LABORATORY WORK ¹ 1
TEST
OF SINGLE PHASE TRANSFORMER
1.1.
Objectives
1. Get acquainted with operation and properties of single
phase transformer;
2. Acquire skills to measure transformer parameters and
operating characteristics.
1.2.
Tasks
1.
Analyze
connection of measuring devices and parameters to be measured.
2. Obtain experimentally operating characteristics and
characteristic parameters of single phase transformer.
3.
Analyze the
primary winding current shape variation at load change from zero to the rated
one.
1.3.
Fundamental Transformer Equations
A transformer is
an electromagnetic device having two or more mutually coupled windings. A
two-winding ideal transformer is shown in Fig. 1.1.
The transformer is ideal in
the sense that its core is lossless, it is infinitely permeable and has no
leakage flux and that its windings have no losses.
The basic
elements of the transformer are: core, primary winding N1, and the secondary winding N2.
If F
is mutual flux linking N1 and N2
then according to Faraday’s
law of electromagnetic induction, EMF’s e1
and e2
are induced in N1 and N2
owing to a finite rate of
change of F
such as:
(1.1)
(1.2)
Fig. 1.1.Two-winding
transformer
The direction of
e1is such that produces a current which opposes the flux change according to
Lenz’s law. The transformer being ideal, e1=
v2, N1=N2, i1=i2.
Thus for current
i1 entering the dotted terminal of coil 1, current i2
must leave the dotted terminal of coil 2 and is equal:
The current
ratio in Eq. (1.1, 1.2) is the reciprocal of the voltage ratio.
Thus, if a transformer steps up voltage applied to
coil 1, it steps down the current entering coil 1 by the same ratio, so that
trans-former remains lossless. Such transformer is said to be ideal.
Fig. 1.2. Loaded
transformer
Quite often
centre-tapped ideal transformers shown in Fig. 1.3 are used. The second coil
has a tap placed in the centre so that the voltage induced across the second
coil:
is divided
equally between two halves of that coil.
Fig. 1.3.Transformer
with a central tap
This transformer
can be used to provide two voltages which are opposite to each other in phase
or polarity. The voltage at terminal a with respect to the centre tap is
opposite in polarity to the voltage of terminal b with respect to the centre tap.
1.4.
Method of testing
1.
Get acquainted
with experimental circuit in Fig. 1.1, 1.2, 1.3 and measuring
devices as well as the purpose of those.
2.
Check the switch
of supply voltage to be in turned off position.
3.
Set the slider
of rheostat to get the greatest value of resistance.
4.
Check the scale
limits of measuring devices: do they fit values of the measured parameters:
voltage 40V and cur-rent 5A.
5.
After fulfilling
requirements of 1.1–1.3 items, connect the experimental circuit to supply
voltage.
6.
Turn on supply
voltage by automatic switch QF1.
7.
Test of
transformer at no-load.
8.
Disconnect one
wire of load rheostat to set up secondary current, equal to zero.
9.
Write readings
of V1r , I10, and V20
into Table.
10. Redraw from measuring device METREL or computer screen
the curves of voltage and current and calculate percentage of the third
harmonic in the primary current.
11. According to the experimental data calculate turns
ratio and core losses.
12. Short-circuit experiment.
13. Set up slider of autotransformer in position to get
the sup-ply voltage equal to zero. Check if the measuring device METREL shows
voltage equal to zero.
14. Connect the wire, being disconnected at no-load test
and slider of the load rheostat setup in position, giving load resistane, equal
to zero.
15. By autotransformer AT slowly increase primary voltage
while the secondary current will reach its rated value 5 A.
16. Write in the table readings V1Sh, I1Sh, P1Sh and I2Sh .
17. Redraw from measuring device METREL or computer screen
the curves of voltage and current and calculate percentage of the third
harmonic in the primary current.
18. After short circuit experiment rebuild experimental
circuit: slider of load rheostat set up in position of the greatest resistance
and by autotransformer set up the rated volt-age 220V.
19. Performance characteristics.
20. Calculate power and efficiency for each load value and
write data to the Table.
21. Observe the variation of primary current shape and
harmonic composition during loading transformer.
22. Turn off the voltage by automatic switch QF after
completing the experiment.
1.5. Content of report
1.
Task of the work
and experimental circuit.
2.
Experimental
data of transformer parameters at no load, short circuit and voltage
regulation.
3.
Voltage
regulation characteristics, plotted in the same figure: 
.
4.
Core losses and
heat losses, turns ratio, relative short circuit voltage and relative voltage
increment.
5.
Shapes of
primary current for three load current values.
6.
Conclusions
about: shape of primary current and its harmonical composition, change of
voltage with load and its relative increment, variation of efficiency and power
factor with load, core and heat losses – how many percents they constitute from
the rated transformer power.
1.6. Control questions
1.
What changes in
the loaded transformer comparing with transformer at no-load?
2.
Why does the
secondary voltage reduce with increasing load?
3.
Plot the
equivalent circuit of transformer and explain the physical meaning of circuit
elements.
4.
Explain, what
will happen with the transformer if there will be an air gap in its core.
5.
Explain, what
will happen with the transformer if its core is made of solid iron.
6.
Why power factor
is not equal to
zero at the load current equal to zero?
7.
What are the
main reasons of primary current distortion at operation on no-load?
8.
What power does
the transformer characterize?
9.
What losses do
appear in the transformer?
10. On what factors
do the core losses depend?
