Question No. 01: Which material is
used for the core of a transformer and why?
Answer: Laminations of specially
alloyed silicon steel (silicon content 4–5 per cent) are used due to its high
electrical resistance, high permeability, non-ageing characteristics and
minimum iron loss.
Question No. 02: What is stacking
factor? What is its approximate value?
Answer: Stacking factor is the
ratio of iron content in the laminated varnished core by volume. Its value is
about 90 per cent, that is, 10 per cent volume is occupied by varnish and air.
Question No. 03: What is the emf
equation of a transformer?
Answer: Erms = 4.44
Fm fN V
Where,
Fm
= maximum value of the flux linkage with both the windings,
f = frequency
of the supply,
N = number
of turns.
Question No. 04: Why are LT windings
placed near the core?
Answer: LT windings are placed
near the core to reduce the total dielectric strength of the insulating
materials provided on the winding.
Question No. 05: What do you mean by
power transformer?
Answer: Transformers that are
used on transmission lines for the transmission and distribution of relatively
large quantities of energy are called power transformers.
Question No. 06: The windings of a transformer
are divided into several coils because?
Answer: It is difficult to wind
as one coil
Question No. 07: What will happen if
the d.c. machine is operated below rated speed?
Answer: This will result in
overheating due to two reasons; First, more field current has to be maintained
in order to produce the rated voltage. Second, decrease in fanning action due
to decrease in speed.
Question No. 08: What will happen if
the d.c. shunt motor opened accidentally running on no-load has its shunt field
winding?
Answer:
- The field will be reduced to only to the value of residual flux.
- The speed will be very high.
- The parts of motor may even fly apart.
Question No. 09: A d.c. shunt motor
is found suitable to drive fans because they require?
Answer: Small torque at start up
and large torque at high speeds
Question No. 10: What is the power
factor of a transformer at no load?
Answer: At no load, the power
factor of a transformer is very low and lagging, whereas the power factor on
load is nearly equal to the power factor of the load, which it is carrying.
Question No. 11: What are the
essential parts of a transformer?
Answer: The essential parts of a
transformer are as follows:
- Magnetic circuit consisting of laminated iron core and clamping structure
- Primary winding
- Tank filled with insulating oil
- HT terminals with bushings
- LT terminals with bushings
- Conservator tank
- Breather
- Vent pipe
- Thermometer
Question No. 12: How is magnetic
leakage reduced?
Answer: Magnetic leakage is
reduced to a minimum by sectionalizing and interleaving the primary and
secondary windings.
Question No. 13: What is the
permissible maximum flux density in transformer core?
Answer: 1.6–1.8 Wb/m2
Question No. 14: What are the two
basic types of transformers?
Answer: The two basic types of
transformers are:
- The isolation type in which the two windings are physically isolated and electrically insulated from each other.
- The auto transformer type in which one coil is used for both the windings.
Question No. 15: What are the types
of windings according to the construction?
Answer: According to the
construction, the types of windings are:
- Sandwich type and
- Cylindrical type
Question No. 16: What is an ideal
transformer?
Answer: A transformer having an
overall efficiency of 100 per cent is called an ideal transformer.
Question No. 17: Oil in transformers
is used to?
Answer: To cool the windings
Question No. 18: What do you
understand by external characteristics of a d.c. generator?
Answer: The graph between the
terminal voltage and load current is known as external characteristics of a d.c.
generator, provided speed and field current remain constant.
Question No. 19: If the rated speed
of a d.c. shunt motor is 1440 r.p.m, which method of speed control would you
suggested to obtain a speed of 1500 r.p.m?
Answer: Field control method of
speed control is suggested.
Question No. 20: What is the normal
phase difference between the voltage and the no-load current in a transformer?
Answer: The no-load current in a
transformer normally lags behind the voltage by about 80°– 85°.
Question No. 21: What is the use of
iron core in a transformer?
Answer: The iron core is used in
a transformer to provide continuous easy magnetic path of low reluctance.
Question No. 22: What is called grain-oriented
laminations?
Answer: Grain-oriented laminations
are cold rolled laminations specially annealed to orient the iron crystals,
that is, the grains in a uniform way in the direction of rolling to get very
high permeability and low hysteresis loss.
Question No. 23: Why is the
frequency not changed during transformation of electrical energy in a
transformer?
Answer: As the same flux having
a definite frequency is responsible for the production of emf on both the
primary and secondary windings, there is no question of change in the
frequency.
