1. D.C.Generator

2. Compound Generator Characteristics
The external characteristics of a cumulatively compounded generator is shown in the figure.
The series excitation aids the shunt excitation.
The degree of compounding depends upon the increase in series excitation with the increase in load current.

3. Compound Generator Characteristics

4. If series winding turns are so adjusted that with the increase in load current ( 𝑰 𝑳 ), the terminal voltage (𝑽 𝑻 ) increases, it is called over-compounded generator.
In such a case, as the load current increases, the series field m.m.f. increases and tends to increase the flux and hence the generated voltage. The increase in generated voltage is greater than the IaRa drop so that instead of decreasing, the terminal voltage increases as shown by curve A.

5. If series winding turns are so adjusted that with the increase in load current, the terminal voltage substantially remains constant, it is called flat-compounded generator.
The series winding of such a machine has lesser number of turns than the one in over-compounded machine and, therefore, does not increase the flux as much for a given load current. Consequently, the full-load voltage is nearly equal to the no-load voltage as indicated by curve B.
If series field winding has lesser number of turns than for a flat compounded machine, the terminal voltage falls with increase in load current as indicated by curve C. Such a machine is called under-compounded generator.

6. NEED FOR Parallel OperatiON OF DC Generators .
Continuity of service
Efficiency
Maintenance and repair
Increasing plant capacity
Non-availability of single large unit

7. Parallel Operation of DC Series Generator
The interesting thing about the parallel operation of DC series generator is that DC series generators are not usually employed for supply of power. Instead DC series motors are arranged in parallel to operate as DC series generators during Electric Braking.  he parallel arrangement of DC series generators are described below:
The two DC series motors are disconnected from the supply mains and are connected in parallel to the resistors. During braking the motors continue to run due to kinetic energy of train and act as self excited dc series generators generating emfs. The kinetic energy is converted in to heat and dissipated in the resistors. This is how DC series generators work while operating in parallel.

8. Equalizing Bar in Parallel Operation of Series Generator
For satisfactory parallel operation of dc series generators, it is necessary to connect the armatures of the two machines through a heavy copper bar, called the equalizing bar.  It is explained below :
Without Equalizing Bar

9. Let two DC machines (I and II) of resistance R (the armature and field resistance), induced emfs E1 and E2 operate in parallel, as shown in the first figure.
When the induced emfs E1 and E2 are equal they will share the equal load. 

10. When the induced emfs E1 and E2 are equal they will share the equal load.  There is a problem when the induced emfs are different in two machines. When one of the induced emf becomes greater than another induced emf a circulating current will flow. If E1 becomes slightly greater than E2, then a current i will circulate in clockwise direction, as shown by dotted lines in the first figure. The magnitude of the circulating current i will be (E1-E2)/R 

11. Now total current supplied by machine I will be (I + i) and of machine II (I - i). So the series field current of machine I increases, also characteristic of a DC series generator is a rising characteristics. Thus the induced emf of machine I (E1) will rise and induced emf of machine II will fall. Thus the difference of the two induced emfs E1 am E2 will increase, which will cause further increase in circulating current. Thus the effect is cumulative and if there were no fuse or automatic switch in the circuit, the current it machine II will be reversed. This would reverse the direction of induced emf E2 and resultant emf in the circuit would be [E - (- E2)] i.e. (E1 + E2) and circulating current (E1+ E2)/2R. 
Thus the two emfs will then act in series around a circuit of very low resistance and conditions are virtually those of a short-circuit on the two machines resulting in damage of the machines. We have to avoid this short circuit problem for the parallel operation of DC series generators.
With Equalizing Bar
Here comes the use of equalizer bars in the parallel operation of DC series generators. The
possibility of reversal of either machine can be prevented by preventing the flow of circulating current produced due to inequalities of induced emfs of the machines through the series field winding. 
This aim can be achieved by connecting a heavy copper bar of negligible resistance across the two machines as shown in the figure. Now the circulating current does not affect the field winding, but it get confined to the armature and the equalizing bars. Now if the armature current increases, the terminal voltage drop occurs and the original condition is restored.