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4/10/2015

PHY 152
(ELECTRICITY AND MAGNETISM)
•ELECTRIC MOTORS (AC & DC)
•ELECTRIC GENERATORS (AC & DC)

INTRODUCTION
Regardless of size, all electrical generators,
whether dc or ac, depend upon the principle of
magnetic induction
...
As long as there is relative motion between
a conductor and a magnetic field, a voltage will
be induced in the conductor
...
That part in which the voltage is
induced is called the armature
...

• Describe the differences between the two
basic types of ac generators
...
The ROTor is the part that
ROTates; the STATor is the part that remains
STATionary
...
In alternators, the armature
may be either the rotor or stator
...
The current in
the coil is supplied via two brushes that make
moving contact with a split ring (commutator)
...
The
forces exerted on the current-carrying wires
create a torque on the coil
...
e ILB times the sine of the angle between B
and I, which would be 90° if the field were
uniformly vertical
...


Fig 1: DC Motor

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The two forces shown above are equal and
opposite, but they are displaced vertically, so
they exert a torque
...


DC Generator
A DC generator is also a DC motor (with
the same components of a motor), but the coil
is turned to generate an e
...
f
...
The value, in Ohms, of
that resistor is reduced in steps as the
speed of the motor increases, until
finally that resistor is removed from
the circuit as the motor reaches close
to its final speed
...
m
...
Let θ be
the angle between B and the normal to the
coil, so the magnetic flux φ is NAB
...

Faraday's law gives:

Fig 2: DC Generator

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AC Motor

As in the DC motor, the ends of the coil connect
to a split ring, whose two halves are contacted by

As in the DC motor case, a current is passed

the brushes
...


'rectify' the e
...
f produced: the contacts are

Since the current is alternating, the motor will run

organized so that the current will always flow in

smoothly only at the frequency of the sine wave
...
More common is the

past the dead spot, where the brushes meet the

induction motor, where electric current is induced in

gap in the ring, the connections between the ends

the rotating coils rather than supplied to them

of the coil and external terminals are reversed
...


One of the drawbacks of the induced AC motor is the
high current which must flow through the rotating
contacts
...
The coils which produce the
magnetic field are sometimes referred to as the "stator",
while the coils and the solid core which rotates is called

Fig 3: AC Motor

the "armature"
...


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AC motors are generally divided into two
categories, induction and synchronous
motors
...
Most AC motors require a starter, or
method of limiting the inrush current to a
reasonable level
...
Since the component of the
velocity perpendicular to the magnetic field
changes sinusoidally with the rotation, the
generated voltage is sinusoidal or AC
...

AC generators are generally called an
alternators
...
This is an

generated back e
...
f will almost balance the

example of energy conversion from mechanical

input voltage and very little current will flow in

to electrical energy and then back to
mechanical energy
...
But if the motor is driving a
heavy load, the back e
...
f will be less and more
current will flow in the motor coil and that electric

also acts as a generator and generates a "back

power being used is converted to the mechanical

emf"
...


motor coil will oppose the change that created
it
...
Rotating – amarture alternators: the rotating-armature
alternator is similar in construction to the dc generator
in that the armature rotates in a stationary magnetic
field
...
Rotating field alternators: the rotating-field alternator
has a stationary armature winding and a rotating-field
winding
...


Alternators

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QUESTIONS
• What is the part of an alternator in which the
output voltage is generated?
• What are the two basic types of alternators?
• What is the main advantage of the rotating
field alternator?

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