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Transformers

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Transformers. 2. Objectives. Describe the components of an ideal transformer. ... Transformers are designated to be most efficient when operated at full rated ... – PowerPoint PPT presentation

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Title: Transformers


1
Chapter 14
  • Transformers

2
Objectives
  • Describe the components of an ideal transformer.
  • Explain the relationship among the voltages,
    currents, and turns ratio in a transformer.
  • Calculate the voltage, current, and power in a
    transformer.

3
Objectives
  • Solve problems involving transformer losses and
    demonstrate the effect they have on efficiency.
  • Show how to connect single-phase transformers to
    produce three-phase transformers.
  • Apply special transformers such as
    autotransformers, current transformers, and
    potential transformers.

4
14-1 Introduction
5
14-2 The Ideal Transformer
  • V1 / V2 N1 / N2
  • a NHS / NLS
  • V Vrated f / frated

6
Simple transformer.
7
Transformer using high side and low side
designation.
8
14-3 Transformer Power Losses
  • Copper Loss
  • Core Loss
  • Hysteresis Loss
  • Eddy Current Loss

9
Copper Loss
  • I2R power loss in the copper wire in the
    transformer windings.
  • Proportional to the square of the winding
    current.
  • Minimal when the transformer is operating with no
    load.
  • Maximum copper loss occurs at rated load.

10
Core Loss
  • Hysteresis Loss
  • Frictional loss caused by the constant change in
    magnetic field polarity in the core.
  • Increases with an increasing voltage or frequency
    applied to the transformer windings.
  • Eddy Current Loss
  • I2R loss in the core material caused by
    circulating induced currents in the core.
  • Proportional to the square of the voltage and the
    square of the frequency applied to the
    transformer windings.
  • Minimized by laminating the core.

11
14-4 Transformer Efficiency
  • ? Pout / Pin
  • Pin Pout Plosses
  • Plosses Pcu Pcore

12
14-5 Effect of Power Factor on Transformer
Performance
  • P V I cos ? V I x P.F.

13
14-6 Impedance Reflection
  • RL(HS) a2 RL(LS)

14
Impedance reflection.
15
14-7 Impedance Matching Transformers
  • ZHS a2 ZLS

16
14-8 Transformer Construction
  • Shell
  • Large
  • Leakage flux
  • E-Core
  • Compact
  • Leakage flux reduced

17
Cores types, (a) shell and (b) E-core.
18
14-9 Three-Phase Transformers
  • Wye-to-Wye
  • Wye-to-Delta
  • Delta-to-Wye
  • Delta-to-Delta

19
Three-phase transformer wye and delta connection
possibilities.
20
14-10 Autotransformers
  • When wired as an autotransformer, the maximum
    current capacity of any winding of a transformer
    is the same as when it is used as a conventional
    transformer.
  • The voltages, currents, and turns ratio still
    conform to the equations given.
  • Although the autotransformer can deliver more
    apparent power (S, in volt-amperes) than the
    transformer alone, the transformer itself cannot
    safely deliver more apparent power than its
    rating.
  • Because autotransformers can be connected in
    several different ways, rather than attempt to
    memorize the equations used to analyze each, it
    is easier to gain a fundamental understanding of
    how a basic autotransformer operates and then
    develop the governing equations as needed.

21
Autotransformer (boost connection).
22
Autotransformer voltages and currents.
23
Autotransformer buck configuration.
24
Variable autotransformer.
25
14-11 Instrument Transformers
  • Potential Transformer
  • Current Transformer

26
Potential transformer and current transformer
application.
27
14-12 Useful Transformer Tips
  • The core losses in a transformer are determined
    by the applied voltage and frequency. Winding
    currents have no bearing on core loss. Reducing
    the load on a transformer will not reduce its
    core loss.
  • The copper losses in a transformer are
    proportional to the squares of the winding
    currents and the square of the frequency. The
    applied voltage has no bearing on the copper
    loss.
  • The maximum current that can be carried safely by
    a transformer winding is determined only by the
    wire gauge of the windings. The dimensions of
    the core and applied voltage have no bearing on
    maximum current capacity. Operating a
    transformer at a reduced voltage will not change
    its maximum current capability.
  • The real power (watts) that can be delivered
    safely by a transformer is the volt-amp rating of
    a transformer times the power factor of the load
    it is powering. For example, a 1-kVA transformer
    can deliver only 700 watts to a 0.7-power-factor
    load.
  • Transformers are designated to be most efficient
    when operated at full rated load (that is, rated
    voltage and rated current). Operating a
    transformer at reduced load current or reduced
    voltage reduces its efficiency.
  • Transformers may be operated at lower frequencies
    if their voltage and apparent power ratings are
    derated by the same ratio as the frequency
    reduction.
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