Transformer – Quantmasters

One of the most important and ubiquitous electrical machines is the transformer.
It is a static electrical machine having no movable parts.
It receives power at one voltage and delivers it at another.
In the process of Power Transmission from one end to another the voltage and current levels may or may not change but the supply frequency remains same.
This conversion aids the efficient long distance transmission of electrical power from generating stations.
Transformer has 2 different coils one is primary coil and the other one is secondary coil.
Source is connected to primary and Load is connected to secondary.
If primary voltage is higher then secondary, it is known as step down type transformer.
If secondary voltage is higher then primary, it is known as step up transformer.
If both the voltages are same, it is known as isolation transformer.
Insulation transformer is used when primary side has to be isolated from secondary.
In its simplest form the transformer consists of the insulated windings wound on a ferromagnetic core and so disposed with respect to each other as shown in a figure that a current through one of the windings will set up a magnetic flux linking more or less completely with the turns of the other.

According to Faraday laws of electromagnetic induction an emf is induced in the secondary which is proportional to the number of turns of the secondary and the flux linking the secondary.
However, the frequency of the secondary voltage is same as that of the source.
To ensure effective magnetic linkage wo windings, the core which serves to support them mechanically as well as to carry their mutual magnetic flux is usually made of a highly permeable iron or steel alloy designed to have a low reluctance.
In some specially designed transformers the core may be of nonmagnetic materials.
Such transformers are known as air cored transformers.
These transformers are normally used in radio devices and in certain types of measuring and testing instruments.

Construction & Principle:

A steel core C consists of laminated sheets, about 0.35 to 0.7 mm thick, insulated from one another.
The purpose of laminating the core is to reduce the eddy current loss.
The vertical portions of the core are referred to as limbs and the top and bottom portions are the yokes.
Coils P and S are wound on the limbs.
Coil P is connected to the supply and therefore termed the primary; coil S is connected to the load and is termed the secondary.

An alternating voltage applied to P circulates an alternating current through P and this current produces an alternating flux in the steel core, the mean path of this flux being represented by the dotted line D.
If the whole of the flux produced by P passes through S, the e.m.f. induced in each turn is the same for P and S.
Hence, if N1 and N2 are the number of turns on P and S respectively
When the secondary is on open circuit, its terminal voltage is the same as the induced e.m.f.
The primary current is then very small, so that the applied voltage V1 is practically equal and opposite to the e.m.f. induced in P.
Since the full load efficiency of a transformer is nearly 100 per cent,
But the primary and secondary power factors at full load are nearly equal.
Based on construction, transformers are of two types: (a) Core type (b) Shell type
The distinction between the two types can be summed up by the fact that in the core type the windings encircle the core whereas in shell type the core encircles the windings.
Out of the two the core type is normally preferred because of additional advantages of permitting visual inspection of the coils in case of fault and of greater ease in making repairs on the site of installation.
It is to be noted that both the windings (primary and secondary or LT and HT) are wound on both the limbs of the core type as well as shell type.
A portion of the LT winding in the form of a coil is placed close to the core from electrical stresses point of view and then over this we have HT coil and so on.
Both the windings will be on both the limbs of the core type construction.

EMF Equation:

Hence, r.m.s. value of e.m.f. induced in primary and secondary are:

The emf induced in each coil depends on the maximum flux produced in the core, the number of turns in each winding and the frequency of supply.

No load Current of Transformer:

The no-load current, I0, taken by the primary consists of two components:
1. A reactive or magnetizing component, I0m, producing the flux and therefore in phase with the flux.
2. An active or power component, I0l, supplying the hysteresis and eddy current losses in the core and the negligible IsquareR loss in the primary winding.
Component I0l is in phase with the applied voltage, i.e. I0l V1 which is core loss.
This component is usually very small compared with I0m, so that the no load power factor is very low

Leakage flux is responsible for the inductive reactance of a transformer.
This flux appears when a load is connected across the secondary coil.
The leakage flux is proportional to the primary and secondary currents and that its effect is to induce e.m.f.s of self-induction in the windings.
The leakage flux can be practically eliminated by winding the primary and secondary, one over the other, uniformly around a laminated ferromagnetic
ring of uniform cross section.
But such an arrangement is not commercially practicable except in very small sizes, owing to the cost of threading a large number of turns through the ring.
The principal methods used in practice are:

Making the transformer window long and narrow.
Arranging the primary and secondary windings concentrically
Sandwiching the primary and secondary windings
Using shell-type construction

Equivalent circuit of a transformer

The behavior of a transformer may be conveniently considered by assuming it to be equivalent to an ideal transformer,
It means a transformer having
- No losses
- no magnetic leakage
- a ferromagnetic core of infinite permeability requiring no magnetizing current
Then allowing for the imperfections of the actual transformer by means of additional circuits or impedances inserted between the supply and the primary winding and between the secondary and the load.

