السبت، 6 أغسطس 2011

Single Turn Alternator

       It consists of a permanent magnet with two poles. A single turn rectangular coil is kept in the vicinity of the permanent magnet. The coil is made up of conducting material like copper or aluminium.
       The coil is made up of the two conductors namely a-b and c-d. Such two conductors are connected at one end to from a coil as shown in the Fig.1
Fig. 1 Single turn coil
       The remaining two ends are to be connected to the rings mounted on the shaft, called slip rings C1 and C2. Slip rings also rotate along with armature of a machine. The two brushes P and Q are resting on the slip rings, just making a contact with slip rings. The brushes P and Q are stationary. The slip ring and brush assembly is required to collect the e.m.f. induced in the rotating coil and make it available to the stationary external resistance. The overall construction is shown in the Fig. 2.
Fig. 2   Single turn alternator
       The coil is rotated in an anticlockwise direction. While rotating the constructions ab and cd cut the lines of flux of the permanent magnet. Due to Faraday's law of electromagnetic induction, e.m.f. gets induced in the conductors. This e.m.f. drives a current through resistance R connected across the brushes P and Q. The magnitude of the induced e.m.f. depends on the position of the coil in the magnetic field. Let us see the relation between magnitude of the induced e.m.f. and various position of the coil. Consider different instants and positions.
Instant 1 : Let the initial position of the coil be as shown in the Fig. 2. The plane of the coil is perpendicular to the direction of the magnetic field. The instantaneous component of velocity of conductors ab and cd, is parallel to the magnetic field as shown and there can not be the cutting of the flux lines by the conductors. Hence no e.m.f. will be generated in the conductors ab and cd and no current will flow through the external resistance R. This position can be represented by considering the front view of the Fig. 2 as shown in the Fig.3(a).
Fig. 3  The different instants of induced e.m.f.
Instant 2 : When the coil is rotated in anticlockwise direction through some angle θ, then the velocity will have two components vsinθ perpendicular to flux lines and vsinθ parallel to the flux lines. Due to vsinθ component, there will be cutting of the flux and proportionally there will be as induced e.m.f. in the conductor ab and cd. This e.m.f. will drive a current through the external resistance R. This is shown in the Fig. 3(b).
Instant 3: As angle 'θ' increases, the component of velocity acting perpendicular to flux lines increases, hence induced e.m.f. also increases. At θ = 90°, the plane of the coil is parallel to the plane of the magnetic field while the component of velocity cutting the lines of flux is at its maximum. So induced e.m.f. in this position, is at its maximum value. This is shown in the Fig. 3(c).
       So as increases from 0° to 90°, e.m.f. induced in the conductors increases gradually from 0 to maximum value. The current through external resistance R also varies according to the induced e.m.f.
Instant 4 ; As the coil continues to rotate further from θ = 90° to 180°, the component of velocity, perpendicular to magnetic field starts decreasing hence gradually decreasing the magnitude of the induced e.m.f. This is shown in the Fig. 3(d).
Instant 5 : In this position, the velocity component is fully parallel to the lines of flux similar to the instant 1. Hence there is no cutting of flux and hence no induced e.m.f. in both the conductors. Hence current through external circuit is also zero.
Instant 6 : As the coil rotates beyond θ = 180°, the conductor ab uptill now cutting flux lines in on particular direction reverses the direction of cutting the flux lines. Similar is the behaviour of conductor cd. So direction induced e.m.f. in conductor ab is opposite to the direction of induced e.m.f. in it for the rotation of  θ = 0° to 180°. Similarly the direction of induced e.m.f. in conductor cd also reverses. This changes in direction of induced e.m.f. occurs because the direction of rotation of conductors ab and cd reverses with respect to the field as θ varies from 180° to 360°. This process continues as coil rotates further. At θ =  270° again the induced e.m.f. achieves its maximum value but the direction of this e.m.f. in both the conductors is opposite to the previous maximum position i.e. θ = 90°. From θ = 270° to 360°, induced e.m.f. decreases without changes in direction and at θ = 360°, coil achieves the starting position with zero induced e.m.f.
Key Point : In general the variations in the magnitude of the induced e.m.f. in a single conductor are alternating in nature as θ varies from 0° to 360°.
       It completes positive half cycle when θ varies from 0° to 180° while it completes negative half cycle when θ varies from 180° to 360°. One such cycle of an alternating induced e.m.f. is shown in the Fig..4.
Fig. 4  Graphical representation of the induced e.m.f.

       It is clear from the above discussion that the induced e.m.f. in a conductor is an alternating in nature. This is tru in case of d.c. generator too. In d.c. generator, such alternating induced e.m.f. is required to be rectified to get unidirectional d.c. e.m.f. This is possibly by replacing slip rings by a device called commutator.
Key Point : A commutator converts internally generated alternating e.m.f. to an unidirectional e.m.f. In an alternator, such a commutator is absent as an alternator is meant for producing an alternating e.m.f.
        The action of commutator is discussed later.

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