The document discusses different types of DC generators and alternators used in aircraft. It describes the key components of DC generators including the armature, field coils, commutator, and brushes. It explains how terminal voltage is produced and factors it depends on. It also summarizes different types of DC generators such as shunt-wound, series-wound, and compound generators as well as how they regulate voltage. Finally, it provides an overview of alternators, describing how they work and how rectifiers are used to convert the AC output to DC.
3. Terminal Voltage
Terminal voltage, as applied to DC
generators, is defined as the voltage that
can be measured at the output of the
generator.
4. 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).
5. 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.
7. Armature
• The purpose of the
armature is to provide
the energy conversion
in a DC machine
• the armature converts
mechanical energy to
electrical energy.
8. 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.
9.
10.
11. 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.
12. A basic DC generator has four
basic parts:
• A magnetic field;
• A single conductor, or loop;
• A commutator; and
• Brushes
13. The magnitude of the voltage
produced is 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
29. 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.
30. A DC generator contains four
ratings.
• Voltage
• Current
• Power
• Speed
31. 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.
35. 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.
39. 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 .
40. 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.
41. 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.
43. 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
44. • 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.
45. 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.
47. 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.