External Characteristic of Cumulative & Differential Compound Generator.

Name of the experiment: Study of the external characteristic of cumulative & differential compound generator.

Theory:

A cumulatively compounded dc generator is a dc generator with both series and shunt fields, connected so that the magnetomotive forces from the two fields are additive. To understand the terminal characteristic of a cumulatively compounded dc generator, it is necessary to understand the competing effects that occur within the machine. If the load on the generator is increased, then as the load increases, the load current IL increases. Since IA = IF + IL, the armature current IA increases too. At this point two effects occur in the generator:

·         As IA increases, the IA(RA + Rs) voltage drop increases as well. This tends to cause a decrease in the terminal voltage VT = EA - IA (RA + Rs).

·         As IA increases, the series field magnetomotive force 91'SE = NSEIA increases too. This increases the total magnetomotive force 91"0' = NFIF + NSEIA i which increases the flux in the generator.1l1e increased flux in the generator increases EA, which in turn tends to make VT = EA - IA(RA + Rs) rise.

These two effects oppose each other, with one tending to increase VT and the other tending to decrease VT.

A differentially compounded dc generator is a generator with both shunt and series fields, but this time their magnetomotive forces subtract from each other. In the differentially compounded dc generator, the same two effects occur that were present in the cumulatively compounded dc generator. This time, though, the effects both act in the same direction. They are -

·         As IA increases, the IA(RA + Rs) voltage drop increases as well. This increase tends to cause the terminal voltage to decrease VT = EA -IA (RA + Rs).

·          As IA increases, the series field magnetomotive force increases too. This increase in series field magnetomotive force reduces the net magnetomotive force on the generator, which in turn reduces the net flux in the generator. A decrease in flux decreases EA, which in turn decreases VT.

Since both these effects tend to decrease VT the voltage drops drastically as the load is increased on the generator.

Required apparatus:

• ·         Dc supply 
• ·         Voltmeter (0-450)v 
• ·         Ammeter (0-5)A
• ·         Wires
• ·         Resistors
• ·         DC Generator (  RPM-1450;      KW-2 ;       FLD AMPS-.467)
• ·         Induction motor ( HP-3;       FL PPM-1420;       FL AMPS -19.1 / 4.55 )    

Circuit diagram:

Fig.:  Circuit diagram of cumulative and differential compound generator.

Data table:

Cumulative compound generator

V	IL	IF
221	0	.66
221	1	.66
221	1.2	.66
221	1.45	.66
221	1.7	.66
218	2.05	.66
216	2.35	.65
216	2.6	.64

Differential compound generator

V	IL	IF
221	0	.66
212	1	.63
210	1.2	.622
207	1.45	.622
204	1.7	.622
200	2.05	.598
197	2.35	.582
191	2.6	.57

------------Cumulative

------------Differential

Precaution:

·        At first, the residual voltage should be taken carefully.

·        Circuit should be checked carefully before supplying power.

·        The reading should be taken carefully.

Discussion:

After plotting the data on the same graph we can see that the characteristics curve of the differential  compound generator is more stipper than the curve of the cumulative compound generator. With the same amount of change in load current, the amount of terminal voltage drop in differential generator is greater than that of the cumulative generator.