Three-Phase to Two-Phase Conversion (Transformers)

Objective:

    · To connect two transformers in such a way as to convert a three-phase system to a two-phase system.

    · To show that a balanced two-phase system gives rise to a balanced three-phase system.

Discussion:

Two-Phase power was wide-spread in industry some time ago, but has since been replaced by three-phase power. However, there still exist requirements for two-phase power, principally in the small motor control field. It is useful, therefore, to know how to convert a three-phase, or from three-phase to two-phase, with or without a change in voltage, by means of special transformers which have the required ratio of transformers. In this regard, the Scott connection is probably the best known, and it is the one we will examine here.

In a balanced, three-phase system, the voltages and currents are all equal and displaced by 120^o from each other. In a balanced two-phase system, the voltages and currents are also equal but are displaced only 90^o from each other. The voltage is, therefore, maximum in one phase when it is zero in the other. Similarly, the current is maximum in one phase when is zero in the other.

Equipment Required:

• A Single Phase Transformer
• Resistive Load
• Power Supply
• AC 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!!!

Figure – 1

1. Using your Single-Phase Transformer, Resistive Load, Power Supply, AC Ammeter and AC Voltmeter, connect the circuit shown in Figure – 1. Note the connections to taps 3 and 6 on T₁ and taps 3, 7 and 4 on T₂. This particular configuration is known as the Scott connection. The winding ratios have been carefully chosen for a balanced two-phase output. The transformer inputs are connected to the variable 380 V 3Ф out put of the power supply, terminals 4, 5 and 6.

2.     a. Adjust the resistance module switches for R₁ = R₂ = 880 Ω.

b. Turn on the power supply and adjust for a line-to-line voltage of 380 V ac.

c. Measure and record the three input line currents I₄, I₅ and I₆.

I₄ = 0.11 A ac; I₅ = 0.11 A ac; I₆ = 0.11 A ac;

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

e. Are the three line currents reasonably well balanced? Yes

3.    a. Remove the two current meters A₅ and A₆ from the input circuit and place one in series with each of the load resistors R₁ and R₂ as shown by the dashed lines in Figure – 1.

b. Turn on the power supply and adjust for a line-to-line voltage of 380 V ac.

c. Measure and record the voltage across and the currents through, the two load resistances R₁ and R₂.

E₁ = 140 V ac; I₁ = 0.155 A ac

E₂ = 135 V ac; I₂ = 0.16 A ac

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

e. Are the load currents and voltages reasonably well balanced? No

4.    a. Calculate the power in each load resistance.

   P₁ = E₁ (140) × I₁ (0.155) = 21.7 W

   P₂ = E₂ (135) × I₂ (0.16) = 21.6 W

   2Ф total power = P₁ + P₂ = (21.7 + 21.6) W = 43.3 W

b. Calculate the apparent power delivered to the transformers from the three-phase source.

   380V × 1.73 × I₄(0.11) = 72.4 VA

   3Ф total power = 72.4 W

Figure – 2

5. Remove the two resistance loads from the circuit and connect the transformer secondaries as shown in Figure – 2. Do not disturb the wiring to the primary windings.

6.     a. Turn on the power supply and adjust for a line-to-line voltage of 380 V ac.

  b. Measure and record the voltage across each winding and the voltage across both windings in series.

     E₁ = 140 Vac; E₂ = 135 Vac; E₃ = 195 Vac

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

Review Question:

1.   How much larger is the apparent power delivered by three-phase power source in procedure 3 than the apparent two-phase power delivered to the load?

Ans: 3Ф total power = 72.4 W and 2Ф total power = 43.3 W. The power difference between source and load = (72.4 – 43.3) W = 29.1 W. We can observe that about 40% larger the apparent power delivered by three-phase power source than the apparent two-phase power delivered to the load.

2.  In the space provided, show how you would connect two Single-Phase Transformers to obtain three-phase output from a two-phase power source. Identify all windings and taps.

Ans: Shown in Figure – 3.

Figure – 3