Podcast
Questions and Answers
What is the primary purpose of a transformer in electrical systems?
What is the primary purpose of a transformer in electrical systems?
Which of the following factors is crucial in determining the efficiency of a transformer?
Which of the following factors is crucial in determining the efficiency of a transformer?
What does the turns ratio in a transformer indicate?
What does the turns ratio in a transformer indicate?
What characterizes auto transformers compared to traditional transformers?
What characterizes auto transformers compared to traditional transformers?
Signup and view all the answers
Which loss is typically associated with transformer operation?
Which loss is typically associated with transformer operation?
Signup and view all the answers
What is the primary purpose of a transformer?
What is the primary purpose of a transformer?
Signup and view all the answers
What happens to a transformer when it operates under load conditions?
What happens to a transformer when it operates under load conditions?
Signup and view all the answers
Which of the following is a factor affecting transformer efficiency?
Which of the following is a factor affecting transformer efficiency?
Signup and view all the answers
How can transformer losses be reduced?
How can transformer losses be reduced?
Signup and view all the answers
What is the relationship between primary and secondary voltage in a transformer with a turns ratio of 1:2?
What is the relationship between primary and secondary voltage in a transformer with a turns ratio of 1:2?
Signup and view all the answers
What component of a transformer receives energy from the AC source?
What component of a transformer receives energy from the AC source?
Signup and view all the answers
Which type of core material is typically used for low frequency transformers?
Which type of core material is typically used for low frequency transformers?
Signup and view all the answers
What is the role of the core in a transformer?
What is the role of the core in a transformer?
Signup and view all the answers
At what frequency is an air-core transformer typically used?
At what frequency is an air-core transformer typically used?
Signup and view all the answers
What happens to the EMFs in auto transformers when calculated from the primary and secondary windings?
What happens to the EMFs in auto transformers when calculated from the primary and secondary windings?
Signup and view all the answers
Which of the following factors does NOT affect the composition of the transformer's core?
Which of the following factors does NOT affect the composition of the transformer's core?
Signup and view all the answers
If 200 volts is applied between points B and C in an auto transformer, what is the secondary voltage available from points A and C?
If 200 volts is applied between points B and C in an auto transformer, what is the secondary voltage available from points A and C?
Signup and view all the answers
Which type of transformer core is best suited for high frequency applications?
Which type of transformer core is best suited for high frequency applications?
Signup and view all the answers
Study Notes
Module 3: Electrical Fundamentals II
- Topic: Transformers
Introduction
- Students should be able to describe transformer construction and operation principles.
- Students should be able to describe transformer losses and methods of overcoming them.
- Students should be able to describe transformer action under load and no-load conditions.
- Students should be able to describe power transfer, efficiency, and polarity markings regarding transformers.
- Students should be able to calculate line and phase voltages and currents.
- Students should be able to calculate power in a three-phase system.
- Students should describe primary and secondary current, voltage, turns ratio, and power.
- Students should describe transformer efficiency.
- Students should describe the construction and operation of autotransformers.
Transformer
- A device composed of two or more coils wound onto a core
- Coils linked by magnetic lines of force to transfer energy
- Available in many shapes and sizes
- Large, high-voltage, and high-current transformers for power distribution in cities
- Miniature transformers for electronic equipment
- Input voltage: 120 V AC
- Output voltage: 17 V AC
Transformer Construction
- Consists of a primary coil (receives energy from AC source), a secondary coil (receives energy from primary and delivers to load), and a core/former to support coils and provide flux path.
Components of a Transformer
- Two coils (windings) wound around a core
- Core material can be air (air-core transformer) or iron (iron-core transformer; more common for higher power)
- Low-frequency transformers often require a low-reluctance core (usually iron)
- Most power transformers are iron-core transformers
Core Characteristics
- Core composition depends on factors such as voltage, current, and frequency.
- Common core materials: air, soft iron, steel
- Air-core transformers are best used for high-frequency voltage sources (frequencies over 20 kHz)
- Iron-core transformers are preferred for low frequencies (below 20 kHz) because they offer better power transfer
- Steel-core transformers use laminated sheets of steel to dissipate heat more readily and efficiently
- Low frequencies need iron/steel cores, providing maximum coupling between primary and secondary.
- Laminated iron cores are sufficient for frequencies up to 20 kHz.
- Eddy currents and power loss become excessive at higher (radio) frequencies, this being overcome by ferrite or air cores.
- Ferrites (ferromagnetic compounds containing iron as principal metallic component) provide negligible losses at frequencies up to 100 MHz.
- Air cores can be used for higher frequencies.
Symbols for Transformers
- Standard symbols used for primary (input) and secondary (output) windings of an iron-core transformer
- Visual representation of winding connections
Laminated Core
- Steel laminations are insulated with varnish and are used in construction of a core
- Takes around 50 laminations to create a one-inch thick core
- Most efficient core allows for maximum flux lines with minimal loss in magnetic and electrical energy.
Transformer Types
- Two main types of cores:
- Shell-core: Most popular type; efficient
- Hollow-core: Core shaped with a hollow centre
- Each layer of the core consists of 'E's and 'I's shaped metal sections that are butted together and insulated
- Laminations pressed together into a core
Transformer Windings
- Transformer with two coils wrapped around a common core
- AC applied to one winding while the load is connected to the other
- Input coil labeled primary
- Output coil labeled secondary
- Windings wound directly on a cardboard form or with insulating material between layers
- Primary is wound, then wrapped in paper or cloth and then secondary is wound on top, covered with insulating paper
- 'E' and 'I' sections of the iron core inserted into and around windings
- Leads connected to AC source and load
Transformer Tappings
- Additional connections to transformer windings, not at the winding ends.
- Tap is located at the centre of the winding.
Transformer Operation
- Depends on impedance of inductors and magnetic coupling between primary and secondary windings
- Magnetic coupling determined by core type and relative position of windings
- Must be used with an AC input voltage
- AC allows voltage and current levels to be increased or decreased by a transformer.
No-Load Condition
- AC applied to the primary only
- No load connected to the secondary winding
- Minimal current flows in secondary winding
- Exciting current flows through primary
No-Load Condition (Factors Determining Exciting Current)
- Voltage applied
- Resistance of primary coil.
- Core losses.
- Inductive reactance.
No-Load Condition (Functions of Exciting Current)
- Used to maintain the magnetic field in the primary.
- Used to overcome the resistance of the wire and core losses, which are dissipated in the form of heat.
No-Load Condition
- When the secondary is unloaded, the primary winding draws very little current, even though it is connected to its source.
- This is because the winding is highly inductive and has a high inductive reactance.
- Primary-winding AC induces a back EMF (counter EMF) in each winding turn.
Producing a Counter EMF
- When applying AC to the primary, a magnetic field is created around the winding.
- The expanding and contracting field induces back EMF on the winding.
- Flux of the applied voltage leaves at the primary's north pole and enters at south pole.
- The back EMF induced in primary has the opposite polarity from the applied voltage.
- Thus, back EMF resists the flow of current in the primary.
- Back EMF limits primary exciting current
Inducing a Voltage in the Secondary
- As the primary current increases, magnetic lines of force outward from the primary wind and cut the secondary.
- Voltage is induced in the coil when magnetic lines cut across it.
- Primary voltage induces secondary voltage.
Primary and Secondary Phase Relationship
- Secondary voltage can be either in phase ('in phase') or out of phase ('out of phase') with the primary voltage, depending on the winding direction and circuit connections.
- In like-wound transformers, secondary voltage is in phase with the primary voltage.
- In unlike-wound transformers, voltages are 180° out of phase.
- Dots are used to indicate points with the same instantaneous polarity.
Coefficient of Coupling
- Depends on the portion of the total flux that cuts both primary and secondary windings.
- Ideally, all primary flux lines would cut the secondary, and all secondary flux lines would cut the primary
- This would result in unity coupling, transferring maximum energy from primary to secondary.
- Leakage flux are lines of flux that do not link with the other winding.
Coefficient of Coupling (Leakage Flux)
- Leakage flux from the primary cannot induce a secondary voltage.
- Voltage induced is lower than it would be if leakage flux did not exist.
- This effect can be duplicated by assuming an inductor is connected in series with the primary winding.
Turns and Voltage Ratios
- Voltage input into secondary is related to the ratio of turns in primary to turns in secondary.
- For a 10-to-one turns ratio, the secondary voltage will be one-tenth of the primary voltage.
- The voltage per turn is the same for both windings.
Turns and Voltage Ratios
- EMF induced into the secondary is the same as that induced in each primary turn.
- If 10 V is applied to a primary, the back EMF in the primary is practically 10 V.
- The induced back EMF in each primary turn is approximately one-tenth of the applied voltage.
- Secondary and primary windings both experience the same flux; each turn will have an EMF and one volt.
Turns and Voltage Ratios (Ratio Equations)
- Secondary voltage to primary voltage is equal to the ratio of secondary turns to primary turns
- Formula is: Es/Ep = Ns/Np
- Using the formula, with known values, you can compute an unknown voltage.
Turns Ratio
- Ratio of voltages/turns is equal to the voltage ratio (e.g. 200 V/10 V = 20:1 = 20 turns for primary/ 1 turn for secondary)
- Fewer turns in secondary than in primary = step-down transformer
- Fewer turns in primary than in secondary = step-up transformer
Voltage Ratio
- Ratio of input primary voltage to secondary voltage, e.g., 30 V / 90 V = 1/3
- Voltage induced in secondary is proportional to the turns ratio.
Effect of a Load
- When the load is connected to the secondary, current flows through the secondary and the load.
- Secondary magnetic field affects the primary magnetic field.
- This interaction results from mutual inductance between primary and secondary windings.
- The same flux links both windings, called mutual flux.
Effect of a Load
- Current flow in secondary creates a flux field in opposition to the primary flux field (Lenz's law).
- Secondary flux cancels some of the primary flux, reducing the back EMF.
- Reduced back EMF increases primary current, generating more flux, which reestablishes the total flux lines.
Turns and Current Ratios
- Number of flux lines in a transformer core is proportional to the magnetizing force of primary and secondary windings.
- Ampere-turn (I × N)measures magnetomotive force (mmf)
- The mmf developed by one amp of current flowing through a one-turn coil is an ampere-turn.
- Flux in the transformer core surrounds the primary and secondary windings; therefore, the ampere-turn values for both windings are the same.
- Formula: Ip(Np) = Is(Ns)
Transformer Efficiency
- Input power to transformer, and output power should be known.
- Input power equals the product of primary voltage and current.
- Output power equals the product of secondary voltage and current.
- Power loss is the difference between input and output.
- Formula to determine efficiency: Efficiency (in %)= Pout/Pin × 100
Transformer Efficiency Calculation
- Efficiency = Output power/Input power × 100 (e.g. 610 / 650 watts = 93.8%)
- Power loss is input power minus output power (e.g., 650 - 610 = 40 watts)
Transformer Efficiency Examples
- Determine the efficiency of a transformer with 650 W input and 610W output (i.e., 93.8%)
- Calculate the power loss of the transformer (i.e., 40W)
Transformer Losses
- Transformers cannot be perfectly efficient, leading to some input-output loss.
- Three losses include copper loss, hysteresis loss, and eddy-current loss.
- Copper losses (I2R losses) are due to the resistance of the windings.
- Minimizing copper loss is accomplished by increasing the diameter of the wire.
- Hysteresis losses are due to the energy used to reverse the magnetism of the core material
- Hysteresis losses are minimized by using a core material with a smaller magnetic area & proper construction.
- Eddy-current losses are due to currents induced in the core by the changing magnetic field.
- Eddy-current losses are minimized by laminating the core (thin, insulated sheets).
Transformer Calculations
- Calculate values for current (primary/secondary), voltage, apparent power for primary/secondary, given values and a transformer diagram.
Autotransformers
- A single coil serves as both the primary and secondary windings.
- Tapped coil allows selection of output voltage.
- Movable tap is in the secondary to select voltages.
- Values of voltage are dependent on the tap location.
Studying That Suits You
Use AI to generate personalized quizzes and flashcards to suit your learning preferences.
Related Documents
Description
This quiz covers the essential concepts of transformers, including their construction, operation principles, and efficiency. Students will learn about transformer losses, calculations for voltages and currents in three-phase systems, and the operation of autotransformers. Prepare to test your knowledge on transformer action under load and no-load conditions!
