Name: Course: BSEE Yr.

& Sec:
Subject: DC Machinery Lab. Time: Room:
Date Date
Instructor: Engr. Romeo de Guzman Fernandez
Performed: Submitted:
Experiment No. 4 THE DC COMPOUND GENERATOR Rating:

OBJECTIVE
1. To study the properties of compound DC generator under no -load and full-load conditions.
2. To learn how to connect both the compound and the differential compound generators.
3. To obtain the armature voltage vs armature current load curves for both the generators.

DISCUSSION
Self-excited shunt generators have the disadvantage in that the changes in their load current
from the no-load to full-load cause their output voltage regulation is due to three factors:
a) The magnetic field strength drops as the armature voltage drops, which further reduces the
magnetic field strength, which in turn reduces the armature voltage etc.
b) The armature voltage drop (I2 R losses) from no- load to full-load.
c) The running speed of the driving motor may change with load. (This is particularly true of
internal combustion engines and induction motors).
The two field windings (shunt and series) on the compound generator are connected so that their
magnetic fields aid each other. Thus, when the load current increases, the current through the shunt
field winding decreases, reducing the strength of the magnetic field. But if the same increase in load
current is made to flow through the series field winding, it will increase the strength of the magnetic
field.
With the proper number of turns in the series winding, the increases in magnetic strength will
compensate for the decrease caused by the shunt winding. The combined magnetic field strength
remains almost unchanged and little change in output voltage will take as the load goes from no-
load to full-load.
If the series field is connected so that the armature current flows in such direction as to oppose
the shunt field, we obtain a differential compound generator. This type of generator has poor
regulation, but is useful in application such as welding and arc lights where maintaining a constant
output current is more important than a constant output voltage. It is the purpose of this Laboratory
Experiment to show these major points.

INSTRUMENTS AND COMPONENTS

Power Supply Module (120/208,3 phase, 120Vdc,0-120Vdc) EMS 8821
DC Metering Module EMS 8412
AC Metering Module EMS 8425
DC motor/Generator Module EMS 8211
Synchronous Motor/Generator Module EMS 8241
Resistance Module EMS 8311
Connection Leads EMS 8941
Timing Belt EMS 8942

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 S
I1
1 1 4
A
0-2.5 Aac
208 Vac
I2
2 A 2 5
0-2.5 Aac
208 Vac
I3
3 A 3 6
7 8
0-2.5 Aac

120 Vdc

Figure 4.1

Caution: The switch in the excitation circuit of the synchronous motor should be closed (up
position) only when the motor is running.

PROCEDURE
Caution: High voltages are present in this Laboratory Experiment! Do not make any connections
with the power on! The power should be off after completing each individual
measurement!
1. Because of this constant running speed, the synchronous motor running 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 4-1.
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 the later Laboratory 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 (Lab. Expt. 3, procedure 6).

0-2.5
Adc IA
A

1 2 0-200
Vdc
EA V

SERIES FIELD RL
3 4

SHUNT FIELD
5 6 7 8

Figure 4-2

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3. a) Using your EMS DC Motor/Generator, DC Metering and resistance Modules, connect the
circuit shown in figure 4-2.
b) Couple the synchronous motor and the dc generator with timing belt.
c) Turn the dc generator field rheostat control knob full cw for minimum resistance
d) Place the resistance switches for no-load (all switches open).

4. a) Turn on the power supply. The synchronous motor should start running
b) If the synchronous motor has switch S, close it at this time.
c) Note if voltage Ea build up.
d)If not, turn off the power supply and interchange any two of the stator connection
leads on the synchronous motor.
e)Measure the open circuit armature voltage.

Ea= ______________Vdc

5. Vary the field rheostat and notice if the armature voltage EA changes .
Explain.______________________________________________________________________
_____________________________________________________________________________
6. Adjust the field rheostat for no-load current (IA = 0A) output voltage EA of 120Vdc.

Do not touch the field rheostat control for the remainder of the Laboratory Experiment!

7. a) Adjust the load resistance to obtain each of the values listed in Table 4-1.
b) Measure and record EA and IA for each of the resistance values listed in the Table.
Note: Although the nominal output current rating of the generator is 1 Adc, it may be loaded up to
1.5 Adc (50% overload) without harm.
c) Turn off the power supply.

RL IA EA POWER
(ohms) (mA) (volts) (watts)

α
600Ω
300Ω
200Ω
150Ω
120Ω
100Ω
80Ω
75Ω

Table 4-1

d) Calculate and record the power for each of the resistances shown in Table 4-1.

8. a) Change the connections to the series field only, so that the armature current flows
through it in the opposite direction.
b) Complete the drawing shown in figure 4-3 showing your proposed circuit change.
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 c) Have your instructor check your circuit and your drawing.

9. a) Turn on the power supply.

b) Adjust the field rheostat for an EA of 120Vdc.
c) Do not touch the rheostat after this.

0-2.5
Adc IA
A

1 2 0-200
Vdc
EA V

SERIES FIELD RL
3 4

SHUNT FIELD
5 6 7 8

Figure 4-3

10. a) Adjust the load resistance to obtain each of the values listed in the Table 4-2.
b) Measure and record EA and IA for each of the resistance values listed in the Table.
c) Turn off the power supply.
d) Calculate and record the power for each of the resistances shown in Table 4-2.

RL IA EA POWER
(ohms) (mA) (volts) (watts)

α
600Ω
300Ω
200Ω
150Ω
120Ω
100Ω
80Ω
75Ω

Table 4-2

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TEST YOUR KNOWLEDGE
1. State which procedure, (7 or 10) is concerned with:
a) The differential compound generator.
Procedure_______
b) The compound generator
Procedure_______

2. Plot the EA vs IA regulation curve on the graph. Use the data from Table 4-2.

140

130

120
EA (volts)
110

100

90

0.5 1.0 1.5

IA (amps)

3. Over what voltage is the armature current nearly constant in the differential compound
generator? `
___________________ Vdc
___________________ Vdc

4. Plot the EA vs IA regulation curve on the graph. Use the data from Table 4-1

5. Calculate the regulation from no-load to full-load (1.0 Adc)__________________________

________________________________________________________________________________
________________________________________________________________________________
____________

Regulation = __________________%

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 140

130
EA (volts)
120

110

100

90

0.5 1.0 1.5

IA (amps)

6. Compare the regulation of the compound regulator, with the regulation of the self-excited
generator and the separately excited
generator.___________________________________________________________
________________________________________________________________________________
________________________________________________________________________________

7. Explain briefly why the voltage does not drop with increasing load for the compound generator.
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
___________________________

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