Islamic University of Gaza
Electric Machines Lab
Faculty of Engineering
Eng. Mohammed S. Jouda
Eng. Omar A. Qarmout
Eng. Amani S. Abu Reyala
Electrical Engineering department
Single-Phase Transformers
Experiment
4
Magnetic circuit and Electric transformer
OBJECTIVE:
When you have completed this Experiment, you should be able to:
Explain and demonstrate important operating characteristics of single-phase transformers.
Connect transformer windings in series-aiding and series-opposing configurations.
Voltage and current measurements along with transformer load curves will be used to study
transformer operation and working characteristics.
Overview:
Transformers are magnetically-operated devices that can change voltage, current, and
impedance values in ac circuits. In its simplest form, a transformer consists of two coils of wire
wound on a common core of ferromagnetic material, such as iron.
Figure 4-1: Single-Phase Transformer
- The first coil is called primary winding, used for the input power, and has number of
turns .
- The second coil is called the secondary winding, connected to the load, and has number of
turns .
- The ratio of the number of turns of wire in the primary winding ( ) to the number of
turns of wire in the secondary winding ( .) is called the turns ratio which set the
relation between the input and the output values of a transformer.
=
=
Transformers are probably the most universal pieces of equipment in the electrical industry, and
range in size from tiny units in transistor radios to extremely large units weighing several tons in
power distribution stations. However, all transformers have basically the same operating
principles and characteristics.
Figure 4-2: Single-Phase Transformer Connected to a Resistive Load.
Transformer principle of operation:
When the secondary is an open-circuit and an alternating voltage V1 is applied to the primary
winding, a small current— called the no-load current Io — flows, which sets up a magnetic flux
in the core. This alternating flux links with both primary and secondary coils and induces in them
e.m.f.’s of E1 and E2 respectively by mutual induction. The induced e.m.f. E in a coil of N turns
is given by E = −N volts,
where d φ_/dt is the rate of change of flux. In an ideal transformer, the rate of change of flux is
the same for both primary and secondary and thus E1 /N1 =E2 /N2, i.e.the induced e.m.f. per turn
is constant. Assuming no losses, E1 =V1 and E2 =V2. Hence
V1/ N1 = V2 / N2 orV1 / V2 = N1 / N2
V1/V2 is called the voltage ratio and N1/N2 the turns ratio, or the ‘transformation ratio’ of the
transformer. If N2 is less than N1 then V2 is less than V1 and the device is termed a step-down
transformer. If N2 is greater then N1 then V2 is greater than V1 and the device is termed a stepup transformer.
PROCEDURE:
PRACTICAL 4.1.A: DETERMINING THE TURNS RATIO USING VOLTAGE RATIO
WITH NO LOAD :
1. Ensure that the power supply (unit 8821-25) is switched off.
2. Make the appropriate connections either for virtual or conventional instrumentation shown in
Figure 4-3 using unit 8341-05.
3. In the virtual instrumentation set the range of ammeters in low mode.
Figure 4-3: Determining the turns-ratio using voltage ratio
4. Ensure the dial on the power supply is set to zero position.
5. Connect E2 between 3&4
6. Switch on the power supply unit and using the output voltage dial set the phase voltage to 220
V as shown on the virtual or conventional voltmeter E1.
7. Read the measurement from the computer and calculate the turns ratio
IPRI = _______A , E1-2 =________V , E3-4 = _________ V
,
=
,
8. Connect E2 between 5&6 and 5&9 then repeat 6 and 7.
E5-6 = _________ V , E5-9 = _________ V
9. Using the measured values in step 7&8, compare these transformer turns ratios with the
corresponding Current ratios.
By using the virtual instrumentation ( LVVL Program ):
Repeat the above procedure by using LVVL Program and Switch on the power supply unit and
using the output voltage dial set the phase voltage to 120 V, Measure and record all above.
PRACTICAL 4.1.B: DETERMINING THE TURNS RATIO USING CURRENT RATIO:
1. Ensure that the power supply (unit 8821-25) is switched off.
2. Make the appropriate connections either for virtual or conventional instrumentation shown in
figure 4-4 using unit 8341-05.
