AC Generators generators are devices which convert mechanical energy into electrical energy

A Simple AC Generator  We noted earlier that Faraday’s law dictates that if a coil of N turns experiences a change in magnetic flux, then the induced voltage V is given by

If a coil of area A rotates with respect to a field B, and if at a particular time it is at an angle  to the field, then the flux (Ф) linking the coil is: Ф= BAcos . A ┴ = Acos  is the effective area of the coil perpendicular to the magnetic field A ┴. The angle θ is the angle between the field and the normal to the plane of the coil.

The rate of change of flux is given by:  Thus for the arrangement shown below

 Therefore this arrangement produces a sinusoidal output as shown below

Simple AC Generator Axis of rotation NS Voltmeter B A C D + _ Slip Rings Carbon Brushes Magnets Coil rotated with energy source External Circuit

Simple AC Generator (student copy for note making) Axis of rotation NS Voltmeter B A C D + _ B A C D

Simple AC Generator (student copy for note making) Axis of rotation NS Voltmeter B A C D + _ B A C D

Main parts of an AC generator  Magnets (These can be electromagnets powered by the generator itself).  Coil of many turns.  Source of energy to rotate coil.  Slip rings which rotate with the coil and transfer current to the external circuit via carbon brushes.  External circuit which contains a load.

Slip Rings  If the loop ABCD is connected to an external circuit as shown in the diagram then the induced EMF will be applied to the external circuit.  To make a continuous connection to the external circuit, the ends of the wire are fastened to Slip Rings mounted on the axis of the rotating loop.  Sliding connectors called brushes complete the circuit. The slip rings rotate with the loop while the non rotating brushes are pressed against the slip rings and allows the induced current to flow in the external circuit.

Slip Rings (Why?)  Wires connected to the rotating coil would get twisted  Therefore we use circular slip rings with sliding contacts called brushes

How do we determine the direction of the induced current?  We can determine the direction of the induced current by applying Lenz’s Law, which states that the induced current will generate a field such that the force will oppose the motion of the moving coil.  If CD is moved downward by mechanical energy then the force generated by the induced current will oppose this motion and hence act vertically upwards.

By applying the Right hand rule:  Palm in the direction of the force generated by the induced current.  Fingers in the direction of the magnetic field.  Thumb will point in the direction of the induced current.

 If AB is moved upwards by mechanical energy then Lenz’s Law implies that a force is generated to oppose this motion (downwards) and by applying the RH Rule current will flow in the direction ABCD.  Note that after half a turn the coil is again horizontal with AB and CD reversed. AB is on the right and CD is on the left. By applying Lenz’s Law we can see that the current direction is reversed.  Note that the slip rings move with the coil.

 Note that the current reaches a maximum when the coil is horizontal and reduces to zero when the loop is vertical.  This is because the number of flux lines being cut by the coil is a maximum when the coil is moving at right angles to the field.  When the coil is vertical no flux lines are being cut since it is moving parallel to the magnetic field and no induced current will be produced.

B A A A A A A B B B B B C C C C C C D D D D D D N S Voltmeter B A C D + _ Voltage 90º 0º0º 180º 270º 360º Note: At 0º,180º,360º Flux lines are being cut at a maximum rate.

 The EMF available to the external circuit will produce a current such that:  I =  R = Resistance of the external circuit in Ohms .  EMF = electromotive force.  I = current in Amps.

Simple DC Generator  A simple generator with two coils N S

 A DC Generator has the slip rings replaced by a split ring commutator.

Comparing DC and AC output voltages

 The ripple can be further reduced by the use of a cylindrical iron core and by shaping the pole pieces  this produces an approximately uniform field in the narrow air gap  the arrangement of coils and core is known as the armature Armature