Ideal and Practical Transformers
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Questions and Answers

What is a characteristic of a practical transformer?

  • It has I2R losses. (correct)
  • It is used in ideal condition.
  • It has no losses.
  • It has 100% efficiency.

    • What type of losses are caused by the resistive heating of conductors in a transformer?

  • Copper losses (correct)
  • Magnetizing losses
  • Iron losses
  • Eddy current losses

    • In a transformer, what does the equivalent circuit represent?

  • The thermal behavior of the transformer
  • The mechanical properties of the windings
  • The behavior equations of the device (correct)
  • The magnetic flux paths

    • Which factor does NOT contribute to core losses in a transformer?

    <p>Skin effect</p> Signup and view all the answers

    What are hysteresis losses in a transformer primarily due to?

    <p>Repeated change in magnetic polarity</p> Signup and view all the answers

    How does frequency affect hysteresis losses in a transformer?

    <p>Hysteresis losses are proportional to frequency.</p> Signup and view all the answers

    Which term refers to the AC current induced in the iron core due to a changing magnetic field?

    <p>Eddy currents</p> Signup and view all the answers

    What is represented by the non-inductive resistance R0 in the equivalent circuit of a transformer?

    <p>Core loss</p> Signup and view all the answers

    What happens to the primary current when the secondary current increases in an ideal transformer on-load condition?

    <p>The primary current increases.</p> Signup and view all the answers

    In an ideal transformer, how does the secondary EMF E2 relate to the secondary terminal voltage V2?

    <p>E2 is equal to V2.</p> Signup and view all the answers

    What causes the secondary current I2 to lag behind the secondary terminal voltage V2 in an ideal transformer?

    <p>The inductive nature of the load.</p> Signup and view all the answers

    Which of the following is NOT a characteristic of a practical transformer?

    <p>There is no leakage flux.</p> Signup and view all the answers

    What effect does a load connected across the secondary winding have on the transformer?

    <p>It causes a load current to flow through the secondary winding.</p> Signup and view all the answers

    What must the primary current develop to maintain the flux in the core of an ideal transformer?

    <p>An mmf that counterbalances the demagnetizing effect of the secondary mmf.</p> Signup and view all the answers

    In an ideal transformer under load conditions, what remains constant?

    <p>The mutual flux.</p> Signup and view all the answers

    What is the lagging angle between the secondary current I2 and the secondary terminal voltage V2 in an inductive load?

    <p>A phase angle ϕ2.</p> Signup and view all the answers

    What determines the direction of the force acting on the armature in a DC motor?

    <p>Fleming's Left Hand Rule</p> Signup and view all the answers

    Which equation represents the relationship for the force exerted on the armature in a DC motor?

    <p>$F = BIL$</p> Signup and view all the answers

    What role does a commutator play in a DC motor?

    <p>It reverses the direction of current flow in armature conductors.</p> Signup and view all the answers

    What is back or counter E.M.F. in a DC motor?

    <p>It is induced voltage that opposes the applied voltage.</p> Signup and view all the answers

    Which of the following factors does NOT affect back E.M.F. in a DC motor?

    <p>Applied voltage (V)</p> Signup and view all the answers

    When does the direction of current flow in an armature conductor need to be reversed?

    <p>When the armature passes through the magnetic neutral axis.</p> Signup and view all the answers

    What happens to the applied voltage (V) in relation to back E.M.F. (Eb) under normal operating conditions of a DC motor?

    <p>$V &gt; Eb$</p> Signup and view all the answers

    What kind of torque is developed when the armature's field attempts to align with the main magnetic field?

    <p>Electromagnetic torque</p> Signup and view all the answers

    What is a key characteristic of the rotor in a squirrel cage induction motor?

    <p>It is permanently shorted at the end rings.</p> Signup and view all the answers

    In a single phase induction motor, what is produced when an alternating current flows through the stator?

    <p>Main flux.</p> Signup and view all the answers

    Why are single phase induction motors described as not self-starting?

    <p>They do not generate sufficient torque initially.</p> Signup and view all the answers

    What type of flux is produced as a result of the rotor current in a single phase induction motor?

    <p>Rotor flux.</p> Signup and view all the answers

    What distinguishes the single phase induction motor from the three phase induction motor in terms of windings?

    <p>Single phase motors have two windings.</p> Signup and view all the answers

    Which of the following statements about the rotor conductors in a squirrel cage motor is correct?

    <p>They are shorted permanently.</p> Signup and view all the answers

    What is the role of the main flux in a single phase induction motor?

    <p>To induce current in the rotor.</p> Signup and view all the answers

    Which of the following statements about the power source for a single phase induction motor is true?

