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| Typical view of Transformer |
What is Transformer?
A transformer is a static or stationary piece of apparatus by means of which electric power in one circuit is transformed into electric power of the same frequency in another circuit. It can raise or lower the voltage in a circuit but with a corresponding decrease or increase in current.
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| Transformer |
Working Principle of a Transformer:
The physical basis of transformer is MUTAL INDUCTION between two circuit linked by a common magnetic flux. In simple form, it consist of two inductive electrical coil which are electrically separated but magnetically linked through a path of low reluctance. Transformer construction is show in the figure.
The two coils possess high mutual inductance. If one coil is connected to a source of alternating voltage, an alternating flux is set up in the laminated core, most of which is linked with the other coil in which it produce mutually induced e.m.f
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| Transformer construction |
i.e According to faradays law of electromagnetic induction e= MdI/dt.
If the second coil circuit is closed, a current flows in it and so electric energy is transferred from the first coil to second coil. The first coil, in which the electric energy is fed from the A.C supply mains, is called primary winding and the other from which energy is drawn out is called secondary winding.
In brief, a transformer is a device that
1. Transfer electric power from one circuit to another circuit.
2. It does so without a change of frequency.
3. It accomplishes this by electromagnetic induction and
4. Where the two electric circuits are in mutual inductive influence of each other.
Transformer Construction:
The simple elements of transformer consists of two coils having mutual inductance and laminated iron core. The two coils are insulated from each other and the steel core.
Other necessary parts are, some suitable container for assembled core and winding's, suitable medium for insulating the core and its winding's from its container, suitable bushing for insulating and bringing out the terminal of winding's from the tank. In all type of transformers, the core is constructed with sheet steel lamination assembled to provide a continuous magnetic path with minimum of air gap included. The steel used is have high silicon content, sometimes heat treated to produce a high permeability and low hysteresis loss at the usual operating flux densities. The eddy current loss is minimized by laminating the core, the lamination being insulated from each other by a light of coat of core plate varnish or by oxide layer on the surface. The thickness of lamination's varies from 0.35mm for frequency of 50Hz 0.5mm for frequency of 25Hz. The core lamination.
Types of Transformer:
According to Construction the transformer are of two general types
1. Core type and
2. Shell type
Another recent development is spiral
core or wound core type, also called as spirakore transformer.
Another means of classifying the transformers is according to the type of cooling employed. The following types are in common use
1. oil-filled self-cooled.
2. oil-filled water-cooled and
3. air-blast type
Average rate of change of flux= Øm /(1/4f)
N2- number of turns in secondary
Øm- maximum flux in core in webers
f- Frequency of input supply voltage in Hz
Losses in a Transformer:
In a static transformer there are no friction and
windage losses. Hence the only losses occurring are
1.core or iron-loss
It includes both hysteresis loss and eddy current loss. Because core flux in the transformer remains practically constant for all loads. The core loss is practically the same at all loads.
These losses are minimized by using steel of high silicon content for the core and by using very thin lamination's. Iron or core loss is found from the O.C test. The input of the transformer when on No-load measure the core losses.
2. Copper-loss
This loss is due to the oh-mic resistance of the transformer wingdings. Total copper loss= I12R01 + I2 2 R02= I12
R1 + I2 2 R2. It is clear that cu.loss is proportional to I2 or KVA2. In other words cu-loss at half the full-load is one-fourth of that at full-load.
Why Transformer Ratings in KVA?
As seen, cu-loss of a transformer depends on current and iron loss on voltage. Hence, total transformer loss depends on Volt-ampere (VA) and not on phase angle between voltage and current i.e dependent of load power factor. That is why rating of transformers is in KVA and not in KW.
Efficiency of a Transformer:
As is the case with other types of electrical machines, efficiency of a transformer at a particular load and power factor is
defines as the output divided by the input – the two being measured in the same units
Efficiency= output/input
But transformer being highly efficient piece of equipment, has very small loss, hence it is impractical to try to measure
transformer, efficiency by measuring output to input These quantity are nearly of the same size. A better method is to determine the losses and then to calculate efficiency form
Efficiency= output/(output+losses)
=output/(output+cu.loss+core loss)
Otherwise efficiency = input-losses/input
=1-losses/input
Efficiency can be computed by determining core loss from no-load or open-circuit test and cu.loss from the short-circuit test.
Condition for Maximum Efficiency:
Cu.loss= I12 R01 or I2
2 R02
Iron loss= Hysteresis loss+eddy current loss=Wh+We=Wi
Copper loss=core loss
It is this value of the output current which will make the cu.loss equal to core loss. By proper design, it is possible to make the maximum efficiency occur at any desired load.
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