AC GeneratorsAC Generators

generatorsgenerators are devices are devices

which convert mechanical which convert mechanical energy into electrical energyenergy into electrical energy

A Simple AC GeneratorA Simple AC Generator

We noted earlier that Faraday’s law dictates that if a coil We noted earlier that Faraday’s law dictates that if a coil of of NN turns experiences a change in magnetic flux, then turns experiences a change in magnetic flux, then the induced voltage the induced voltage VV is given by is given by

t

ΦNV

d

d

If a coil of area If a coil of area A A rotates with respect to a rotates with respect to a field field BB, and if at a particular time it is at an , and if at a particular time it is at an

angle angle to the field, then the flux ( to the field, then the flux (ФФ) ) linking linking the coil is: the coil is: ФФ== BAcos BAcos..

AA┴ ┴ == AcosAcos is the effective area of the coil is the effective area of the coil perpendicular to the magnetic field Aperpendicular 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:The rate of change of flux is given by:

Thus for the arrangement shown belowThus for the arrangement shown below

tΦ

NVdd

Therefore this arrangement produces a Therefore this arrangement produces a sinusoidal output as shown belowsinusoidal output as shown below

Simple AC GeneratorSimple AC GeneratorAxis of rotation

N S

Voltmeter

B

A

C

D

+_Slip Rings

Carbon Brushes

Magnets

Coil rotated with energy source

External Circuit

Simple AC Generator (student copy Simple AC Generator (student copy for note making)for note making) Axis of

rotation

N S

Voltmeter

B

A

C

D

+_

B

A

C

D

Simple AC Generator (student copy Simple AC Generator (student copy for note making)for note making) Axis of

rotation

N S

Voltmeter

B

A

C

D

+_

B

A

C

D

Main parts of an AC generatorMain parts of an AC generator

Magnets (These can be electromagnets Magnets (These can be electromagnets powered by the generator itself).powered by the generator itself).

Coil of many turns.Coil of many turns. Source of energy to rotate coil.Source of energy to rotate coil. Slip rings which rotate with the coil and Slip rings which rotate with the coil and

transfer current to the external circuit via transfer current to the external circuit via carbon brushes.carbon brushes.

External circuit which contains a load.External circuit which contains a load.

Slip Rings Slip Rings If the loop ABCD is connected to an external If the loop ABCD is connected to an external

circuit as shown in the diagram then the induced circuit as shown in the diagram then the induced EMF will be applied to the external circuit.EMF will be applied to the external circuit.

To make a continuous connection to the external To make a continuous connection to the external circuit, the ends of the wire are fastened to Slip circuit, the ends of the wire are fastened to Slip Rings mounted on the axis of the rotating loop. Rings mounted on the axis of the rotating loop.

Sliding connectors called brushes complete the Sliding connectors called brushes complete the circuit. The slip rings rotate with the loop while circuit. The slip rings rotate with the loop while the non rotating brushes are pressed against the the non rotating brushes are pressed against the slip rings and allows the induced current to flow slip rings and allows the induced current to flow in the external circuit.in the external circuit.

Slip Rings (Why?)Slip Rings (Why?)

Wires connected toWires connected tothe rotating coilthe rotating coilwould get twistedwould get twisted

Therefore we useTherefore we usecircular circular slip ringsslip ringswith slidingwith slidingcontacts calledcontacts calledbrushesbrushes

How do we determine the direction How do we determine the direction of the induced current?of the induced current?

We can determine the direction of the We can determine the direction of the induced current by applying induced current by applying Lenz’s Law, Lenz’s Law, which states that the induced current will which states that the induced current will generate a field such that the force will generate a field such that the force will oppose the motion of the moving coiloppose the motion of the moving coil..

If CD is moved downward by mechanical If CD is moved downward by mechanical energy then the force generated by the energy then the force generated by the induced current will oppose this motion induced current will oppose this motion and hence act vertically upwards.and hence act vertically upwards.

By applying the Right hand rule:By applying the Right hand rule:

Palm in the direction of the force Palm in the direction of the force generated by the induced current.generated by the induced current.

Fingers in the direction of the magnetic Fingers in the direction of the magnetic field.field.

Thumb will point in the direction of the Thumb will point in the direction of the induced currentinduced current..

If AB is moved upwards by mechanical energy If AB is moved upwards by mechanical energy then Lenz’s Law implies that a force is then Lenz’s Law implies that a force is generated to oppose this motion (downwards) generated to oppose this motion (downwards) and by applying the RH Rule current will flow in and by applying the RH Rule current will flow in the direction ABCD.the direction ABCD.

Note that after half a turn the coil is again Note that after half a turn the coil is again horizontal with AB and CD reversed. AB is on horizontal with AB and CD reversed. AB is on the right and CD is on the left. By applying the right and CD is on the left. By applying Lenz’s Law we can see that the current direction Lenz’s Law we can see that the current direction is reversed.is reversed.

Note that the slip rings move with the coil.Note that the slip rings move with the coil.

Note that the current reaches a maximum when Note that the current reaches a maximum when the coil is horizontal and reduces to zero when the coil is horizontal and reduces to zero when the loop is vertical. the loop is vertical.

This is because the number of flux lines being This is because the number of flux lines being cut by the coil is a maximum when the coil is cut by the coil is a maximum when the coil is moving at right angles to the field.moving at right angles to the field.

When the coil is vertical no flux lines are being When the coil is vertical no flux lines are being cut since it is moving parallel to the magnetic cut since it is moving parallel to the magnetic field and no induced current will be produced.field and no induced current will be produced.

B

A

A

A

A

AA

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

90º0º 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 The EMF available to the external circuit will produce a current such that:produce a current such that:

I = I =

R = Resistance of the external circuit in Ohms R = Resistance of the external circuit in Ohms ..

EMF = electromotive force.EMF = electromotive force. I = current in Amps.I = current in Amps.

EMF/R

Simple DC GeneratorSimple DC Generator

A simple generator with two coilsA simple generator with two coils

N

S

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

Comparing DC and AC output Comparing DC and AC output voltagesvoltages

The ripple can be further reduced by the use of The ripple can be further reduced by the use of a cylindrical iron core and by shaping the pole a cylindrical iron core and by shaping the pole piecespieces this produces anthis produces an

approximatelyapproximatelyuniform field in theuniform field in thenarrow air gapnarrow air gap

the arrangementthe arrangementof coils and coreof coils and coreis known as theis known as thearmaturearmature Armature