Transformer Notes

1. SSDN
FUNDAMENTALS OF ELECTRICAL ENGINEERING
(BEE 201)
SESSION – April 2025 – July 2025
Dr. Pavan Khetrapal
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UNIT - 3

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UNIT - 3: TRANSFORMERS
Contact
Hours
Magnetic circuits, ideal and practical
transformer, equivalent circuit, losses
in transformers, regulation and
efficiency.
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MAGNETIC
CIRCUIT
In 1820, Oersted noticed something
strange while setting up an
experiment. When he placed a
compass near an electrical current, the
needle moved. Oersted’s discovery
proved that electricity could create a
magnetic field. It also proved that
there is a relationship between
electricity and magnetism. His work
paved the way for other scientists to
investigate electromagnetism.

4.  MAGNETISM is the force of attraction or repulsion of a magnetic material due to the
arrangement of its atoms, particularly its electrons.
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Loadstone
Magnetite
Magnetic
Lines of
Force

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Magnetic Lines of Force

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Electromagnets are a different
from permanent magnets.
Electromagnets are made of coils of
wire with electricity passing through
them. Moving charges create magnetic
fields, so when the coils of wire in an
electromagnet have an
electric current passing through them,
the coils behave like a magnet. When
the electricity stops flowing, the coils
don’t act like a magnet anymore.
Air Core
Inductor
Air Core
Inductor
Iron Core
Inductor Iron Core
Inductor Iron Core
Toroid
Iron Core
Toroid
Air Core
Toroid

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Magnetic Field of a Coil
Magnetic Effect of Current

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MAGNETIC CIRCUIT
The complete closed path followed by magnetic lines of force or magnetic flux is referred to as a magnetic
circuit.
MAGNETIC CIRCUIT

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DEFINITIONS PERTAINING TO MAGNETIC CIRCUIT
r


 
 0
Absolute
Permeability
Permeability of Free
Space or Vacuum
= 4π × 10 -7
H/m
Relative
Permeability

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COMPARISON OR ANALOGY BETWEEN ELECTRIC CIRCUIT AND MAGNETIC CIRCUIT
SIMILARITIES
MAGNETIC CIRCUIT ELECTRIC CIRCUIT
1. The closed path for magnetic flux is
called a magnetic circuit.
1. The closed path through which an
electric current either flows or is
intended to flow is called as electric
circuit
3. Magnetomotive Force (mmf) = N × I 3. Electromotive Force (emf) = V × I
6. Magnetic Field Intensity 6. Electric Field Intensity
7. Permeance 7. Conductance
8. Permeability 8. Conductivity
DISSIMILARITIES
1. Energy is required in creating the
flux, and not in maintaining it.
1. When current flows through an
electric circuit, energy is expended
so long as the current flows.
2. The magnetic lines of force appears
to flow, actually it does not
2. Electrons actually flows thus
constitute the current
3. There is no perfect insulator for flux. 3. Air is perfect insulator for current

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DEFINITION OF TRANSFORMER:
WORKING PRINCIPLE OF TRANSFORMER:
IDEAL TRANSFORMER:
EMF EQUATION OF TRANSFORMER:
IDEAL TRANSFORMER ON NO LOAD:
IDEAL TRANSFORMER ON - LOAD:

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𝑰𝒆

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j

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RESISTIVE LOAD
(UPF Load)
INDUCTIVE LOAD
(Lagging pf Load)

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Equivalent Circuit
Referred to
Primary Side
𝑽 𝟐=𝑬𝟐
𝒌/ 𝑲 𝒐𝒓 𝒂=
𝑵𝟐
𝑵𝟏

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Equivalent Circuit
Referred to
Secondary Side
𝒌/ 𝑲 𝒐𝒓 𝒂=
𝑵𝟐
𝑵𝟏

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TRANSFORMER VA / KVA / MVA Rating is Given
𝑷𝒓𝒊𝒎𝒂𝒓𝒚 𝑪𝒖𝒓𝒓𝒆𝒏𝒕 ( 𝑰𝟏)=
𝒌𝑽𝑨×𝟏𝟎𝟎𝟎
𝑽 𝟏
𝑨𝒎𝒑 .
𝑺𝒆𝒄𝒐𝒏𝒅𝒂𝒓𝒚 𝑪𝒖𝒓𝒓𝒆𝒏𝒕 ( 𝑰𝟐)=
𝒌𝑽𝑨×𝟏𝟎𝟎𝟎
𝑽𝟐
𝑨𝒎𝒑 .

