QUESTION
1. Distinguish between a generator and
a motor.
GENERATOR- mechanical energy is
converted to electrical energy
MOTOR- it converts electrical energy to 7. How many lines of force must be cut
mechanical energy per second if 1 volt is to be generated
in a moving conductor?
2. What is an armature? A field?
1 volt is generated for every 100,000,000
Armature- the laminated steel core (108) lines cut per second.
containing current-carrying copper wires
8. In actual generator, what effects has
- the part of an electric machine that inclu the number of parallel paths in the
des the main current-carrying winding a armature winding upon terminal
nd in which the voltage?
electromotive force is induced.
Field-the electrically conducting circuit, The number of parallel paths determines
usually a number of coils wound on the current rating of a generator, where
individual poles and connected in series that as the number of series conductor per
produces the magnetic field in a motor or path is a measure of the terminal
generator. voltage of the machine.
3. What two important requirements are 9. What effect has the number of
necessary before generator action is armature paths upon the current-
possible? carrying ability of a generator?
Presence of magnetic line of force
Motion of conductors cutting the flux 10. Is the power output of generator
affected by the number of parallel
4. What two important requirements are paths in the armature winding? Give
necessary before motor action is the reason for your answer.
possible?
No, because it says that, the power
Presence of magnetic lines of force rating is independent in the manner of
Current through conductors lying in the which the cells or conductors are
magnetic field connected.
5. State faradays law. 11. Derive the fundamental voltage
equation for the dc-generator.
The magnitude of generated voltage is
directly proportional to the rate at which
conductor cuts magnetic lines of force.
6. In what two ways, is it possible to
generate a higher voltage in a moving
conductor?
15. Using the fictitious rubber band
comparison, explain how the
direction of the generated voltage
may be determined.
12. Write the fundamental voltage
equation as a function of the total
number of armature conductors and
the number of armature winding
circuits.
PrpmZ -8
Eg= a60 *10 16. State Lenz law
The direction of the generated voltage in the
Where: coil is such that it tends to produce a current
Eg= Total generated voltage flow opposing a change of flux through the
= flux per pole Maxwells coil.
Z = armature slot * armature conductor
P = no. of poles
Rpm = speed of armature, revolution per 17. Using Lenzs law, explain how the
minute direction of the generated voltage
Z = total number of armature conductors may be determined.
effectively used to add to resulting voltage
a = no. of armature paths connected in
parallel
13. What two factors determine the
direction of the generated voltage in
a conductor through a magnetic
field? 18. What kind of current flows in the
armature conductors of a dc-
The direction of the flux,whichis,of generator?
course determined by magnet polarity
The direction of motion of a conductor or 19. What is an alternating current? What
coil is meant by the frequency of an
alternating current?
14. Considering a dc-generator, in what
two ways can the polarity (plus and Alternating current- a current whose
minus) of the brushes can be magnitude and direction change
changed? periodically
Frequency- it is the cycles per second of
an alternating current
20. What is a commutator? Describe its
construction.
Commutator-a cylindrical ring or disk
assembly of conducting members,
individually insulated in a supporting
structure with an exposed surface for
contact with current-collecting brushes
and mounted on the armature shaft, for moment of a force or system of forces
changing the frequency or direction of tending to cause rotation.
the current in the armature windings.
-a device for reversing the direction of a 26. What factors determine the force
current. exerted by a conductor on the
armature of a d-c motor?
21. What factors determine the frequency
of a generated alternating voltage? The strength of the main field
The value of the current through the
conductor
27. Will a force be exerted by a
conductor carrying a current when it
is placed parallel to a magnetic field?
Explain carefully.
22. Carefully describe commutation
process, using appropriate sketches
to illustrate your answer.
28. What happens to the existing uniform
field if a conductor carrying a current
is placed in this field?
23. Why is it desirable to have many
coils of wire and commutator
segments on the armature of a dc
generator?
29. Using a sketch showing a two-pole
When they are a great many armature
motor with a single coil placed with
coils, the external voltages between
its two sides under the pole centers,
brushes approaches a smooth
describe how torque is developed by
unvarying line that approximates the
the coil.
pure dc voltage supplied by a battery
When the number of coils increased the
voltage waves becomes smoother
24. What kind of current of current is
theoretically delivered to a load by a
dc generator?
30. Why is no torque developed by those
conductors occupying positions in
A dc generator does not deliver a pure the inter-polar spaces of a motor?
direct current, as does a storage battery
25. What is meant by torque?
Torque- something that produces or
tends to produce torsion or rotation; the
31. What is the essential difference use extremely powerful alnico
between commutation in a dc permanent magnets for the field poles.
generator and commutation in dc Where permanent magnets are
motor? employed, the main field flux is constant
in magnitude; no flux control is possible.
In the dc generator the commutator and
brushes function to change the internally 35. Make sketches showing two ways in
generated alternating current to a load which the two field windings, shunt
applied direct current and series, may be connected for
In the dc motor the commutator and compound generator or motor
brushes perform an inverse function by operation.
changing the externally applied direct
current to alternating current flowing in
the armature conductors
32. Describe the commutation process in
a dc-motor.
33. What two types of field windings are
used in dc machines? How do they
differ from each other with regards to
the number of turns of wire? The size
of wire? The manner in which they
are connected?
Shunt- a main field winding with many
turns of fine wire, the winding is
connected directly across the brush
terminals where full voltage exists.
Series a main field winding with few
turns of heavy wire, the winding is
connected in series in one of the line
wires where the full armature current
flows.
34. When is it desirable and permissible
to use permanent magnet in d-c
machines? What kind of material is
generally used for permanent
magnets?
In some small generators and motors it
has sometimes been found desirable to
