Lab Info. -- SYNCHRONOUS MACHINE EXPERIMENTS
EXPERIMENT NO. 24
SYNCHRONOUS IMPEDANCE METHOD OF
PREDICTING VOLTAGE REGULATION
PURPOSE:
To determine the synchronous impedance of the alternator by performing open-circuit and shortcircuit tests and to use it to predict voltage regulation.
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 0 to 300 Volt AC Voltmeter
5. One 0 to 0.5 Amp AC Ammeter
6. One 0 to 150 Volt/1 Amp DC Power Supply
7. One 0 to 125 Volt/5 Amp DC Power Supply
8. One 0 to 1.0 Amp DC Ammeter
9. One 0 to 75 Volt DC Voltmeter
PROCEDURE:
1. Couple the Alternator to the DC motor and make the connections shown in Figure 34. Adjust
the motor's rheostat to its minimum resistance, full 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.
3. Perform an open-circuit test on the alternator. Record the field current and armature volts in
Table 35.
The Open-Circuit Test: A separately excited (no-load) magnetization curve is obtained for the
alternator, operated at synchronous speed. A DC Ammeter is connected in the field circuit to
record the field current, and an AC Voltmeter is connected across any two stator leads to record
the line voltage, V1. A sufficient number of readings are taken starting with zero field current,
both below and above the knee of the curve. In each case, the field current, If, and the generated
phase voltage, Egp, (i.e.
), are recorded, and a saturation curve is plotted. As with the
DC magnetization curve, the results should be taken in one direction to avoid minor hystersis
loops.
4. Perform a short-circuit test on the alternator using the connections shown in Figure 35. Record
the field current and armature amps in Table 36.
The Short-Circuit Test: The short-circuit characteristic is taken by connecting ammeters in the
line to record the phase current. The field current is adjusted to zero, and the alternator is brought
up to speed. Readings are taken of DC field current versus AC short-circuited armature current.
5. Calculate the synchronous impedance from the following equation:
Egp = Ia Zp
where; Ia = full load current per phase
Egp = open-circuit voltage produced by the same field current that caused the rated short-circuit
current
Zp = the synchronous impedance per phase
6. Determine the D-C resistance per phase of the stator using the volt-ammeter method as shown
in Figure 36.

The A-C resistance per phase, Ra, is obtained by multiplying the D-C resistance by a factor
which varies from 1.2 to about 1.8 depending on the frequency, quality of insulation, size,
capacity, etc. For the purposes of this experiment, use a factor of 1.5 in computing the effective
(A-C) armature resistance per phase. Record these values in Table 37.
FIGURE 34
FIGURE 35
FIGURE 36