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LOSSES IN TRANSFORMER
 When a magnetization force is applied to a magnetic material, the molecules in the
material align in a particular direction. However, when this magnetizing force(H) is
reversed in the opposite direction, the internal friction of the molecular magnets
resists the reversal of magnetism(B). Therefore, the magnetic field(B) lags the
magnetizing force(H), and this phenomenon of a magnetic field lagging with a
magnetizing force is called hysteresis.
 In order to overcome the internal resistance within a magnetic material, a portion
of the magnetizing force is employed. However, this process generates heat due
to work done by the magnetizing force, which results in energy loss in the form of
heat. This loss of energy is known as hysteresis loss, and it is directly
proportional to the area of the B-H curve.

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LOSSES IN TRANSFORMER

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EDDY CURRENT LOSS
 When magnetic flux links to a closed circuit, an electromotive force (EMF) is
induced. The EMF causes the current to flow in the circuit. The magnitude of the
current depends on the EMF and the resistance of the circuit. The transformer
core is made of a conducting material and has finite resistance. The induced EMF
causes the currents to flow in the core material, and these currents are called
eddy currents.
 The eddy currents do not perform any useful work; they result in energy loss in the
magnetic material. The loss caused by eddy currents is called Eddy Current
Loss.

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MINIMIZATION: Transformer cores are made using thin laminations to reduce eddy current
loss. The laminations are insulated from one another to limit the flow of eddy currents.

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EFFICIENCY:

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 When a transformer is loaded, the secondary terminal voltage decreases
due to a drop across secondary winding resistance and leakage reactance.
This change in secondary terminal voltage from no load to full load
conditions, expressed as a fraction of the no-load secondary voltage is
called regulation of the transformer.
VOLTAGE REGULATION

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 A transformer in its basic form consists of two main parts:
1. Magnetic Circuit
2. Electric Circuit
CONSTRUCTIONAL FEATURES OF TRANSFORMERS
ELECTRIC CIRCUIT MAGNETIC CIRCUIT
Primary Winding:
• This is the input coil.
• It is connected to the AC supply.
• When current flows through it, it creates a magnetic
field.
Secondary Winding:
• This is the output coil.
• It is connected to the load.
• Electrical energy is induced in this coil due to the
magnetic field created by the primary coil.
Magnetic Core:
• Made of laminated silicon steel sheets to reduce
eddy current losses.
• Provides a low reluctance path for the magnetic
flux.
• The alternating flux flows through this magnetic
path and links both windings.
Laminations:
• The core is made of thin laminated sheets, not a
solid block.
• Laminations are insulated from each other to
reduce eddy currents, improving efficiency.

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Ques: The primary and secondary windings of a 600 kVA transformer have
resistance of 0.4 Ω and 0.001 Ω respectively. The primary and secondary
voltages are 6000 V and 200 V respectively. The iron loss is 3200 W. Calculate
the efficiency on full load at the load power factor being 0.8. [March 2025]
Ques: A single phase 100 kVA, 6.6kV/230, 50 Hz transformer has 90% efficiency
at 0.8 lagging power factor both at full load and also at half load. Determine
iron and copper loss at full load for transformer. [2020-21]
Ques: In a 25 kVA, 2000/200 V transformer, the constant and variable losses are
350 W and 400 W respectively. Calculate the efficiency on unity power factor:
(a) full load and (b) half load. [2016-17; 2019-20]