Basic_Electrical_Engineering_4th_edition

hundreds ofohms. Let E be the induced emf in case of generators and Eb the back emf for a de motor,Ia armature current, IL load current, Ish the shunt field current. Following relations canbe obtained forthe generators and motors after referring to the circuit diagram shown in Fig. 6.8. Fig. 6.8 (a) is for series generator

where Visthe terminalvoltage afterloadRr,is connected. Infactwithout connectingloadin case

of de series generator no voltage can be induced as series field circuit is open. Fig. 6.8 (b) is for shunt generator

Replacing the generator by its emf and internal resistance (armature resistance) and motor by its back emf and internal resistance we have the equivalent circuits for the series, shunt generators and motor as shown in Fig. 6.9.

Series, shunt and compound generators are self-excited de generators i.e. they do not re­ quire any external source for excitation. However, as the name suggests separately excited de generatorrequires an external de source to excite its field winding.

Before we studyconditionsforself-excitationwefirstobtainmagnetisationcharacteristics ofthese machines.

Terminal voltage

v

(a)

Fig. 6.13. (a) Connection diagram for series generator (b) characteristic.

It is seenthatthe terminalvoltage ofthegeneratorincreases as the loadcurrentincreases. Sincethe terminalvoltageincreases withincreaseinloadcurrent, a series generatoris suitedfor special applications. One such application is arc welding. As welding electrodes make contact with each other, before welding starts, a high current flows. As the welding electrodes separate, there is a steep rise in terminal voltage because armature reaction decreases due to slight de­ crease incurrent. Thehighterminalvoltage ensures thatarcwillbe maintained in air acrossthe contacts so that weldingwill be accomplished.

Another applicationis as a series booster. It's connectedin series withthe line to compen­ sate forthe series voltage drop as the currentin the line increases.

6.6.2Load Characteristics of Compound Generator

Fig. 6.14 (a) shows the connection diagram and 6.14 (b) the characteristics.

Acumulative compound generator iscapable ofmuchbettervoltage regulation. Since the load current flows throughthe seriesfield, the pole mmfactuallyincreaseswithload and so does the generated voltageE.

Ifthe increase inEis not sufficientto overcomeJ(Ra +Rs) drops inthe armature and series field resistances at fullload, therewill be positive voltage regulation andthe machine is saidtobe undercompounded.

Ifthe increase inE exactly compensates for the armature and series fieldf(Ra +Rse) drops at full load, the voltage regulation will be zero, and the generator is said to be flat compounded.

Ifthe terminalvoltage at fullloadis greaterthanthat at no load (negative regulation) the generatoris overcompounded.