21

Why the secondary amp – turns are called as
demagnetizing ampere  turns in the transformer?


Demagnetizing
ampere turns
 When the current is passing through secondary
winding of the transformer, secondary flux is produced which oppose the main
flux therefore the main flux is reduced.
 As the secondary amp – turns reduces the main flux,
it is called as demagnetizing ampere – turns.

22

Why core losses are constant in the transformer
under all load conditions?


Core losses
 The core losses depend upon the supply voltage. As
the supply voltage remains constant, the core loss remains constant in spite
of any load conditions.

23

What is meaning of the leakage flux?


Leakage
flux
 The flux which does not follow its main path is
known as leakage flux.
OR
 The flux which completes its path through air is
called as leakage flux.

24

Describe the effect of load current on the leakage
flux.


Effect
of load current on leakage flux
 The primary and secondary ampere – turns is small at
no load condition hence leakage fluxes are negligible.
 The leakage fluxes associated with primary and
secondary current increases as the load current is increased.

25

What is approximately percentage leakage reactance
drop of the supply voltage in the transformer?


The percentage leakage reactance drop is
approximately 1 to 2% of the supply voltage in the transformer.

26

Which parameters can find out from open circuit test
and short circuit test?


The following parameters can find out from
Open
circuit test
 No load power factor cos Ф_{o}
 Iron loss W_{i}
 Magnetizing current I_{µ}
 Working component of the no load current I_{w}
 No load reactance X_{o} and resistance R_{o}
Short
circuit test
 Copper loss at full load as well as other load condition
 Equivalent impedance Z_{01} or Z_{02}

27

Which winding is generally kept open in the open
circuit test in the transformer?


The high voltage winding is generally kept open in
the open circuit test in the transformer.

28

Which loss is neglected in the open circuit test of
the transformer?


Open circuit test
 The normal ( or name plate ) voltage is applied to
the primary of the transformer during open circuit test.
 This will set up normal flux in the core of the
transformer hence the watt meter indicates total iron loss.
 However the no load current is very small ( 1 to 5%
of full load current ) therefore copper loss is negligible.

27

How to find out voltage transformation ratio by
performing open circuit test in the transformer?


Open circuit test
 The high resistance voltmeter is connected across
secondary winding of the transformer.
 The voltage transformation ratio of the transformer
is finding out by measuring the voltage across primary and secondary winding.

28

Which winding is generally short circuited in the
short circuit test in the transformer?


The low voltage winding is generally short circuited
test in the transformer and the supply is given to the high voltage winding.

29

What should be applied voltage to the transformer
winding during short circuit test?


The voltage is applied to the primary of the
transformer in the short circuit test such that full load current flows in
the either primary or secondary winding. This voltage is generally 5 to 10%
of the supply voltage.

30

Why the rating of the transformer is given in kVA?


Rating of transformer
 The copper loss in the transformer depends upon
current and iron loss depends upon voltage therefore total loss in the
transformer depends upon the product of VA therefore the rating of the
transformer is given in kVA.
 The manufacturer of the transformer does not know
the power factor of the load therefore the rating of the transformer is given
in the kVA not in KW ( kVA × Power factor ).

31

Why do you mean by ‘voltage regulation’? Describe
its significance.


Voltage regulation
 The voltage regulation of the transformer is defined
as difference between no load voltage to the full load voltage as a
percentage of no load voltage.
% Voltage regulation = ( V_{o} – V_{f }) / V_{o }× 100%
Where
V_{o} = No load voltage
_{ }V_{f} = Full load
voltage
 The voltage regulation indicates percentage of
voltage drop at full load condition.
 Lower value of the voltage regulation indicates
better transformer than that of higher value of the voltage regulation.
 Higher value of voltage regulation means lower value
of short circuit current and vice versa.

32

Explain the terms : Per unit resistance and Per unit
reactance


Per unit
resistance
 It is resistance drop with respect to primary /
secondary voltage when full load current flows through it.
%R
= I_{1}R_{01} / V_{1}
= I_{2}R_{02} / V_{2}
Per unit
reactance
 It is reactance drop with respect to primary /
secondary voltage when full load current flows through it.
%X
= I_{1}X_{01} / V_{1}
= I_{2}X_{02} / V_{2}

33

At what value of power factor, the voltage
regulation of the transformer becomes minimum?


The voltage regulation of the transformer has
 Positive value for lagging power factor
 Negative value for leading power factor and
 Minimum value at unity power factor

34

Which parameters can find out by performing back to
back test in the transformer?


Back to back test
 Efficiency at any load
 Voltage regulation
 Temperature rise

35

Why efficiency of the transformer is higher than
that of rotating machines?


There are no rotating losses in the transformer
therefore the efficiency of the transformer is higher than that of the
induction motor.

36

Which parameters greatly affect the value of copper
loss? Voltage or current?


Copper loss
 The current in the winding greatly affect the value
of the copper loss ( Copper loss in the winding = I^{2}r , where I is
the current passing through the winding and r is the resistance of the
winding. )

37

What is condition for maximum efficiency in the
transformer?


The iron loss is equal to copper loss is the
condition for maximum efficiency in the transformer.

38

Why the distribution transformer is designed for low
value of core loss?


Distribution transformer
 The distribution transformer primary is supplied throughout
the day ( 24 hours ) therefore the iron losses occur throughout the day
whereas the copper losses depend upon the duration and amount of load.
 Therefore the design of the distribution transformer
is made such that iron losses become minimum.

39

Define: All day efficiency


All day efficiency
 It is defined as the output in the kWh to the input
in kWh. The all day efficiency depends upon the load cycle.

40

Describe the condition for zero regulation in the
transformer.


Condition for zero regulation in the transformer
Where r = Resistance of the winding and
x = Reactance of the winding

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