LOAD TEST OF A D.C. SHUNT GENERATOR
OBJECT: To study the internal & external characteristics of a D.C. shunt generator.
APPARATUS:
SR. NO. ITEMS RANGE NO. REGD. REMARKS
1 D. C. Shunt generator
2 D.C. Shunt motor with starter
3 Rheostate
4 Ammeter
5 Volt meter
6 Techometer
7 Load resistance
THEORY: The external characteristic is the graph of terminal voltage of a generator plotted against load current. The internal characteristic is the graph induced E.M.F. of a generator plotted against load current.
In a shunt generator field winding is connected in parallel to the armature. Compared to series field winding the shunt field winding is supposed to carry small current so that the generator is not unnecessarily loaded & the losses are not excessive. Due to this a shunt field winding is wound with thin wire & large no of turns.
As the shunt generator is started, a small amount of voltage is induced in the armature due to residual magnetic field. Since the field winding is connected across the armature a small current flows (Produced by residual magnetism) through it due to this small voltage. This field current produces same more flux which in turn increases the induced E.M.F. The increased induced E.M.F again increases the flux & this cumulative process continues till a steady value of e.m.f. Is decided by the field resistance & magnetization curve.
The current & flux depend upon the terminal voltage. As the load current increases, hence the terminal voltage decreases. Due to the reduction in the terminal voltage there is slight decreases in the field current, which in turn reduces the flux & hence the induced e.m.f. Also decreases due to the armature reaction.
From the above, it is clear that both the external & internal characteristics are having drooping nature.
The induced e.m.f in a shunt generator can be calculated as follows:
E = V + Ia R a = V + (IL +ISh )Ra ( Ia = IL + ISh )
PROCEDURE:
1. Set up the connections as per circuit diagram.
2. Start the generator at its normal speed with the help of prime mover. (d.c. Motor)
3. Adjust the terminal voltage to the rated value with the help of field rheostate.
4. Gradually increase load current of generator with the help of load rheostate.
5. At each time note down terminal voltage & field current of generator with help of voltmeter & ammeter respectively.
6. Make calculations & draw graphs.
(i) Armature voltage drop Ia Ra V/S load current IL.
(ii) Terminal voltage V V/S load current IL.
(iii) Induced e.m.f E V/S load current IL.
OBSERVATION TABLE:
Sr No. Load current IL Shunt field current in amps Armature current in amps Ia=IL+Ish Terminal voltage in volts at normal speed V Armature voltage in volts IaRa Induced e.m.f. in volts E=V+IaRa
1.
2.
3.
4.
5.
6.
7.
8.
CALCULATIONS:
CONCLUSION: