Lecture 3: Transformers I
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Questions and Answers

What is the primary reason for using transformers in electric power systems?

  • To change AC voltage levels without affecting the power. (correct)
  • To improve the efficiency of the electrical appliances.
  • To convert AC power to DC power.
  • To increase the resistance in the circuit.

    • Which of the following statements is true regarding the windings in a transformer?

  • The primary winding is always at a lower voltage than the secondary winding.
  • The low voltage side always has more turns than the high voltage side.
  • The windings must always be made of copper.
  • The high voltage side can function as either primary or secondary. (correct)

    • What happens to the current when a transformer steps up the voltage?

  • The current decreases to keep power constant. (correct)
  • The current increases to maintain power.
  • The current remains unchanged.
  • The current fluctuates based on power demand.

    • In a step-down transformer, which side acts as the primary winding?

    <p>The side with higher current. (D)</p> Signup and view all the answers

    How are transformers typically constructed?

    <p>With windings wrapped around a rectangular or three-legged laminated core. (A)</p> Signup and view all the answers

    What characterizes a high voltage side in a transformer?

    <p>It has more turns than the low voltage side. (B)</p> Signup and view all the answers

    In the context of transformers, a core form construction has windings wrapped around how many sides?

    <p>Two sides. (A)</p> Signup and view all the answers

    What defines a shell form transformer?

    <p>It features windings around only the central leg of the core. (D)</p> Signup and view all the answers

    What happens to the induced emfs E1 and E2 when a load is connected to the secondary terminals of a transformer?

    <p>They both decrease. (D)</p> Signup and view all the answers

    How is the total primary current (I1) related to the load current (I2') and the magnetizing current (Io)?

    <p>I1 is the sum of Io and I2'. (A)</p> Signup and view all the answers

    What must occur when the difference between V1 and E1 appears in a transformer?

    <p>The primary current must increase. (B)</p> Signup and view all the answers

    In the equation N1I2' = N2I2, what do N1 and N2 represent?

    <p>The number of turns in the primary coil. (D)</p> Signup and view all the answers

    Which of the following statements is true regarding the magnetizing flux when load current (I2) flows in the transformer?

    <p>The magnetizing flux will decrease. (D)</p> Signup and view all the answers

    What is one of the assumptions of an ideal transformer regarding the core?

    <p>The core has no losses. (B)</p> Signup and view all the answers

    How does the current on one side of an ideal transformer relate to the turns ratio?

    <p>It is inversely proportional to the turns ratio. (B)</p> Signup and view all the answers

    What does impedance transformation in a transformer allow?

    <p>Impedance can be transferred from one side to another. (A)</p> Signup and view all the answers

    According to Faraday’s law, what remains constant in an ideal transformer?

    <p>The voltage per turn. (A)</p> Signup and view all the answers

    Which of the following is NOT an assumption of an ideal transformer?

    <p>The windings have energy losses. (C)</p> Signup and view all the answers

    In a single-phase power system with a load ZL = 4+j3, what represents the total load impedance seen by the generator?

    <p>It combines load and transmission line impedance. (D)</p> Signup and view all the answers

    What aspect of an ideal transformer minimizes leakage flux?

    <p>The problem of insulating the high-voltage winding from the core. (D)</p> Signup and view all the answers

    What is the frequency of the generator supplying the load in the example provided?

    <p>60 Hz (A)</p> Signup and view all the answers

    What is the primary function of a transformer?

    <p>To change AC electric power from one voltage level to another (B)</p> Signup and view all the answers

    What components make up a transformer?

    <p>Two or more coils of wire wrapped around a magnetic core (C)</p> Signup and view all the answers

    What issue does the transformer address in electric power systems?

    <p>Handling large currents at low voltages over long distances (D)</p> Signup and view all the answers

    What is the typical voltage rating range for transformers?

    <p>From few VA to hundreds MVA (D)</p> Signup and view all the answers

    What role did transformers play in the development of electric power systems?

    <p>They enabled the creation of long-distance power transmission lines (B)</p> Signup and view all the answers

    Which of the following statements about transformers is false?

    <p>Transformers are directly connected to their coils. (D)</p> Signup and view all the answers

    What historical development necessitated the invention of transformers?

    <p>The need for efficient transportation of low-voltage power (A)</p> Signup and view all the answers

    In what way do transformers improve the efficiency of power systems?

    <p>By minimizing voltage drops and power losses during transmission (A)</p> Signup and view all the answers

    What is the current that flows in the primary circuit of a transformer when the secondary circuit is open?

    <p>No load current (D)</p> Signup and view all the answers

    Which component of the no load current is responsible for overcoming the reluctance of the core?

    <p>Magnetizing current (B)</p> Signup and view all the answers

    What is a characteristic of practical transformers that differs from ideal transformers?

    <p>Non-zero reluctance (C)</p> Signup and view all the answers

    In practical transformers, what type of losses occur within the core?

    <p>Core losses (B)</p> Signup and view all the answers

    How is the magnetizing current expressed in relation to the magnetizing flux in practical transformers?

    <p>$N_1 I_m = \phi R$ (B)</p> Signup and view all the answers

    What happens to the induced voltage in the secondary winding when the primary current is increased?

    <p>It increases (B)</p> Signup and view all the answers

    Which component is NOT part of the no load current (Io) in a transformer?

    <p>Resistive current (D)</p> Signup and view all the answers

    What is the impact of leakage flux in practical transformers?

    <p>It reduces efficiency (B)</p> Signup and view all the answers

    Flashcards

    Transformer

    A device that changes AC voltage levels without affecting the power supplied. It can increase or decrease voltage based on the number of turns in its windings.

    Primary winding

    The winding connected to the AC power source in a transformer.

    Secondary winding

    The winding connected to the load in a transformer.

    Step-down transformer

    A transformer where the high voltage side acts as the primary coil, resulting in a lower voltage output.

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    Step-up transformer

    A transformer where the high voltage side acts as the secondary coil, resulting in a higher voltage output.

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    Core form transformer

    A transformer with a rectangular laminated core and windings wrapped around two sides.

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    Shell form transformer

    A transformer with a three-legged laminated core and windings wrapped around the central leg.

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    Power conservation in transformers

    The amount of power supplied by the transformer stays constant, even if the voltage is changed. This means that if the voltage increases, the current decreases proportionally.

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    Ideal Transformer

    An idealized, lossless transformer with zero core reluctance, no magnetizing current, and no core or winding losses. It also assumes no leakage flux, making it a simplified theoretical model.

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    Voltage Transformation Ratio

    The ratio of the voltage induced in the secondary winding to the voltage applied to the primary winding, which is also equal to the ratio of the turns in each winding.

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    Current Transformation Ratio

    The ratio of the current flowing in the primary winding to the current flowing in the secondary winding, which is inversely proportional to the turns ratio.

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    Impedance Transformation

    The property of a transformer to transform an impedance connected to the secondary winding to an equivalent impedance on the primary side, allowing for impedance matching.

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    Inductive Coupling

    The ability of a transformer to transfer energy from one circuit to another without direct electrical connection, allowing for efficient power transmission.

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    Turns Ratio

    The ratio of the number of turns in the primary winding to the number of turns in the secondary winding.

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    Leakage Flux

    The magnetic flux that does not link both the primary and secondary windings, representing a loss of efficiency in the transformer.

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    Magnetizing Current

    The current drawn by the primary winding to create the magnetic field in the core, which is minimized in an ideal transformer but exists in real transformers.

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    What is a transformer?

    A device that changes AC electric power from one voltage level to another using a magnetic field.

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    What are the main components of a transformer?

    Two or more coils wrapped around a ferromagnetic core, with no direct connection between them.

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    What is an ideal transformer?

    A transformer that operates without any losses, transferring all power efficiently between coils.

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    What is the turns ratio of a transformer?

    The ratio of the number of turns on the secondary coil to the number of turns on the primary coil.

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    What is the law of conservation of power in a transformer?

    The power in the primary coil is equal to the power in the secondary coil.

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    Why are transformers important in electrical power systems?

    Transformers are essential for efficient transmission of electricity over long distances. They allow power to be sent at high voltage and low current, minimizing losses.

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    How are transformers used in power distribution?

    Transformers are used to increase voltage for efficient transmission and then decrease voltage for safe use in homes and businesses.

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    How is the power handling capacity of a transformer measured?

    Transformers are rated in VA (Volt-Amperes) or kVA (kilo-Volt-Amperes), indicating their power handling capacity.

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    No-load current (Io)

    The current that flows in the primary winding of a transformer when the secondary winding is open circuited.

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    Magnetizing current (Im)

    A component of no-load current that is responsible for creating the magnetic flux in the core.

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    Core loss current (Ic)

    A component of no-load current that is responsible for the losses in the magnetic core.

