Jul 11, 2013

Working Principle and Construction of DC Generator

Principle Of Generator


An electrical generator may be a machine that converts mechanical energy into electrical energy. The energy conversion is predicated on the principle of the assembly of dynamically induced e.m.f. As seen from fig 1, whenever a conductor cuts the magnetic flux, the dynamically induced emf is created in it consistent with Faraday's Laws of magnetic force Induction. This e.m.f. causes a current to flow if the conductor circuit is closed.

Hence, 2 basic essential elements of an electrical generator are a field and a conductor or conductors which might therefore move on cut the flux.


In the above fig is shown a single-turn rectangular copper coil ABCD rotating regarding its own axis in a very magnetic field provided by either magnet is or electromagnets. The two ends of the coil are joined to 2 slip-rings ‘a’ and “b” that are insulated from one another and from the central shaft. Two collection brushes (of carbon or copper) press against the slip-rings. Their function is to gather this elicited within the coil and to convey it to the external load resistance R. The rotating coil is also known as ‘armature’ and also the magnets as ‘field magnets’.


Imagine the coil to be rotating in clock-wise direction shown in fig 2. Because the coil assumes sequential positions within the field, the flux joined with it changes. Hence, an e.m.f. is induced in it that is proportional to the speed of amendment of flux linkages (e = NdΦ dt).

Once the plane of the coil is at right angles to lines of flux i.e. once it's in position, 1, then flux joined with the coil is most however rate of amendment of flux linkages is minimum.

It is thus as a result of during this position, the coil sides AB and CD don’t cut or shear the flux, rather they slide on them i.e. they move parallel to them. Hence there’s no induced e.m.f. within the coil. Allow us to take this no-e.m.f. or vertical position of the coil because the beginning position. The angles of rotation or time are measured from this position. As the coil continues rotating any, the speed of amendment of flux linkages will increase, until position three is reached wherever θ = 90º. Here, the coil plane is horizontal i.e. parallel to the lines of flux. As seen, the flux joined with the coil is least however rate of amendment of flux linkages is most. Hence, maximum e.m.f. is induced within the coil once during this position shown in the fig.3.

In the next quarter revolution i.e. from 90º to 180º, the flux joined with the coil step by step will increase however the speed of amendment of flux linkages decreases. Hence, the induced e.m.f. decreases step by step until in position five of the coil, it's reduced to zero price.

So, we discover that within the half revolution of the coil, no e.m.f. is induced in it once in position 1, most once in position three and no e.m.f. once in position five. The directions of this induced e.m.f. are often found by applying Fleming’s Right-hand rule which provides its direction from A to B and C to D. Hence, the direction of current flow is ABMLCD. The present through the load resistance R flows from M to L throughout the primary revolution of the coil.

In the next revolution i.e. from 180º to 360º, the variations within the magnitude of e.m.f. square measure almost like those within the half revolution. Its price is most once coil is in position seven and minimum once in position 1. However it'll be found that the direction of the induced current is from D to C and B to A as shown in Fig. 1. Hence, the trail of current flow is on DCLMBA that is simply the reverse of the previous direction of flow. Therefore, we discover that the current that we tend to get from such a straightforward generator reverses its direction when each revolution. Such a current undergoing periodic reversals is thought as alternating current. It is, obviously, completely different from a right away current that unendingly flows in one and therefore the same direction. It ought to be noted that electrical energy not solely reverses its direction; it doesn't even keep its magnitude constant whereas flowing in anybody direction. The 2 half-cycles are also known as positive and negative half-cycles severally.


For making the flow of current simplex within the external circuit, the slip-rings square measure replaced by split-rings shown in fig 4 above. The split-rings square measure created out of a conducting cylinder that is turn over 2 halves or segments insulated from one another by a skinny sheet of translucent substace or another insulating material which is shown in fig 5.

As before, the coil ends square measure joined to those segments on that rest the carbon or copper brushes. From the fig 6 we have seen that within the half revolution current flows on (ABMNLCD) i.e. the brush No. 1 to bear with phase ‘a’ acts because the positive finish of the availability and ‘b’ because the negative finish. Within the next revolution, the direction of the induced current within the coil has reversed. However at constant time, the positions of segments ‘a’ and ‘b’ have additionally reversed with the result that brush No.1 comes involved with the phase that is positive i.e. phase ‘b’ during this case. Hence, current within the load resistance once more flows from M to L. The wave of the present through the external circuit is as shown in Fig. 7. This current is simplex however not continuous like pure electricity.

It ought to be noted that the position of brushes is thus organized that the amendment over of segments ‘a’ and ‘b’ from one brush to the opposite takes place once the plane of the rotating coil is at right angles to the plane of the lines of flux. It’s thus as a result of in this position, the induced e.m.f. within the coil is zero.

Another vital purpose price memory is that even currently the present induced within the coil sides is alternating as before. It’s solely attributable to the rectifying action of the split-rings (also known as commutator) that it becomes simplex within the external circuit. Hence, it ought to be clearly understood that even within the coil of a D.C. generator, the induced voltage is alternating.


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