3. In the virtual instrumentation set the range of ammeters in low mode.
Figure 4-4: Determining the turns-ratio using current ratio
4.
5.
6.
7.
Ensure the dial on the power supply is set to zero position.
Connect I2 between 3&4
Switch on the power supply unit and using the output voltage dial set the current I2 0.1A.
Read the measurement from the computer and calculate the turns ratio
, = ______,
, = ______
= ______,
,
=
,
8. Connect I2 between 5&6 and 5&9 then repeat 6 and 7 but set I2 0.2A.
, = ______
, = ______,
9. Using the measured values in step 7&8, compare these transformer turns ratios with the
corresponding Current ratios.
By using the virtual instrumentation ( LVVL Program ):
Repeat the above procedure by using LVVL Program and Switch on the power supply unit
and using the output voltage dial set the phase voltage to 120 V, Measure and record all above.
Practical aspects:
1. Most transformers are used to provide either a set up or a step down of secondary voltage.
2. Large electricity supply transformers are used to change the relatively low voltage produced
by the power station generators to much higher level for transmission of power over long
distance; e.g. power may be generated at 11kV and transmitted at 27.5kV.
3. At sub-station near to a town or city where the electricity will be used several step-down
transformers will be connected between the transmission lines and the distribution system.
4. Further step-down transformers reduce the main supply to the nominal value specified by the
supply authorities; for example 240 volt in domestic and light industry applications.
5. In many types of electrical equipments such as industrial electronics instruments, step-down
transformer are used to provide the low voltage power supplies required for transistors and
integrated circuits
6. Stepped ratio transformers enable a small motor or possibly heating element to be powered
from a range of different supply voltages. In these cases, a mains voltage selector switch may
by connected to tapping points on the primary winding for 100, 120, 200 or 240V.
THE RATED VOLTAGE AND SATURATION:
An exciting current is needed in order to generate alternating magnetic flux across the
transformer windings. The exciting current, which is directly related to the alternating
magnetic flux, increases in direct proportion to the applied voltage until core saturation sets
in. This occurs when the applied voltage exceeds the rated value of the primary, and then the
linear relationship between the primary voltage and the exciting current breaks down. The
curve of primary voltage versus exciting current flattens and smaller increases in primary
voltage lead to larger increases in exciting current as shown in Figure 4-5. The exciting
current is only a few milliamperes in the EMS Single-Phase Transformer module, and
generally its value is a small percentage of the nominal current of a transformer.
Figure 4-5: Saturation Curve of a Transformer
PROCEDURE:
PRACTICAL 4.2: THE RATED VOLTAGE AND SATURATION:
1. Ensure that the power supply (unit 8821-25) switched off.
2. Make the appropriate connections either for virtual or conventional instrumentation shown in
figure 4-6.
Figure 4-6: Effect of Core Saturation on Exiting Current
3. In the virtual instrumentation set the range of ammeters in low mode.
4. Ensure the dial on the power supply is set to zero position.
5. Switch on the power supply unit and using the output voltage dial, take the results after
reaching 10% from the indicator.
6. Repeat 5 after increasing another 10% (Use data table on the virtual program to record the
results) and continue repeating until reaching 100% (Table 4-1).
7. Display the Graph window, select E1 as the X-axis parameter, and E2 as the Y-axis
parameter. Make sure the line graph format and the linear scale are selected.
8. Switch off supplies.
9. Observe the curve of primary voltage versus exciting current represented by E1.
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
Table 3-1
Exercise 4.1:
- Does the exciting current increase more rapidly after the rated voltage is exceeded? Why?
- Does the curve illustrate that the transformer core becomes saturated?
- Review the measured data to determine how the primary-to-secondary voltage ratio was
affected when the transformer core became saturated.
- The turns ratio for a transformer with 225 turns of wire in its primary winding and 675 turns in the
secondary is
a. 1 : 3.
b. 3 : 1.
c. NS / NP.
d. N2 / N1.
