Load Characteristics of DC Shunt Generator

LOAD CHARACTERISTICS OF DC SHUNT GENERATOR

AIM:

To obtain internal and external characteristics of DC shunt generator.

APPARATUS REQUIRED:

S.No.	Apparatus	Specification	Quantity
1	Ammeter
2	Voltmeter
3	Rheostats
4	Loading Rheostat
5	Tachometer

MACHINES DETAILS:

THEORY:

In a shunt generator, the field winding is connected in parallel with the armature winding so that terminal voltage of the generator is applied across it.The shunt field winding has many turns of fine wire having high resistance. Therefore, only a part of armature current flows through shunt field winding and the rest flows through the load.

      For a DC Shunt GeneratorE₀=V + I_aR_a

I_a= I_L + I_sh

I_sh= V/R_sh

Where, E₀ is the induced emf

           V is the terminal voltage

           I_a is the armature current

            I_L is the load current

Internal characteristics:  it is the plot of the induced voltage E with armature current. Drop in voltage is due to armature reaction which increases with increase in load current.

External characteristics:    It is the plot of terminal voltage with load current at constant field resistance and speed. As the load on the machine is varied the terminal voltage drops it is due to

1. The drop in voltage across the armature resistance, I_aR_a drop.
2. Armature reaction the air gap flux decreases which will reduced the induced emf.
3. The drop in terminal voltage due to 1 & 2 results in decrease field current which further reduces the induced emf.

PRECAUTIONS:

• The field rheostat of motor should be at minimum position.
• The field rheostat of generator should be at maximum position.
• No load should be connected to generator at the time of starting and stopping.

PROCEDURE:

• Connections are made as per the circuit diagram.
• After checking minimum position of DC shunt motor field rheostat and maximum position of DC shunt generator field rheostat, DPST switch is closed and starting resistance is gradually removed.
• Adjust the motor field rheostat to adjust the speed to rated speed of generator.
• Under no load condition, Ammeter and Voltmeter readings are noted, after bringing the voltage to rated voltage by adjusting the field rheostat of generator.
• Load is varied gradually until rated current and for each load, voltmeter and ammeter readings are noted.
• Then the generator is unloaded and the field rheostat of DC shunt generator is brought to maximum position and the field rheostat of DC shunt motor to minimum position, DPST switch is opened.

FORMULAE:

E_g          =          V + I_a R_a (Volts)                    

I_a          =          I_L + I_f (Amps)

CIRCUIT DIAGRAM:

PROCEDURE TO FIND Ra:

• Connections are made as per the circuit diagram.      
• Supply is given by closing the DPST switch.
• Readings of Ammeter and Voltmeter are noted.
• Armature resistance in Ohms is calculated as R_a = (Vx1.5) /I

TABULAR COLUMN:

S.No.	Voltage V (Volts)	Current I (Amps)	Armature Resistance Ra (Ohms)

S.No.	Field Current If (Amps)	Load Current IL (Amps)	Terminal Voltage (V) Volts	Ia = IL + If (Amps)	Eg=V + Ia Ra (Volts)

SAMPLE  GRAPH:

RESULT: