EEE363 (Exp 1) Study of The Open Circuit Characteristics (OCC) of Separately Excited DC Shunt Generator
EEE363 (Exp 1) Study of The Open Circuit Characteristics (OCC) of Separately Excited DC Shunt Generator
EEE363 (Exp 1) Study of The Open Circuit Characteristics (OCC) of Separately Excited DC Shunt Generator
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Objective:
•To study the properties of the separately excited DC shunt generator under no load.
Theory:
A DC machine can run either as a motor or as a generator. A motor Converts electrical power
into mechanical power while a generator converts mechanical power into electrical power. A
generator must, therefore, be mechanically driven in order that it may produce electricity.
Since the field winding is an electromagnet, current must flow through it to produce a magnetic
field. This current is called the excitation Current, and can be supplied to the field winding in
One of two ways, it can Come from a separate, external DC source, in which case the generator
is called a separately excited generator, or it can come from the generator's Own output, in which
case the generator is called a self-excited generator.
The voltage induced in the coils (and, therefore, the DC voltage at the brushes) depends only
upon two things - the speed of rotation and the strength of the magnetic field. If the speed is
doubled, the voltage doubles. If the field strength is increased by 20%, the voltage also increases
by 20%.
Although separate excitation requires a separate DC power source, it is useful in cases where a
generator must respond quickly and precisely to an external control source, or when the output
voltage must be varied over a wide range.
With no electrical load connected to the generator, no current flows and only a voltage appears at
the output. However, if a resistance load is connected across theoutput, current will flow and the
generator will begin to deliver electric power to load.
Equipments required:
1. DC motor/generator module.
2. Power supply module.
3. DC ammeter module.
4. DC voltmeter module.
5. Synchronous motor/generator module.
6. AC ammeter module
Procedure:
CAUTION!
High voltages are present in this Experiment. Do not make any connections with the power on.
The Power should be turned off after completing each individual measurement.
No Load Characteristics
1. Because of its constant running speed, the synchronous motor will be used to mechanically
drive the DC generator. Using your Power Supply, AC Ammeter and Three-Phase Synchronous
Motor/Generator, connect the circuit shown in Figure 7-1
2. Terminals 1, 2 and 3 on the power supply provide fixed three-phase power for the three stator
windings. (Three-phase power will be covered in later Experiments). Terminals 8 and N on the
power supply provide fixed DC power for the rotor winding. Set the rheostat control knob to its
proper position for normal excitation (Experiment 1, procedure 6).
b. Connect the shunt field of the generator, terminals 5 and 6, to the variable DC output of the
power supply, terminals 7 and N, while connecting the 300 mA meter in series with the positive
lead.
c. Connect the 300 V dc meter across the generator output [armature (terminals 1 and 2)].
d. Couple the synchronous motor and the DC generator with the timing belt.
CAUTION !.
The switch in the excitation circuit of the synchronous motor should be closed (1) only when the
motor is running.
4 a. Turn on the power supply. The synchronous motor should start running.
b. Close the Switch S.
c. Vary the shunt field Current by rotating the voltage control knob on the power supply.
Note the effect on the generator output (armature voltage) E, as indicated by the 300 V dc meter.
d. Measure and record in Table-1 the armature voltage E. for each of the listed field Currents.
DATA TABLE-1:
IF(mA) EA (Volts)
0
25
50
75
100
125
150
175
200
275
300
e. Return the voltage to zero and turn off the power supply.
f. Can you explain why there is an as mature voltage even when the field current is zero?
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Questions:
1. State two ways by which the Output polarity of a shunt DC generator can be
Changed.
2. If a DC generator delivers 180 W to a load, what is the minimum mechanical power (in watt )
4. Define Critical Resistance. From the OCC curve, find the value of critical resistance.
5. Draw OCC curve of the DC shunt generator at new speed at 1000 rpm when old speed is 1500
rpm.
6. What is the difference between self and separately excited DC shunt generator?