LOAD CHARACTERISTICS OF SELF EXCITED

DC SHUNT GENERATOR

AIM:

To obtain the internal and external characteristics of the given self excited DC generator by conducting the load test.

THEORY:

A DC generator works on the principle of Faraday’s Law of Electromagnetic induction, which says that, “Whenever a conductor is moved in magnetic field, an EMF is generated in it”. “The magnitude of induction EMF is directly proportional to the rate of change of flux”.

The voltage equation for a DC shunt generator is given by VL=Eg-IaRa. Under No Load Condition, since Ia is negligibly small, from the above equation, the terminal voltage(VL),is the noload induced EMF(Eg). As the load on the generator increases , the load current and hence the armature current increases. Due to armature reaction the induced EMF in the armature decreases. Also increased armature current causes increase in IaRa drop. Hence the terminal voltage decreases with increase load. The plot between the terminal voltage (VL) and load current (IL) is known as the external or load characteristics. The plot between the induced EMF (Eg) and the armature current (Ia) is known as the internal or total characteristics. The type of graph of internal and external characteristics is shown in model graph.

NAME PLATE DETAILS:

DC generator DC Motor as Prime mover

Rated current : 16A 18.6A Rated voltage : 220V 220VPower rating : 3.5KW 3.5KW Rated speed : 1500 RPM 1500RPM Excitation* Voltage : 220V 220V* Current : 0.8A 0.95A

APPARATUS REQUIRED:

S.No. Apparatus Range Type Quantity

1 Ammeter(0-2)A dc

(0-20) A dc

MC

MC

1

1

2 Voltmeter (0-300)V dc MC 1

3 Rheostats 1200, 0.8A Wire Wound 2

4 Loading Rheostat 5KW, 230V - 1

5 Tachometer (0-1500)rpm Digital 1

6 Connecting Wires 2.5sq.mm. Copper Few

PRECAUTIONS:

1. The field rheostat of motor should be at minimum position.

2. The field rheostat of generator should be at maximum position.

3. The DPST switch on the load side is kept open at the time of starting the experiment. 4. No load should be connected to generator at the time of starting and stopping.

Fuse Calculation:

125% of the motor rated current.(125 * 18.6) /100 = 23.25 A

PROCEDURE:

1. The circuit connections are made as per the circuit diagram. 2. Keeping the motor field rheostat in minimum position, generator field rheostats inmaximum position and the starter in its off position, the main supply is switched ON to thecircuit.3. The motor is started using the 3-point starter by slowly and carefully moving the starter handle from its OFF to ON position. 4. The motor is brought to its rated speed by adjusting its field rheostat and checked with the help of the tachometer. 5. With the DPST switch open, the potential divider is slowly varying until generator voltage is equal to its rated value (220V). The terminal voltage and the field current are noted in thetabular column.6. Load is varied gradually and at each step the speed of the generator is checked and maintained constant at its rated value by adjusting the field rheostat of the motor. For each load the terminal voltage(VL), field current(IF) and the load current (IL) are noted down in the tabular column.7. This procedure is continued until the generator is loaded to 120%of its rated value.8. Once the experiment is completed the load on the generator is gradually decreased, the various rheostats are brought back to their original position in sequence and the main supply is switched OFF.

DETERMINATION OF ARMATURE RESISTANCE:

PROCEDURE:

1. Connections are made as per the circuit diagram.

2. Supply is given by closing the DPST switch.

3. Readings of Ammeter and Voltmeter are noted.

4. Armature resistance in Ohms is calculated as Ra = (Vx1.5) /I

TABULAR COLUMN:

S.No. Voltage

V (Volts)

Current

I (Amps)

Armature Resistance

Ra (Ohms)

TABULAR COLUMN:

S.No.Field Current

If (Amps)

Load Current

IL (Amps)

Terminal Voltage

(V) Volts

Ia = IL + If

(Amps)Eg =V + Ia Ra

(Volts)

FORMULAE:

Eg = V + Ia Ra (Volts)

Ia = IL + If (Amps)

Eg : Generated emf in Volts

V : Terminal Voltage in Volts

Ia : Armature Current in Amps

IL : Line Current in Amps

If : Field Current in Amps

Ra : Armature Resistance in Ohms

MODEL GRAPH:

RESULT:

Thus the load characteristics of self excited DC shunt generator were obtained.

VIVA QUESTIONS:

V L, E

(Vol

ts)

If, IL (Amps)

V Vs IL

(Ext Char)

E Vs IL

(Int Char)

1. What is the principle of generator?

2. What is meant by residual magnetism?

3. What is critical field resistance?

4. What is meant by saturation?

5. What is the difference between a separately excited dc generator and shunt generator?

6. If a DC shunt generator fails to build up voltage, what may be the probable reasons?

7. Why does the terminal voltage decrease as the load current increases?

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