KVA Rating of Transformer:

Manufacturers design transformers based on the voltage and current required for the transformer operation.
They then specify this on the transformer nameplate in terms of VA (Volt Amps).
This is referred to as the rating of a transformer.
All electrical devices are rated based on the maximum power it can consume or generate or transfer.
When few of them are rated in KW (kilowatts) or watts, few others are rated in kVA (kilo volt ampere) or VA (volt ampere).
Transformers are rated in kVA or VA and not in kilowatts.
Why is a transformer rated in KVA but not in KW?
KVA stands for kilo volt ampere, which basically is the unit of electric power.
While calculating kVA of any piece of equipment the power factor is not taken into account.
Power in kVA = Voltage x Current.
This means kVA is the unit of measurement for that equipment in which the output power is independent of power factor.
For example Rating of Alternators, Transformers, and UPS etc.
As the transformer runs on very high efficiency, its losses can be neglected and hence rated input in kVA at the primary = rated output in kVA at secondary

Losses in Transformer:

Since distribution transformers has no rotating parts, it has no mechanical losses.
These losses appear in the form of heat and produce an increase in temperature and a drop in efficiency.
Losses can be classified into two categories: copper losses and core losses.
Core Loss:
Core losses are otherwise known as Iron Loss.
These Losses occur in Core of transformer.
Theses are of two different types: (a) Eddy Current Losses (b) Hysteresis Loss
Eddy Current Losses:
– Eddy currents are caused by the alternating current inducing a current in the core of the transformer.
– Eddy current losses within a transformer core cannot be eliminated completely, but they can be greatly reduced and controlled by reducing the thickness of the steel core.
– Instead of having one big solid iron core as the magnetic core material of the transformer or coil, the magnetic path is split up into many thin pressed steel shapes called laminations.
– The laminations used in a transformer construction are very thin strips of insulated metal joined together to produce a solid but laminated core.
– These laminations are insulated from each other by a coat of varnish or paper to increase the effective resistivity of the core thereby increasing the overall resistance to limit the flow of the eddy currents.
– The eddy current losses are kept to a minimum by using laminated cores.
– Eddy currents increase with frequency; they are directly proportional to the square of the AC frequency.
Hysteresis Loss:
– occurs in all ferromagnetic transformer cores, but especially in laminated iron.
– Hysteresis is the tendency for a core material to act sluggish in accepting a fluctuating magnetic field.
– Air cores essentially never exhibit this type of loss.
– In fact, air has lowest overall loss of any known transformer core material.

Copper Loss:

Transformer Copper Losses are mainly due to the electrical resistance of the primary and secondary windings.
Most transformer coils are made from copper wire which has resistance.
This resistance opposes the magnetizing currents flowing through them.
When a load is connected to the transformers secondary winding, large electrical currents flow in both the primary and the secondary windings, electrical energy and power ( or the I square R ) losses occur as heat.
Copper losses vary with the load current & it is also proportional to KVA rating of Transformer.
Copper loss is almost zero at no load, and maximum at fullload when current flow is at maximum.

Efficiency of Transformer:

Efficiency of a transformer can be defined as the output power divided by the input power.
Efficiency = output / input.

For a load at a given power factor, the output of transformer is given by:
= Secondary Voltage x Secondary current x Cosine of the angle between voltage and current

At any load other than full load output of the transformer will vary, Copper losses will vary.
The iron loss will remain constant.
For a load factor x (secondary current at any load/ full load secondary current)

Condition for Maximum Efficiency:

For maximum efficiency, the derivative of efficiency with respect to primary or secondary current must be equal to zero.
When Copper loss = core loss, the efficiency will be maximum as per the condition.

Voltage Regulation:

The voltage regulation is the percentage of voltage difference between no load and full load voltages of a transformer with respect to its full load voltage.
Expression of Voltage Regulation of Transformer, represented in percentage, is: