.
Both the generators must have
equal voltage.
The change of voltage with the
change of load should be of the same character.
The positive and negative of the
bus bar must be appropriately connected with positive and negative terminals of
the generator.
There must be stable and similar
rotational speed characteristics for the prime movers driving the generator.
23) What are the reasons for the
parallel operation of transformers?
The main reasons for operating
transformers in parallel are as follows:
This is an economical method for
large loads.
Parallel operation of transformer
allows us to add extra load if required in future.
The space capacity of the
substation can be reduced by parallel operation if we connect transformers of
standard size.
The parallel connection maximizes
the electrical power system availability as we can shut down any system for
maintenance without affecting other system performance.
24) What are the significant parts
of DC Generator?
The significant parts of DC
generator are:
Yoke Poles.
Armature.
Field winding.
Armature Winding.
Commutator brushes.
25) What is a Commutator?
The device which converts the
alternating induced emf of a generator to the DC emf is called Commutator. In
the case of the motor, it produces unidirectional torque.
26) What is meant by reactance
voltage?
When a coil undergoes Commutation
in a coil then an EMF is induced, and this induced EMF is called reactance
voltage.
An armature coil has certain
self-inductance, and when the coil undergoes commutation then there is a change
in current (di) in that coil, and the commutation takes certain time (dt) thus
the coil undergoing commutation has self-induced EMF which is given by
L(di/dt).
27) What are Electric Motors?
An electromechanical device that
converts electrical energy into mechanical energy is called an electric motor.
Almost all the motors operate by
the interaction between the magnetic field and current carrying conductors to
generate force.
28) On what principle Electric
motor works?
When a current carrying conductor
is placed in an external magnetic field perpendicular to the conductor, the
conductor experiences a perpendicular force to itself and the external magnetic
field.
The main principle behind the
production of mechanical force is the interaction between the electric current
and magnetic field.
To find the direction of the force
experienced on a conductor can find out by the right-hand rule for the force.
29) How many types of electric
motors are there?
There are mainly two types of
electric motors:
A.C Motors.
D.C Motors.
30) What is an AC motor?
AC motors are electric motors that
are driven by an alternating current. It consists of two parts an outside
stationary stator and inside the rotor. The outer stators have coils that are
supplied by an alternating current to produce a rotating magnetic field and
inside rotor is attached by output shaft, and the torque is provided to it by
the rotating field.
31) What is a DC motor?
The machine that transforms
electrical energy into mechanical energy in the form of rotation is called DC
motor. Its movement is produced by the physical behavior of the
electromagnetism. The magnetic field used to generate movement is produced by
the inductors inside them, or we can say that DC motors are mechanically
commutated electric motors that are driven by Direct Current (DC).
32) What are the applications of
the Electric motors?
Applications of Electric motors
are:
Industrial fans.
Machine tools.
Household Applications.
Record Players.
Disk Drives.
Power tools.
Blowers and pumps etc.
33) What is the Universal motor?
The motor which is designed to
operate on AC and DC power both is known as Universal motor or Series wound
motor. It performs very well on AC because in case of AC the current in both
the field and the armature alternates in synchronism manner and hence, the
resulting mechanical force occurs in the constant direction of rotation.
34) What are the advantages and
disadvantages of the AC motors?
Advantages of AC motor are as
follows:
AC motors have a low cost.
These motors have a long life.
These motors have high efficiency.
AC motors are available in high
rating.
It is available in standardized
types.
Disadvantages of AC motor are as
follows:
Variable frequency source is
required for speed control.
High starting inrush current is
there.
35) What are the advantages and
disadvantages of the DC motors?
Advantages of DC motor are as
follows:
DC motors have long Lifespan.
These motors require low
maintenance.
DC motors have high efficiency.
DC motors have high holding
torque.
Precision positioning is there.
Disadvantages of DC motor are as
follows:
Its initial cost is higher.
They require a controller for its
operation.
DC motors are costlier.
36) What is the slip speed?
Slip speed is the difference
between the synchronous speed and the actual speed of the rotor. The formula
for Slip is:
S ≜
(NS-Nr)/NS Per unit (p.u)
37) What are the advantages of
skewing of cage rotor conductors?
The advantages of skewing of cage
rotor conductors are as follows:
Skewing of cage rotor conductors
helps to reduce the noise during operation.
The locking of the stator and
rotor teeth is reduced because of skewing of cage rotor conductors.
38) How many types of losses occur
in a transformer?
Two types of losses occur in a
transformer:
Iron loss or Core loss Pi
Copper loss or I2R loss Pc
39) What is an iron loss or core
loss Pi?
Iron loss is the sum of hysteresis
loss (Ph) and eddy current loss (Pe). This type of loss mainly occurs in the
magnetic core of the transformer.
Pi = Ph + Pe
40) What is a Copper loss or I2R loss
Pc?
The loss which takes place in the
primary and secondary winding of the transformer because of the winding
resistance is called the Copper loss or I2R loss.
The Total copper loss in the
transformer = Primary winding copper loss + Secondary winding copper loss
41) What is Dielectric Loss?
The losses that occur in
insulating materials, that is, in the transformer oil and the solid insulation
of transformer are known as a dielectric loss.
42) Why do we perform an
Open-circuit test and Short-circuit test on the transformer?
The Open-Circuit test and
Short-Circuit test are performed to determine the circuit constant, efficiency
and regulation of a transformer, without loading the transformer. The power
consumption of these loads is very less.