[Q:-01]
What is a Transformer ?
Ans:
A transformer is a static piece of
apparatus which transforms electrical energy from one circuit to another
circuit without changing the frequency either stepping up or stepping down the
voltage.
[Q:-02]
On what theory the principle of operation of a transformer is based ?
Ans :
The principle of operation of a
transformer is based upon the meaty of mutual induction between two electrical
circuits linked by a common magnetic flux.
[Q:-03]
What do you understand by the rating of a transformer ?
Ans :
The rating of a transformer is the maximum
power which can be drawn from it without the temperature rise in the winding
exceeding the safe limits for the particular class of insulation employed.
[Q:-04]
How is the rated capacity of a transformer expressed and why ?
Ans :
The rated capacity of a transformer is
expressed in kVA not in kW. Generally the rating of a transformer is determined
by its temperature rise. The temperature rise is caused by the losses in the
machine. Copper loss depends upon the value of the load current and iron loss
on voltage. Therefore, total loss of a transformer depends on volt ampere (VA)
and is independent of load power factor. A certain amount of current will
produce the same I2R loss at any value of power factor. This loss
limits the output of the machine. The output in kilowatts is proportional to
the power factor. For a given kW load if the power factor decreases, the load
current increases proportionately causing more losses and temperature rise in
the machine. For the above reason transformers are generally rated in kVA not
in kW.
[Q:-05]
What is the power factor of a transformer ?
Ans :
The power factor of a transformer is very
low and lagging on no load. But the power factor on load is nearly equal to the
power factor of the load which it is carrying.
[Q:-06]
What is the normal phase difference between the voltage and the no load current
in a transformer ?
Ans :
The no load current in a transformer
normally lags behind the voltage by about 75°.
[Q:-07]
What are the essential parts of a transformer ?
Ans :
The essential parts
are as follows :—
(i) Magnetic circuit
consisting of laminated iron core and clamping structures.
(ii) Primary winding.
(iii) Secondary
winding.
(iv) Tank filled with
insulating oil.
(v) H.T. terminals
with bushings.
(vi) L.T. terminals
with bushings.
(vii) Conservator
tank.
(viii) Breather.
(ix) Vent pipe.
(x) Thermometer.
[Q:-08]
What is the name of the winding to which the supply is given ?
Ans :
Primary winding.
[Q:-09]
What is the name of the winding from which the supply is taken for load
connections ?
Ans :
Secondary winding.
[Q:-10]
What material is used for the cores of a transformer and why ?
Ans :
Laminations of specially alloyed silicon
steel (silicon content 4 to 5%) are used due to its high electrical resistance,
high permeability, non-ageing characteristics and minimum iron loss.
[Q:-11]
What types of silicon steel are used as magnetic material and which one has
better magnetic properties?
Ans :
Two types of silicon steel are used for
magnetic frame e.g. (i) Hot rolled silicon steel & (ii) Cold rolled grain
oriented silicon steel. Cold rolled grain oriented silicon steel has better
magnetic properties.
[Q:-12]
What is the use of iron core in a transformer ?
Ans :
The iron core is used in a transformer to
provide continuous easy magnetic path of low reluctance.
[Q:-13]
What are the important properties of core material of a transformer and when it
will be called as properly designed core?
Ans :
The important
properties of transformer core are
(i) high
permeability,
(ii) high resistivity
and
(iii) low coercive
force. Core having minimum iron losses and no load current is mentioned as
properly designed.
[Q:-14]
Why stepped cores are generally used for transformer?
Ans :
As low voltage and high voltage coils are
circular in shape stepped cores are used for better utilization of space and
reducing the mean length of the L.V. and H.V. turns in order to achieve the
ultimate savings in copper material.
[Q:-15]
What are the advantages and disadvantages of using comparatively higher flux
density in design of core?
Ans:
Advantages :
(i) For same output
the core and yoke section are reduced.
(ii) Mean length of
low voltage and high voltage turns is reduced resulting saving in copper
materials.
(iii) The overall
size and weight of the transformer are reduced.
(iv) The overall cost
of the transformer is ultimately reduced.
Disadvantages :-
(i) Magnetizing
current and iron losses are increased.
(ii) Saturation of
magnetic material happens.
(iii) Due to higher
no load losses the efficiency is reduced.
(iv) The temperature
rise becomes more.
[Q:-16]
Why cross section of the yoke is taken greater than the core section?
Ans :
To reduce the flux density in the yoke
thereby reducing iron losses and no load current for yoke section.
[Q:-17]
How is magnetic leakage reduced ?
Ans :
The magnetic leakage is reduced to a
minimum by sectionalizing & interleaving the primary and secondary
windings.
[Q:-18]
Why should the joints in iron core be staggered ?
Ans :
The joints in iron core should be staggered
to avoid clear air gap in the magnetic circuit as the air gap reduces the
magnetic flux due to its nigh reluctance.
[Q:-19]
Why are iron cores made laminated in the trans-former ?
Ans :
Iron cores are made
laminated to reduce the eddy current loss.
[Q:-20]
What determines the thickness of the lamina or stamping ?
Ans :
Frequency.
[Q:-21]
Why are the laminations insulated from each other ?
Ans :
The laminations are insulated from each
other by insulating varnish or thin paper to break the path of eddy currents
thus to reduce the eddy current loss.
[Q:-22]
What is stacking factor ? What is its approximate value ?
Ans :
Stacking factor is the ratio of iron content
in the laminated varnished core by volume.
Its value is about
90% i.e. 10% volume is occupied by the varnish and air.
[Q:-23]
What is called grain oriented laminations ?
Ans :
The grain oriented laminations are such
cold rolled laminations specially annealed to orient the iron crystals i.e the
grains in a uniform direction in the direction of rolling which produce very
high permeability and low hysteresis loss. These laminations are punched and
assembled in such a way that the grains of different lamina lie in the same
direction as the magnetic field and not across it.
[Q:-24]
What is the permissible maximum flux density in transformer core ?
Ans:
(i) With HRSS — 1.4
Weber / m2
(ii) With CRGOSS —
1.8 Weber / m2
[Q:-25]
Why is special care taken during tightening of Iron cores ?
Ans :
To make it free from
vibration and humming sound.
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