- To perform no load test on a dc shunt generator.
- To obtain the magnetization characteristics at rated speed under separate excitation.
- To obtain the Critical Field Resistance.
A dc shunt generator is a machine which converts mechanical energy to electrical energy. This conversion of energy is based on the principle of the production of a dynamically induced emf. In a D.C. shunt generator, the field is on the stator and the armature is on the rotor. The field is excited by a dc supply. With the help of a prime mover, the rotor is rotated thereby cutting the flux produced by the field coils. This rate of change of flux-linkage with the conductor induces an alternating voltage, which is rectified through a commutator converting it into a d.c. output which is given by
where, j=Flux per pole,
Z=Total number of conductors,
P= Number of poles,
A=Number of parallel paths,
N=Speed in rpm.
The magnetic circuit in a dc machine contains ferromagnetic material viz, iron having silicon content sheet. Hence, except at low values of field current, F shows a nonlinear relation with the field current, but it gets saturated as field current rises further. Again, the magnetic material has some retentivity i.e., residual magnetism. So there may be some armature voltage even at no field current. All these can be reflected by a plot at a fixed speed called the no load magnetization characteristic. The voltage due to residual magnetism is used to circulate current through shunt field winding in a direction (so connected across d.c. m/c armature) to aid further magnetization or flux and hence more built up voltage. This is known as self-building up process. However, because of B-H characteristic or magnetic saturation, this self-building up again depends on the total maximum field circuit resistance (known as critical field resistance) at certain specified speed and also on speed (known as critical speed) at certain field circuit resistance.
- The connections are made as shown in the circuit diagram.
- The d.c. shunt motor is started with maximum armature resistance and minimum field circuit resistance through a 3 point starter.
- The speed is set to rated value by adjusting the motor armature rheostat.
- The generated voltage and motor speed at zero field current of the generator field winding are noted.
- The field current of the generator is gradually increased in steps keeping the speed at its rated value. The corresponding values of armature voltage and field current are also noted down.
- The field current of the generator is reduced to zero and the motor generator set is switched off.
Experimental Data Table:
- Plot Generator voltage (Eg) Vs. Field current (If) at N= rated
- Draw the critical field circuit resistance line which is a straight line passing through the origin and touching the maximum portion of the straight (approximate) part of the plot and get its slope in ohm. This is the value of the critical field resistance.