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Question : 24
Total: 26
SECTION -E
(a) Draw a labelled diagram of AC generator. Derive the expression for the instantaneous value of the emf induced in the coil.
(b) A circular coil of cross-sectional area 200
OR
(a) Draw a labelled diagram of a step-up transformer. Obtain the ratio of secondary to primary voltage in terms of number of turns and currents in the two coils.
(b) A power transmission line feeds input power at
Solution:
(a) Labelled diagram of AC generator Expression for instantaneous value of induced emf.
(b) Calculation of maximum value of current
When the coil is rotated with constant angular speed the angleθ t
θ = ω t ,
Therefore, magnets flux,( ϕ B )
ϕ B = B A cos ω t
Induced emfe = − N
e = N B A ω s i n ω t
e = e 0 s i n ω t
wheree 0 = N B A ω
(b) Maximum value of emf
e 0 = N B A ω
= 20 × 200 × 10 − 4 × 3 × 10 − 4 × 50 V
= 600 mV
Maximum induced current
i 0 =
=
mA
OR
(a) Labelled diagram of a step up transformer
Derivation of ratio of secondary and primary voltage
(b) Calculation of number of turns in the secondary
(a)
When ac voltage is applied to primary coil the resulting current produces an alternating magnetic flux, which also links the secondary coil.
The induced emf, in the secondary coil, having Ns turns, is
e s = − N s
This flux, also induces an emf, called back emf, in the primary coil.
e s = − N s
Bute p = V p
ande s = V s
⇒
=
For an ideal transformer
l p V p = i s V s
⇒
=
(b)
=
=
∴ N s = 300
(b) Calculation of maximum value of current
When the coil is rotated with constant angular speed the angle
Therefore, magnets flux,
Induced emf
where
(b) Maximum value of emf
Maximum induced current
OR
(a) Labelled diagram of a step up transformer
Derivation of ratio of secondary and primary voltage
(b) Calculation of number of turns in the secondary
(a)
When ac voltage is applied to primary coil the resulting current produces an alternating magnetic flux, which also links the secondary coil.
The induced emf, in the secondary coil, having Ns turns, is
This flux, also induces an emf, called back emf, in the primary coil.
But
and
For an ideal transformer
(b)
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