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.
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.
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.
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.
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.
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 Фo
Iron loss Wi
Magnetizing current Iµ
Working component of the no load current Iw
No load reactance Xo and resistance Ro
Short circuit test
Copper loss at full load as well as other load condition
Equivalent impedance Z01 or Z02
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.
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 wattmeter indicates total iron loss.
However, the no load current is very small (1 to 5% of full load current)
therefore copper loss is negligible.
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.
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.
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.
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).
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 = ( Vo – Vf ) / Vo × 100%
Where Vo
= No load voltage
Vf = 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.
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 = I1R01
/ V1
= I2R02
/ V2
Per unit reactance
It is reactance drop with respect to primary / secondary voltage when
full load current flows through it.
%X = I1X01
/ V1
= I2X02
/ V2
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
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
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.
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 = I2r , where I is the current passing
through the winding and r is the resistance of the winding. )
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.
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.
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.
Describe the condition for zero regulation in the transformer.
Condition for zero regulation in the transformer
tan φ = r / x
Where r = Resistance of the winding and
x =
Reactance of the winding
No comments:
Post a Comment