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Question : 24
Total: 26
SECTION - E
(i) An a.c. source of voltage
(ii) In a series
OR
(i) Write the function of a transformer. State its principle of working with the help of a diagram. Mention various energy losses in this device.
(ii) The primary coil of an ideal step up transformer has 100 turns and transformation ratio is also 100 . The input voltage and power are respectively
Calculate :
(a) number of turns in secondary
(b) current in primary
(c) voltage across secondary
(d) current in secondary
(e) power in secondary
Solution:
(i) Obtaining expression for impedance & phase angle
Condition of current being in phase with voltage
Naming of circuit condition
(ii) Calculation of
(i)
From Figure
=
L +
R +
C
where|
R | = i m R
|
L +
C | = V C m − V L m
= i m ( X C − X L )
⇒ V m 2 = V R m 2 + ( V C m − V L m ) 2
l m 2 Z 2 = l m 2 R 2 + I m 2 ( X C − X L ) 2
Z = √ R 2 + ( X C − X L ) 2
From Figuretan ϕ =
=
ϕ = tan − 1 (
) 1 ∕ 2
Condition for current and voltage are in phase:
V L = V C or X L = X C
Circuit is called Resonant circuit.
(ii) Power factorP 1 =
=
=
(as X L = R ) 1 ∕ 2
Power factor when capacitorC X C = X L
P 2 =
=
= 1
as Z = R at resonance
∴
=
=
OR
(i) Function of transformer
Working principle and diagram
Various energy losses (two)
(ii) Calculation of part (a), (b), (c), (d) & (e)
(i) Conversion of ac of low voltage into ac of high voltage & vice versa.
Mutual induction : When alternating voltage is applied to primary windings, emf is induced in the secondary windings.
Energy losses:
(a) Leakage of magnetic flux
(b) Eddy currents
(c) Hysteresis loss
(d) Copper loss
(ii)N p = 100
Transformation ratio= 100
(a) Number of turns in secondary coil
N S = 100 × 100 = 10000
(b) Input Power= ×
1100 = 220 × I p
⇒ I p = 5 A
(c)
=
= 100
⇒ V S = 2.2 × 10 4 volts
(d)
=
= 100
⇒ I s =
= 0.05 A
(e) Power in secondary = Power in Primary= 1100 W
Condition of current being in phase with voltage
Naming of circuit condition
(ii) Calculation of
(i)
From Figure
where
From Figure
Condition for current and voltage are in phase:
Circuit is called Resonant circuit.
(ii) Power factor
Power factor when capacitor
∴
OR
(i) Function of transformer
Working principle and diagram
Various energy losses (two)
(ii) Calculation of part (a), (b), (c), (d) & (e)
(i) Conversion of ac of low voltage into ac of high voltage & vice versa.
Mutual induction : When alternating voltage is applied to primary windings, emf is induced in the secondary windings.
Energy losses:
(a) Leakage of magnetic flux
(b) Eddy currents
(c) Hysteresis loss
(d) Copper loss
(ii)
Transformation ratio
(a) Number of turns in secondary coil
(b) Input Power
(c)
(d)
(e) Power in secondary = Power in Primary
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