Download t6_generators

DO PHYSICS ONLINE
MOTORS AND GENERATORS
GENERATORS
Generators are used to convert mechanical energy (rotation) into electrical
energy. A simple generator consists of a loop of wire that is made to rotate in a
uniform magnetic field. The force causing the rotation is applied externally. As
the loop is turned the magnetic flux through it changes, thus creating A
motionally induced current.
As the loop turns, the point marked D spends half its time to the left of point E
and the other half to the right of E. While D is to the left of E, the current flows
in the direction CDEF (right hand palm rule). When the loop is in the vertical
direction, with D directly above E the angle between the magnetic field and the
normal to the loop is zero, so no current is flowing. During the rest of the
revolution, D is to the right of E the current flows in the direction FEDC. Thus,
the direction on which the current flows around the loop is reversed for half of
each revolution.
magnetic field in +x direction
n
D

N
E
B
S
C
F
slip rings
y
brush contacts
direction of
rotation
clockwise
around z axis
(out of page)
A
B
to external circuit
x
z
Fig. 1. A simple generator AC generator with a split ring commutator .
As always with rotary devices (motors and generators) special arrangements
have to be made to connect the rotating components to a fixed external circuit.
In figure (1), each end of the rotating loop is attached to a metal hoop called a
slip ring. The slip rings turn with the loop, but as they turn they rub against two
electrical brush contacts. The brushes are fixed and carry the current produced
in the rotating loop into the external circuit. This arrangement ensures that the
direction of the current supplied to the external circuit will also be reversed for
each half of a revolution. This is an AC generator. This is different with the kind
of current produced by a battery (DC current – constant in value and does not
reverse direction).
Generator parameters:
number of winding of coil N
rotation speed  [rad.s-1]
rotation frequency f =  / 2 [Hz]
period of rotation T = 1/f = 2/ [s]
angle between magnetic field and normal to the area of the coil
   t [rad]
magnetic flux (constant magnetic field) B  B A cos( )  B A cos( t ) [T.m2]
induced emf
emf   N
d B
 N  B A sin( t )  N  2 f  B A sin(2 f t )
dt
Figure (2) shows a series of diagrams explaining how the emf is induced by a
rotating coil in a uniform magnetic field that points out of the page.
magnetic field B out of page
up
v

down
v 
F on e- = 0
B

v
B
v
out of page
F on eF on e-
area n
 = 270o
area n 
out of page
= 0

FB = q v B sin
A
1
B
4
v
B = BAcos
B+
n
B
2
A-
3
v
F on eF on e-
up
v

F on e- = 0
down
v 
v
B
out of page
A+
B-
area n
 = 90o
B
Fig. 2a Rotating coil in magnetic field produces an emf.
 ac generator –slip ring commutator
 DC generator – split ring commutator
A
area n 
into page
 = 180o
magentic flux B = B A cos = B A cos(2 f t)
1
1
1
B

(0,0)
2
4
 2 4
3
time t
3
emf (A wrt B)  = - dB/dt = -(2  f) B A{-sin(2 f t)} = + 2  f A B sin(2  f t)
Fig. 2b.ABAs the
an ac emfis
induced.
 2  4 2  4
AB
2
 2 coil rotates,
+
+
(0,0) 
3 1
1
_

time t
(0,0) 
_
1

3

1

1
3
time t
4
4
B
1
3
magentic flux B = B A cos = B A cos(2 f t)
1
1
1

(0,0)
2
4
 2 4
time t
Fig. 2c. Brushes are used to make the connections between the generator
and external circuit.
For a3 DC generator a single split ring commutator. is
3
used.
emf (A wrt B)  = - dB/dt = -(2  f) B A{-sin(2 f t)} = + 2  f A B sin(2  f t)
AB
slip ring
brushes
+
+
(0,0) 
_
2  4 B 2  4
A
AB
2
2
1
3 1

1
3
time t
(0,0) 
_
1
3

1

3

1
time t
load
4
4
ac voltage across load
slip rings
rotation of axle
(hand or petrol
motor etc)
coil windings
attached to the slip
rings and turned by
the rotation of the
axle
brushes for connections to external circuit:
brushes remain stationary as commutator rotates
Fig. 2d. Brushes are used to make the connections between the generator
and external circuit. For a ac generator a pair of slip rings are used.
Motors and generators are structurally the same:
 A generator has a mechanical input and voltage output
 A motor has a voltage input and mechanical output
ac generator –slip ring commutator
dc generator – split ring commutator
Advantages and disadvantages of ac and DC generators
 AC easier to transform (increase or decrease voltage) using transformers
than DC.
 AC high voltage transmission – smaller currents – smaller heating losses.
 AC voltages emits electromagnetic radiation which can interfere with
electrical / electronic equipment.
 AC produces eddy currents which lowers power output.
 DC not does induced voltages in nearby electrical devices or wires.
 DC generators less reliable due to sparking (causes electrical interference)
and wear across the split ring commutator.
Counter torque in a generator
Principle of conservation of energy  more mechanical input needed to
produce greater electrical output.
Generator not connected to an external circuit  emf exists at the terminals
 no current flows  little effort required to rotate armature  zero
current  zero counter torque
Generator connected to an external circuit  emf exists at the terminals 
induced current flows in coils of armature  current carrying coil in a magnetic
field  torque experience by coil which opposes the rotation  counter
torque
Greater current drawn from generator  greater counter torque  greater
applied torque to keep generator turning.
Do problems
P6998