<Dr Ahmed El-Shenawy>

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Electric Engineering II EE 326

Lecture 4 & 5

<Dr Ahmed El-Shenawy>

Electric Engineering II “Dr. Ahmed El-Shenawy”

Transformers

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٢ Transformers

Electrical transformers have many applications:

• Step up voltages (for electrical energy transmission with minimized losses)

• Step down voltages to suite load requirements

• Provide electrical isolation between different grids

• Provide impedance matching for maximum power transfer (XL= XC)

• Provide reduced AC voltages and currents for protection and metering purposes

1. Magnetic sheet steel to reduce eddy currents 2. Primary windings (receive power from source) 3. Secondary windings (delivers power to load)

Transformers Construction

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٣ Transformers Construction

The typical transformer has two windings insulated electrically from each other.

These windings are wound on a common magnetic core made of laminated sheet steel. The principal parts of a transformer and their functions are given in table.

Types of transformer construction: A. Core type; B. Shell type

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There are two main shapes of cores used in laminated-steel-core transformers.

One is the CORE Type, so named because the core is shaped with a hollow square through the center. Figure illustrates this shape of core. Notice that the core is made up of many laminations of steel and transformer windings are wrapped around both sides of the core.

Core type transformer

Shell type transformer

The most popular and efficient transformer core is the SHELL. As shown, each layer of the core consists of E- and I-shaped sections of metal. These sections are butted together to form the laminations. The laminations are insulated from each other and then pressed together to form the core.

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٥ Types of Transformer Applications

There are many different types of transformers.

Transformers are a type of passive electronic component that are used to step up or step down the voltage in a circuit or system.

The voltage transformer

The power transformer

The current transformer

The auto transformer

A TRANSFORMER is a device that transfers electrical energy from one circuit to another by electromagnetic induction (transformer action).

The electrical energy is always transferred without a change in frequency, but may involve changes in magnitudes of voltage and current. Because a transformer works on the principle of electromagnetic induction, it must be used with an input source voltage that varies in amplitude. There are many types of power that fit this description; for ease of explanation and understanding, transformer action will be explained using an ac voltage as the input source.

Theory of Operation

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٦ Faraday’s law of electromagnetic induction,

The magnetic field produced by the first coil will induce a voltage in the second coil. This is transformer action. To ensure that the coils are closely coupled the coils should be wound on an iron core, which will provide a path for the mutual flux linking the coils(Q: Why will this be better than air?), to further improve the coupling the second coil should be wound on top of the first coil. For now the coupling between the coils will be assumed to be ideal.

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The current that produces the mutual flux will be a sine wave that lags the voltage by 90 degrees the flux will be in phase with the current. In the case of theideal transformerthe reluctance of the core will be zero(μ_r= ∞)and the current required to produce the mutual flux will be zero.

Ideal Transformers

•Nocore or copper losses

•Noleakage fluxes

•Infinite core permeability

•Same m.m.f for both sides

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With reference to the figure and by applying Faraday’s and Lenz’s laws, we can say:

Theory of Operation

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٩ The Real Transformer

In practice the transformer windings will have resistance they will not be perfectly coupled, the transformer core will not have zero reluctance and the alternating flux in the core will result in core losses. In a real transformer these all need to be included in the analysis. To take these factors into account the equivalent circuit of the transformer will be of the form shown in figure

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Winding Resistance (R1and R2)

Both the primary and the secondary winding will have resistance. These are represented by R₁andR₂in the equivalent circuit. There will be a voltage drop and a power loss associated with these resistances. The power loss is often referred to as the copper loss of the transformer.

Leakage Inductance (l1and l2)

As the windings are not perfectly coupled some of the flux in the primary will not link the secondary and some of the flux in the secondary will not link the primary. This can be taken into account by introducing the primary and secondary leakage inductance’sl₁andl₂. These represent a voltage loss (loss of flux) but not a power loss.

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Magnetising Inductance (Lm)

As the transformer core will have a finite value of reactance (μ_r≠ ∞) then the self inductance of the windings will be finite. This will result in a magnetising current flowing inL_mto produce the mutual flux.

Magnetising Resistance (Rm)

The process of producing an alternating flux in the transformer core produces loses in the core. These losses are a result of the hysteresis of the core material and the production of eddy currents in the core. The combined loss is called the iron loss of the transformer (Q: What form will the iron loss take?). This is represented in the equivalent circuit by a resistanceR_m.

Hysteresis: Figure shows the form of the hysteresis loop for a transformer core. When the transformer winding is connected to an alternating supply the flux in the core will alternate at the same frequency as the supply. For each cycle of the supply the flux density in the core will traverse the hysteresis loop. The area enclosed by this loop is proportional to the loss associated with hysteresis. Transformers use core materials that minimise the size of this loop(Q: What are B and H?).

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Eddy Currents: The alternating flux in the transformer core can produce currents in the core material through transformer action. These unwanted currents are referred to as eddy currents and they produce losses in the core. To minimize eddy currents the transformer core is laminated. Each lamination will be less than 0.5 mm thick and a layer of insulating material separates each lamination.

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Referred Equivalent Circuit

To make performing calculations easier it is usual to refer the equivalent circuit parameters to either the primary or the secondary. In the next figure the equivalent circuit is referred to the primary winding using the following relations that can be verified by considering the relation between the primary and secondary voltage and current.

The equivalent circuit can be further simplified by moving the magnetising branch to the primary input terminals. This will introduce some error in the representation but, as the magnetising current is small compared to the load current, the error will not be large

Parameter Measurement

It is possible to obtain the approximate equivalent circuit parameters of a transformer by conducting two tests, an open-circuit test and a short-circuit test. As the names of these tests imply they are performed with the secondary of the transformer connected either in open-circuit or short-circuit.

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Regulation

From the above exercise it can be seen that the output voltage of the transformer when loaded is not the same as for the no-load condition. Thevoltage regulationis defined as:

This is often quoted as a percentage evaluated at the full load condition. Loads with a lagging or unity power factor will have a positive regulation (output voltage reducing), for loads with a leading power factor the regulation may be negative. For high power transformers the regulation can be better than 1%, smaller transformers will have a higher value which could be greater than 40%. In some instances the leakage reactance of the windings is designed to be high, resulting in poor regulation, to prevent excessive current if the transformer is accidentally short- circuited.

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<Dr Ahmed El-Shenawy>

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Electric Engineering II EE 326

Lecture 4 & 5

<Dr Ahmed El-Shenawy>

Transformers

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Transformers

Transformers Construction

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Transformers Construction

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Core type transformer

Shell type transformer

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Types of Transformer Applications

Theory of Operation

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Faraday’s law of electromagnetic induction,

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Ideal Transformers

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Theory of Operation

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The Real Transformer

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