Friday, May 14, 2010

D.C. Generators #1

Defn:
An electrical generator is a machine that converts mechanical energy into electrical energy.
This conversion is based on the production of dynamically induced emf whenever a conductor cuts magnetic flux –Faraday’s Law of Electromagnetic Induction.
Essential Parts of an Electrical Generator
----Magnetic Field
----Conductor/Conductors which move so as to cut the flux


Construction
This diagram shows a single turn rectangular coil ABCD rotating about its own axis in a magnetic field provided by either permanent magnets or electromagnets. Collecting brushes are made of copper or carbon and press against the slip rings. This function is to collect current (induced from the coil to the load resistor R.


Principle of Operation
Let’s assume the coil is rotating in clockwise direction. As the coil assumes position in the field, the flux linked with it changes. Hence, the induced emf is proportional to the rate of change of flux linkages.ie

When the plane of the coil is at right angles to the lines of flux i.e. when it is in position 1, then the flux linked with the coil is maximum but rate of change of flux linkages is minimum. It is so because in this position, the coil sides AB and CD do not cut or shear the flux, rather they slide along them i.e. they move parrarell to them. This is the starting position. Angle of rotation or time will be measured from this position.
As the coil rotates further, the rate of change of flux linkage (and hence emf in it) increases till position 3 where it is 90.here the coil plane is horizontal i.e. Parrarell to the lines of flux.
As seen, the flux linked with the coil is minimum but the rate of flux linkage is maximum. Hence maximum emf is induced in the coil when in this position.
In the next quarter revolution i.e. from 90-180, flux linked gradually increases but the rate of change of flux linkage decreases. Hence the induced emf decreases gradually till position 5 of the coil.it is reduced to zero value.

Flemming’s Right-Hand Rule gives the direction of induced emf from A to B and C to D. Hence the direction of current flow is ABMLCD. The current through the load resistance R flows from M to L during the first revolution of the coil.
In the next half revolution i.e. from 180 to 360, the variations in the magnitude of emf are similar to those ij the first half revolution. Its value is maximum when the coil is in position 7 and minimum when in position 1.But the direction of the induced current is from D to C and B to A. Hence, the path of current flow is along DCLMBA.

Therefore we find that the current obtained from a simple generator reversed its direction after every half revolution aka Alternating Current. It should be noted that A.C not only reverses its direction, it does not even keep its magnitude constant while flowing in any one direction.
For making the flow of current in the external circuit. The slip ring are replaced by split-rings. The split-rings are made of a conducting cylinder which is cut into two halves or segments insulated from each other by a thin sheet of mica or some other insulated material.

It is seen that in the first half revolution current flows along ABMNLCD i.e. Brush 1 is in contact with segment ‘a’ acts as the +ve end of the supply and ‘b’ acts as the –ve end. in the next half revolution, the direction of the induced current in the coil has reversed. But at the same time, the positions of segment ‘a’ and ‘b’ have also reversed such that No1 touches the segment which is positive i.e. ‘b’ hence the current is unidirectional but not continues like pure direct current.

Another important point worth remembering is that even now the current induced in the coil sides is alternating as before. it is only due to the rectifying action of the split-rings (also called commutator) that it become unidirectional in the external circuit. Hence it should be clearly understood that even in the armature of a DC generator, the induced voltage is AC.

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