• This is having only one winding; part of this winding is common to both primary and secondary.
• In 2-winding transformer both primary and secondary windings are electrically isolated, but this is not in the case of autotransformer.
• The operations of both the transformers are similar.
• It is cheaper than the 2-winding transformer because usage of copper is less.
• It is used where the transformation ratio is less than the unity.
• It is smaller in size but has higher efficiency and superior in voltage regulation compared to 2-winding transformer.
• In a step down autotransformer, BC is the primary winding having N1 number of turns CE is the secondary winding having N2 number of turns.
• If we apply a voltage V1 to the coil BC, alternating magnetic flux will be produced in the core
• When load is connected between terminals E and C power will be supplied to it.
Neglecting iron losses and no load current,
• The current in section CE is the vector difference of current in the section BE (I1) and load current (I2).
• Since these two currents are practically in phase opposition.
• The resultant current is (I2 – I1) where I2 is greater than I1.
Apparent power supplied to the load = V2 I2 volt-ampere
= V2 I1 + V2 (I2 – I1) VA.
Here V2I1 is the portion of the apparent power supplied to the load through the portion BE by conduction and portion V2 (I2 – I1) is the power supplied to the load through the portion CE by induction.
Advantages of Auto transformer
1. Only one winding is used in the autotransformer. Therefore weight of the core material and volume of copper required reduces which results in low cost.
2. Power losses are less. So efficiency will be high
3. Higher KVA rating
4. Lower percentage reactance. Hence better voltage regulation
5. Can be used for obtaining variable voltage supply.
Disadvantages of Auto transformer
1. In autotransformers, the high voltage winding and low voltage winding is not electrically isolated. There is a electrical connection between those two windings.
2. If there is an accidental open circuit in the low voltage winding the voltage of the high voltage winding affects the load side.
3. Short circuit current due to external short circuit in auto transformers will be twice that of short circuit current in two winding transformer.
4. As a result of higher short circuit currents, the auto transformer winding need higher mechanical strength than that of the two winding transformer.
5. Autotransformer winding need more insulation than that of two winding transformer.
Applications of auto transformer
Autotransformers are used:
1. To give small boost to a transmission line to correct the voltage drop.
2. To interconnect two grids which have different voltage ratings (3 phase auto transformers)
3. As starters for 3 phase induction motors.
4. To give smooth variation of voltage to test circuits in the laboratories.
5. As furnace transformers for getting convenient supply to suit the furnace winding from a 230 volt supply.
6. As control equipment for single phase and 3 phase electric locomotives.
All day efficiency
Large capacity transformers used in power systems are classified broadly into Power transformers and Distribution transformers. The former variety is seen in generating stations and large substations. Distribution transformers are seen at the distribution substations.
The basic difference between the two types arise from the fact that the power transformers are switched in or out of the circuit depending upon the load to be handled by them. Thus at 50% load on the station only 50% of the transformers need to be connected in the circuit. On the other hand a distribution transformer is never switched off. It has to remain in the circuit irrespective of the load connected. In such cases the constant loss of the transformer continues to be dissipated. Hence the concept of energy based efficiency is defined for such transformers. It is called 'allday' efficiency. The allday efficiency is thus the ratio of the energy output of the transformer over a day to the corresponding energy input. One day is taken as duration of time over which the load pattern repeats itself. This assumption, however, is far from being true. The power output varies from zero to full load depending on the requirement of the user and the load losses vary as the square of the fractional loads. The no-load losses or constant losses occur throughout the 24hours.Thus,the comparison of loads on different days becomes difficult. Even the load factor, which is given by the ratio of the average load to rated load, does not give satisfactory results. The calculation of the all day efficiency is illustrated below with an example. The graph of load on the transformer, expressed as a fraction of the full load is plotted against time in Fig. 27. In an actual situation the load on the transformer continuously changes. This has been presented by a stepped curve for convenience.
Hence a better option would be to keep the constant losses very low to keep the allday efficiency high. Variable losses are related to load and are associated with revenue earned. The constant losses on he other hand has to be incurred to make the service available. The concept of all day efficiency may therefore be
more useful for comparing two transformers subjected to the same load cycle. The concept of minimizing the lost energy comes in to effect right from the time of procurement of the transformer. The constant losses and variable losses are capitalized and added to the material cost of the transformer in order to select the most competitive one, which gives minimum cost taking initial cost and running cost put together. Obviously the iron losses are capitalized more in the process to give an effect to the maximization of energy efficiency. If the load cycle is known at this stage, it can also be incorporated in computation
of the best transformer.
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