Transformer are extensively used for transmission and distribution of power and transform energy from one voltage level to another voltage. Conventional transformer do not suit for many application and for every specific purpose some modifications are required to be made according to the needs. In such case special transformer are used in account. There are several special transformer according to the area of application one of them is Auto Transformer.
Auto transformer has same operating principle and construction as that of two-winding transformer. The auto-transformer differs from a conventional two-winding transformer in a way in which the primary and secondary are interrelated.In case of conventional two-winding transformer the primary and secondary are completely insulated from each other but are magnetically linked by a common core. But in case of auto transformer primary as well as secondary are connected electrically as well as magnetically, a part of the single continuous winding is common to primary and secondary where the single winding is wound on a laminated silicon steel core and both primary and secondary sections of this one winding are on the same magnetic circuit.
AB=primary winding consisting of whole turns between terminals A and B i.e N1
V1=voltage applied across primary terminal A and B
Winding AB is tapped at C.
BC=secondary winding consisting of turns between terminals B and C ie.N2
V2=voltage applied across secondary terminal B and C
Neglecting the internal impedance drop,voltage per turn is V1/N1.
voltage across BC is
Equivalent circuit of Auto transformer
The equivalent circuit of auto transformer is shown above. Where
Hence the ratio of current in the parts of winding is (k/1-k).
The ratio of induced emfs in the parts of the winding CB and AC is
Hence the auto transformer with a turn ratio of K is equivalent in its transformation to ordinary transformer with a ratio of k/1-k.
Copper saving in Auto transformer
Since “cross section of conductor(copper or aluminium) α current to be carried out”. Also “Length of the conductor in a winding α number of turns”.
Therefore, “Weight of the conductors material in the winding α product of cross-sectional area and length of the conductor α product of number of turns and current to be carried outs”
The total weight of conductor material required is proportional to
The top section Ac has (N1-N2) number of turns and carries current I1 whereas bottom section BC has N2 number of turns and carries current I2-I1.
so weight of conductor material in auto transformer is proportional to
Therefore the nearer the ratio of transformation is to unity,the greater is the economy in conductor material requirement. If V2=V1 then K is equal to unity then for auto transformer no conductor material is needed. If k is to be made very small there will be a little saving in the conductor material.
Advantage of Auto transformer
- For the same core area the size and weight of the core of the auto transformer is reduced.
- For transformation ratio = 2, the size of the auto transformer would be approximately 50% of the corresponding size of two winding transformer. 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. For transformation ratio of 1/2, approximately 50 percent saving in conductor material and cost might be 65 to 70 percent of that of a two winding transformer of the same rating.
- Auto -transformer has lower losses and therefore higher efficiency than those of the two-winding transformer of the same output.
- Auto transformer has the lower value of leakage impedance and has better voltage regulation than a two-winding transformer of the same volt amperes output.
Disadvantage 0f Auto transformer
- If an open circuit develops in common winding BC, the supply voltage would be impressed directly upon the external load circuit in series with the AC portion of the winding. With a step-down transformer this could results in burden out secondary loads/serious shock hazard, if voltage ratio V2/V1 is quite low.
- Auto transformer is never used for interconnecting hv and lv systems. If an auto transformer is used to supply a low voltage system from an high voltage system, it is necessary to earth the low voltage side.
Applications of Auto transformer
- Auto transformer are used when transformation ratio k is nearly equal to unity ranging from 0.4 to 1.
An auto transformer used in above is balance coil. The balance coil is cheaper and more efficient than balancer set. If the load on the upper half of the system is greater than on the lower half, the winding BC acts as the primary and winding AC acts as secondary to supply the extra load required by the upper half of the system.
- Auto -transformer with a number of tapping are used for starting induction motors . Auto transformer used for this purposes are auto-starters.
- A continuously variable Auto transformer are used in electrical testing laboratory.
- Auto transformer are used as regulating transformers.
- Auto-transformers are also used as boosters to raise the voltage in an ac feeder.
- Auto transformer are mainly used for interconnecting systems that are operating at roughly the same voltage .For example, the interconnection of EHV system(220kv amd 132kv) by auto transformer results in considerable saving of bulk and cost as compared to the conventional two winding transformers.