BICOL STATE COLLEGE OF APPLIED SCIENCES

AND TECHNOLOGY
COLLEGE OF ENGINEERING AND ARCHITECTURE

ELEN114L-ELECTRICAL MACHINES I
THE DC SELF-EXCITED SHUNT GENERATOR
LABORATORY REPORT NO. 2

Leader: Edwin Jethro M. Oliveros

Members:
Mariel G. Camasis
Mike Jayson A. Malate
Mark Jomel B. Mangampo
Justin Jay S. Ruiz
BSEE3A
 LABORATORY REPORT NO.: 2
DC Self-Excited Shunt Generator

OBJECTIVES:
1. Understand how a DC generator operates.
2. Determine the voltage of a given load when a generator operates.

DISCUSSION
Modern DC generators with field coils are self-excited generators which get started with the initial
current in the field coils. When generator is switched off, a small magnetism is developed in rotor iron
which induced electromotive force in the armature due to which the current is produced in the field
windings. Initially, weak magnetic field creates less current in the coil, but to sustain self-excitation, the
additional magnetic flux increases the electromotive force in the rotor, due to which voltage keep on
increasing until the machine takes the full load.
The small amount of magnetism is present in the rotor iron. The residual magnetic field of the main
poles, induced an electromotive force in the stator coils, which produces initial current in the field
windings. Due to flow of small current in the coil, an increase in magnetic field occurs. As a result, voltage
output increases, in turns, increases the field current. This process continues as long as the electromotive
force in the armature is more than the voltage drop in the field winding. But, after a certain level, field
poles get saturated and at that point electric equilibrium is reached, and no further increase in armature emf
and increase in current. The resistance of the field winding has certain fixed value, at which self-excitation
can be achieved. This resistance value may vary according to electric parameters of the generator.

PROCEDURE
1) Because of its constant running speed, the synchronous motor will be used to mechanically drive the dc
generator. Using your EMS Power Supply, AC Metering and Synchronous Motor Modules, connect the
circuit shown in Figure.

2) Terminals 1, 2, and 3 on the power supply provide fixed three-phase power for three stator windings.
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,
3) Using the EMS DC Motor/ Generator, DC Metering and Resistance Modules, connect the circuit shown
in Figure 2.
4) Couple the synchronous motor and the dc generator with the timing belt.
5) Turn the DC Generator field rheostat control knob full clockwise for minimum resistance.
6) Make sure the brushes are in their neutral position.
7) Place the resistance switches for no-load (all switches open)

ELEN114L – ELECTRICAL MACHINES 1 LABORATORY EXPERIMENT NO. 2

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 8) Turn the power supply. The synchronous motor should start running.
9) If the synchronous motor has switch S, close it at this time.
10) Note if voltage EA, builds up. None
11) If not, turn off the power supply and interchange the shunt field leads at terminals 5 and6
12) Measure the open circuit armature voltage.

EA 222 V dc

13) Vary the field rheostat and notice if the armature voltage EA changes.
Explain.
~ The E A decreases when the rheostat knob turns clockwise. While, it increases when it is turns
counter-clockwise.
14) Place the resistance switches so that the total load resistance is 120 ohms. Adjust the field rheostat. Until
the generator is delivering an output voltage of 120 Vdc. The ammeter IA should indicate 1Adc.
15) This is the correct setting of the field rheostat control of the rated power output (120V x 1A = 120W) of
the dc generator.
16) Adjust the load resistance to obtain each of the values listed in the Table 1.
17) Measure and record EA and IA for each of the resistance values listed.
18) Turn off power supply.
19) Calculate and record the power for each resistance shown in the table.

Table 1
R L (ohms) I A ( Amps) E A (Volts) Power (Watts)
∞ 0 300 0
600 0.46 290 133.4
300 0.46 284 130.64
200 0.74 223 165.02
150 1.05 158 165.9
120 1 120 120
100 0.83 83.6 69.388
80 0.61 50 30.5
75 0.41 43.9 18

20) Reverse the rotation of the driving motor by interchanging any two of the stator lead connection
(terminals 1, 2, or 3) to the synchronous motor.
21) Remove the generator load by opening all resistance switches.
22) Turn on the power supply.
23) Does the Voltage build-up? No
Explain

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 ~ The rotational direction and the field are directly proportional. Therefore, when the motor is in
reverse there is no voltage build up.

24) Turn off the power supply.

25) Plot the EA vs IA regulation curve on the graph of the figure below.

Current and Voltage Relationship

1.2

0.8

0.6

0.4

0.2

0
300 290 284 223 158 120 83.6 50 43.9

Ia

QUESTIONS

1) If a self-excited generator has lost all of its residual magnetism, can it build up an output voltage?
~ No, a self-excited generator cannot build up an output voltage when it lost all of its residual
magnetism because of the reduction of the residual flux. Due to reduction of residual flux, there is no
increase in field current.

2) How would you get a generator to work after it had lost all of its residual magnetism?

~ To make a generator work again after it lost all of its residual magnetism is connecting a DC
source separately, which makes it a separately excited generator.

3) Does a generator slowly lose its residual magnetism with time?

~ A generator slowly lose its residual magnetism with time due to inactivity of the machine.

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DOCUMENTATION

CONCLUSION
I therefore conclude that it is important to learn and understand this experiment because this experiment
tackles how to generate electrical energy from mechanical energy. Thus, by learning this experiment we
can understand how electrical energy is formed by using mechanical energy. Moreover, I observed the
relationship between the voltage, current, and resistance in this experiment. As seen in the graph, the peak
of the graph is the 120V and 1A at 120-ohms resistance. But as the resistance decreases the voltage and
current also decreases.

ELEN114L – ELECTRICAL MACHINES 1 LABORATORY EXPERIMENT NO. 2

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