Ans: – A generator is device that converts mechanical energy into electrical energy.
Ans: – The basic principle of a DC generator is electromagnetic induction, i.e. when a conductor cuts across the magnetic field, an e.m.f. is induced in it.
Ans: – Armature is a rotating part of a DC machine where conversion of energy takes place.
Ans: – The armature serve the following purposes: – (i) It houses the conductors in the slots. (ii) It provides an easy path for magnetic flux. 5. What is the function of field system?
Ans: – Field system produces the main magnetic flux.
Ans: – It serves two purposes; (i) It provides mechanical protection to the inner parts of the machine. (ii) It provides a low reluctance path for magnetic flux.
7. How will you make yoke for small and large DC machines?
Ans: – The yoke is made of cast iron for smaller machines and for large machines, it is made of cast steel or fabricated steel.
8. What are the main functions of pole shoes?
Ans: – Pole shoes serve the following purposes: (i) They support the field coils. (ii) They spread out the magnetic flux over the armature periphery more uniformly. (iii) Since pole shoes have larger cross section the reluctance of magnetic flux is reduced.
9. What are field coils?
Ans: – The pole coils or field coils consist of enameled copper wire and are former wound.
10. What is the function of collecting brushes?
Ans: – The collecting brushes are pressed upon the commutator and form the connection between the armature winding and external circuit.
11. Give the EMF equation of DC generator.
Ans: – EMF Equation;
Eg = generated emf in volts
N = speedin rpm
Φ = flux in Weber
Z = No. of armature conductors
P = No. of poles
A = no. of parallel paths
12. What do you understand by separately excited generators?
Ans: – A DC generator whose field magnet winding is supplied from an independent external DC source is called separately excited generator.
13. What are self excited generators?
Ans: –A DC generator whose field magnet winding is supplied from the output of the generator itself is called a self excited generator.
14. What are the advantages of carbon brushes?
Ans: – Because carbon is conducting material and at same time in powdered form provides lubricating effect on the commutator.
15. Why is the armature made up of silicon steel?
Ans: – Since armature of a rotating part of a Dc machine, reversal of flux takes place in the core, hence hysteresis losses are produced. To minimize these losses silicon steel is used for its construction.
16. Why is the armature winding placed on the rotor of a DC machine?
Ans: – The armature winding of a DC machine is placed on the rotor to facilitate commutation i.e. to convert the alternating voltage produced in the winding into direct voltage at brushes.
17. Why do we use slotted armature in a DC machine?
Ans: – The slotted armature has the following advantages: (i) The conductors can be properly placed so that there is little fear of them being displaced. (ii) The air-gap can be reduced to a value required for mechanical clearance. This reduces the reluctance of the magnetic circuit.
18. Armature core of a DC machine always laminated, why?
Ans: – To reduce eddy current losses.
19. why are the brushes staggered?
Ans: – The brushes are staggered in order to prevent ridge formation on the surface of commutator.
20. State Faraday’s first law of electromagnetic induction.
Ans: – Whenever a conductor cuts by the magnetic flux an emf is induced in the conductor.
21. State Faraday’s second law of electromagnetic induction.
Ans: – The magnitude of induced emf is directly proportional to the rate of change pf flux linkage.
22. What are the factors that may affect the voltage build up of a DC shunt generator?
Ans: – The following factors affect voltage build up of a DC shunt generator; (i) The field connections are incorrect. (ii) The speed of the generator is too low. (iii) There is insufficient residual magnetism.
23. Why is series generator not generally used?
Ans: – The terminal voltage of the DC series generator increases with increase in load. For this reason, a DC series generator has limited applications.
24. What happens if a DC machine is operated at a speed below the rated speed?
Ans: – If a DC machine is operated at a speed below that rated value, the machine may be overheat due to: (i) Increase field current necessary to produce rated voltage. (ii) Decrease in fanning action because of decrease in speed.
25. Why are ‘equaliser’ not used in wave winding?
Ans: – There is no need ‘equaliser’ in a wave wound machine because conductor in each of two parallel paths pass under all N and S poles successively.
26. What do you mean by armature reaction?
Ans: – The effect of magnetic field set up by the armature current on the distribution of flux under main poles of a generator.
27. What is an over-compounded generator?
Ans: – An over-compounded generator is one whose voltage rise with the application of load so that its full load voltage exceeds its no-load voltage.
28. What is reactance voltage?
Ans:- Reactance voltage is a self induced emf in coil under going commutation.
29. In lap wound DC machines equalizer rings are provided, Why?
Ans: – To avoid unequal distribution of current at the brushes thereby assist in achieving sparkles commutation.
30. Name the best suited material for commutation segments?
Ans: – The best suited material is silvered copper containing about 0.05% silver.