LOAD TEST OF A D.C. COMPOUND GENARATOR
OBJECT: To study the internal & external characteristic of a d.c. compound generator ( Long shunt cumulatively compounded).
APPARATUS:
SR. NO. NAME RANGE NO. REQD. REMARKS
1 D.C. compound machine
2 Volt meter
3 Ammeter
4 Rheostate
5 Leading Rheostate
6 Techometer
7 D.P.S.
8 D.C. shunt motor as a prime mover
THEORY:
A generator provided with both series & shunt field winding is known as compound generator.
1. In these compound generator the shunt field is stronger than the series field. These ways can be connected with armature in four ways as shown in the following diagram.
2. When series field adds to the shunt field, generator is said to be cumulatively compounded. On the other hand, if series field opposes the shunt field, the generator is said to be differentially compounded.
3. Cumulative compounded generator.
(i) short shunt generator: (ii)Long shunt generator:
4. Differentially compounded generator:
(i) Short shunt generator: (ii) Long shunt generator:
5. In the long shunt cumulatively compound d.c generator, flux due to series field is added in the flux due to shunt field. Since the armature current is passing through the series field winding as the load current (IL) increases, flux due to series field also increases. This additional series field flux can be adjusted to give constant voltage at all loads, under this condition the compound generator is called �Level Compounded�. If the value of series field flux is more then the total flux then, it is called �Over Compounded� & if its value is less then this total flux, it is called �Under Compounded�. In any case, cumulatively compounded generator has field dropping characteristics than that of d.c. Shunt generator. So his generator is more advantageous compared with series & shunt generator. A specially designed compound generator can supply a regulated voltage.
The induced e.m.f (E) for the long shunt cumulatively compound d.c. Generator can be given by:
E = V+ Ia (Ra + Rse)
Where:- V = Terminal voltage in volts.
Ia = Armature current in amp.
RA = Armature resistance in ohm.
Rse = Series field resistance in ohm.
PROCEDURE:
1. Set up the connections as per circuit diagram.
2. Adjust the no load terminal voltage to the rated value at the rated speed with help of field rheostate.
3. Gradually load the generator with keeping the primover (motor) speed constant.
4. Note down readings of load current IL, terminal voltage V & shunt field current Ish.
5. Make necessary calculation.
6. Draw graphs:-
(i) Terminal voltage V v/s IL.
(ii) Induced e.m.f E v/s load current
(iii) Armature voltage drop Ia (Ra+ Rse) v/s load current IL.
OBSERVATION TABLE: Take Ra + Rse = 1.6 ohm
Sr.
No Load current IL in amp Field current Ish in amp Armature current Ia=IL+Ish in amp Terminal voltage (at normal speed) V in volts Armature & series drop Ia(Ra+Rse) in volts Induced e.m.f. in volts E=V+Ia(Ra+Rse)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
CALCULATION:-
CONCLUSION: