Electra

CONSTRUCTION of a TRANSFORMER

               Transformer consists of two coils that have got mutual inductance and a laminated steel core, at which the coils are wound. The coils are insulated from each other and also from the steel core.

 This construction has to be kept in a suitable container with proper insulation for the core and the windings from the container.

 The terminals of the coils are taken out through suitable bushings, which are also used for insulation purpose. The bushing used can be porcelain, capacitor-type or oil-filled depending on the application.

Core in the transformer has got greater strength as they are made of steel. The steel sheets are arranged with a minimum air gap between them. The core is laminated to minimize the eddy current loss. The sheet laminations are insulated from each other using varnish.

 Lamination needs to be of certain thickness, it is varied from 0.35mm for 50Hz frequency to 0.5mm for a 25Hz frequency. The steel laminations are alternatively joined through staggered joints. These staggered joints are said to be imbricated.

Transformer is of two types based on the construction – core type and shell type. In core type, core is surrounded by the coil whereas in shell type, windings are surrounded by core but only a considerable portion of the coil.

Know about CURRENT TRANSFORMERS

                         Current transformers are used to measure the large currents. In high-voltage alternating current circuits, it is impracticable to measure the current.

For measuring the current in such circuits, current transformers are connected to the circuit with low-range ammeters.

The primary coil of the current transformer is connected in series with the line. Ammeter is connected across the secondary side of the transformer. Current transformer is a step-up transformer with respect to the voltage.

 When voltage gets increased in the transformer, it is obvious that the current will get reduced. When the transformer has got a 100:10 primary to secondary current ratio, the voltage steps up by 10 times, whereas the current gets step-down by 1/10^th of the actual value.

The current transformation ratio is calculated as the ratio of the primary current to the secondary current. Now the ammeter reading is noted down.

Then the line current is calculated by the product of the current transformation ratio and the ammeter reading.

Know about POTENTIAL TRANSFORMERS

                Potential transformers are used for measuring high alternating voltages. This is a step-down transformer with accurate ratio.

The true voltage on the high voltage side can be calculated by this transformer. A standard low-range voltmeter is connected to the transformer.

 The deflection in the voltmeter is measured and when it is divided by the voltage transformation ratio, it gives the true voltage on the high voltage side of the transformer.

The power rating of the potential transformer is usually small in the range of 40 to 100W and they are of shell-type. The type of potential transformer used varies depending on the voltage, for voltages up to 5000, dry type of potential transformer are used.

Dry-type or oil-immersed is used for the voltage range of 5000 to 13800, but above 13800 only oil-immersed type is used. Primary is of high voltage, so the secondary should be completely insulated from the primary for safety operation. The secondary is also grounded for the protection of the operator.

PARALLEL OPERATION OF TRANSFORMERS- a overview

            The following conditions must be satisfied to perform parallel operation in single phase transformers

1.       The primary windings of the two transformers must be of same voltage rating and frequency.

2.       The polarity of the two transformers must be connected properly, else it may lead to some serious problems.

3.       The transformation ratio of both the transformers needs to be identical.

4.       Circulating current should be avoided, else the load sharing will differ from the KVA ratings of the transformers. This is achieved by having same X/R ratio.

5.       Circulating current can be avoided by having the equivalent impedance inversely proportional to the KVA rating. This is to be carried out when the two transformers have different KVA rating.

In case of three-phase transformers, the above said conditions along with the following conditions are to be satisfied

1. 1.       Same phase sequence must be maintained.
2. 2.       Individual transformer in the three-phase transformer needs to be of same construction which can be either core type or shell type.
3. 3.       Transformers must have same phase displacement between the primary and the secondary voltages.
4. 4.       Terminal voltage of the primary and the secondary must be used for the voltage ratio.

Testing a TRANSFORMER

                 Two tests are carried out in transformers namely open-circuit test and short-circuit test.

 These tests are carried out to calculate certain parameters of the transformer – equivalent resistance and equivalent leakage reactance with respect to the primary winding, core-loss conductance and magnetic susceptance.

Without loading the transformer the above parameters can be calculated, hence the tests are very much economical. The performance of the transformer is calculated on these parameters.

Open-circuit test is done to determine core loss in the transformer. In this test, the high voltage winding is kept open as the other winding is connected to the supply.

In the low voltage winding, the wattmeter, ammeter and voltmeter are connected. Voltage is applied to the winding producing iron loss which gets recorded in the wattmeter. The no-load primary current is very small, so no copper loss. Hence the wattmeter reading represents only the core loss.

In short-circuit test, the low voltage winding is short circuited and applied with low voltage and then gradually increased to reach full load current. The core loss is considerably small as the mutual flux is very small compared with its normal value. Hence, the wattmeter reading represents the full-load copper loss.

What is AUTO TRANSFORMER

                  Auto transformer is similar to an ordinary two winding transformer but the difference between the two transformer is auto transformer has got only one winding.

 In two winding transformer, the two windings are electrically separated, whereas the two windings are electrically so as to form a single coil.

The operation and the theory are same for both the transformers. As the auto transformer got only one winding, it uses less copper with the reduction in their cost. Hence they are cheaper than the two winding transformer.

Here in this transformer, the total winding is considered as the primary winding and the secondary winding is from the load contact to the other end of the coil. The transformation ratio gets slightly away from unity but the auto transformer has got higher efficiency when compared with the two winding transformer for the same output.

 The size of the auto transformer is also small with improved voltage regulation.

Transformation ratio is the ratio of number of turns in the secondary to the number of turns in the primary and  it can also be defined as the ratio of the secondary voltage to the primary voltage.