experiment4 single phase transformer...
TRANSCRIPT
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Experiment4
Single Phase Transformer
Objectives:
- To study the characteristics of the single phase transformer.
- To study how transformers work.
- To study the voltages and currents curves when the operating single phase transformer.
Theory:
A transformer is an electrical device that transfers electrical energy between two or more circuits
through electromagnetic induction. Commonly, transformers are used to increase or decrease the
voltages of alternating current in electric power applications.
The coil to which the source supplies the power is called the primary winding. The coil that
delivers power to the load is called the secondary winding.
Since the induced emf in a coil is proportional to the number of turns in a coil, it is possible to
have a higher voltage across the secondary than the applied voltage to the primary. In this case,
the transformer is called a step-up transformer.
A step-up transformer is used to connect a relatively high-voltage transmission line to a
relatively low-voltage generator. On the other hand, a step-down transformer has a lower voltage
on the secondary side.
Figure 1: Single phase transformer
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Construction of a Transformer
In order to keep the core loss to a minimum, the core of a transformer is built up of thin
laminations of highly permeable ferromagnetic material such as silicon-sheet steel. Silicon steel
is used because of its properties and low magnetic losses.
In all types of transformer construction, the central iron core is constructed from of a highly
permeable material made from thin silicon steel laminations. These thin laminations are
assembled together to provide the required magnetic path with the minimum of magnetic losses.
The resistivity of the steel sheet itself is high, thus reducing any eddy current loss by making the
laminations very thin.
Figure 2: thin laminations from steel
Firstly// Ideal Transformer
A two-winding transformer with each winding acting as a part of a separate electric circuit. Let
N1, and N2 be the number of turns in the primary and secondary windings.
Figure 3: ideal transformer
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Let us first consider an idealized transformer in which there are no losses and no leakage flux. In
other words, we are postulating the following:
1- The core of the transformer is highly permeable.
2- The core does not exhibit any eddy-current or hysteresis loss.
3- All the flux is confined to circulate within the core.
4- The resistance of each winding is negligible.
A transformer turns ratio (a):
The total induced voltage in each winding is proportional to the number of turns in that winding
and the current is inversely proportional to both voltage and number of turns.
E1: primary voltage
I1: primary current
E2: secondary voltage
I2: secondary current
N1: primary turns
N2: secondary turns
a: turns ratio
Secondly//A Non-ideal Transformer
Practical transformers can be called non ideal transformer because not all of the magnetic flux
produced by the primary winding will link with the secondary winding can be losses induced within
transferring power from primary to secondary.
Winding Resistances
However small it may be, each winding has some resistance. Nonetheless, we can replace a non-ideal
transformer with an idealized transformer by including a lumped resistance equal to the winding
resistance of series with each winding. As shown in Figure 4, R1 and R2 are the winding resistances of
the primary and the secondary.
Figure 4: An ideal transformer with winding resistances
a = 𝑁1
𝑁2 =
𝑉1
𝑉2 =
𝐼2
𝐼1
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Leakage Fluxes
Not all of the flux created by a winding confines itself to the magnetic core on which the winding is
wound. Part of the flux, known as the leakage flux, does complete its path through air.
Figure 5: Transformer with leakage fluxes.
XI and X2 are the leakage reactances of the primary and secondary windings as shown in figure 6.
Figure 6: An ideal transformer with winding resistances and leakage fluxes
Finite Permeability
The core of a non-ideal transformer has finite permeability and core loss, known as the
excitation current from the source Iⱷ, is the sum of two currents: the core-loss current Ic and
the magnetizing current Im.
Iⱷ = Ic + Im
The core-loss component of the excitation current accounts for the magnetic loss (the
hysteresis loss and the eddy-current loss) in the core of a transformer, RC is the equivalent
core-loss resistance, Xm called magnetizing reactance can be represent the magnetizing
component of the excitation current.
Figure 7: Equivalent circuit of a transformer including winding resistances, leakage reactance,
core-loss resistance, magnetizing reactance, and an ideal transformer
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Exact equivalent circuit:
Figure 8The exact equivalent circuit as viewed from the primary side
Figure 9 The exact equivalent circuit as viewed from the secondary side
Approximate Equivalent Circuits:
Figure 10: Approximate equivalent circuit of a transformer as viewed from the primary side
Figure 11: Aapproximate equivalent circuit of a transformer as viewed from the secondary side
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Experimental Procedures:
You can find transformer turns ratio (a) by two methods: 1- Transformation voltage ratio
2- Transformation current ratio
Find the turn’s ratio between nodes 1, 2 primary side and nodes 3, 4 secondary side.
Part1 : Determine the turn’s ratio using voltage ratio
1- Connect the circuit as shown in figure below.
Figure 12 Determine the turn’s ratio using voltage ratio
2- Open data table window and record data step by 10% from 0% - 100%
Figure 13 Data table for voltage ratio
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3- Open graph window from data table to find slope of the line between E1 at Y-axis and E2
at X-axis.
Figure 14 Graph window for voltage ratio
4- Find slope of the line from figure above.
a = 1
2 =
1 12
3 2 = 0.55
Part2: Determine the turn’s ratio using current ratio
1- Connect the circuit as shown in figure below.
Figure 15 Determine the turn’s ratio using current ratio
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2- Open data table window and record data step by 10% from 0% - 100%
Figure 16 Data table for current ratio
3- Open graph window from data table to find slope of the line between I2 at Y-axis and I1
at X-axis.
Figure 17 Graph window for current ratio
4- Find slope of the line from figure above.
a = 2
1 =
1 1
2 1 = 0.55
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Exercise:
1- Repeat the above two methods to find transformer ratio (a) between:
a) Primary side 1,2 and secondary side 5,6
b) Primary side 7,8 and secondary side 5,6