    <p>It operates only on single phase power.</p> Signup and view all the answers

    What is the primary purpose of the stator in a single-phase induction motor?

    <p>To produce a rotating magnetic field</p> Signup and view all the answers

    Which material is primarily used for making the stamping of the stator to reduce hysteresis losses?

    <p>Silicon steel</p> Signup and view all the answers

    How does the stator core of a single-phase induction motor differ from that of a three-phase induction motor?

    <p>It features concentric coils for easier adjustment</p> Signup and view all the answers

    What type of rotor construction is used in a single-phase induction motor?

    <p>Squirrel cage rotor</p> Signup and view all the answers

    Why are the rotor slots in a single-phase induction motor slightly skewed?

    <p>To prevent magnetic locking of stator and rotor teeth</p> Signup and view all the answers

    What is the relationship between supply voltage frequency, the number of poles, and synchronous speed in an induction motor?

    <p>Synchronous speed is given by the formula $N_s = \frac{120f}{P}$</p> Signup and view all the answers

    What are the components of the rotor in a squirrel cage rotor design?

    <p>Conducting end rings and rotor conductors</p> Signup and view all the answers

    What is the key disadvantage of using shaded pole motors among other types of single-phase induction motors?

    <p>Low starting torque</p> Signup and view all the answers

    Study Notes

    Ideal Transformer On-Load

    • When a load is connected across the secondary winding, the transformer is considered loaded.
    • A load current flows through the secondary winding and the load.
    • The secondary EMF (E2) causes a current (I2) to flow.
    • For an ideal transformer, E2 is equal to the secondary terminal voltage (V2).
    • An inductive load will cause I2 to lag behind E2 (or V2) by an angle (ϕ2).
    • The primary current (I1) must flow to counter-balance the demagnetizing effect of the secondary mmf (N2I2).
    • The primary winding draws enough current to neutralize the demagnetizing effect of the secondary current, keeping the main flux in the core constant.
    • When the secondary current (I2) increases, the primary current (I1) also increases, maintaining a constant mutual flux (ϕm).
    • The secondary current I2 lags behind the secondary terminal voltage V2 by an angle of ϕ2.

    Practical Transformer

    • The primary and secondary windings have finite resistances.
    • Leakage flux exists, meaning not all flux is confined to the magnetic circuit.
    • The magnetic core has finite permeability, requiring considerable MMF to establish flux.
    • Losses are present, mainly due to winding resistances, hysteresis, and eddy currents.
    • The efficiency of a practical transformer is less than 100%.

    Losses in Transformer

    • Copper Losses (Winding Resistance):
      • Current flowing through the windings causes resistive heating.
      • Skin effect and proximity effect contribute to increased losses at higher frequencies.
    • Core or Iron Losses:
      • Hysteresis Losses:
        • Power losses due to repeated magnetic polarity changes.
        • Losses are proportional to frequency and peak flux density.
      • Eddy Current Losses:
        • AC current induced in the iron core due to a changing magnetic field.

    Back or Counter E.M.F. (Eb)

    • When the armature of a DC motor rotates, an EMF is induced in the armature conductors.
    • This induced EMF acts in the opposite direction to the applied voltage (Lenz's Law) and is known as back or counter EMF (Eb).
    • Eb is always less than the applied voltage (V).

    Single Phase Induction Motor

    • Stator:
      • Stationary part.
      • Contains the stator windings which are connected to a single-phase AC supply.
      • The stator core is laminated to reduce eddy current losses.
    • Rotor:
      • Rotating part.
      • The rotor is of squirrel cage type with aluminum or copper bars short-circuited at both ends by conducting end rings.
      • Slots in the rotor are slightly skewed to prevent magnetic locking and noise.
    • Working Principle:
      • The stator current produces a main flux.
      • The main flux induces an EMF in the rotor conductors.
      • Rotor current is produced due to a closed circuit.
      • The rotor current produces its own flux (rotor flux).
      • The interaction of the main flux and the rotor flux generates torque to rotate the motor.

    Comparison of Single and Three Phase Induction Motors

    • Single-phase induction motors are not self-starting while three-phase motors are self-starting.
    • Single-phase motors require additional starting mechanisms.
    • Single-phase motors typically have a higher starting current than three-phase motors.

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    Transformer and Electrical Machines PDF

    Description

    Explore the principles of ideal and practical transformers in this quiz. Understand the relationship between load, current, and voltage in transformer operations, including the effects of inductive loads and leakage flux. Test your knowledge of key concepts to see how transformers maintain mutual flux.

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