This document discusses three-phase transformers and auto transformers. It provides details on their construction, working principles, types of connections, advantages, disadvantages, applications and limitations. Three-phase transformers use three cores arranged 120 degrees apart to provide three-phase power with zero net flux. Auto transformers have a single winding that acts as both the primary and secondary sides, requiring less copper. They are more efficient but present safety issues for step-down voltages due to the direct connection between primary and secondary.

1.  Introduction
 Construction
 Principal
 Working
 Advantages
 Disadvantages

2.  Generation of electrical power of usually three phase and at
higher voltage Like 13.2Kv,22Kv Or some what higher
 Similarly transmission of electrical power is also Very high
voltage like 110Kv, 132Kv,400Kv
 To setup the Generated voltage for transmission Purpose it is
necessary to have three phase Transformers

4.  The three cores are arranged at 120 degree from each other.
Only primary windings are shown On the cores for the
simplicity
 The primary connected to to the three phase supply
 The three fluxes are zero at any instant

5.  Hence the centre leg does not carry any flux
 So if centre leg is removed, any two legs Provide the return
path of the current and ,hence The flux in the third leg
 This is the general principle used in the design of 3 phase core
type transformer

6.  The primary and secondary winding of 3 phase transformers
as three phase windings can be connected in different ways
Such as in star or in delta with suitable connection the voltage
Can be raised or lowered
 In this section Some commonly user connections For 3 phase
transformer are discussed

7.  Star-star connection
 Delta-Delta connection
 Star-Delta connection
 Delta-Star connection

14.  It Occupies less space
 Its weight is Less
 Its cost is comparatively less
 It is much easy to transport
 It is more efficient
 Its core size is comparatively small

15.  When One of the phases of a Single unit 3 phase transformer
becomes faulty,The entire Unit of three phase transformer
Needs to removed from the Supply for repair
 But in the case of bank Of the three phase transformer, only
the faulty transformer is removed From the supply for the
repair While the other two transformers Remains connected
in the system Without interrupting The supply completely

16.  Introduction
 Types of auto transformers
 Advantages of auto transformers
 Disadvantages of auto transformers
 Applications of auto transformers
 Limitations of auto transformers

17. Introduction
• An auto transformer is an electrical
transformer with only one winding
• In an auto transformer,Portions of the same
winding,Act as both Primary and
secondary Sides of the transformers

18.  Step up autotransformers
 Step down auto transformers

19. • An Auto transformer in which Ns>Np is called step up auto transformer
• An Step up auto transformer Is a transformer in which convert low alternating current
into high alternating current

20. • An auto transformer in which Np>Ns is called step down auto transformer
• An step down autotransformer is an transformer which converts low low alternating
voltage into high alternating voltage

21.  An auto transformer requires Less copper Then two windings
transformers Similar ratings
 An auto transformers operates at a higher efficiency Then two
windings transformers of similar ratings
 An auto transformer has better rating of voltage regulation then
two winding transformer of similar rating
 An auto transformer have a smaller size then two winding
transformer of similar rating
 An auto transformer requires less exciting current,than Two
winding transformer of similar rating

22.  There is a direct connection between primary and secondary
windings, therefore The output is no longer D.C Is isolated
from the input
 An auto transformer is not safe for Stepping down high
voltage into low voltage
 The short circuit current Is much larger than for two windings
Transformers of same rating,So that the short circuited
secondary Part of the primary also To be short circuited. This
reduces the effective resistance and reactance

23.  Auto transformers are used to compensate For voltage in
transmission and distribution line. When used for this purpose
they are also called as booster transformers
 Auto transformer are used to to reducing the the voltage
supplied to AC motors during starting time
 Auto transformers are used for continuously varing supply
 In control equipment for or one phase and three phase
locomotive electricals

24.  IT requires both fever windings and smaller core , an auto
transformer for power application is typically lighter And less
costly then two winding transformer upto the voltage ratio as
3:1
 Like multiple winding transformers,Auto transformers operates
on time varying Magnetic fields and so will not function in DC
 A failure of the installation of the windings of the transformer
Can result in full input voltalge Applied to output .also a break
In the part of the winding That is used as both primary and
secondary Will result in the transformer acting As an inductor
with load