1. DC Generators

2. Common terms

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.

6. Commutation
commutation is the mechanical conversion from AC to DC at the brushes of a DC machine

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.

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
The angle at which the conductor cuts the

18. DC from Four Armature Loops

19. Eg = KFN (4-1) Where Eg = generated voltage; K = fixed constant;
F= magnetic flux strength;
N  = speed in RPM

20. 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).

21. 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

22. 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.

23. 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.

24. Figure 4-9. Self-excitation Generator

25. 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.

26. 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.

27. Parameter of DC Generator

28. Terminal Voltage

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.

32. DC Generator Construction

33. TYPES OF DC GENERATORS Shunt-Wound DC Generators
Series-Wound DC Generators
Compound Generators

34. Shunt-Wound DC Generators

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.

36. Series-Wound DC Generators

37. Series Generator A series generator has poor voltage
regulation, and, as a result, series
generators are not use for fluctuating loads.

38. Compound Generators

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.

42. ALTERNATORS

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.

46. Simple Alternator

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.

48. Alternator Circuit