AUTO TRANSFORMER

1) What is auto transformer? What is the difference between an auto transformer and a two winding transformer?

Auto transformer also called as auto-former is an electrical transformer where primary and secondary shares same common single winding. So basically it’s a one winding transformer

The main difference between conventional two winding transformer and auto-transformer is their working principle and hence application.  In an auto -transformer energy transfer is mainly through conduction process and only a small part is transferred inductively

Auto-transformer works on the principle of induction and conduction together.

(Two winding transformer works on principle of electro-magnetic induction only)

NOTE:

Conduction involves a flow of electric charge due to an electric field. To conduct a current from one conductor to another, the two conductors should be in contact. In conduction, current is generated when an electric field passes through the conductor.

In induction, a current can be made to flow in a conductor by keeping it in the vicinity of another conductor, carrying a constantly varying current. In induction, current is generated when a magnetic field changes around a conductor.

2) Why is an auto transformer not used as distribution transformers as it is more efficient than a normal transformer?

ANS) Distribution transformers are normally 11 kV/ 400 Voltage level transformer. Auto-transformer is typically used when the voltage ratios are close to 1. (Max < 2). They are used for stepping down voltage from such as 220kv to 132kv. As the ratio on distribution side is too high from (11000 to 400v) therefore auto transformer is not used on distribution side.

3) Why autotransformer is used when voltage ratio is maximum 2?

ANS) 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 of cost is appreciable when the ratio of transformer is low, that is lower than 2.

4) What is transformation ratio in auto transformer?

Transformation ratio is the ratio of secondary voltage to the primary voltage and is equal to the ratio of the number of turns in the secondary winding to the number of turns in the primary winding

K = N2/N1 = E2/E1

K = N2/N1 = E2/E1

5) Why the short circuited current in an auto transformer is more compared to a two winding transformer?

ANS) 2 winding transformer has got electrical isolation. That is there is no electrical short between primary and secondary winding in auto transformer. Let’s take an example:

Consider a 10-kva transformer rated at 2300/230 volts which will have a leakage impedance of say 4 %. If it is connected as an auto transformer stepping down from say 2530 to 2300 volts which corresponds to a transformation ratio of 1.1 (2530/2300=1.1) , the impedance drop will be:

4*(0.1/1.1)= 0.364%

Its leakage impedance is 9.1 % of the leakage impedance of 2 winding transformer

Therefore short circuit current in an auto transformer is more compared to a two winding transformer and in order to protect auto transformer from the hazards of high amount of short circuit current, a current limiting is inserted.

6) What are the Advantages of auto transformer over distribution transformer?

ANS) Autotransformer is a transformer where primary and secondary shares same common single winding. So basically it is a one winding transformer.

The cost and as well as size is reduced. They are cheaper, lighter in weight, lower leakage reactance, lower losses and lower excitation current.

Autotransformers are clearly much cheaper than conventional double wound transformers of the same VA rating. When deciding upon using an autotransformer it is usual to compare its cost with that of an equivalent double wound type.

This is done by comparing the amount of copper saved in the winding. If the ratio “n” is defined as the ratio of the lower voltage to the higher voltage, then it can be shown that the saving in copper is: n.100%. For example, the saving in copper for the two autotransformers would be:

7) What are Disadvantages of Auto transformer?

·        The main disadvantage of an autotransformer is that it does not have the primary to secondary winding isolation of a conventional double wound transformer. Then an autotransformer can not safely be used for stepping down higher voltages to much lower voltages suitable for smaller loads.

·        If the secondary side winding becomes open-circuited, current stops flowing through the primary winding stopping the transformer action resulting in the full primary voltage being applied to the secondary terminals.

·        If the secondary circuit suffers a short-circuit condition, the resulting primary current would be much larger than an equivalent double wound transformer due to the increased flux linkage damaging the autotransformer.

·        Since the neutral connection is common to both the primary and secondary windings, earthing of the secondary winding automatically Earth’s the primary as there is no isolation between the two windings. Double wound transformers are sometimes used to isolate equipment from earth.

8) what are the applications of auto transformer?

Autotransformer has many uses and applications including the starting of induction motors, used to regulate the voltage of transmission lines, in audio applications and impedance matching between such as a low impedance microphone and a high impedance amplifier input.

Auto transformer is used as variac in laboratory or where continuous variable over broad ranges are required.

9) Should there be only one tapping from auto transformer?

Autotransformer can also be constructed with more than one single tapping point. Auto-transformers can be used to provide different voltage points along its winding or increase its supply voltage with respect to its supply voltage VP as shown.