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What is the difference between a transformer and auto transformer ?

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Question added by Avinash pradeep , Electrical engineer , orion battery
Date Posted: 2016/02/04
Shankar Kumar
by Shankar Kumar , Senior Testing and commissioning Engineer , ENGSOL

TRANSFORMER PRINCIPLE MUTUAL INDUCTION

AUTO TRANSFORMER SELF INDUCTION

Ahmed Fathy Mohamed Abdel Rahim
by Ahmed Fathy Mohamed Abdel Rahim , Electrical Engineer - Operation and Maintenance , “KFUPM” King Fahd University of Petroleum and Minerals

What is the difference between autotransformer and two winding transformer?

Definition

Autotransformer is a transformer having only one winding, a part of which acts as a primary and the other as a secondary.

Two winding Transformer is a static machine which transfers electrical energy from one end to another without changing frequency.

Number of Windings / coils

Autotransformer has only one winding wound on a laminated core.

Two winding transformer has two separate winding, i.e., primary and secondary winding.

Sharing of winding

In Autotransformer primary and secondary share same winding.

In two winding transformer primary and secondary have separate windings.

Windings/Phase

Autotransformer consists of single winding per phase .

Two winding transformer consists of pair of winding per Phase

Amount of Winding Material

In Autotransformer Winding Material requirement is less. Two winding transformer Winding Material requirement is more

Methods of varying output voltage

In Autotransformer the output voltage for a given constant input voltage can be varied from zero to the maximum level by simply varying the number of secondary turns.

In Two winding transformer output voltage can be varied only by varying the input voltage or by tap changing.

Methods of Energy transfer

In Autotransformer energy transfer occurs by both conduction and induction.

In Two Transformer energy transfer is only through induction.

Excitation current Requirement

In Autotransformer Excitation current requirement is small.

In two winding Transformer excitation current requirement is larger

Source & load connection

Load connected to the auto transformer is electrically connected with source. load connected to

transformer is electrically isolated from the source.

winding material

winding material requirement is less in an auto transformer.

winding material requirement is more in Two winding Transformer.

Efficiency

Autotransformer is more efficient and economical.

Two winding Transformer are less efficient than auto transformer.

Insulation of windings

In Autotransformer the primary and secondary windings are not electrically insulated.

In two winding Transformer primary and secondary winding are electrically insulated from each other.

Working principle

Autotransformer works on self induction rule.

In two winding Transformer works on mutual Induction

Weight & Size

Autotransformer are Lighter in weight and smaller in physical size as it requires fewer windings and a smaller core.

In two winding transformers has Large weight and big in physical size as it requires many windings and a big core.

Power Transfer

In Autotransformer power transfer takes place partly by transformation and partly by direct electrical connection.

In two winding Transformer power transfer takes place by transformation

Voltage Regulation

In Autotransformer Voltage Regulation is better.

In two winding Transformer Voltage Regulation is good.

Primary & secondary connection

In Autotransformer The primary and secondary winding are connected both electrically and magnetically

In two winding Transformer The primary and secondary winding circuits are connected magnetically.

Load Connection

In Autotransformer load connection depends upon the tapping.

In two winding Transformer tappings are connected directly to the load

Leakage flux and resistance

In Autotransformer it is low.

In two winding Transformer it is high

Impedance

In Autotransformer the leakage flux between the primary and secondary windings is small hence the impedance is low.

Two winding Transformer The leakage flux between the primary and secondary windings is high hence the impedance is high.

Output voltage

In Autotransformer it is variable.

In two winding Transformer it is constant.

Cost

Autotransformers are cheap.

Two winding Transformer are costly

Applications

Autotransformer are used as a starter for an induction motor, as a voltage regulator, in railways, in a laboratory.

Two winding Transformer are used in power system for step up and step down the voltage.

Industrial use

Autotransformer are not commonly used in industries because of it's hazard to the load as it directly connects load when failure comes & they have low current ratings and high voltage rating.

Two winding Transformer don’t have such problem

Common Input & output terminal

In Autotransformer there will be one terminal that is common between the input and output.

In two winding Transformer There is no common terminal between input and output.

Voltage ratio & power level

Autotransformer are used only for a small ratio or small power levels.

Two winding Transformer are used only for a large ratio or large power levels.

Inrush current

Autotransformer has lesser inrush current due to self induction.

Two winding Transformer has large inrush current due to mutual induction.

Voltage & current ratings

Autotransformer have low current ratings and high voltage rating.

Two winding Transformer have high current rating, less voltage rating but same power 

Frame size

It should be known that the provision of tapping on an auto transformer increases considerably the frame size of the transformer.

There is no such problem in two winding transformer

Variable current or voltage

Auto Transformer is a variable voltage and variable current Transformer.

Two winding Transformer supply fixed voltage and current

Phase angle

In Auto Transformer connections on primary and secondary sides needs to be same. This introduces complications due to changing primary and secondary phase angle particularly in the case of delta/delta connection.

In two winding transformers there is no such problem

Electro magnetic balance

In Auto Transformer it is more difficult to maintain the electromagnetic balance of the windings when voltage adjustment tapping are provided.

In two winding transformers it is not difficult to maintain the electromagnetic balance of the winding when voltage adjustment tapping are provided.

Isolation between primary & secondary

In autotransformer there is no isolation between primary and secondary

In two winding transformers there is isolation between primary and secondary

Winding insulation failure

In autotransformer a failure of the winding insulation will result in full input voltage applied to the output.

In two winding transformers there is no such problems

Electrocution

This problem may exist in auto transformer.

This problem doesn't exist as two winding transformer consists of a primary and a secondary coil winding that are electrically separated. This limits the risk of electrocution when the active parts and the earth are touched simultaneously.

Design consideration

In autotransformer because of electrical conductivity of the primary and secondary windings the lower voltage circuit is liable to be impressed upon by higher voltage. To avoid breakdown in the lower voltage circuit, it becomes necessary to design the low voltage circuit to withstand higher voltage.

There is no such consideration in two winding transformer.

Efficiency

An auto transformer has higher efficiency than two winding transformer. This is because of less ohmic loss and core loss due to reduction of transformer material.

Two winding transformer are less efficient compared to auto transformer.

Large transformation ratio application

Auto transformer cannot be used in such application.

Two winding transformer can be used in such application.

Short circuit current

Auto transformer has low impedance, this results into a high short circuit currents under fault conditions.

Two winding transformer has high impedance, this results into a low short circuit currents under fault conditions.

 

 

Thabet Almezwk
by Thabet Almezwk , QC/QA Engineer , Alfanar Electrical system

the number of coils, auto transformer has one coil

auto transformer

1) It consists of  single winding per phase (2) primary and secondary share same winding. (3) the output for given constant input voltage can be varied from zero to the maximum level by simply varying the number of secondary turns.(4) energy transfer occurs by both conduction and induction.(5) excitation current requirement is small.(6)load connected to  the auto transformer is electrically connected with source .(7) winding material requirement is less in an auto transformer. (8( it is more efficient and economical . (9) it is mostly used for voltage range abovekV.

 

   Transformer( two winding transformer)    

(1)  It consists of pair of winding per phase.(2) primary and secondary have separate  windings.(3) the output can be varied only  by varying the input  voltage or by tap changing.(4) energy transfer is only through induction.(5) excitation current is larger.(6) load connected is electrically isolated from the source.(7) winding material requirement is more.(8)  less efficient than auto transformer.                                                                                                    

Muhammad Saad Khan
by Muhammad Saad Khan , Internee , SMEDA

Auto transformer has single winding which acts both as primary and secondary

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