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TRANSFORMER PRINCIPLE MUTUAL INDUCTION
AUTO TRANSFORMER SELF INDUCTION
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.
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.
Auto transformer has single winding which acts both as primary and secondary
