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Applied Electricity

Outline Syllabus

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1. Transformers2. Induction motors3. DC motors4. Special purpose motors5. Solid state control6. Electrical lighting7. Heating and welding8. Electrical wiring9. Economics of power utilization

Learning Outcomes

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Calculating transformer & motor performance under variety of load conditions

Selection of electrical motors for a given applications

Basic knowledge in electricity utilization in the areas of lighting, heating and welding

Understanding wiring regulationsCarry out simple voltage drop calculations for

cablesEstimate monthly electricity bill for an installationMethods of minimizing the cost of electricity

Electrical Machines

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Applied Electricity – EE 2802

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Transformers

Contents

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1. Introduction2. Working principle3. Turns ratio4. Emf equation5. Equivalent circuit & phasor diagram6. Voltage regulation7. Losses & efficiency8. Cooling9. Tests on transformers10. Three phase transformers11. Transformer ratings & nameplate12. Other types of transformers

1) Introduction

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A transformer is a static device, that changes ac electric power at one voltage level to another voltage level of the same frequency through the action of a magnetic filed

Why transformers are important? Ideally voltage changes without affecting the

power supplied Losses (in power transmission lines) α I2

Electric power generated at one location Voltage stepped up (current reducing) Transmitted with very low losses Voltage stepped down for final use

2) Working Principle(Single-phase power transformers)

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2 windings Primary - connected to the alternating

voltage source Secondary - delivers the power to the load

Ideal Transformer

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A lossless device

3) Turns Ratio

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Stepped up and down transformer depends on the turns ratio

For an ideal transformer,

Magneto-motive force due to primary current

= Magneto-motive force due to secondary current

4) emf Equation

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Sinusoidal primary current produces a sinusoidal flux

emf induced in the primary winding

maximum value of e1

effective value of the primary emf

Similarly, the effective value of the secondary emf

5) Equivalent Circuit and Phasor Diagram

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Practical Transformers

Windings of practical transformers have resistance

Core is not infinitely permeable

Flux is not completely transferred to the secondary

Iron cores produce eddy-current and hysteresis losses

Practical Transformer

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Rm (core loss resistance) - represents iron losses Xm (magnetizing reactance) - measure of the

permeability of the transformer core

Equivalent Circuit & Phasor Diagram

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Referring Impedances

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Resistance in the primary becomes , when referred to the secondary

Resistance in the secondary becomes , when referred to the primary

Equivalent Circuit

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Referred to primary

Referred to secondary

Approximate Equivalent Circuit

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Referred to primary

Referred to secondary

Example 1

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A 100kVA single phase transformer has 400 turns in the primary and has 80 turns in the secondary. In primary side X1=1.1Ω and R1=0.3Ω. In secondary side, X2=0.035Ω and R2=0.01Ω. Supply voltage is 2200V. Determine the equivalent impedance referred to primary.

6) Voltage Regulation

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When the transformer is loaded -> secondary voltage changes due to drop in the internal impedance

Voltage regulation is a term that is used to identify this variation

Transformer taps- To permit small changes in the turns ratio- Possible to maintain the voltage up to ± x% of

the rated value

7) Losses & Efficiency

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Efficiency is a function of secondary current

For maximum efficiency;

Losses:- Copper loss- Core loss (eddy current loss +

hysteresis loss)

8) Cooling

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Indoor transformers below 200kVA Cooled by the natural flow of the air Placed inside a metallic housing which

is having ventilating louvers

Indoor larger transformers Built the same way Forced circulation of clean air is

provided

Distribution transformers below 200kVA Immersed in mineral oil and enclosed

in a steel tank Heat is dissipated by radiation and

convection to the outside air

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Large distribution transformers External radiators are added to

increase the cooling surface of the oil filled tank

Oil circulates around the transformer windings and moves through the radiator

Heat is released to the surrounding air

For still higher ratings Cooling fans blow air over the radiators

For transformers in the megawatt range Cooling may be effected by an oil-water

heat exchanger

9) Tests on Transformers

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Open Circuit Test

High voltage side kept open Wattmeter reading is the core loss

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Short Circuit Test

Low voltage side short circuited Wattmeter reading is the copper loss

Example 2

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A 2400V/240V, 24kVA transformer gave the following results. High voltage side open - 240V, 2A, 200W Low voltage side closed- 100V, 10A, 150W

The transformer is supplying an load of 0.8 power factor lagging at full load.

Find the equivalent circuit parameters. Calculate operating efficiency and maximum efficiency.

10)Three Phase Transformers

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Made either by:1. 3 single phase transformers connected in a 3–

phase bank 2. 3 windings are wound on a common core

1st approach – each unit coult be replaced individually

2nd approach - cheaper, smaller and lighter

Three-phase transformer connectionsDepending on the connections of primary and secondary coils;

Y-Y Y-Δ Δ-Y Δ-Δ

Y-Y connection

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Rarely used for large amount of power transmission

Can be used for special types of applications

Y-Δ connection

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Used at substations where the voltage is stepped down

Secondary coil current is 57.7% of the load current

Δ-Y connection

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Used at the generator station, where the voltage is stepped up

Δ-Δ connection

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Using 2 transformers

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Reduction in the power handling capacityEconomical

1.Open Δ ( V-V)

2.Open Y – Open Δ3. Scott – T 4. 3-phase T

Example 3

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Three identical single-phase transformers are needed to connect a 6 kVA, 120V, three-phase load to a 4800V, three-phase transmission line.

For a Y/Δ connection, determine the followings of each single-phase transformer.

I. Power ratingII. Voltage ratingIII. Current ratingIV. Turns ratio

11) Transformer ratings & nameplate

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4 major ratingsApparent powerVoltageCurrentFrequency

- To protect winding insulations from breakdown due to high voltages

- To prevent high heating loss- If operated on a different frequency, voltage

should also be changed

11) Other types of Transformers

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1) Auto Transformer For small changes in voltage small rating transformer with variable

output Used in educational laboratories Has one winding -> common for primary

and secondary

Advantages: Cheaper initial investment Low leakage reactance Low losses Requires low excitation current

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2) Instrument Transformers

Voltage Transformer (Potential Transformer) Ratio of primary to secondary voltage is known HV primary & LV secondary Low power rating Provide a sample of the power system’s

voltage to the instruments measuring it

Current Transformer Ratio of primary to secondary current is known Sample the current in a line & reduce it to a

safe and measurable level