CHAPTER 1
INTRODUCTION
This chapter covers the motivation, objectives and general introduction to the thesis.
1.1 Problem Statement
Electrical power is being transmitted from one place to another in the whole world.
Generally, the transmission voltages are kept high to keep the transmission losses
minimum. This is because the current is low at high voltages. Lower current means lower
copper loss or heat loss or in simple words, saves our electrical power.
Voltages are being varied for different reasons. We also know that high power
consumption devices take more current. When more current is being drawn, then voltages
are also dropped. This can be observed from the equation:
𝑉 = 𝐸 − 𝐼𝑅
Here, V is the output voltage of the source, or the supply voltage, E is the internally
generated EMF, I is the current being drawn and R is the internal resistance or the internal
resistance plus the resistance to support end. As the current drawn increases, the value of
IR increases and so, receiving end voltages are dropped.
Now this drop-in voltage at the receiving side, is a problem for the equipment as
well as the transmission. The equipment may be voltage sensitive. Low voltages can cause
burnouts and many other problems. As the power to be consumed is an approximate
constant i.e.
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𝑃 = 𝑉𝐼
Where P is the power rating of the equipment, V is the voltage rating of the equipment and
I is the current that will flow due to the load impedance. As V drops, I must increase in
order to feed the load its required power. Higher I not only, is bad for the equipment but is
also bad for transmission. As seen above, higher currents cause source voltages to drop,
and now higher voltage drops with occur in the transmission line as well and energy will
be lost in the form of heat. More energy will be lost in the transmission line than before.
Moreover, the transmission line’s current rating might get exceeded and the wire may burn
or get damaged.
As seen from the above examples, low voltages at the receiving end are a big
problem. The problem can be solved by the use of Autotransformers in particular.
1.2 Transformer
A transformer is a static (no moving parts involved) electrical device that transfers
electrical power from one circuit to another. It works on the principle of mutual induction.
A transformer can either be used to step up the voltage (step-up transformer) or reduce
voltage levels (step-down transformer). In some cases, the transformer may also be used
for isolation purposes. This is done by keeping the same turn ratio on primary and
secondary side. The voltage level is directly proportional to the turns on a certain coil. Such
a transformer is also termed as one to one transformer. The electrical power transferred
from one circuit to another is the same, the variation is just in the voltage levels. If voltage
increases, current decreases and if voltage decreases, current increases. The power being
transmitted, is the same in both cases.
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This is the device that is used to increase AC voltage levels at the supply end to
reduce transmission losses. The transformer used at the supply end is called a step-up
transformer and consequently, a step-down transformer is used at the receiving side. There
is no electrical link between the primary and secondary sides of a transformer. Both sides
are linked through magnetic flux. A general transformer schematic is presented in Figure
1. The transformer power relation can be seen from the equation below.
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𝑁𝑠
𝑉𝑠
𝐼𝑝
=
=
𝑁𝑝
𝑉𝑠
𝐼𝑠
Here,
Ns = Number of turns of secondary coil
Np = Number of turns of primary coil
Vs = Secondary voltage level
Vp = Primary voltage level
Ip = Primary circuit current
Is = Secondary circuit current
ZL= Load Impedance
Figure 1-1: Simple Transformer Schematic
1.3 Auto Transformer
Auto transformer is a transformer with one winding only. The same winding acts both, as
primary and secondary winding. Auto transformers have several electrical contact points also
called tappings or taps. An auto transformer has at least 3 taps. The more taps a transformer has,
the more voltage levels can it be adjusted to. Taps are made on certain turns of the coil. As turns
are directly proportional to voltage levels, connecting to each turns gives a different voltage level.
The most basic structure of an auto transformer can be found in Figure 1-2.
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Figure 1-2 Structure of an Auto Transformer
As seen in Figure 1-2, the transformer has only one winding and has several taps on the same
winding. The secondary side may be connected to any of the taps and that is how voltage levels
are varied with the use of an Auto transformer.
1.4 Variable Transformer (Variac)
A variable transformer, also called a Variac, is a device that is used to provide smooth
varying output voltages. Variac (Variable AC) has just a single primary winding as it is an auto
transformer. On top of that winding, is a tapping or a contact point connected to a knob that can
be moved over the primary winding.
The knob is the variable selector switch that can slide across any portion of the primary winding.
The voltages at output side will be continuously updated as the position of the knob changes.
The Variac may go up to 300% or even more of their mains value. This means that the
output voltage levels can be anywhere between zero and three times the value of supply voltage.
The difference between Variac and having different taps, is that of output voltage variation. Using
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taps, the transformer can only vary between specific voltage levels and cannot transition smoothly
from one voltage level to another. Variac can do the same job smoothly and the exact desired
voltage level can be obtained that lies within the working capacity of the Variac.
Figure 1-3: VARIAC Schematic
Here,
Vp = Primary Supply Voltage
Np = Number of turns of primary coil
Ip = Current flowing from primary coil
Ns = Number of turns of secondary coil
Vs = Secondary Output Voltage
Isp = Current in primary coil due to secondary tapping
RL Load = Resistive Inductive Load
The secondary tapping point can be moved on the primary turns smoothly. This results in
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a continuously updated varying output voltages. The contact point tapping is a carbon brush which
has its own wear and tear. When a current Ip flows through the single winding, the secondary
current Is flows in a direction opposite to Ip. A portion of this winding generates the Vs. The
current flowing in the portion is therefore Ip – Is.
Like conventional transformers, there is no electrical isolation in auto transformer as there
is only one physical winding. Also, short circuits in an auto transformer are worse than short circuit
in a normal transformer.
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