Electricity & Concept of Transformers
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Transcript of Electricity & Concept of Transformers
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Applied Electricity
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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
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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
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Electrical Machines
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Applied Electricity – EE 2802
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Transformers
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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
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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
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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
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Ideal Transformer
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A lossless device
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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
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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
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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
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Practical Transformer
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Rm (core loss resistance) - represents iron losses Xm (magnetizing reactance) - measure of the
permeability of the transformer core
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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
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Equivalent Circuit
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Referred to primary
Referred to secondary
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Approximate Equivalent Circuit
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Referred to primary
Referred to secondary
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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.
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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
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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)
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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
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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
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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.
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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
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Y-Y connection
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Rarely used for large amount of power transmission
Can be used for special types of applications
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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
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Δ-Y connection
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Used at the generator station, where the voltage is stepped up
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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
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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
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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
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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