11
Question
Describe the principal, construction and working of a single-phase AC generator
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Solution
Principle : Single-phase a.c generator is based on the principle of electromagnetic induction, according to which an emf is induced in a coil when it is rotated in a uniform magnetic field.
Essential parts of an AC generator:
(i) Armature : It is a rectangular coil consisting of a large number of loops or turns of insulated copper wire wound over a laminated soft iron core or ring. The soft iron core not only increases the magnetic flux but also serves as a support for the coil.
(ii) Field magnets : The necessary magnetic field is provided by permanent magnets in the case of low power dynamos. For high power dynamos, field is provided by electromagnet. Armature rotates between the magnetic poles such that the axis of rotation is perpendicular to the magnetic field.
(iii) Slip rings : The ends of the armature coil are connected to two hollow metallic rings R1 and R2 called slip rings. These rings are fixed to a shaft, to which the armature is also fixed. When the shaft rotates, the slip rings along with the armature also rotate.
(iv) Brushes : B1 and B2 are two flexible metallic plates or carbon brushes. They provide contact with the slip rings by keeping themselves pressed against the ring. They are used to pass on the current from the armature to the external power line through the slip rings.
Working : Whenever, there is a change in orientation of the coil, the magnetic flux linked with the coil changes, producing an induced emf in the coil. The direction of the induced current is given by. Fleming's right-hand rule.
Suppose the armature ABCD is initially in the vertical position. It is rotated in the anticlockwise direction. The side AB of the coil moves downwards and the side DC moves upwards (figure). Then according to Fleming's right-hand rule the current induced in arm AB flows from B to A and in CD it flows from D to C. Thus the current flows along DCBA in the coil. In the external circuit the current flows from B1 to B2.
On further rotation, the arm AB of the coil moves upwards and DC moves downwards. Now the current in the coil flows along ABCD. In the external circuit the current flows from B2 of B1. As the rotation of the coil continuous, the induced current in the external circuit keeps changing its direction for every half a rotation of the coil. Hence the induced current is alternating in nature (fig). As the armature completes n rotations in one second, alternating current of frequency v cycles per second is produced. The induced emf at any instant is given by e=Eosinωt.
The peak value of the emf, Eo=NBAω
where N is the number of turns of the coil,
A is the area enclosed by the coil,
B is the magnetic field and
ω is the angular velocity of the coil.
Essential parts of an AC generator:
(i) Armature : It is a rectangular coil consisting of a large number of loops or turns of insulated copper wire wound over a laminated soft iron core or ring. The soft iron core not only increases the magnetic flux but also serves as a support for the coil.
(ii) Field magnets : The necessary magnetic field is provided by permanent magnets in the case of low power dynamos. For high power dynamos, field is provided by electromagnet. Armature rotates between the magnetic poles such that the axis of rotation is perpendicular to the magnetic field.
(iii) Slip rings : The ends of the armature coil are connected to two hollow metallic rings R1 and R2 called slip rings. These rings are fixed to a shaft, to which the armature is also fixed. When the shaft rotates, the slip rings along with the armature also rotate.
(iv) Brushes : B1 and B2 are two flexible metallic plates or carbon brushes. They provide contact with the slip rings by keeping themselves pressed against the ring. They are used to pass on the current from the armature to the external power line through the slip rings.
Working : Whenever, there is a change in orientation of the coil, the magnetic flux linked with the coil changes, producing an induced emf in the coil. The direction of the induced current is given by. Fleming's right-hand rule.
Suppose the armature ABCD is initially in the vertical position. It is rotated in the anticlockwise direction. The side AB of the coil moves downwards and the side DC moves upwards (figure). Then according to Fleming's right-hand rule the current induced in arm AB flows from B to A and in CD it flows from D to C. Thus the current flows along DCBA in the coil. In the external circuit the current flows from B1 to B2.
On further rotation, the arm AB of the coil moves upwards and DC moves downwards. Now the current in the coil flows along ABCD. In the external circuit the current flows from B2 of B1. As the rotation of the coil continuous, the induced current in the external circuit keeps changing its direction for every half a rotation of the coil. Hence the induced current is alternating in nature (fig). As the armature completes n rotations in one second, alternating current of frequency v cycles per second is produced. The induced emf at any instant is given by e=Eosinωt.
The peak value of the emf, Eo=NBAω
where N is the number of turns of the coil,
A is the area enclosed by the coil,
B is the magnetic field and
ω is the angular velocity of the coil.
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