Auto Transformer

It is a transformer with one winding, in which part of winding is common to both primary and secondary. Primary and secondary are not electrically isolated from each other as in case with a 2 winding transformer. But its theory and operation are similar to those of a two-winding transformer. Because of one winding, it uses less copper and hence is cheaper. It is used where transformation ratio differs from unity. Figure 1.56(a) and 1.56(b) shows step down and step-up auto transformer.

Figure 1.54(a) AB is primary winding having N, turns and BC is secondary winding having N₂ turns. Neglecting iron losses and no-load current.

V₂ / V₁ = N₂ / N₁ = I₁ / I₂ = K

The current in section CB is difference of I₁ and I₂. But as the two currents are practically in phase opposition, the resultant current is (I₂ – I₁). (i.e) I₂ >I₁. As compared to an ordinary 2-winding transformer of same output, an auto transformer has higher efficiency but smaller size. Its voltage regulation is much better.

Saving of Cu: Volume and weight of cu, is proportional to the length and area of the cross section of the conductors. Now length of conductor is proportional to the number of turns and cross section depends on current. Weight is proportional to the product of the current and number of terms.

Weight of Cu in section AC is α (N₁ - N₂) I₁;

Weight of Cu in section BC is α N₂ (I₂ − I₁).

Total weight of Cu in Auto-transformer α (N₁ - N₂) I₁ + N₂ (I₂ – I₁).

If a two-winding transformer were to perform the same duty, then weight of Cu on its primary α N₁I₁ ; ωt of Cu on secondary α N₂I₂.

Total weight of Cu α N₁I₁ + N₂I₂

Hence, saving will increase as K approaches unity. It can be proved that power transformed inductively input (1 - K). The rest of the power = (K × input) is conducted directly from the source to the load. i.e., It is transferred conductivity to the load.

Uses:

1. To give small boost to a distribution cable to correct the voltage drop.

2. As auto-starter transformers to give upto 50 to 60% of full voltage to an induction being motor during starting.

3. As furnace transformers for getting a convenient supply to suit the furnace winding from a 230 V supply.

4. Interconnecting transformers in 132 kV/330 kV system.

5. In control equipment for 1-phase and 3-phase electrical locomotives.

Advantages of using Auto Transformer

1. 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 in cost of the material is of course not in the same proportion. The saving of cost is appreciable when the ratio of transformer is low, that is lower than 2. Thus auto transformer is smaller in size and cheaper.

2. 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.

3. Auto transformer has better voltage regulation as voltage drop in resistance and reactance of the single winding is less.

Disadvantages of using Auto Transformer

1. Because of electrical conductivity of the primary and secondary windings the bar 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.

2. The leakage flux between the primary and secondary windings is small and hence the impedance is low. This results into severer short circuit currents under fault conditions.

3. The connections on primary and secondary sides have necessarily needs to be same, except when using interconnected starring connections. This introduces complications due to changing primary and secondary phase angle particularly in the case of delta/ delta connection.

4. Because of common neutral in a star/star connected auto transformer it is not possible to earth neutral of one side only. Both their sides should have their neutrality either earth or isolated.

5. It is more difficult to maintain the electromagnetic balance of the winding when voltage adjustment tappings are provided. It should be known that the provision of tapping on an auto transformer increases considerably the frame size of the transformer. If the range of tapping is very large, the advantages gained in initial cost is lost to a greatevent.

Three Phase transformers are widely used as Power transformers, Distribution transformers and in Electrical Grids.

1. When we generate the Power using an alternator(AC), the voltage at which it is generated is of mostly 11KV( Sometimes a bit more than that but not too high due to insulation constraints). For transmitting purpose we install a transformer just after the generator so that it can step up/down tge voltage). Moreover, it is better to generate in 3 phase rather than 1 phase due to many advantages. Therefore, the transformer you have to use should be a 3 phase one.

2. Some loads such as industrial, and commercial require 3 phase transformers to meet their demand.

3. The transmission voltage such as 132/220 KV, we need step up/down in the both ends of Transmission lines. There are lots of applications of 3 phase transformers, I am just giving an idea of its major application. Hope it'll help you.