Chapter 4 DC Generators
Common terms
Terminal Voltage
Terminal voltage, as applied to DC
generators, is defined as the voltage that
can be measured at the output of the
generator.
Counter-Electromotive Force
(CEMF)
In a generator using a rotating armature, the
conductors cut the magnetic lines of force in
the magnetic field. Voltage is induced in the
armature conductors. This induced voltage
acts counter to applied voltage; therefore, it
is called counter-electromotive force (CEMF).
Applied Voltage
Applied voltage is defined as the voltage
that is delivered across the load. This
voltage should be the same as terminal
voltage; however, various circuit faults and
losses may reduce the terminal voltage.
Commutation
commutation is the
mechanical conversion
from AC to DC at the
brushes of a DC machine
Armature
• The purpose of the
armature is to provide
the energy conversion
in a DC machine
• the armature converts
mechanical energy to
electrical energy.
Armature
• The purpose of the field in a DC
machine is to provide a
magnetic field for producing
either a voltage (generator) or a
torque (motor).
• The field in a DC machine is
produced by either a permanent
magnet or an electromagnet.
Summary
• The purpose of the armature is to provide the energy
conversion in a DC machine.
• The purpose of the rotor is to provide the rotating
element in a DC machine.
• In DC machines, the purpose of the stator is to
provide the field.
• The purpose of the field in a DC machine is to
provide a magnetic field for producing either a
voltage or a torque.
A basic DC generator has four
basic parts:
• A magnetic field;
• A single conductor, or loop;
• A commutator; and
• Brushes
The magnitude of the voltage produced i
s dependent on a number of factors:
• The strength of the magnetic field
• The speed at which the conductor cuts the
magnetic field
• The length of the conductor within the
magnetic field
• The angle at which the conductor cuts the
magnetic field
DC from Four Armature Loops
Eg = KN (4-1)
Where
Eg = generated voltage;
K = fixed constant;
= magnetic flux strength;
N = speed in RPM
The magnitude of the EMF induced in a
conductor by electro-magnetic induction
is dependent upon the following factors:
• The rate at which the conductor is cut by the lines
of magnetic flux (in this case speed of armature
rotation).
• The length of the conductor (determined by the
number of turns in the armature winding).
• The flux density (the strength of the magnetic field).
• The aircraft generator, whether AC or DC, is
driven by the aircraft engine and therefore its
speed of rotation is variable, especially in the
case of the piston engine aircraft.
• The EMF induced in the armature windings
of a generator will vary directly with the
speed of rotation of the armature
By this means the EMF induced in the
generator armature, and therefore the
generator output voltage, can be controlled
regardless of generator speed or electrical
load by varying the current supplied to the
core winding of the electro-magnet.
When the generator field current is supplied from an
external source of direct current, as in this case, this
is known as external, or separate excitation.
Figure 4-9. Self-excitation Generator
Residual magnetism
• The soft iron of the electro-magnets retains
a small amount of magnetism, known as
residual magnetism, even when there is no
field current.
• This residual magnetism is sufficient to
induce an EMF in the armature of the
generator when it first starts to rotate, which
initiates a current flow from the generator.
Residual magnetism
• Residual magnetism may be lost, or its
polarization reversed, due to excess heat,
shock or reversal of field current flow.
• The residual magnetism can be restored by
briefly passing a current through the field.
This is known as field flashing, or flashing
the field.
Parameter of DC
Generator
Terminal Voltage
DC generator output voltage is
dependent on three factors
• The number of conductor loops in series in
the armature
• Armature speed, and
• Magnetic field strength.
A DC generator contains four ratings.
• Voltage
• Current
• Power
• Speed
DC Generator Construction
• The Yoke is a cylinder of cast iron, which supports the pole
pieces of the electromagnetic field.
• The Armature is driven by the aircraft engine, and holds the
windings (in which the output voltage of the machine is
induced) and the commutator.
• The Commutator changes the AC voltage induced in the
armature into DC voltage.
• The Quill Drive is a weak point, which is designed to shear and
protect the engine if the generator seizes.
• The Suppressor reduces radio interference, which may result
from sparking between the brushes and commutator.
DC Generator Construction
TYPES OF DC GENERATORS
• Shunt-Wound DC Generators
• Series-Wound DC Generators
• Compound Generators
Shunt-Wound DC Generators
Shunt-Wound DC Generators
The shunt-wound generator, running at a
constant speed under varying load
conditions, has a much more stable voltage
output than does a series-wound generator.
Series-Wound DC Generators
Series Generator
A series generator has poor voltage
regulation, and, as a result, series
generators are not use for fluctuating loads.
Compound Generators
The change in output voltage from no-load
to full-load is less than 5 percent. A
generator with this characteristic is said to
be flat-compounded .
For some applications, the series winding is
wound so that it overcompensates for a change
in the shunt field. The output gradually rises
with increasing load current over the normal
operating range of the machine. This type of
generator is called an over-compounded
generator.
The series winding can also be wound so
that it undercompensates for the change in
shunt field strength. The output voltage
decreases gradually with an increase in load
current. This type of generator is called an
under-compounded generator.
ALTERNATORS
Alternators
• Alternators used in many light single and
twin-engined aircraft
• Alternators are lighter than DC generators
• Alternators do not suffer from the problems of
arcing produced by commutation
• The armature winding is in the stationary
casing of the machine and the generator
field windings and their electro-magnets are
on the rotor.
• Only the relatively small field current need
be passed through brushes and slip rings to
the rotating field windings.
In aircraft alternators, the rotating magnetic field
cuts through the stationary conductors of the
armature winding, inducing EMF. The armature
winding is connected to the output terminals of
the alternator, from which the load current is
supplied to the distribution bus bars through a
rectification system that converts the AC output
to DC.
Simple Alternator
Rectifier
A rectifier is a static semiconductor device
that permits current flow in one direction
only and thereby converts bi-directional AC
into unidirectional DC.
Alternator Circuit
END OF CHAPTER 4
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