Lab Info. -- SYNCHRONOUS MACHINE EXPERIMENTS
EXPERIMENT NO. 22
LOAD CHARACTERISTICS OF AN ALTERNATOR
LAGGING POWER FACTOR
PURPOSE:
To determine the load characteristics of an alternator when operated into a .8 lagging power
factor load.
APPARATUS REQUIRED:
1. One SM-100-3 Synchronous Machine as Three-Phase Alternator
2. One DM-100 DC Machine as Motor
3. One SLA-100 Strobe-Tachometer (or other tachometer)
4. One RLC-100 Load Bank
5. One 0 to 300 Volt AC Voltmeter
6. Two 0 to 0.5 Amp AC Ammeters
7. One 0 to 150 Volt/1 Amp DC Power Supply
8. One 0 to 125 Volt/5 Amp DC Power Supply
9. One 0 to 1.0 Amp DC Ammeter
PROCEDURE:
1. Couple the Alternator to the DC motor and make the connections shown in Figure 32. Adjust
the motor's rheostat to its minimum resistance, fully CCW position. Adjust the 125 volt DC
supply to 125 volts.
2. Have the instructor check your connections and then start the motor. Adjust the motor's speed
to 1800 RPM by adjusting the motor's rheostat. Adjust the alternator's output to 220 volts by
adjusting the 150 Volt DC Supply.
3. Perform a .8 lagging PF load test on the alternator from no load to 0.4 amps load. A .8 PF load
can be approximated using a minimum of instruments by utilizing the graph. First, apply a
resistive load step. Using the ammeter reading obtained, travel vertically on the graph to the
point of intersection with the 0.8 PF line. Next, travel horizontally from this point to the reactive
current value. Adjust the reactive load to yield this inductive current to obtain the 0.8 PF load. It
will be necessary to adjust the motor's rheostat after each load step is applied to maintain 1800
RPM. Record in Table 33 the armature volts, the load resistive and inductive currents, field
current, and speed.
4. Determine the armature amps for each load step. (Use the graph).
5. Calculate the apparent power (volt-amperes) for each load step.
6. Calculate the actual power (watts) for each load step.
SUGGESTIONS FOR CONCLUSION:
Using the data in Table 33, plot the armature volts, output volt-amperes, and output watts as
ordinates versus the armature amps as abscissa. Compare the voltage regulation of the alternator
at .8 lagging power factor to the regulation at unity power factor (Exp. No. 30). Discuss the
validity of the method used in this experiment to obtain a .8 power factor load.
QUESTIONS:
1. How could you use a wattmeter and an ammeter to adjust the load to a .8 power factor?
2. Why is it more important that automatic voltage regulators be used with alternators than with
DC generators.
TABLE 33
FIGURE 32