Characteristics of DC Shunt, DC Series and DC Compound Generator

The most important characteristics of DC generators are given below.

Magnetizing or open circuit characteristics ( O.C.C ) ( E_o – I_f )

• It shows the relation between no load emf in armature for different value of field current. 
• The magnetizing characteristic is practically same for all types of generators.

Internal characteristics ( E – I_a )

• It shows the relation between emf actually induced in the armature and armature current I_a . 
• The induced emf E is obtained after considering the effect of armature reaction.

External characteristics ( V – I_L )

• It shows the relation between terminal voltages V for different value of load current I_L. 
• The value of terminal voltage V is obtain by subtracting

         (i) Armature reaction drop from E_O

         (ii) Armature resistance drop I_aR_a from E_O

 Magnetizing characteristics

( 1 ) Self Excited Generator

• The field winding of the DC generator ( whether shunt or series ) is disconnected from the machine and connected to DC source in order to obtain the magnetizing characteristics of the DC generator. 
• The armature is connected to DC voltmeter to measure DC voltage for different value of field current.

• The field current is increased step by step and corresponding value of armature voltage Eo are to be noted for constant speed of the machine. 
• It should be noted that some emf is generated even when field current is zero due to residual magnetism of the poles. 
• The total reluctance is given by air gap reluctance ( Reluctance of the iron path is neglected due to high permeability ) which is constant at low value of the flux densities. 
• Therefore the generated emf is directly proportional to field current.
• The iron path reluctance has some appreciable value at high value of the flux densities therefore E_o and If does not hold linear relationship ( Saturation of pole starts at point Q ). 
• It should be noted that the open circuit characteristic for speed above normal speed would lie above this curve and for a lower speed would lie below it.

( 2 ) Separately Excited Generator

• As we know that the back emf of DC generator is given by

         E_b = FZNP / 60A

         Therefore the back emf ( E_b ) a f

• The generated emf is directly proportional to flux for low value of field current. 
• It is a straight line represented by OP. The poles becomes saturated at higher value of field current therefore emf Eo and If does not hold linear relationship.
•  It means that high value of field current is required to produce a given increase in voltage then on a lower part of the curve (OP).

External and Internal Characteristics

( 1 ) DC Shunt Generator

• The connection diagram to obtain external and internal characteristics of the DCshunt generator is shown in the Figure C. 

• The load is applied gradually after building up voltage and reading of voltmeter V, field current I_sh and load current I_L are to be noted. 
• The relation between V – I_L and E – I_L represents external and internal characteristics respectively. 
• As the DC generator supplies load for lighting purpose, its terminal voltage remains practically constant in spite of load current changes. 
• The terminal voltage V is gradually decreased as the load current increases as shown in the Figure C. 
• The terminal voltage decrease due to 

      ( i ) Armature resistance drop

• The terminal voltage ( V = E – I_aR_a ) decreases as the armature resistance increases due to drop in the armature resistance I_aR_a.

     ( ii ) Armature reaction drop

• The field flux decreases and therefore the induced emf is also decreases due to demagnetizing effect of armature reaction. 
• Therefore the terminal voltage V is equal to 

                E_o – armature reaction drop – armature resistance drop

• The DC shunt generator has drooping characteristics because the terminal voltage decreases as the load current increases.

( 2 ) DC Series Generator

• The field winding is connected in series with the load in the DC series generator therefore it builds up some voltage only after load is applied. 
• The external and internal characteristic of the DC series generator is shown in the Figure D.

• It should be noted that the DC series generator has rising characteristics because its terminal voltage increases as the load current increases. 
• However the terminal voltage starts to decrease at high loads due to excessive demagnetizing effect of armature reaction.

( 3 ) Differential Compound Generator

• The characteristics of the DC compound generator depend upon number of series field turns and number of shunt field turns. 
• The net flux in the differential compound generator decreases as the load increases.

           F = F_sh – F_se

• Therefore the terminal voltage is decreases as the load current increases.

( 4 ) Cumulative compound generators

Under compound generator

• Number of series field turns < Number of shunt field turns
• As the number of series field turns are less that the shunt field turns, the net flux decreases as the load current increases. 
• The terminal voltage decreases as the load current increases due to decrease in flux.

Level Compound Generator

• Number of series field turns = Number of shunt field turns
• The terminal voltage remains constant for any value of load current which is within the limits.

Over compound generator

• Number of series field turns > Number of shunt field turns
• As the series field turns are more than the shunt field turns, the net flux and hence the terminal voltage increases as the load current increases. 

Why the DC Compound generator is superior to that of the shunt generator for lighting loads?

• The shunt generator has drooping characteristics it means that its terminal voltage decreases as the load current increases. 
• This decrease in voltage is objectionable particularly for lighting loads due to fact that slight change in terminal voltage makes appreciable change in candle power of the incandescent lamps. 
• The series field turns in the compound generator is designed so that it will compensate the voltage drop (which is occurs due to armature reaction drop and armature resistance drop) therefore the terminal voltage remains constant for a given load conditions.

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