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    Reluctance

    The ability of a material to resist the formation of a magnetic field.

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    Turns ratio (N1/N2)

    The ratio of the number of turns in the primary winding to the number of turns in the secondary winding.

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    Magnetizing current equation

    The magnetizing current is proportional to the reluctance of the magnetic core and the flux.

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    Faraday's Law of Electromagnetic Induction

    The induced voltage in a winding is directly proportional to the rate of change of magnetic flux through the winding.

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    Induced voltage relationship

    The induced voltages in the primary and secondary windings are proportional to the number of turns in each winding.

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    Max Induced EMF

    The maximum value of the induced electromotive force (EMF) in a transformer is directly proportional to the rate of change of the magnetic flux.

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    RMS Induced EMF

    The root-mean-square (RMS) value of the induced EMF in a transformer is calculated by dividing the maximum induced EMF by the square root of 2.

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    Load Effect on Induced EMFs

    When a load is connected to the secondary winding of a transformer, the secondary current creates a flux that opposes the primary flux. This decreases the net flux and reduces the induced EMFs.

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    Primary Current Increase Due to Load

    The increase in the primary current needed to compensate for the load current in the secondary winding is denoted as 𝐼2′.

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    Relationship between Primary and Secondary Current Increase

    The ratio of the primary current increase (𝐼2′) to the secondary current (𝐼2) is equal to the inverse of the turns ratio (N2/N1).

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    Study Notes

    Lecture 3: Transformers I

    • Transformers change AC electric power at one voltage level to another voltage level using a magnetic field
    • Transformers consist of two or more coils of wire wrapped around a ferromagnetic core
    • Coils are (usually) not directly connected; the connection is the magnetic flux within the core
    • Transformer ratings range from few VA to hundreds MVA
    • Transformers are needed to step up voltage for efficient long-distance transmission and step down voltage for final use; initial systems operated at 120 V DC

    Introduction to Transformers

    • A transformer ideally changes one AC voltage level to another without affecting the actual power supplied.
    • If voltage steps up, current decreases to maintain constant power.
    • AC power can be generated centrally, stepped up for efficient transmission, and stepped down for final use.

    Construction of Transformers

    • One type uses a rectangular laminated steel core with windings around two sides (core form)

    • One winding is the primary, connected to the supply; the other is the secondary, connected to the load

    • Transformer windings can be classified as high voltage and low voltage sides based on the number of turns; the high voltage side has more turns than the low voltage side

    • Another type uses a three-legged laminated core with windings around the central leg (shell form)

    • The shell form simplifies insulation of the high voltage winding from the core

    Ideal Transformer

    • Ideal transformers are theoretical lossless devices with assumptions:

      • Infinite core permeability (zero reluctance)
      • No magnetizing current required
      • Core lossless (no hysteresis or eddy current losses)
      • Lossless windings
      • No leakage fluxes

    • Voltage relation (no load): induced EMF in primary and secondary windings is proportional to turns ratio

    • Voltage per turn is constant for primary and secondary

    • Voltage is directly proportional to N (turns), Current is inversely proportional to N

    • Current relation (load): the product of turns and current is equal across primary and secondary

    • Input power equals output power

    • Impedance transformation: impedances can be transferred from one side to the other

    Practical Transformer

    • Practical transformers have:

      • Non-infinite permeability (reluctance not zero)
      • Magnetizing current
      • Core losses
      • Winding resistances
      • Leakage fluxes

    • No-load current: a current flows in the primary circuit even with an open secondary circuit; it consists of magnetizing and core loss currents

    • Magnetizing current: produces magnetizing flux in the core inducing EMF in primary and secondary

    • Core loss current: overcomes core losses (hysteresis and eddy currents)

    • Induced voltages: -Max and RMS values of induced voltages can be calculated. -Induced EMF is directly proportional to number of turns and frequency

    • Secondary current flows if a load is connected across the secondary terminals

    • Secondary current produces flux which decreases the magnetizing flux produced by the primary currents.

    • Induced EMF in the primary and secondary will decrease.

    • Difference between primary voltage and induced voltage.

    • Load current increase primary current (I').

    • Total current is equal to no-load current + I'

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    Lecture 3-2 Transformers I PDF

    Description

    This quiz covers the fundamentals of transformers, including their construction, functionality, and role in changing AC voltage levels. Learn about how transformers operate, their ratings, and the significance of magnetic fields in their operation. Understand the importance of transformers in efficient long-distance power transmission.

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