External characteristic of DC generator.

Experiment No.: – 05

Aim of the Experiment: –

External characteristic of DC generator.

Apparatus Required: –

Sl. No.	Name	Specification	Quantity
1	DC Motor Generator Set	Motor: 110V, 18A, 1440rpm, Generator: 110V, 18A, 1440rpm	1 no.
2	Voltmeter	(0-150) V, PMMC	1 no.
3	Ammeter	(0-20) A, PMMC	1 no.
4	Rheostat	(0-80) Ω, 5A	1 no.
5	Connecting wire	PVC Insulated copper	As per required
6	Tachometer	Digital type	1 no.

Circuit Diagram: –

Theory: –

In a DC generator, the output voltage is determined by the interaction between the magnetic field produced by the field winding and the armature current drawn by the load. The external characteristic graphically represents how the generated voltage responds to changes in the load current.

The external characteristic of a DC generator typically exhibits the following characteristics:

No-load or Open-circuit Region: At no load or when the load current is zero, the generator’s output voltage is at its maximum value. This is the point where the generator is not supplying any current to an external load, and the generated voltage is solely determined by the magnetic field strength and speed of the generator.

Drooping Region: As the load current increases from zero, the generator’s output voltage starts to decrease. This decrease in voltage is due to the armature reaction and the voltage drop across the armature resistance and reactance. The characteristic curve shows a gradual decline or droop as the load current increases.

Knee Point: The knee point is the point on the external characteristic curve where the rate of voltage decrease starts to accelerate significantly. It indicates that the generator is reaching its maximum capacity and cannot maintain the desired voltage level for higher load currents. At this point, the generator is operating at its rated load.

Saturation Region: Beyond the knee point, the external characteristic curve becomes almost horizontal or slightly curved. In this region, further increase in the load current has a minimal impact on the generated voltage. The generator has reached its maximum output capacity, and the voltage regulation becomes poor.

The external characteristic curve provides important information for understanding the behaviour of the DC generator under varying load conditions. It helps determine the voltage regulation capability of the generator and assess its suitability for specific applications.

Procedure: –

1. Connect the DC generator to the power supply using suitable connecting wires as per circuit diagram.
2. Ensure that the power supply is switched off and Set the load resistors to their maximum resistance value.
3. Switch on the power supply and observe the reading on the voltmeter and note down the no-load voltage, which is the voltage observed when no load current is drawn from the generator.
4. Gradually decrease the resistance of the load resistors to increase the load current and at each load current setting, note down the corresponding voltage reading on the voltmeter and the load current reading on the ammeter.
5. Take measurements at regular intervals, increasing the load current step by step and ensure the generator is operating under stable conditions and wait for the readings to stabilize before recording.

Precautions: –

1. Ensure all connections are properly secured and insulated.
2. Do not touch the generator or any moving parts while it is in operation. Bottom of Form

Observation Table: –

Sl. No.	Load current (IL) in Amp.	Induced Voltage (V) in Volts.	Speed (N) in rpm
1
2
3
4
5

Plot the graph between Load current and induced voltage.

Conclusion: –

                              To be written by student.