Module 3 : Transformers
Transformer windings - EMF equation -
equivalent circuit - phasor diagram -
efficiency and voltage regulation -
parallel operation - transformer testing
28.03.24 Lecture 1_Module 4_BEEE215L 1
Construction details of a transformer
• Transformer is a static device that transfers
electric power from one circuit to another
circuit without change of frequency
• It consists of two inductive coils; primary
winding and secondary winding.
• The coils are electrically separated but
magnetically linked to each other.
• A laminated core provides magnetic path for
the flux, to get linked with the secondary
winding.
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Construction details of a transformer
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Types of transformer
• Based on the construction –core type and
shell type
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Types of transformer
• Based on the supply – 1-ph and 3-ph
• Based on the output voltage level – step up
and step down
• Based on the usage – power transformer,
distribution transformer, instrument
transformer
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Core type transformer
• The windings surround a considerable part of
the core
• Cylindrical coils
• Cruciform core section
• Different layers are insulated from each other
• Low voltage winding is placed nearer to the
core
• Low voltage applications
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Shell type transformer
• Each winding is subdivided into subsections in
order to reduce the leakage
• Better mechanical support
• Suitable for high voltage applications
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Transformer on No load
• When the primary is excited with supply
source with voltage V1, the primary winding
draws a current Io to produce the necessary
flux in the core and also to supply the copper
losses.
• The flux producing component of the no load
current is called magnetizing current Im or Iµ.
• The component Iw supplies the core losses
• Io = Iµ + Iw
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Transformer on No load
• The magnetizing component of no load current
Iµ = Io sin φo
• The core loss component of no load current
Iw = Io cos φo
• The no load current magnitude
Io = √ (I2µ + I2w)
• Total power input on no load
Wo = Vo× Io × cos φo
where Φo no load angle between voltage and
current.
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Equivalent circuit at no load
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Transformer on load
• When the load is connected to the transformer,
the secondary current starts flowing through the
load.
• Depends on the load connected at the secondary,
the magnitude and phase of the secondary
current I2 are varied.
• Due to the secondary mmf N2I2, secondary
current I2 set up the magnetic flux Φ2 in the core.
• This secondary flux opposes the main flux Φ1 and
the EMF induced in the primary E1 reduces.
• The primary draws more current from the supply.
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Transformer on load
• This load component current is anti phase with
I2 and produces the flux Φ2’ in order to
neutralize the effect of Φ2.
• Hence the mmf N1I2’ balances the mmf N2I2.
• Thus, the net flux in the transformer is
constant
• N1I2’ = N2I2
• I2’ = (N2/N1)I2
• I2’ = K I2
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Transformer on load
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Transformer on load
• The net or resultant flux in a transformer is
constant under different load conditions
Inference:
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Ideal transformer
• Winding resistances are negligible.
• All flux set up by primary links the secondary
windings (no leakage flux).
• Core losses are negligible.
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Transformer on load (with winding
resistance and leakage reactance)
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Equivalent circuit of a transformer
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Equivalent circuit of a transformer
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Equivalent circuit of a transformer
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Equivalent circuit of a transformer
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Equivalent circuit of a transformer
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Transformer on load
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Transformer on load (with winding
resistance and leakage reactance)
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Transformer on load (with winding
resistance and leakage reactance)
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Losses in a transformer
• Core losses or Iron losses
– Hysteresis loss
– Eddy current loss
• Copper loss
• Total losses = core loss + copper loss
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Efficiency
• Efficiency = output/input
• Efficiency = (Input – losses)/input
• Efficiency = 1-(losses/input)
• Maximum efficiency occurs when copper loss
is equal to core loss.
• All day efficiency
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Voltage Regulation
The voltage regulation is the percentage of
voltage difference between no load and full load
voltages of a transformer with respect to its full
load voltage or no load voltage.
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Voltage Regulation
The voltage regulation is the percentage of
voltage difference between no load and full load
voltages of a transformer with respect to its full
load voltage or no load voltage.
• Lagging power factor
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Voltage Regulation
• Leading power factor
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Transformer testing
• Polarity test
• OC test and SC test
• Load test
• Sumpner’s test (Back to back test)
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Transformer testing (OC test)
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Transformer testing (SC test)
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Polarity test
Similar polarity ends of the two windings of a
transformer are those ends that acquire
simultaneously positive or negative polarity of emfs
induced in them
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Sumpner’s test (Back to back test)
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Parallel operation of transformers
Conditions
• Same voltage ratio of transformer
• Same percentage impedance (same X/R ratio)
• Same polarity
• Same phase sequence (three phase)
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Parallel operation of transformers
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Equal voltage ratios
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Auto transformers
• Part of the winding is common for both
primary and secondary
• Electrically not isolated
• Weight of conductor in auto transformer =
(1-k) weight of conductor in 2-winding
transformer
• Saving of copper = k times the copper material
in a 2-winding transformer
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All day efficiency
• Power transformers – max. efficiency at full
load
• The load on a distribution transformer varies
over a wide range during a 24 hours/day
• Distribution transformers are designed with
low core loss
• Distribution transformers – max. efficiency at
half or 3/4th of its full load
• All day efficiency = total energy output in a
day/total energy input in a day (24 hrs)
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Effect of power factor on efficiency
Max. efficiency occurs at the same load current
independent of power factor
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