# Voltage and current relationship of an auto transformer

### What is Auto Transformer ?

Figure1 – Boosting autotransformer connection When the secondary winding delivers a load current of I2 Ampere the demagnetizing ampere turns is I2T2. voltages in relationship to windings in an autotransformer. The use of a 2-winding transformer as an autotransformer. 7. The connection of three identical single phase transformers to be used in 3-phase system. to the coil (between points B and C), voltage per turn will be / = 2 V. If we take out a much current can be supplied to a load at V? From the given rating of . ampere! This ingenious thought led to the invention of an auto transformer. Fig. When the secondary winding delivers a load current of I2 ampere the demagnetizing ampere transformer type of connection can be used with advantage, to obtain higher output. primary winding, the output voltage becomes (V1 + V2) volt.

It is used quite extensively in bulk power transmission systems because of its ability to multiply the effective KVA capacity of a transformer. The connection is shown in Figure 1 below.

Figure1 — Boosting autotransformer connection The primary and secondary windings of a two winding transformer have induced emf in them due to a common mutual flux and hence are in phase. This prompted the use of a part of the primary as secondary. This is equivalent to common the secondary turns into primary turns. Total number of turns between A and C are T1. At point B a connection is taken.

Section AB has T2 turns. As the volts per turn, which is proportional to the flux in the machine, is the same for the whole winding, V1: T2 When the secondary winding delivers a load current of I2 Ampere the demagnetizing ampere turns is I2T2. The cross section of the wire to be selected for AB is proportional to this current assuming a constant current density for the whole winding. Thus some amount of material saving can be achieved compared to a two winding transformer. The magnetic circuit is assumed to be identical and hence there is no saving in the same.

To quantify the saving the total quantity of copper used in an autotransformer is expressed as a fraction of that used in a two winding transformer as: This ratio therefore the savings in copper. As the space for the second winding need not be there, the window space can be less for an autotransformer, giving some saving in the lamination weight also. The larger the ratio of the voltages, smaller is the savings.

As T2 approaches T1 the savings become significant. Thus autotransformers become ideal choice for close ratio transformations.

## Autotransformer

Figure 2 — Close ratio transformations The autotransformer shown in Figure 2 above is connected as a boosting autotransformer because the series winding boosts the output voltage. In two-winding transformers, the primary voltage is associated with the primary winding, the secondary voltage is associated with the secondary winding, and the primary voltage is normally considered to be greater than the secondary voltage.

In the case of a boosting autotransformer, however, the primary or high voltage is associated with the series winding, and the secondary or low voltage is associated with the common winding; but the voltage across the common winding is higher than across the series winding. Limitation of the autotransformer One of the limitations of the autotransformer connection is that not all types of three-phase connections are possible.

The Y-Y connection must share a common neutral between the high-voltage and low-voltage windings, so the neutrals of the circuits connected to these windings cannot be isolated.

• What is Auto Transformer ?

The phase shift is a function of the ratio of the primary to secondary voltages and it can be calculated from the vector diagram. Advantages of the autotransformer There are considerable savings in size and weight. There are decreased losses for a given KVA capacity.

Using an autotransformer connection provides an opportunity for achieving lower series impedances and better regulation.

## Auto Transformer

Audio system[ edit ] In audio applications, tapped autotransformers are used to adapt speakers to constant-voltage audio distribution systems, and for impedance matching such as between a low-impedance microphone and a high-impedance amplifier input.

To increase the distance between electricity Grid feeder points, they can be arranged to supply a split-phase kV feed with the third wire opposite phase out of reach of the train's overhead collector pantograph. The 0 V point of the supply is connected to the rail while one 25 kV point is connected to the overhead contact wire. This system increases usable transmission distance, reduces induced interference into external equipment and reduces cost. A variant is occasionally seen where the supply conductor is at a different voltage to the contact wire with the autotransformer ratio modified to suit.

Variable autotransformers[ edit ] A variable autotransformer, with a sliding-brush secondary connection and a toroidal core. Cover has been removed to show copper windings and brush. Variable Transformer - part of Tektronix Curve Tracer By exposing part of the winding coils and making the secondary connection through a sliding brusha continuously variable turns ratio can be obtained, allowing for very smooth control of output voltage.

The output voltage is not limited to the discrete voltages represented by actual number of turns. The voltage can be smoothly varied between turns as the brush has a relatively high resistance compared with a metal contact and the actual output voltage is a function of the relative area of brush in contact with adjacent windings.

### Current and auto transformers - Transformers working principle

Typically the primary connection connects to only a part of the winding allowing the output voltage to be varied smoothly from zero to above the input voltage and thus allowing the device to be used for testing electrical equipment at the limits of its specified voltage range.

The output voltage adjustment can be manual or automatic. The manual type is applicable only for relatively low voltage and is known as a variable AC transformer often referred to by the trademark name Variac.

These are often used in repair shops for testing devices under different voltages or to simulate abnormal line voltages. The type with automatic voltage adjustment can be used as automatic voltage regulatorto maintain a steady voltage at the customers' service during a wide range of line and load conditions. Another application is a lighting dimmer that doesn't produce the EMI typical of most thyristor dimmers.

Variac Trademark[ edit ] From toVariac was a U. InInstrument Service Equipment applied for and obtained the Variac trademark for the same type of product.