Direct Current Generators

Basic Type of Direct Current Machine

The direct current machines are generally alternate current machines but equipped with a device called commutator which would convert alternating current into direct current under certain conditions.In the early stages the conditions for the commutator to work was so complicated so an idea of using homo polar machine came into the scene but it had no much advantages over commutator machines so in the earlier periods of this century D.C. commutator machines were developed which could meet the strigent demands.So nowadays we use only D.C. commutator machines and homopolar amachines are used in rare cases or when needed.In the present time large amount of D.C. machines like D.C. motors and D.C. generators are made so as to meet the industrial applications.And the working similar behind these devices are much similar.

Principle of a Generator

The basic principle behind generator is Faraday’s law itself when ever a conductor cut a magnetic flux am emf is induced in it and if the circuit is closed a current will through it.Therefore the essential parts of a generator include magnetic field and conductors to cut the magnetic flux produced by the field.

Elementary Generator

Let us see how a generator work or voltage builds up in a generator with the help of a single turn coil.Checkout the diagram below which shows the position of the coil at various angle in the field.In this figure this coil has got 4 conductors or 4 coil sides but here we are considering the top and bottom coils with respect to the position ‘A’ in he figure.

Lets see how the voltage builds up ,at first the coil is moving parallel to the field at position ‘A’ and few magnetic flux lines get cut by it so the voltage developed will be nearly zero.When the coils moves to position ‘B’ it cut the maximum amount of flux or get associated with maximum ie at 90 degree rotation it cuts the maximum flux and hence the voltage produced will be the maximum and then coil moves to position ‘c’ ie to 180 degree from position ‘A’ ie position ‘c’ is as same as ‘A’ and so the rotation is frmom maximum flux cutting region to minimum where the flux lines are relatively parallel to the coil so the voltage back to zero.Position ‘D’ is same as that of ‘B’ but the flux cutting in the reverse direction so the voltage builds up to maximum in the reverse direction according to Lenzes law and at the next stageback to the initial position ‘D’ which is same as that of ‘A’.And from the graphs its clear that the the voltage generated is proportional to the sine of angle between the conductor and field.

In the figure above the voltage produced is twice the actual voltage since two conductors are placed there opposite to each other now there may be doubt why an emf is not produced in the two vertical coils the figure?Actually an emf is produced in these coils also but it gets canceled because since the two conductors or coils move in same direction.And in first case the two coils move in opposite direction and voltage build up happens.

A sinusoidal wave form is obtained only if the magnetic field is uniform but it is not easy to obtain a uniform field so it would rather resemble a square wave in the practical case.And the position ‘A’ and position ‘c’ is known as the interpolar or neutral zones of the dynamo.

Written by John on September 11th, 2008 with 4 comments.
Read more articles on Direct Current Machines and Electrical Machines.

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