Experiment no. 3: DC Compound Generator – Self-Excited

VALENZUELA, Maria Michaela P., Group IV, EE153L/B6

Objectives:

The main objective of the experiment is to investigate the relationship between the output voltage and the output current of a DC generator with a shunt-connected field and a cumulatively connected series field, and driven at constant speed..

Theoretical Background:

Compound generators can be either cumulatively compounded or differentially compounded. Normally, they are connected as cumulative compound so that the magnetomotive force of the series field windings aids that of the shunt field windings. The latter way of compounding is rarely used in practice. A compound generator that is operated cumulatively has its series field assisting the shunt field while when it is operated differentially, the series field has to oppose the shunt field. The shunt field windings usually contain many turns of relatively small wire while the series field windings, wound on the outside, consist of a few turns of comparatively heavy conductor because it must carry the full load armature current of the machine. The voltage of a cumulative compound generator may also be controlled over reasonable limits through a rheostat in the shunt field circuit.

The relative strengths of the shunt and series field windings dictate the external characteristic of a cumulative compound generator. As the load current increases, the series field magnetomotive force increases and tends to increase the flux thereby increasing the generated voltage as well. The series excitation aids the shunt excitation and the degree of compounding depends upon the increase in series excitation with the increase in the load current. If the series winding turns are adjusted in such a way that the terminal voltage increases with the increase in load current, the generator is said to be over-compounded. In this case, as the load current increases, the series field magnetomotive force increases the flux and the generated voltage. The increase in the generated voltage is greater than the IARA drop so that instead of decreasing, the terminal voltage increases. When the terminal voltage substantially remains constant with the increase in load current, the generator is said to be flat-compounded. In this case, the full-load voltage is nearly equal to the no load voltage. Lastly, if the series field winding has lesser number of turns than for a flat-compounded machine, the terminal voltage falls with increase in the load current. Generators that possess such characteristics are said to be under-compounded wherein the full load terminal voltage is less than the no load.

Procedure:

Position the FH50 Mimic diagram over the machine access sockets of the test bed. Mount the test generator FH50 on the right-hand machine position and the prime mover FH50 on the left-hand position. Insert the 16-way plugs of the two machines in their respective sockets on the test bed. Set up the equipment and connect the given wiring diagram. Switch on the FH2MkIV at the main switch and press the green push button to engage the contactor. Set up the equipment and connect the diagram in Fig1.1. Start the prime mover by rotating the armature rheostat clockwise. Set the armature rheostat so that the machines rotate at 1500rev/min. Adjust the 2000-Ω rheostat to give an output current of 120mA. Allow the machine to warm up for 15 minutes. Set the 50-Ω rheostat to 10Ω. Turn the 2000-Ω rheostat of R1 to zero then to maximum. Gradually decrease 2000-Ω rheostat to produce the current values shown in the results table and record corresponding values of output current and output voltage. Repeat the whole procedure with the R1 50-Ω rheostat to 40 Ω and then zero.

GenVL

AL

Decade Resistor(Set to 100 ohms)

Series Field

Diverter R1 (50 ohms)

Shunt Field

IAIF

IL

Results and Discussion:

Fig1.1

Output Current, I-

L (mA)Output Voltage, VL (V)

Diverter at ∞

Diverter at 40Ω

Diverter at 10 Ω

0 16.3 18.8 21.750 25.2 27.7 28

100 27.3 30 30.4150 29.2 31.2 31.1200 30.7 31.3 31.4250 30.1 31.2 31.2300 29.6 30.8 30.5350 29.1 30.4 30.1400 28.6 29.5 29.5450 27.9 28.6 28.6500 26.8 27.9 27.8

Table1.1

Output Current, IL

(mA)

Output Voltage, VL

(V)Diverter at 0

Ω0 10

30 8.350 7.370 6.8

100 5.2110 4.3120 3.3130 2.9120 3.6110 3.980 5.160 6.1

Table1.2

0 50 100 150 200 250 300 350 400 450 5000

5

10

15

20

25

30

35 Compound Generator

Diverter at ∞Diverter at 40ΩDiverter at 10 Ω

Out

put V

olta

ge (V

)

0 30 50 70 100 110 120 1300

2

4

6

8

10

12

Generator without compounding

VLSe-ries3

Output Current, IL (mA)

Out

put V

olta

ge, V

L

Fig 1.2

As shown in table1.1, the output current is varied from 0mA to 500mA with intervals of 50mA. Increasing this current means lowering the resistance at the load branch. The output voltage that corresponds to each output current and diverter resistance is also determined. For both diverters, the output voltage increases then decreases as the output current increases. It states that although the generated voltage increases with the output current, the increase in the voltage drops across the parallel combination of the diverter and the series field winding and the load is greater than the increase in the generated voltage. As a result, the load voltage decreases even if the generated voltage increases. Initially at 0mA output current, the output voltages at RD=40Ω and RD=10Ω are 18.8V and 21.7V respectively. VL at RD=10Ω is greater than that of RD=40Ω since the voltage drop at the parallel combination of the series field winding and the divider resistance is lower when the diverter has a smaller value. Hence, most of the voltage coming from the generated voltage will be absorbed by the load itself.

In table 1.2, the diverter resistance is at 0Ω. Because of this, the series field winding is somehow bypassed. Similarly with the previous results, the output voltage decreases as the output current increases. Using Kirchhoff’s voltage rule, the load voltage will somehow approximate the generated voltage. Most likely, the difference between their values can be associated with the voltage drop across the armature resistance of the generator.

Based from the results, the machine in part a exhibits under-compounding since the terminal voltage declines with an increasing current.

Conclusion:

Compound generators are generators that exhibit characteristics of both series and shunt generators. They are normally operated cumulatively wherein the series fields aid the shunt field. As the load current increases, the generated voltage also increases.

Over-compounded generators exhibit a characteristic wherein the terminal voltage increases with an increasing load current. Flat-compounded generators produce a terminal voltage that somehow remains constant even with an increasing output current. Under-compound generators produce a terminal voltage that declines with an increasing load current.

References:

Principal of electrical machinery and optimization of electrical power, SuthipongThanasansakorn, Southeast Asia Fishery Development Center, Training Department Thailand June 2010

Electric Machinery Fundamentals, Fourth Edition, Stephen J. Chapman