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Control System Engineering

by Anna Superkings
Type: PracticalInstitute: Anna university Specialization: Electronics and Communication EngineeringViews: 13Uploaded: 19 days agoAdd to Favourite

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Control System Engineering by Anna Superkings

Anna Superkings
Anna Superkings

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Anna Superkings
Anna Superkings
EXPT.NO: 1(a) OPEN CIRCUIT CHARACTERISTICS OF SELF EXCITED DC SHUNT GENERATOR DATE: AIM: To obtain the O.C.C of the given self excited DC shunt generator and hence to determine i) Critical field resistance ii) Critical speed iii) O.C.C at the specified speed NAME PLATE DETAILS: D.C. GENERATOR: D.C. MOTOR : Rated Voltage : Rated Voltage : Rated Current : Rated Current : Rated Speed : Rated Speed : Power Rating : Power rating : APPARATUS REQUIRED: S.no. Apparatus Range Type Quantity THEORY: A D.C. generator requires an excitation circuit to generate an induced voltage. Depending on whether the excitation circuit consumes power from the armature of the machine or from separately required power supply, the generators may be classified as self excited or separately excited generators respectively 2
The induced EMF in a DC generator is given by the equation E g = PZ N/ 60A volts. Since P,Z and A are constants the above equation can be rewritten as E g = K N. If the speed of the generator is also maintained constant then Eg =K1 , but the flux is directly proportional to the field current, hence E g = K2 If. From the above equation it is clear that the induced EMF is directly proportional to the field current when speed is maintained constant. The plot between the induced EMF and the field current is known as open circuit characteristics of the DC generator. The typical shape of this characteristic is shown in figure. The induced EMF when the field current is zero is known as residual voltage. This EMF is due to the presence of a small amount of flux retained in the field poles of the generator called residual flux. Once the O.C.C is obtained the parameters such as critical field resistance, critical speed and the maximum voltage to which the machine can build up can be determined. If required the O.C.C at a different speeds can also be obtained. Critical speed is minimum speed below which the generator shunt fails to excite. PRECAUTIONS: (Not to be included in the Record) 1. Remove the fuse carriers before wiring and start wiring as per the circuit diagram. 2. Keep the motor field rheostat at minimum resistance position and generator field rheostat at maximum resistance position. 3. The SPST switch is kept open at the time of starting the experiment. 4. Fuse calculations. As the test is a no-load test the required fuse ratings are 20% of motor rated current. 5. Replace the fuse carriers with appropriate fuse wires after the circuit connections are checked by the staff-in-charge. 3
PROCEDURE: 1. The circuit connections are made as per the circuit diagram in the shown figure. 2. Keeping the motor field rheostat in its minimum position, generator field rheostat in maximum position; and the starter in its OFF position, the main supply is switched ON to the circuit. 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 rheostat and checked with the help of a tachometer. 5. With the SPST switch open, the residual voltage is noted. 6. Now the SPST switch is closed and the Potential divider is varied in steps and at each step the field current (If) and the corresponding induced EMF (E g) are recorded in the tabular column. This procedure is continued until the generator voltage reaches 120% of its rated value the speed of the machine is maintained constant. 7. After the experiment is completed the various rheostats are brought back to their original position in sequence and then main supply is switched OFF. Table for OCC S. No. Field current If (amps) Generator voltage Eg (volts) Table for Measuring Rf Vf (volts) If (amps) Rf (ohms) 4

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