Transcript 06_AC Generators (martin).

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
dΦ
V  N
dt
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
V N
dΦ
dt
 Therefore this arrangement produces a
sinusoidal output as shown below
Simple AC Generator
Axis of
rotation
Magnets
B
C
N
S
A
Slip Rings
Coil rotated with
energy source
D
_
Carbon
Brushes
+
Voltmeter
External
Circuit
Simple AC Generator (student copy
for note making)
Axis of
rotation
B
C
N
S
A
D
_
+
Voltmeter
Simple AC Generator (student copy
for note making)
Axis of
rotation
B
C
N
S
A
D
_
+
Voltmeter
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
C
N
S
A
D
_
Note: At 0º,180º,360º
Flux lines are being cut
at a maximum rate.
+
Voltmeter
Voltage
90º
0º
270º
180º
B
C
B
C
D
C
D
D
D
C
B
B
A
A
B
C
C
B
360º
A
A
A
A
D
D
 The EMF available to the external circuit will
produce a current such that:
 I = EMF/R
 R = Resistance of the external circuit in Ohms
.
 EMF = electromotive force.
 I = current in Amps.
Simple DC Generator
 A simple generator with two coils
S
N
 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