application of auto-transformer and it's comparison with two winding transformer

application of auto-transformer

Auto-transformers is using were no problem when the electrical connection between primary and secondary and transformation ratio K is nearly equal to unity. Hence Trey is wildly used for ratio ranging from at 0.4 to 1.0.

Balance Coil

1. To get neutral in a 3-wire ac distribution system in the same way as a balancer set is used in a 3-wire dc distribution. The connection diagram is shown in below fig.If the load in the lower half of system is greater than on the upper half, the winding AC acts as the primary and winding BC acts as secondary to supply the extra load required by the lower half of the system. This phenomenon is known as balance coil.
2. To start induction motors and synchronous motors auto-transformer is used in which a number of tappings are there. Used in this application transformer called as auto-starters.
3. With a large number of voltage level, auto-transformer are used in a testing laboratory.
4. Auto-transformer has the biggest sphere of usefulness as regulating transformer.
5. They are used in raise the voltage of ac feeder like a booster.
6. As furnace transformers for getting a convenient supply to suit the furnace winding from normal 230 V ac supply.

3-phase auto-transformer

Auto-transformers are mainly used for inter connecting systems those are operating at roughly the same voltage. The inter connection for EHV system by the auto-transformer results in considerable saving of bulk and cost as compared to the conventional two winding transformers. Of course, a 3-phase auto-transformers will be required.

comparison of auto-transformer with two winding transformer

VA rating

See the fig. of a two windings transformer is connected as an auto-transformer, now compare the volt-ampere of the two.

Voltage rating of two windings transformer = (V₁ – V₂) I₁ = (I₂ - I₁) V₂

Voltage rating of auto-transformer = V₁ I₁ = V₂ I₂

Voltage rating of two windings transformer / Voltage rating of auto-transformer =

                                                                                                                   =V₁ I₁ / (V₁ – V₂) I₁

                                                                                                                                                                                                                                                                                                                                      = V₂ I₂ / (I₂ - I₁) V₂

                                                                                                                                                                                                                                                                                                                                      = 1/ (1 - K)        

From the above equation, it is seen that a two-winding transformer of a given volt-ampere rating when connected as an auto-transformer can handle high volt-amperes. This is because in the auto-transformer connection part of VA is conducted directly from the primary to the secondary.

Conductor material requirements

The X-section of conductor is proportional to the current be carried and length of the conductor in a winding is proportional to the number of turns. Hence the weight of the conductor material in the winding being proportional to product of cross-sectional area and length of the conductor, is proportional to the product of number of turns and current to be carried.

 In an ordinary transformer, the total weight of conductor material required is proportional to

(N₁ I₁ + N₂ I₂ ) α 2N₁I₁ because N₂I₂ =N₁I₁

In an auto-transformer the top section AC has (N₁ – N₂) turn and carries current I₁ and bottom section BC has turns N₂ and carries current I₂ - I₁.

So weight of conductor material in an auto-transformer having the same input (V₁I₁), the same output (V₂ I₂) and the same voltage ratio (V₂ / V₁) as that of the two winding transformer is proportional to 2(N₁ – N₂) I₁

So weight of conductor in an auto-transformer/ weight of conductor in a two winding transformer = 2(N₁ – N₂) I₁/ 2N₁I₁ =1- N₂/N₁ = 1-K

Hence saving in conductor material required for winding affected by using an auto-transformer

            = K× weight of conductor in a two winding transformer

From the above equation, it is obvious that nearer the ration of transformation is to unity, the greater is the economy in conductor material requirement. If V₂ = V₁ i.e. when K is equal to unity, the auto-transformer, according to equation (a), needs no conductor material, for actually no transformer is required. If voltage is to be transformed from very high voltage to very low voltage i.e. when K is to be very small, there will be a little saving in the conductor material and in such cases it is preferable to use an ordinary two-winding transformer.

core size

The voltage per turn, and, therefore the flux, in the same so the X-section of the core is not affected, however, the reduction in conductor material means a smaller widow area and, therefore, reduced core length. Thus, for the same core area the size ad weight of the core of the auto-transformer is reduce. Accordingly, the size and weight of the auto-transformer is reduced.

cost 

Saving in both conductor and core materials is affected by using an auto-transformer, as explained above in 2 point. The saving in material and cost, is, of course, less than that indicated, on account of the fact that the core is nearly as large as in case of the two-winding transformer. For transformation ration of 1/2, approximately 50% saving in conductor material would be affected, and the cost might be 65% to 70% of that of a two-winding transformer of the same rating.

losses and efficiency

Owing to the reduction in conductor and core materials, the ohmic losses in conductor and the core loss are lowered. Thus an auto-transformer has lower losses and, therefore, higher efficiency than those of the two-winding transformer of the same output.

Reference

Theory & Performance of electrical machine by J. B. Gupta