Question No. 24: What are the types
of transformers?
Answer: Types of transformers
are:
- Step up transformer
- Step-down transformer.
Question No. 25: How does a
transformer contribute towards the widespread popularity of AC system over DC?
Answer: High voltage of AC
system can be obtained using a transformer for transmission of electrical
power. Using a transformer, AC voltage can be increased or decreased without
any power loss.
Question No. 26: What is the most
important precaution in any experiment with d.c. shunt motor?
Answer: Before switching on d.c.
supply, a sufficient resistance should be put in series with the armature of
the d.c. shunt motor.
Question No. 27: Does the direction
of rotation of d.c. shunt motor would get reversed if the armature current and
field current both are reversed?
Answer: No.
Question No. 28: What is the
difference between cylindrical-type and sandwich-type winding?
Answer: In cylindrical-type winding,
the length of the coils is equal to the length of the core limb. The primary
and secondary windings are placed one over the other, placing low-voltage winding
nearer the core, whereas in sandwich-type winding HT and LT windings are placed
lengthwise one above and other like a sandwich.
Question No. 29: What is the name of
the winding to which supply is given?
Answer: The name of the winding is
Primary winding.
Question No. 30: What is the name of
the winding from which the supply is taken for load connections?
Answer: Secondary winding.
Question No. 31: Why are iron cores
in transformers made laminated?
Answer: Iron cores are made
laminated to reduce eddy current loss.
Question No. 32: What determines the
thickness of the lamination or stamping?
Answer: Frequency determines the
thickness of the lamination or stamping.
Question No. 33: Why are the
laminations insulated from each other?
Answer: The laminations are
insulated from each other by insulating varnish or thin paper to break the path
of eddy currents and thus reduce eddy current loss.
Question No. 34: What is the phase
relationship between the primary and secondary voltages of a transformer?
Answer: The primary and
secondary voltages of a transformer are 180° out of phase.
Question No. 35: What is turn ratio
of a transformer?
Answer: The ratio of the number
of turns in the primary to the number of turns in the secondary-windings is
called the turn ratio or the ratio of transformation of the transformer, which
is indicated by a constant.
Question No. 36: What is voltage
ratio of a transformer?
Answer: Voltage ratio is the
ratio of the voltage between the line terminals of one winding to that between
the line terminals of another winding at no load.
Question No. 37: What are the types
of transformers according to the arrangement of iron cores?
Answer: There are three types:
- Core type,
- Shell type and
- Distributed core or Berry type.
Question No. 38: What magnetic
circuit is formed in Berry-type constructions and why?
Answer: Distributed magnetic
circuits are formed in Berry-type construction because of its distributed
cores.
Question No. 39: What is called limb
of a transformer?
Answer: The vertical portion of
the iron core where the windings are placed is called limb of a transformer.
Question No. 40: What do you mean by
step-up and step-down transformers?
Answer: When a transformer
converts low voltage to high voltage, it is called a step-up transformer and
when the transformer converts high voltage to low voltage it is called a step-down
transformer.
Question No. 41: What do you mean by
distribution transformers?
Answer: When transformers are
used for distributing the energy from transmission lines as well as networks for
local consumption and the secondaries are directly connected to the consumer’s load,
they are called distribution transformers.
Question No. 42: What do you mean by
lighting transformer?
Answer: A transformer used to
supply a distribution circuit having no motors connected to it is called
lighting transformer.
Question No. 43: When the required
thickness of lamination in a transformer decreases?
Answer: When the applied voltage
increases
Question No. 44: The size and
construction of bushings in a transformer depend upon the?
Answer: Size of tank and voltage
supplied.
Question No. 45: What is the
resistance of the field winding of a D.C. shunt generator kept low?
Answer: If the field resistance
of a D.C. generator is more than particular value (critical resistance), the
generator will fail to build up the voltage. For this reason, the field
resistance of a D.C. shunt generator is kept low.
Question No. 46: What range of speed
can you get with the field control method of speed control of d.c. shunt motor?
Answer: Speed higher than rated
speed can be obtained by using this method.
Question No. 47: What range of speed
can you get with the armature control method of speed control of d.c. shunt
motor?
Answer: Speed lower than the
rated speed can be obtained by the armature control method.
Question No. 48: What is the most
essential condition for the voltage build up for a d.c. shunt generator?
Answer: There should be a residual magnetism in the
poles of the DC shunt generator.