2. ELECTRICAL MACHINES
An electrical machine consumes electrical energy to do a
specific work or it converts electrical energy to other forms
like mechanical energy, light energy etc. In this module, we
will be discussing the principle, working, construction,
application, merits and demerits of various electrical machines
used in our day to day life. The following are the various
machines which we will be dealing with:
3. Cont. ..
Transformer
• Principle of operation
• Construction
• EMF equation and transformer ratio
• Losses in a transformer
• Efficiency
• Applications
4. Cont. ..
DC machines
• Construction
• Principle of operation and working of DC Machines
• Back EMF
• Voltage, power and Torque of DC Machines
• Necessity of a starter
• Types of DC machines
• Applications
5. Cont. ..
Three phase induction motors
• Construction
• Principle of operation and working
• Applications
Single phase Induction motors
• Types
• Applications
Universal Motor
6. 1. TRANSFORMER
Transformer is a static AC machine (i.e. it contains no moving
parts) which is used to increase or decrease the AC voltage
without changing the frequency. It consists of 2 windings,
primary and secondary windings which are wound on the same
magnetic core as shown below.
7. I. Principle of Operation
The working principle of transformer is Faradays Laws of
electromagnetic Induction. The two windings; primary and
secondary are electrically separated but wound on the same
magnetic core. When AC voltage is applied to primary
winding, it increases alternating magnetic flux ( ) which
links with the secondary winding. This alternating magnetic
flux will induce an EMF in the secondary winding. The
magnitude of induced EMF in the secondary can be increased
or decreased by increasing or decreasing the number of turns.
m
8. Cont. ..
The transformer which increases the voltage is called step up
transformer
For a step up transformer
The transformer which decreases the voltage is called step down
transformer
For step down transformer
Where;
N1 and N2 are number of turns in primary and secondary
windings respectively
V1 and V2 are voltages in primary and secondary respectively
1 2 1 2
N N _and _V V
1 2 1 2
N N _and _V V
9. II. Construction of a Transformer
The main parts of a transformer are:
• Core: the core is made of laminated silicon steel. Each lamination have a
thickness of 0.35mm to 0.5mm and is coated with thin layer of vanish. Several
numbers of such lamination are pressed together to form the core. The
function of core is to provide low reluctance path to the magnetic flux , the
transformer is of two types; core type and shell type.
• In core type transformer, the windings are wound on the side limbs of the
core as shown in the figure below. In core type transformer the high voltage
winding (HV) is wound over the low voltage winding (LV)
• In shell type transformer, both the LV and HV windings are wound on the
central limb in sandwich pattern as shown beow
11. Cont. ..
The main parts of a transformer are:
• Windings: These are made of enameled copper wire. Each turn
of the winding is insulated from each other.
• Transformer tank: This tank stores the oil needed for cooling
and insulation of transformer. It is mounted on the top of the
transformer.
• Conservator tank: This is a small tank connected to the main
tank. This accommodated the change in oil level during the
heating and cooling of transformer oil.
12. Cont. ..
The main parts of a transformer are:
• Terminal Bushing: These are used to insulate the output
terminals of the transformer
• Breather: the breather provides the passage of air in to the
transformer during contraction of oil during cooling. The
breather consists of silica gel, through which the air is passed.
The silica gel absorbs the moisture content in the air
14. III. EMF Equation of Transformer
Let an alternating voltage V1 with frequency f be applied to the
primary winding of the transformer as shown above. Let the number
of turns in the primary be N1. the alternating voltage will set up a
flux given by,
Where:
Is the maximum value of flux
m sin t
2 f
m
15. Cont. ..
By Faradays Law, induced EMF,
1 1
1 1
1 1
1 1
1 1
2
2 90
m
m
m
m
d
e N
dt
d
e N ( sin t )
dt
i.e.
e N cos t
e N f cos t
e N f sin( t )
16. Cont. ..
In the above equation e1 attains maximum value when
therefore the maximum value E1 is given by
90 1
sin( t )
1 1
1 1
2 2
2
4 44
4 44
m
m
m
E N f
E . N f
Similary;
E . N f
18. V. Losses in a Transformer
The losses in a transformer are of two types;
i. Core losses or Iron Losses
ii. Copper Loss
Core or Iron Loss: is occurring due to alternating flux in the core. It
mainly consist of eddy currents and hysteresis loss. The core loss or
iron loss is constant for a transformer and does not vary with load.
Eddy Current loss is due to eddy current occurring in the core of
transformer. The core is laminated to reduce the eddy current loss.
Eddy current loss is given by
2 2 2
e e at
P K B f t V
19. Cont. ..
Hysteresis loss is due to frequent reversal of magnetic flux in the core. It is
given by
Where; Ke = Eddy current loss constant
= Hysteresis coefficient
f = frequency
V = volume of core
t = thickness of lamination
1 6
.
h m
P B fV
i e h
P P P
20. Cont. ..
Copper Loss: is due to resistance of the winding. It is the loss
occurring in the winding of transformer.
Where;I2 and I1 are secondary and primary current respectively
R1 and R2 are primary and secondary winding resistance
respectively
TOTAL LOSS ON TRANSFORMER = CORE LOSS + COPPER LOSS
2
I R
2 2
1 1 2 2
c
P I R I R
21. VI. Efficiency of a transformer
Efficiency is maximum when core loss = copper loss
Output _ power
Efficiency
Input _ power
Or
Output _ power
Efficiency
Output _ power Losses
24
24
kWh _ output _in _ hrs
Efficiency
kWh _input _in _ hrs
23. Advantages of Autotransformers
An autotransformer
Requires less Cu than a two winding transformer of similar rating
Operates at a higher efficiency than a two winding transformer of
similar rating
Has better voltage regulation than a two winding transformer of the
same rating
Has smaller size than two winding transformer of the same rating
Requires smaller exciting current than a two winding transformer of
the same rating
24. VII. Instrument Transformer
The original magnitude can be determined by just multiplying the
result with the transformation ratio. Such specially constructed
transformers with accurate turn ratio are called Instrument
transformer
These instruments transformers are of two types
i. Current transformer (CT)
ii. Potential Transformer (PT)
27. Questions
What will happen if the primary of a transformer is connected
on DC supply?
Why do we need transformers?
What are Applications of Transformer
28. 2. DC MACHINES
A transformer works on alternating current, while a DC
machine works on Direct current. A DC machine can be a DC
motor or DC generator. It has two parts:
i. Stator - it is the stationary part
ii. Rotor - it is a rotating part