- Transformer saturation occurs when the
a. primary current is greater than the rated value.
b. secondary winding is short-circuited.
c. secondary voltage is lower than the rated value.
d. primary voltage is greater than the rated value.
- Why is it necessary to apply a low voltage to the primary winding instead of the rated voltage when
evaluating the current ratio of a transformer?
a. To ensure that rated current will flow in the secondary.
b. To ensure that the current rating of the primary is not exceeded.
c. To ensure that the voltage rating of the secondary is respected.
d. To ensure that exciting current is maximum.
Practical aspects:
Practically, it is necessary to use the transformer at its rated values or fewer than it in order to
preserve the transformer from damaging.
THREE PHASE SEQUENCE DETERMINATION:
When you have completed this exercise, you will be able to determine and use transformer
polarities to properly connect separate windings so that the voltages add (series-aiding) or
subtract (series-opposing).
When the primary winding of a transformer is energized by an ac source, an alternating
magnetic flux is established in the iron core. This alternating flux links, or couples, the turns of
each winding on the transformer and induces ac voltages in the windings. However, when two or
more windings are connected together, their relative instantaneous polarities have a significant
effect on the resulting net voltage. If the voltage in one winding is at its maximum positive peak
when the voltage in another winding is at its maximum negative peak, i.e. they are 180 out of
phase, they will oppose each other and the resulting voltage will be the difference between the
two. For this reason, standards have been adopted for marking the polarity of transformer leads.
When we speak of the polarity of transformer windings, we are identifying all terminals that
have the same polarity, either negative or positive, at any instant in time. The dots used in Figure
4-7 indicate that at a given instant in time, when terminal 1 is positive with respect to terminal 2,
then terminal 5 is positive with respect to terminal 6.
Figure 4-7: Dot indicator to specify the Transformer Polarity
This means that if the terminals 1 and 5 are connected to gather then the voltage between the terminals 2
and 6 is equal to the deference between , and , , and if the terminals 1 and 6 are connected to gather
then the voltage between the terminals 2 and 5 is equal to the total of , and , .
PROCEDURE:
PRACTICAL 4.3: TRANSFORMER POLARITY DETERMINATION
1. Ensure that the power supply (unit 8821-25) switched off.
2. Set up the transformer circuit in Figure 4-8, and connect terminals 1 and 5 together as shown.
Note that the ac input power in this circuit is connected at winding 3-4.
Figure 4-8. Transformer Windings Connected in Series.
3. Turn on the power and adjust the voltage control to set ES at exactly 50% of the rated voltage
for winding 3-4. Note that the rated voltage is the sum of the intermediate winding voltages
between terminals 3 and 4. Measure and record the voltages at transformer windings 1-2, 5-6,
and 2-6. Note that E2-6 is obtained from programmable meter A using the function E2+E3.
E1-2 =________ V, E5-6 = _______V, E2-6 =_________ V
4. Are the windings connected in series-aiding, or series-opposing?
5. Return the voltage control to zero and turn off the Power Supply. Disconnect terminals 1 and
5, and connect terminals 1 and 6 together. Reverse connections to meter E3. If this new
connection is series-aiding, what will be the value of E2-5 when the same voltage of step 6 is
applied to winding 3-4?
6. Turn on the power and once again set ES at exactly 50% of the rated voltage for winding 3-4.
Measure and record the voltages at transformer windings 1-2, 5-6, and 2-5. Note that E2-5 is
obtained from programmable meter A.
E1-2 =________ V, E5-6 = _______V, E2-5 =_________ V
PRACTICAL ASPECTS
1. In most transformer applications, it is not necessary to know the polarity of the windings.
However, when two transformers are required to be connected in parallel, it is essential to
establish correct polarity; otherwise destructively large currents would circulate in the
secondary windings. This is of particular importance in power distribution systems.
2. In most transformers supplying industrial loads, e.g. motors, heaters etc., two or more
secondary windings may be connected in series or in parallel to provide a range of voltage
and current levels. Again, the polarity of the secondary winding must be known