12 March, 2007

Electrical Generator,Generator construction and working

An electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, or any other source of mechanical energy.

In 1831-1832 Michael Faraday discovered that a potential difference is generated between the ends of an electrical conductor that moves perpendicular to a magnetic field. He also built the first electromagnetic generator called the 'Faraday disc', a type of homopolar generator, using a copper disc rotating between the poles of a horseshoe magnet. It produced a small DC voltage, and large amounts of current.

The Dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic principles to convert mechanical rotation into an alternating electric current. A dynamo machine consists of a stationary structure which generates a strong magnetic field, and a set of rotating windings which turn within that field. On small machines the magnetic field may be provided by a permanent magnet; larger machines have the magnetic field created by electromagnets.

The energy conversion in generator is based on the principle of the production of dynamically induced e.m.f. Whenever a conductor cuts magneticic flux , dynamically induced e.m.f is produced in it according to Faraday's Laws of Electromagnetic induction.This e.m.f causes a current to flow if the conductor circuit is closed. Hence, two basic essential parts of an electrical generator are (i) a magnetic field and (ii) a conductor or conductors which can so move as to cut the flux.

Simple loop Generator construction :
To see the construction and working of a simple loop generator Click here
Simple loop generator is having a single-turn rectangular copper coil rotating about its own axis in a magnetic field provided by either permanent magnet or electro magnets.In case of without commutator the two ends of the coil are joined to slip rings which are insulated from each other and from the central shaft.Two collecting brushes ( of carbon or copper) press against the slip rings.Their function is to collect the current induced in the coil. In this case the current waveform we obtain is alternating current ( you can see in fig). In case of with commutator the slip rings are replaced by split rings.In this case the current is unidirectional (observe in fig).

Generator working :
In figure see the case when the coil is rotating in anticlock-wise direction with out commutator. As the coil assumes successive positions in the field, the flux linked with it changes.Hence, an e.m.f is induced in it which is proportional to the rate of change of flux linkages (e=-N dΦ/dt). When the plane of the coil is at right angles to lines of flux then 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 do not cut or shear the flux, rather they slide along them i.e they move parallel to them.Hence,there is no induced e.m.f in the coil.Generaly this no e.m.f is taken as the starting position i.e zero degrees position.The angle of rotation or time wil be measured from this position.

As the coil continues rotating further, the rate of change of flux linkages (and hence induced e.m.f in it ) increases till the coil rotates 90° from its startinig position. Here the coil plane is vertical (see in fig) i.e parallel to the lines of flux.As seen, the flux linked with the coil is minimum but rate of change of flux linkages is maximum. Hence , maximum e.m.f is induced in the coil when in this position.

In the next quarter revolution i.e from 90° to 180°,the flux linked with the coil gradually increases but the rate of change of flux linkages decreases.Hence,induced e.m.f decreases gradually till it becomes zero.

So,we find that in the first half revolution of the coil, no e.m.f is induced in it at 0°, maximum when the coil is at 90° position anno e.m.f when coil is at 180°.The direction of this induced e.m.f can be found by applying Fleming's Right hand rule.

In the next half revolution i.e from 180° to 360°, the variations in the magnitude of e.m.f are similar to those in the first half revolution.Its value is maximum when coil is at 270° and minimum when the coil is at 360° position.But it wil be found that th direction of induced current is reverse of the previous direction of flow.

Therefore,we find that the current which we obtain from such a simple generator reverses its direction after every half revolution.Such a current undergoing periodic reversals is known as alternating current.It should be noted that alterating current not only reverses its direction, it does not even keep its magnitude constant while flowing in any one direction.The two half- cycles may be called positive and negative half-cycles respectively.

Now see when the coil is rotating with commutator.In this case the slip rings are replaced by split rings.The split rings are made out 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 insulating material (you can see in fig). As before, the coil ends are joined to these segments on which rest the carbon or copper brushes.

In case of split rings, the positions of the segments of split rings have also reversed when the current induced in the coil reverses i.e when the curent direction reverses the brushes also comes in contact with reverse segments as that of positive half-cycle.Hence, this current is unidirectional.It should be noted that the position of the brushes is so arranged that the change over of segments from one brush to other takes place when the plane of the rotating coil is at right angles to the plane of the lines of flux.It is so because in that position, the induced e.m.f in the coil is zero.You can observe this in two cases by pausing the waveform.

Another important point is that now the current induced in the coil is alternating as before.It is only due to the rectifying action of the split-rings (also called commutator) that it becomes unidirectional in the external circuit.

To better understand the operation of generator Click here

Fleming's Right hand Rule
Fleming's right hand rule (for generators) shows the direction of induced current flow when a conductor moves in a magnetic field.

The right hand is held with the thumb, first finger and second finger mutually at right angles, as shown in the diagram .
  • The Thumb represents the direction of Motion of the conductor.
  • The First finger represents the direction of the Field.
  • The Second finger represents the direction of the induced or generated Current (in the classical direction, from positive to negative).

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