Electrical machines siskind

25. What is meant by pitch factor? - Pitch factor is the ratio of the voltage generated in the fractional pitch coil to the voltage generated in the full pitch coil. 26. What effect does the pitch factor have upon the generated voltage of an alternator? - Fractional pitching reduces harmonics in the air gap field. And in turn reduces parasitic losses in the windings and core and gives a better output voltage waveform. 27. Why are distributed windings generally used in alternators? -When several coils in a group are connected in series, total voltage generated by that group is not equal to the voltage per coil multiplied by the number of coils in the group. It is always less than this because the coils are displaced di splaced from each other, which means that the voltages generated in the several coils ar not in phase with each other. 28. What is meant by distribution factor? - Measure of resultant emf of a distributed winding is compared to a concentrated winding. It is expressed as ratio of the phasor sum of the emfs induced in all the coils distributed in a number of slots under one pole to the arithmetic sum of the emfs induced. 29. What effect does the distribution factor have upon the generated voltage of an alternator? - The distribution factor reduces the output voltage for a given number of winding turns but for more it reduces the harmonics in the output voltage. Current harmonics in an electricity supply are highly undesirable but would inevitably result from the presence of voltage harmonics. 30. Define alternator Regulation? - Alternator Regulation. Before an a-c generator is ready to function to deliver electrical load, It must be brought up to synchronous speed by its prime mover it must be separately excited from a d-c source; it must have its terminal voltage adjusted to the correct value by proper manipulation of the field rheostat. 31. In a given alternator, upon what does the regulation depend? -Loading an alternator will affect its terminal voltage, just as it does a dc generator, but the manner in which it does so will depend upon the character load. 32. What general limiting values can be designed to the regulation of alternators delivering unity power factor loads? Lagging power factor loads? - A lagging factor load, such as induction motors ( to be discussed subsequently ) electrical welders fluorescent lighting, and electromagnetic devices, will cause the terminal voltage of the alternator to drop as much as 25 to 50 percent below the no load value.

33. What three voltage drops occur in the alternator? - The armature winding resistance, Armature reactance and the armature reaction. First of this always causes a voltage drop, exactly as in the dc generator. The second and third factors between the two extremes can have either effect upon the terminal emf. 34. In making calculations for the regulations, How much the three voltage drops is used? - In the study of dc generator operations, it was learned that two factors are responsible for the change in the voltage as load is applied. Resistance drop in the armature ci rcuit and change in flux. The same two factors are responsible for the change in votage of an alternator but, in additions there is a third factor, the resistance voltage drop. 35. Why does the armature winding possesses reactance? Upon what two factors does it depend? Exactly how much the terminal will drop only or rise will depend upon, the magnitude of the load and the actual over all power factor of the combined loads. 36. Under what condition of loading is it possible for the generated emf in the alternator to be greater at full load than at no load? - The minus (negative) regulation calculated for the leading power factor load merely indicates that the full load voltage is more than the no load voltage.