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UNIVERSITI MALAYSIA PERLIS
ELECTRICAL ENGINEERING TECHNOLOGY
EMT 113/4
EXPERIMENT 2
SINGLE PHASE TRANSFORMER
– Voltage and Current Ratios
MARKS
Q6
Q7
Q8
Q9
Q11
Q12
Q13
Q14
Q15
9
1
2
1
3
1
1
3
1
T 1-1
Q19
G19
Q20
Q21
Discussion
Conclusion
Total
%
33
1
5
1
2
4
5
73
100
NAME
PROGRAMME
MATRIK #
GROUP
DATE
EMT113/4
Single Phase Transformer-Voltage and Current Ratios
EXPERIMENT 2
TITLE : SINGLE PHASE TRANSFORMER – VOLTAGE AND CURRENT RATIOS
OBJECTIVES
1.
2.
To familiar with voltage and current characteristics of a single phase transformer.
To be able to use the transformer turns ratio to predict the voltage and current
that will flow in the secondary winding.
EQUIPMENTS
EMC Workstation Model 8110, Single Phase Transformer Model 8341, Power
Supply Model 8821 and Data Acquisition Interface Model 9062.
INTRODUCTION
The windings of a standard single-phase transformer are called the primary
winding and the secondary winding. The primary winding is the power input
winding and this is the side that is connected to the ac power source. The
secondary winding is connected to the load and is physically and electrically
isolated from the primary. The voltage and current that flow in the secondary are
related to the primary voltage and current by the transformer turns ratio N1/N2 (or
Np/Ns) through a very simple relationship. The ratio of primary voltage to
secondary voltage equals N1/N2, while the ratio of the primary to secondary is
equal to the inverse of the turns ratio, N2/N1. This result in the following,
EPRI = N1
ESEC
N2
which gives:
and
which gives:
ESEC
= EPRI
N1
X
N2
IPRI = N2
ISEC
N1
ISEC
= IPRI
N1
N2
X
Transformers are designed with fixed ratios between primary and secondary
voltages and are widely used to step-up (increase) or step-down (decrease) load
voltages and currents. The Single-Phase Transformer module used in these
exercises has its nominal ratings silk-screened on the front panel and most
transformers have markings to indicate their nominal characteristics. Also, many
transformers have intermediate taps, or connection terminals on the secondary
side, so that different voltage ratios can be obtained using a single transformer.
Determining a transformer’s voltage ratio is simple matter. With no load
connected to the secondary winding, only small exciting current necessary to
create the magnetic flux inside the transformer flows in the primary winding.
Transformer losses are minimum and the ratio of primary to secondary voltage
equals the turns ratio. The turns ratio can be found by measuring the unloaded
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
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EMT113/4
Single Phase Transformer-Voltage and Current Ratios
secondary voltage with nominal voltage applied to the primary. The current ratio
can be evaluated by measuring the short-circuit secondary current with a small
ac voltage applied to the primary. The voltage applied to the primary must be low
enough to ensure that the nominal current in the primary winding is not exceeded.
Otherwise, the windings may overheat and be damaged.
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 the primary voltage versus exciting current flattens
and smaller increases in primary voltage lead to larger increases in exciting
current as shown in Figure 2-1. 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 2-1: Saturation Curve of a Transformer
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
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EMT113/4
Single Phase Transformer-Voltage and Current Ratios
PROCEDURE
Figure 2-2: Single –Phase Transformer Measurements
1. Install the Power Supply, Data Acquisition Interface (DAI) and Single-Phase
Transformer modules in the EMS Workstation.
2. Make sure that the main switch of the Power Supply is set to the O (OFF)
position, and the voltage control knob is turned fully counter clock-wise (ccw). Set
the voltmeter select switch to the 4-N position.
3. Ensure that the DAI LOW POWER INPUT is connected to the main Power
Supply, set the 24V-AC power switch to the 1 (ON) position.
4.
Display the Metering application.
5. Set up the transformer circuit shown in Figure 2-2. Connect E1 and I1 as shown
and use E2 to measure the different secondary voltages.
6. Turn ON the power and adjust the voltage control for the value of Es given in
Figure 2-2. Measure the transformer primary current and the different terminal
voltages. After recording the measurements, rotate the voltage control fully ccw
and then turn OFF the power.
Note: When measuring the various voltages, turn OFF the Power Supply before
modifying the connections of the DAI to the circuit.
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EMT113/4
Single Phase Transformer-Voltage and Current Ratios
Name:______________________________
Date:_______________________________
Matric No:____________________
Course:______________________
IPRI = ________ A
E1-2 = _________ V
E3-4 =__________ V
E5-6 = ________ V
E3-7 = _________ V
E7-8 =__________ V
E8-4 = ________ V
E5-9 = _________ V
E9-6 =__________ V
9 marks
7. Do the secondary voltages compare well with the rated values written on the
front panel?
