A Transformer, in which a part of the winding is common to both the Primary and Secondary circuit, is called an Auto Transformer. It shall be noted that in Two Winding Transformer, Primary and Secondary windings are electrically isolated but in Auto Transformer, the two windings are not electrically isolated.
Why do we need to go for Auto Transformer?
We have some advantages of auto-transformer over normal two winding transformers.
- Autotransformers usually smaller in size, because one winding is eliminated.
- As size is small cost also low(so cheap in cost)
- As the winding is same so leakage reactance will be less.
- Increased kVA rating.
Principle of operation of Auto Transformer
The principle of operation of the transformer is the same as the one of the common transformer, and then the relation between input and output voltages and input and output currents and the ratio of number of turns between the primary and the secondary winding is the same.
The currents of the primary and secondary windings are flowing on the opposite directions, so the total current flowing through the common part of the winding is equal to the difference between the current on the low-voltage winding and the current on the high-voltage winding.
A simplified diagram of a Step-down Auto Transformer is shown in figure below
As shown in the figure above, N1 and N2 are the number of turns between winding AB and AC respectively. If a voltage V1is applied across AB, an exciting current will start flowing through the full winding. Therefore, voltage per turn in winding AB is V1/N1and hence voltage across CB is (V1/N1)N2. As the load current is I2and the current taken from the source is I1, neglecting losses
Input Power = Output Power
V1I1CosØ1= V2I2CosØ2……………………….(1)
Assuming internal impedance drop and losses to be negligible, power factor for primary and secondary will be almost same.
CosØ1 = CosØ2
Therefore from equation (1),
V1I1 = V2I2
So, V2 / V1= I1 / I2 = N2 / N1 =k
Advantages and Disadvantages of of Auto transformer
Advantages:
Less costly
Better regulation
Low losses as compared to ordinary two winding transformer of the same rating
Disadvantages:
There are various advantages of the auto transformer, but then also one major disadvantage, why auto transformer is not widely used, is that
The secondary winding is not insulated from the primary winding. If an auto transformer is used to supply low voltage from a high voltage and there is a break in the secondary winding, the full primary voltage comes across the secondary terminal which is dangerous to the operator and the equipment. So the auto transformer should not be used to for interconnecting high voltage and low voltage system.
Used only in the limited places where a slight variation of the output voltage from input voltage is required.
Applications of Auto transformer
It is used as a starter to give upto 50 to 60% of full voltage to the stator of a squirrel cage induction motor during starting.
It is used to give a small boost to a distribution cable, to correct the voltage drop.
It is also used as a voltage regulator
Used in power transmission and distribution system and also in the audio system and railways.
