The Three-Phase Alternator

Objective:

• To obtain the no-load saturation curve of the alternator.
• To obtain the short-circuit characteristics of the alternator.

Discussion:

The terms alternating current generator, synchronous generator, synchronous alternator and alternator are commonly used interchangeably in engineering literature. Because synchronous generators are so much more commonly used than induction generators, the terms alternator, as often used, and as used here, applies only, to synchronous generators.

Alternators are, by far, the most important source of electric energy. Alternators generate an AC voltage whose frequency depends entirely upon the speed of rotation. The generated voltage value depends upon the speed, the DC field excitation and the power factor of the load.

As the DC field excitation of an alternator is increased, its speed being held constant, the magnetic flux and hence, the output voltage will also increase in direct proportion to the current. However, with progressive increase in DC field current, the flux will eventually reach a high enough value to saturate the iron in the alternator.

There will be a smaller increase in flux for a given increase in DC field current. Because the generated voltage is directly related to the magnetic flux intensity, it can be used as a measure of the degree of saturation.

The three phase of the alternator are mechanically spaced at equal intervals from each other, and therefore, the respective generated voltage are not in phase, but are displaced from each other by 120 electrical degrees.

When an alternator delivering full rated output voltage is suddenly subjected to a short-circuit, very large currents will initially flow. However, these large short-circuit currents drop off rapidly to save values if the short-circuit is maintained.

Equipment Required:

• Three-Phase Synchronous Motor/Generator
• Four-Pole Squirrel-Cage Induction Motor
• Power Supply
• DC Voltmeter/Ammeter
• AC Voltmeter
• Wires

Procedure:

CAUTION!!!

High voltages are Present In the Experiment! Do not make any connections with the power on! The power should be turned off after completing each individual measurement!!!

Three-Phase Synchronous Motor/Generator

1. Using your Three-Phase Synchronous Motor/Generator, Four-Pole Squirrel-Cage Induction Motor, Power Supply, AC Voltmeter and DC Voltmeter/Ammeter, connect the circuit shown in Figure-1. The squirrel-cage motor will be used to drive the synchronous motor/generator as an alternator. Its speed will be assumed constant during this Experiment. Note that the squirrel-cage motor is connected to the fixed 380 V 3Ф output of the power supply, terminal 1, 2 and 3. The rotor of the alternator is connected to the variable 0-220 output of the power supply, terminal 7 and N.

2.   a. Couple the squirrel-cage motor to the alternator with the timing belt.

b. Set the alternator field rheostat at its full CW position (for zero resistance). Open switch S.

c. Set the power supply voltage control at its full CCW position (for zero DC voltage).

3.  a. Turn on the power supply. The motor should be running.

     b. With zero DC excitation measure and record E₁, E₂ and E₃ (use the lowest ranges of the voltmeters.

         E₁ = 10 Vac, E₂ = 10 Vac, E₃ = 10 Vac

     c. Explain why there is an AC voltage generated in the absence of DC excitation.

       Ans: In a Synchronous Generator Rotor; there is some residual magnetism. For this reason we have found an AC voltage generated without DC excitation.

4.   a. Close the switch S.

b. Gradually increase the DC excitation from zero to 0.05 Adc.

c. Measure and record in Table – 1 the three generated voltages E₁, E₂, E₃.

d. Repeat (b) for each of the DC current listed in Table – 1

e. Return the voltage to zero and turn off the power supply.

Table – 1

5. Calculate and record in Table – 1 the average output of the alternator for each of the listed DC currents.

6.   a. Turn on the power supply and adjust the DC excitation until E₁ = 380 Vac. Measure and Record     E₂ and E₃.

    E₁ = 380 Vac, E₂ = 380 Vac, E₃ = 380 Vac

b. Turn on the power supply without touching the voltage adjusts control.

c. Reconnect the three AC voltmeters so they will measure the voltages across each of the three stator windings.

d. Turn on the power supply. Measure and record the generated voltages across each of the wye connected stator windings

     E_{1 to 4} = 220 Vac, E_{2 to 5} = 220 Vac, E_{3 to 8} = 220 Vac

e. Return the voltage to zero and turn off the power supply.

Figure – 2

7. Using your Synchronizing Module, connect the circuit shown in Figure – 2. Note that the switch is wired to present a dead shot across the alternator windings when it is closed.

8.   a. Set the synchronizing switch to its open position.

b. Turn on the power supply and adjust the DC excitation until E₁ = 380 Vac. The motor should be running and the three lamps on the synchronizing module should be illuminated.

c. measure and record the DC exciting current I₁.

    I₁ = 0.25 Adc

d. Apply a short-circuit to your alternator by closing the synchronizing switch and note the behavior of the AC current I₂.

e. To what approximate peak value did I₂ increase?

    I₂ = 0.11 Aac

f. What is the final steady-state value of I₂ and I₁?

    I₁ = 0.25 Adc and I₂ = 0.11 Aac

g. Return the voltage to zero and turn off the power supply.

Review Questions:

1. Plot your recorded average voltage values vs DC current values from Table – 1 on the graph of Figure – 3.

2. Draw a smooth curve through your plotted points.

3. Up to what voltage is the curve a reasonably straight line?
    E = 220 Vac

4. Where would you say is the knee of the saturation curve?
    E = 300 Vac

5. Explain why the voltage increases less rapidly as the DC current increases.

Figure – 3

Ans: Alternators generate an AC voltage whose frequency depends upon the speed of rotation, DC field excitation and power factor of the load. If the DC field excitation is increased; its speed constant, the magnetic flux increased and the output voltage will also increased in direct proportion to the current. So there are a smaller increased in flux for a given increase in DC field current; the output voltage increases.