2. 2
Syllabus
• DC Generator: Construction, principle of operation and classification, armature
winding, voltage build up process, armature reactions and commutation, load-voltage
characteristic, voltage regulation, losses and efficiency.
• DC Motors: Operation, construction, back emf and torque, types, torque speed
characteristics, starting, speed control, braking, performance and applications.
• Single Phase Transformers: Construction and principle of operation, cooling,
loading, equivalent circuit, regulation, PU systems, losses and efficiency, tests, parallel
operation, harmonics and inrush current.
• Three Phase Transformers: Connections, harmonic suppression, vector groups,
parallel operation and testing.
• Autotransformer: Construction, principle of operation instrument and applications
of auto-transformers.
• Introduction to Stepper motors, Permanent magnet DC motors, Brushless DC
motors.
EEE-233: Electrical Machines - I
3. 3
Introduction
WHAT IS TRANSFORMER ?
• A transformer is a static piece of
apparatus by means of which an electrical
power is transferred from one alternating
current circuit to another electrical circuit.
• It is one of the most useful electrical
devices ever invented.
• It can change the magnitude of
alternating voltage or current from one
value to another without any change in
frequency.
• A transformer is a static device – it has
no moving parts. It is rugged and durable
in construction.
• They also have a very high efficiency —
about 99%.
4. 4
Introduction
WHAT IS TRANSFORMER?
(contd..)
• It essentially consists of two
windings, the primary and
secondary, wound on a common
laminated magnetic core.
• The winding connected to the
a.c. source is called primary
winding and the one connected
to load is called secondary
winding.
• There is no electrical contact
between them. The transfer of
energy takes place through the
magnetic field.
5. 5
Introduction
• During the second half of the 19th century the alternators, transformers and
induction motors were invented.
• Transformers can be found operating in the frequency range starting from a few
hertz going up to several mega hertz.
• Power ratings vary from a few mill watts to several hundreds of megawatts.
• One could choose a moderate voltage for the generation of a.c. power, a high
voltage for the transmission of this power over long distances and finally use a
small and safe operating voltage at the user end. All these are made possible by
transformers.
WHAT IS TRANSFORMER? (contd..)
7. 7
Introduction
FUNCTIONS OF TRANSFORMER
• The transformer is extremely important as a component in many different
types of electric circuits, from small-signal electronic circuits to high voltage
power transmission systems.
The most important function performed by transformers are:
Changing voltage and current level in an electric system.
Matching source and load impedances for maximum power transfer in
Electronic and control circuitry.
Electrical isolation.
8. 8
Introduction
BASIC PRINCIPLES
• Faraday’s law states that a voltage appears
across the terminals of an electric coil when the flux
linkages associated with the same changes. This
emf is proportional to the rate of change of flux
linkages. Putting mathematically,
𝒆 = 𝒅𝝍
𝒅𝒕
• Where, e is the induced emf in volt and 𝝍 is the
flux linkages in Weber turn.
• Fig. 1 shows a coil of N turns. All these N turns
link flux lines of 𝝓 Weber resulting in the N𝝓 flux
linkages. In such a case,
𝝍 = 𝐍𝝓 and
𝒆 = 𝐍 𝒅𝝓
𝒅𝒕 volt
Fig. 1: Flux linkages of a coil
9. 9
Introduction
• The change in the flux linkage can be brought about in a variety of way -
coil may be static and unmoving but the flux linking the same may change
with time. (transformer)
flux lines may be constant and not changing in time but the coil may move
in space linking different value of flux with time. (d.c machines and
synchronous machines)
both 1 and 2 above may take place. The flux lines may change in time with
coil moving in space. (a.c machines such as induction machines and also in
a.c. commutator machines)
BASIC PRINCIPLES (contd..)
10. 10
Introduction
Fig. 2 shows a region of length L m, of uniform
flux density B Tesla, the flux lines being normal to
the plane of the paper. A loop of one turn links
part of this flux. The flux linked by the turn is
BLX Weber
Here X is the length of overlap in meters.
• If B does not change with time and the loop is
unmoving then no emf is induced in the coil as
the flux linkages do not change. Such a condition
does not yield any useful machine.
• On the other hand if the value of B varies with
time a voltage is induced in the coil linking the
same coil even if the coil does not move.
Fig. 2: Static coil
BASIC PRINCIPLES (contd..)
11. 11
Introduction
• The magnitude of B is assumed to be varying sinusoidally, and can be expressed as,
𝑩 = 𝑩𝒎𝒔𝒊𝒏𝝎𝒕
where 𝑩𝒎 is the peak amplitude of the flux density. 𝝎 is the angular rate of change
with time.
• The instantaneous value of the flux linkage is given by,
𝝍 = 𝑵𝝓 = 𝑵𝑳𝑿𝑩𝒎𝒔𝒊𝒏𝝎𝒕
• The instantaneous value of the induced emf is given by,
𝒆 =
𝒅𝝍
𝒅𝒕
= 𝑵𝝓𝒎𝝎𝒄𝒐𝒔𝝎𝒕 = 𝑵𝝓𝒎𝝎𝒔𝒊𝒏 𝝎𝒕 +
𝝅
𝟐
; Here, 𝝓𝒎 = 𝑳𝑿𝑩𝒎
• The peak value of the induced emf is 𝒆𝒎 = 𝑵𝝓𝒎𝝎
• The rms value is given by 𝑬 =
𝑵𝝓𝒎𝝎
√𝟐
=
𝑵𝝓𝒎𝟐𝝅𝒇
√𝟐
= 𝟒. 𝟒𝟒𝑵𝝓𝒎𝒇 volt
• This induced emf has a phase difference of 𝝅/𝟐 radian with respect to the flux
linked by the turn. This emf is termed as ‘transformer’ emf and this principle is used in
a transformer.
BASIC PRINCIPLES (contd..)
