chapter2.1

QUESTION 1.

Distinguish between a generator and a motor. GENERATOR- mechanical energy is converted to electrical e nergy MOTOR- it converts electrical energy to mechanical energy

2.

What is an armature? A field? 

Armature- the laminated steel core containing current-carrying copper wires -the part of an electric machine that includes the main currentcarrying winding and in which the electromotive force is induced.



Field- the electrically conducting circuit, usually a number of coils wound on individual poles and connected in series that produces the magnetic field in a motor or generator.

3.

4.

What two important requirements are necessary before generator action is possible? 

Presence of magnetic line of force



Motion of conductors cutting the flux

What two important requirements are necessary be fore motor action is possible? 

Presence of magnetic lines of force



Current through conductors lying in the magnetic field

5. State faradays law. 

The magnitude of generated voltage is directly proportional to the rate at which conductor cuts magnetic lines of force.

6.

How many lines of force must be cut per second if 1 volt is to be generated in a moving conductor? 

7.

8

1 volt is generated for every 100,000,000 (10 ) lines cut per second.

Write the fundamental voltage equation as a function of the total number of armature conductors and the number of armature winding circuits.

Eg=

 

-8 *10

where: Eg= Total generated voltage

= flux per pole Maxwell’s Z=armature slot * armature conductor

P= no. of poles Rpm= speed of armature, revolution per minute Z= total number of armature conductors effectively used to add to resulting voltage a= no. of armature paths connected in parallel

8.

What two factors determine the direction of the generated voltage in a conductor through a magnetic field?  

9. State Lenz law

The direction of the flux,which is,of course determined by magnet polarity The direction of motion of a conductor or coil



The direction of the generated voltage in the coil is such that it tends to produce a current flow opposing a change of flux through the coil.

10. What is an alternating current? What is meant by the frequency of an alternating current? 

Alternating current- a current whose magnitude and direction change periodically



Frequency- it is the cycles per second of an alternating current

11. 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 ar mature shaft, for changing the frequency or direction of the current in the armature windings. -a device for reversing the direction of a current.

12. What kind of current of current is theoretically delivered to a load by a dc generator? 

A dc generator does not deliver a pure direct current, as does a storage battery

13. What is meant by torque? 

Torque- something that produces or tends to produce torsion or rotation; the moment of a force or system of forces tending to cause rotation.

14. What factors determine the force exerted by a conductor on the armature of a d-c motor? 

The strength of the main field



The value of the current through the conductor

15. What two types of field windings are used in dc machines? How do they differ from e ach 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.

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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.

16. When is it permissible to use permanent m agnet 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 use extremely powerful alnico permanent magnets for the field poles. Where permanent magnets are employed, the main field flux is constant in magnitude; no flux control is possible.

17. What is the essential difference between commutation in a dc generator and commutation in dc motor? 

In the dc generator the commutator and brushes function to change the internally generated alternating current to a load applied direct current



In the dc motor the commutator and brushes perform an inverse function by changing the externally applied direct current to alternating current flowing in the armature conductors

18. Why is it desirable to have many coils of wire and commutator segments on the armature of a dc generator? 

When they are a great many armature coils, the external voltages between brushes approaches a smooth unvarying line that approximates the pure dc voltage supplied by a battery



When the number of coils increased the voltage waves becomes smoother

19. In actual generator, what effects has the number of parallel paths in the armature winding upon terminal voltage? 

The number of parallel paths determine the current rating of a generator, whereas the number of series conductor per path is a measure of the terminal voltage of the machine

20. Is the power output of generator affected by the number of parallel paths in the armature winding? Give the reason for your answer. 

No, because it says that, the power rating is independent in the manner of  which the cells or conductors are connected