Download Auto transformer

GOKUL TECHNICAL CAMPUS
HANSABA COLLEGE OF ENGINEERING &
TECHNOLOGY, SIDHPUR
•Subject: DC Machine & Transformer (2130904)
Name:-Sajidali-130980109030,
Hardik 130980109026,
Manish:-130980109081,
Nishant-130980109047
Branch: Electrical Engineering
Division:- C1
Semester: 3rd
Year: 2014
Introduction Auto
Transformer
• Auto transformer is kind of electrical transformer where
primary and secondary shares same common single winding.
• The Normal Transformer has separate Primary and secondary
windings.
• But the autotransformer is a special number in which a part of
winding is common for the primary and secondary winding.
• The construction of an Autotransformer is shown in the fig.
• It consist of only one winding wound one a laminated
magnetic core with a rotary movable contact.
• So Voltage Per Turn Winding is V1
N1
Hence, the voltage across the portion BC of the winding, will be,
• V1*N2 And from the Figure above This voltage is V2
• N1
• Hence,V1*N2 =V2
N1
• V2=N2=constant=K
V1 N1
• Thus autotransformer can operate as a step down a step up
transformer.
• (1)Step up Transformer
• (2)Step Down Transformer
• The Connection of autotransformer as a step Down
Transformer is shown fig.
• It shows That the two fixed Terminals A and B are
connected to single phase AC supply V1, The winding
AB acts as a Primary Winding.
• A Part of the complete winding i.e. CB acts as
secondary Winding Across Which the load is
connected.
• The Operating Principle of an autotransformer is same
that of the normal transformer
•
•
•
•
V2=N2*V1
N1
There for the load voltage for this configuration is
given by.
Where N2=number of turns coresponding to
secondary i.e CB
N1=number of turns coresponding to
primary i.e AB
As the number of turns coresponding to winding
CB i.e. n2 is less then that of winding AB i.e.n1 this
configuration operates as step down transformer
• If we neglate an the losses the megnetistisng
circuit an the leckage reactaence then the
transformer ratio defined as
K=V2=N2=I2
V1 N1 I1
• I1= primary current
• I2=load current
• The connection of an autotransformer for
operating it as the step up transformer.
• Note that the part CB of the complete winding
acts as the primary winding. The ac input voltage
V1 is applied between these terminal.
• As the number of turns of winding AB is higher
than the number of turns of winding CB, the
autotransformer now acts as a step up
transformer.
• Neglecting the loss the magnetizing current and
leakage reactances, the load voltage V2 is given by,
V2=N2*V1
N1
• As only one winding is uses the copper required
for the transformer is very less
• The size and hence the cost is reduced as
ccompared to the conventional transformer
• The losses taking place in the winding are
reduced the efficiency is higher then the
conventional transformer
• Due to reduced resistance the voltage regulation
is better then the conventional transformer
• There is no electrical isolation between the primary
and secondary windings. This can prove to be
dangerous for high voltage applications.
• It posses a low impedance, hence if the secondary
circuit is short circuited, then a large current will flow
on the secondary side.
• If The common Part of CB The winding (windingCB)
breaks (open Circuit) Then The transformer action is
lost and full primary Voltage Apperas across the
secondry.
Applications of an Autotransformer
• It can be used as a variable, i.e. Variable ac supply to
vary the ac voltage applied to the load smoothly from
0V to about 270 V.
• In order to start the ac machine such as induction
motors or synchronous motors.
• To vary the supply voltage(as per requirements) of a
furnace.
• As a dimmerstat : when the variac autotransformer is
used to control the intensity of lamps in the cinema
halls etc., it is called as the dimmerstat.
Copper Savings in Auto Transformer
• Now we will discuss the savings of copper in auto transformer
compared toconventional two winding transformer.
• We know that weight of copper of any winding depends upon
its length and cross - sectional area. Again length of conductor
in winding is proportional to its number of turns and cross sectional area varies with rated current.
• So weight of copper in winding is directly proportional to
product of number of turns and rated currentof the winding.
• Therefore, weight of copper in the section AC proportional to,
(N1-N2)I1
and similarly, weight of copper in the section BC
proportional to,
N2(I2-I1)
Hence, total weight of copper in the winding of auto transformer
proportional to,
(N1-N2)I1+N2(I2-I1)
N1I1-N2I1+N2I2-N2I1
N1I1+N2I2-2N2N1
2N1I1-2N2I1
2(N1I1-N2I1)
and similarly, weight of copper in the section
BC proportional to,
Hence, total weight of copper in the winding of auto
transformer proportional to,
• In similar way it can be proved, the weight of
copper in two winding transformer is proportional
to,
N1I1 + N2I2⇒ 2N1I1
• (Since, in a transformer N1I1 = N2I2)
Let's assume, Wa and Wtw are weight of copper in
auto transformer and two winding transformer
respectively,
Auto transformer employs only single winding per
phase as against two distinctly separate windings in
a conventional transformer. Advantages of using
auto transformer. For transformation ratio = 2, the
size of the auto transformer would be
approximately 50% of the corresponding size of
two winding transformer. For transformation ratio
say 20 however the size would be 95%. The saving
in cost is of course not in the same proportion. The
saving of cost is appreciable when the ratio of
transformer is low, that is lower than 2.