Yes
No
1 mark
8. The transformer windings between terminals 1 and 2 and between terminals 5
and 6 each have 500 turns of wire. The number of turns in the winding between
terminals 3 and 4 is 865. Calculate the turns ratios between the primary and
secondary windings for each case.
N1-2 = ________
N5-6
N1-2 = ________
N3-4
2 marks
9. Using the measured values in step 6, compare these transformer turns ratios
with the corresponding voltage ratios. Are they approximately the same?
Yes
No
1 mark
Figure 2-3: Determining the ratio of Primary Current to Secondary Current
10. Connect I2 as shown in Figure 2-3 and note that it short-circuits secondary
winding 5-6. Turn on the power and slowly adjust the voltage control to obtain the
value of current Is given in Figure 2-3.
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
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EMT113/4
Single Phase Transformer-Voltage and Current Ratios
Name:______________________________
Date:_______________________________
Matric No:____________________
Course:______________________
11. Record the values of primary voltage and current and the value of the shortcircuit secondary current in winding 5-6.
EPRI = ________ V
IPRI = _________ A
ISEC =__________ A
3 marks
12. Return the voltage control to zero and turn off the power. Calculate the ratio of
primary current to secondary current.
IPRI = ________
ISEC
1 mark
13. Is the ratio approximately equal to N2/N1?
Yes
No
1 mark
14. Connect I2 so that it now short-circuits secondary terminals 3-4. Turn on the
Power Supply and slowly adjust the voltage control knob for the same value of
current used in step 10. Once again, record the values of primary voltage and
current and the secondary winding current.
EPRI = ________ V
IPRI = _________ A
ISEC =__________ A
3 marks
15. Return the voltage control to zero and turn off the power. Again, calculate the
ratio of primary current to secondary current. It is equal to N2/N1?
Yes
No
1 mark
Figure 2-4: Effect of Core Saturating on Exciting Current
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
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EMT113/4
Single Phase Transformer-Voltage and Current Ratios
Name:______________________________
Date:_______________________________
Matric No:____________________
Course:______________________
16. Set up the transformer circuit shown in Figure 2-4. It will be used to show how
exciting current is affected when the transformer core becomes saturated. Since
the exciting current is so small, the corresponding voltage across a sense
resistor R will be used to illustrate its variation. Connect the transformer primary
terminals to Power Supply terminals 4 and 5 through sense resistor R. Connect
E1, E2 and E3 to measure the transformer voltages.
17. Turn on the power and use the output voltage control to obtain values for E2
equally spaced at about 10% intervals over the complete control knob range. At
each voltage adjustment, click the Record Data button to enter the
measurements in the Data Table. Then fill in the data into Table 2-1 below.
Interval,
%
0
Sensed Voltage
(E1), V
Primary Voltage
(E2), V
Secondary Voltage
(E3), V
10
20
30
40
50
60
70
80
90
100
33 marks
Table 2-1 Transformer Voltages
18. When all data values have been recorded, rotate the voltage control fully ccw,
and turn off the Power Supply.
19. Display the Graph screen, select E1 as the X-axis parameter and E2 as the Yaxis parameter. Click the Line Graph button to observe the curve of primary
voltage versus exciting current, represented by E1. Then copy this graph into the
graph in the RESULT Section. Does the exciting current increase more rapidly
after the rated voltage is exceeded?
Yes
No
1 mark
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EMT113/4
Single Phase Transformer-Voltage and Current Ratios
Name:______________________________
Date:_______________________________
Matric No:____________________
Course:______________________
20. Does the curve illustrate that the transformer core becomes saturated?
Yes
No
1 mark
21. Review the measured data to determine how the primary-to-secondary voltage
ratio was affected when the transformer core become saturated.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
2 marks
22. Ensure that the Power Supply is turned off, the voltage control is fully ccw and
remove all leads and cables.
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EMT113/4
Single Phase Transformer-Voltage and Current Ratios
Name:______________________________
Date:_______________________________
Matric No:____________________
Course:______________________
RESULT
5 marks
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
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OP-AMP SINEWAVE 'NON-INVERT'
output voltage input
EMT113/4
Single Phase Transformer-Voltage and Current Ratios
Name:______________________________
Date:_______________________________
Matric No:____________________
Course:______________________
DISCUSSION
The primary winding in standard single-phase transformer is _____________________
time
______________________________________________________________________
______________________________________________________________________
2 marks
The secondary winding in standard single-phase transformer is ___________________
______________________________________________________________________
______________________________________________________________________
2 marks
CONCLUSION
The current ratio can be evaluated by _______________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
5 marks
PUSAT PENGAJIAN KEJURUTERAAN MIKROELEKTRONIK
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