The Single-Phase Transformer

Objective:

• To study the voltage and current ratios of a transformer.
• To learn about transformer-exciting currents, volt-ampere capacity and short-circuit currents.

Discussion:

Transformers are probably the most universally-used pieces of equipment in the electrical industry. They range in size from miniature units in transistor radios to huge units, weighing tons, used in central power distributing stations. However, all transformers have the same basic properties which you are about to examine.

When mutual induction exists between two coils or windings, a change in current through one induces a voltage in the other. Every transformer has a primary winding and one or more secondary windings. The primary winding receives electrical energy from a power source and couples this energy to the secondary winding by means of a changing magnetic field. The energy appears as an electromotive force across the secondary winding, and if a load is connected to the secondary, the energy is transferred to the load. Thus, electrical energy can be transferred from one circuit to another, with no physical connection between the two. Transformers are indispensable in AC power distribution, since they can convert electrical power at a given current and voltage into an equivalent power at some other current and voltage.

When a transformer is in operation, AC currents flow in its windings and an alternating magnetic field is set-up in the iron core. As a result, copper and iron losses are produced which represents active power (watts) and causes the transformer to heat up. Establishing a magnetic field requires reactive power (var) which is drawn from the power line. For these reasons the total power delivered to the primary winding is always slightly larger than the total power delivered by the secondary winding. However, we can say, to a good approximation, that in most transformers:

a) Primary Power (watts) = Secondary Power (watts)

b) Primary Volt-amperes (VA) = Secondary Volt-amperes (VA)

c) Primary vars = Secondary vars.

When the primary voltage is raised beyond its rated value, the iron core (laminations) begins to saturate, and the magnetizing (exciting) current increases rapidly.

Transformers are subject to accidental short-circuits caused by natural and man-made disasters. The short-circuit currents can be very large and, unless interrupted, will quickly burn out a transformer. It is the purpose of this Experiment to show these major points.

Equipment Required:

• A Single Phase Transformer
• 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!!!

o 1. Examine the construction of the Single-Phase Transformer paying particular attention to the transformer, connection terminals and the wiring.

1. The transformer core is made up of thin sheets (lamentations) of steel. Identify it.


2. Note that the transformer windings are brought out to terminals mounted on the transformer coil.
3. Note that these windings are then wired to the connection terminals mounted on the module face.

o 2. Identify the three separate transformer windings maked on the module face:

1. List the rated voltage for each of the three windings:

Terminals 1 to 2 = 220 V ac

Terminals 3 to 4 = 380 V ac

Terminals 5 to 6 = 220 V ac

2. List the rated voltage between the following connection terminals:

     Terminals 3 to 7 = 190 V ac

     Terminals 7 to 8 = 139 V ac

     Terminals 8 to 4 = 51 V ac

     Terminals 3 to 8 = 329 V ac

     Terminals 7 to 4 = 190 V ac

     Terminals 5 to 9 = 110 V ac

     Terminals 9 to 6 = 110 V ac

3. List the rated current for each of the following connections:

Terminals 1 to 2 = 0.25 A ac

Terminals 3 to 4 = 0.15 A ac

Terminals 5 to 6 = 0.25 A ac

Terminals 3 to 7 = 0.15 A ac

Terminals 8 to 4 = 0.15 A ac

o 3. Using the lowest range of our ohmmeter, measure and record the DC resistance of each winding:

Terminals 1 to 2 = 880 Ω

Terminals 3 to 4 = 2533.333 Ω

Terminals 3 to 7 = 1266.667 Ω

Terminals 7 to 8 = 926.667 Ω

Terminals 8 to 4 = 340 Ω

Terminals 5 to 6 = 880 Ω

Terminals 5 to 9 = 440 Ω

Terminals 9 to 6 = 440 Ω

o 4. Now we measure the unloaded secondary voltages with 220 V ac applied to the primary winding.

a. Connect the circuit shown in Figure-1.

b. Turn on the power supply and adjust for 220 V ac as indicated by the voltmeter across the power supply terminals 4 to N.

c. Measure and record the output voltage E₂.

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

e. Repeat ( b, c and d) measuring the output voltage E₂ for each of the listed windings.

f. Winding 1 to 2 = 220 V ac

Winding 3 to 4 = 380 V ac

Winding 5 to 6 = 220 V ac

Winding 3 to 7 = 185 V ac

Winding 7 to 8 = 138 V ac

Winding 8 to 4 = 50 V ac

Winding 5 to 9 = 108 V ac

Winding 9 to 6 = 106 V ac

o 5. a. We measured voltage correspond well with the rated values. But we have very something difference between the rated values and the measured values due to different kind of losses which is negligible.

b. The current for which the magnetic flux produced is called magnetizing current. The current I₁ is associated with the magnetic flux so here I₁ is magnetizing current. We measure it.

o 6. Windings 1 to 2 and 5 to 6 each have 1100 turns of wire. Winding 3 to 4 has 1900 turns. Calculate the following turn ratios:

o 7. a. Connect the circuit shown in Figure-2. Notice that current meter I₂ short-circuits winding 5 to 6.

b. Turn on the power supply and gradually increase the voltage until the short-circuit current I₂ is 0.2 A ac.

c. Measure and record E₁, I₁, and I₂.

I₁ = 0.4 A ac

E₁ = 30 V ac

I₂ = 0.2 A ac

4. Return the voltage to zero and turn off the power supply.
5. Calculate the current ratio:

6. For a transformer the turn and current ratio are inversely equal. So theoretically the voltage and current ratio is not equal. Because-

Figure - 3

o 8. a. Connect the circuit shown in Figure-3. Notice that winding 3 to 4 is short-circuited by the current meter I₃.

b. Turn on the power supply and gradually increase the voltage until the current through the primary winding I₁, is 0.2 A ac.

c. Measure and record I₃ and E₁.

I₃ = 0.12 A ac

E₁ = 18 V ac

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

e. Calculate the current ratio:

o 9. Now we determine the effect of core saturation upon the exciting current of a transformer.

a. Connect the circuit shown in Figure-4. Notice that power supply terminals 4 and 5 are now being used. These terminals will furnish variable 0 to 380 V ac.

b. Turn on the power supply and adjust for 50 V ac as indicated by the voltmeter across power supply terminals 4 to 5.

c. Measure and record the exciting-current I₁ and the output voltage E₂ for each of the input voltages listed in Table-1.

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

E1 V ac	I1 mA ac	E2 V ac
50	0	45
100	0	95
150	0	145
200	0	198
250	0.1	240
300	0.2	298
350	0.3	340
400	0.5	378

o 10. a. Plot our recorded current values on the graph of Figure-5. Draw a smooth curve through our plotted points.

b. Note that the magnetizing current increase rapidly after a certain input voltage has been reached.

c. When the voltages increase regularly; the current is increase. One time they are in saturation point.