AC generator, slip rings are used, such that each end of the coil remains connected to one ring at any given point of time. Moreover these rings remain connected to only one brush at any instant. By doing so, the change in direction of current is also received in the connected circuit.
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An AC generator works on the principle of electromagnetic induction and is used to convert mechanical energy to electrical energy. Here we shall discuss a simple working model of an AC generator.
As shown in the figure we have a rectangular loop in between two poles of a magnet. The ends of the wire are connected to two circular metal rings, which is connected to the external circuit by two graphite brushes. The axle of the rectangular coil is connected to a small water turbine. Water from tape is used to give a controlled rotation to the turbine.
When the faucet of the tap is opened, the flow of water rotates the turbine. As a result the magnetic flux through the rectangular coil changes. This change in magnetic flux induces a current in the rectangular loop.
Derivation:
A is the area of the rectangular loop. B is the strength of the uniform magnetic field.
Initially, the plane of the coil is perpendicular to the magnetic field. The magnetic flux through the coil in this position is,
Ø = B.A = BA cos0 = BA
After time t the coil rotates through an angle θ = ωt. The flux now is,
Ø = BA cos ωt
Using Faraday’s Law, the emf induced in the loop is given by,
The emf varies sinusoidally with time with an angular frequency ω, hence with a time period T = 2π/ω.
For N turns of the coil, the induced emf is,
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Hi,
We have a rectangular coil of area A placed in between magnetic poles. When the coil rotates there is a change in magnetic field lines passing through the area of the coil. We know a changing magnetic field produces an EMF. So, a current is induced within the rectangular coil. As the number of field lines through the coil is changing with the rotation of the coil, the induced EMF is also changing. The rotational motion of the coil is periodic. That is, the coil returns to its original position after every complete rotation. So, the EMF induced is also periodic. Hence, we have an alternating current within the coil. This is the working principle of an AC generator. For mathematical understanding, the relevant knowledge will be provided to you in higher standard
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After half a rotation, arm AB starts moving down and the arm CD upward. Therefore, directions of induced currents in these arms change. Thus net induced current now becomes in the direction DCBA. In the external circuit now current flows from B1 to B2. Thus after every half rotation current changes its direction and an alternating current is obtained from the generator.
Function of Brushes: Brushes are kept pressed on the two slip tings separately. Outer ends of the brushes are connected to the galvanometer (or the external load). Thus brushes help in transferring current from the coil ABCD to the external circuit.
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The principle of working of an electric generator is that when a loop is moved in a magnetic field, an electric current is induced in the coil. It generates electricity by rotating a coil in a magnetic field. The following figure shows a simple AC generator.
MNST ? Rectangular coil
A and B ? Brushes
C and D ? Two slip rings
X ? Axle, G ? Galvanometer
If axle Xis rotated clockwise, then the length MN moves upwards while length ST moves downwards. Since the lengths MN and ST are moving in a magnetic field, a current will be induced in both of them due to electromagnetic induction. Length MN is moving upwards and the magnetic field acts from left to right. Hence, according to Fleming?s right hand rule, the direction of induced current will be from M to N. Similarly, the direction of induced current in the length ST will be from S to T.
The direction of current in the coil is MNST. Hence, the galvanometer shows a deflection in a particular direction. After half a rotation, length MN starts moving down whereas length ST starts moving upward. The direction of the induced current in the coil gets reversed as TSNM. As the direction of current gets reversed after each half rotation, the produced current is called an alternating current (AC).
To get a unidirectional current, instead of two slip rings, two split rings are used, as shown in the following figure.
In this arrangement, brush A always remains in contact with the length of the coil that is moving up whereas brush B always remains in contact with the length that is moving down. The split rings C and D act as a commutator.
The direction of current induced in the coil will be MNST for the first rotation and TSNM in the second half of the rotation. Hence, a unidirectional current is produced from the generator called DC generator. The current is called AC current.
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It works on electromagnetic induction. the magnetic flux through a coil changes induced current is produced.
Construction -
Armature - alternating currents generator consists of a rectangular coil ABCD . Coil consists of large number of turns of insulated copper wire around over a soft iron core the coil is kept between the poles of a strong magnet.
Diagram -
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