Ee-416 (dc Generator Characteristics)

1. Name the three general types of dc generator?  Answer : 1. Shunt 2. Series 3. Compound 2. How does the construction field is differ in the three types of generator?  Answer: If the excitation is produced by a field winding that is connected to full line voltage, the machine is known as a shunt generator. If the excitation originates in a field winding connected in series with the armature so that the flux depends upon the current delivered to the load the machine called a series generator. If the excitation is produced by two field windings, one connected to the full, line voltage and the other excited by the line or armature current, a comparatively large current, is called a compound generator.

3. In a compound generator, why7 is the series-field coil wound over the shunt-field coil?  Answer: The series-field coil is wound over the shunt-field coil because the series field, carrying high value of current, is kept coil more readily when placed on the outside. Another point to recognize is that the shunt field contains many turns of comparatively fine wire , while the series field is wound with few turns of heavy wire. 4. Why is it not desirable to operate a generator at a speed lower than the one for which it is signed?  Answer: It should be desirable to operate at some higher or lower speed the field rheostat must be adjusted for the new speed. 5. Why is it important to keep the armature resistance of generator as low as possible?  Answer: It should be clearly understood that the generated voltage, which depends upon the flux is always greater than the terminal of load voltage be exactly the amount of the voltage drop in the armature circuit. 6. What is meant by voltage regulation?  Answer: Is the percent change in output voltage from no load to full load. 7. How is the terminal voltage of a generator controlled?  Answer: If the voltage full with increased in load, the field-rheostat resistance must decreased, this raises the flux and with it the generated emf. If the load changes gradually, or if the expense of automatic regulating equipment is not warranted, the field rheostat may be adjusted manually with each change in terminal voltage.

8. What are meant by the following terms when applied to compound generators or compound; over compound? Under compound? Flat-compound? Differential-compound? Cumulative-compound?  Answer: If the full-load voltage is higher than the no-load value, it is called an over compound generator. If the fullload voltage is less than zero-load voltage, it is referred to as an under compound machines. If the terminal voltages at no load and full load are equal, the machine is said to be flat-compound. If the action of the series field must oppose, that is “buck”, that shunt field, the machine is referred to a differential-compound. The shunt and series-field coils around each of the main poles should be connected that they create flux in the same direction if the tendency of the generator to lose voltage is to be counteracted, is said to be cumulative-compound. 9. How can degree of compounding be adjusted?  Answer: A very low resistance shunt connected directly across the series field, point a and b, the no-load voltage may be brought up to almost any desired value to meet individual demands. 10. Name several practical applications of the series generator?  Answer: If the latter is higher than the required value a diverter is connected across the series field and the full-load test is repeated. If the full-load voltage is still too high, the diverter resistance is reduced. If the full loadvoltage is less than desired, the diverter resistance is increased. 11. What is meant by armature reaction?  Answer: It means when load is added to the armature then current is passed through armature conductors then armature creates flux. The armature reaction is that the generated voltage is actually reduced. 12. What is meant by the frequency of an alternating current? 13. Describe the commutation process in a dc motor? 14. Why is it desirable to have many coils of wire and commutator segments on the armature of dc generator? 15. What is meant by torque? 16. What factors determine the force exerted by a conductor on the armature of a dc motor? 17. Will a force be exerted by a conductor carrying a current when it is placed parallel to a magnetic field? Explain. 18. What happens to the existing uniform field if a conductor carrying a current is placed in this field? 19. Why is no torque developed by those conductors occupying positions in the interpolar spaces of a motor? 20. What two types of field winding are used in dc machines and how do they differ from each other?