Electrical knowledge

WHAT IS TRANSFORMER?

The transformer is a static device (means that has no moving parts) that consists of one, two or more windings which are magnetically coupled and electrically separated with or without a magnetic core. It transfers the electrical energy from one circuit to the other by electromagnetic induction principle. The winding connected to the AC main supply is called primary winding and the winding connected to the load or from which energy is drawn out is called as the secondary winding. These two windings with proper insulation are wound on a laminated core which provides a magnetic path between windings.

When the primary winding is energized with alternating voltage source, an alternating magnetic flux or field will be produced in the transformer core. This magnetic flux amplitude depends on the applied voltage magnitude, frequency of the supply and the number of turns on the primary side. This flux circulates through the core and hence links with the secondary winding. Based on the principle of electromagnetic induction, this magnetic linking induces a voltage in the secondary winding. This is called as mutual induction between two circuits. The secondary voltage depends on the number of turns on the secondary as well as magnetic flux and frequency.

Transformers are extensively used in electrical power systems to produce the variable values of voltage and currents at the same frequency. Therefore, by an appropriate primary and secondary turns proportion desired voltage ratio is obtained by the transformer.

Transformer Construction

The main parts of a transformer are core, windings, container or tank, bushings and conservator and radiators.

Core

For high power applications, transformer core is made with high permeability material which provides the low reluctance path for the magnetic flux. The cross section of the core would be square or rectangular.

Generally the iron core transformers provide better power transformation compared with air core transformers. Air core transformers are used for high-frequency application (above 2 KHz) whereas , for low-frequency applications (below 2 KHz) iron core transformers are employed.

In all types of transformers, the core is made up of silicon steel or sheet steel laminations which are assembled to provide a continuous magnetic path for the flux. With this laminated core eddy current losses are minimized.

The thickness of these laminated sheets of steel are 0.35 to 5 mm and are insulated with a varnish, or oxide, or phosphate and then formed as a core.

For better magnetic properties, Hot rolled grain oriented (HRGO) steel, or Cold Rolled Grain Oriented (CRGO) steel, or High B (HiB) laminations are used. In case of small transformers, the core is constructed with hot rolled silicon steel laminations in the form of E and me, C and I or O are used.

Windings

Generally, the (two winding) transformer has two windings namely primary and secondary windings which are made up of high-grade copper.

The voltage connected to the primary winding is called primary voltage whereas the induced voltage in the secondary is called as secondary voltage. If the secondary voltage is more than the primary, it is called as a step-up transformer and if less, it is called as a step-down transformer. Therefore, the windings are designated as HV and LV windings based on the voltage level.

Compared to the LV winding, HV winding needs more insulation to withstand high voltages, also needs more clearance to the core and the body.

The transformer coils can be concentric or sandwiched coils. Concentric coils are used in core type transformers whereas sandwiched coils are used in shell type transformers. In the concentric arrangement, LV winding is placed near to the core and HV winding is placed around the LV winding for low insulation and clearance requirements. The most commonly used coils for the transformer include helical, sandwiched, disc and crossover coils.

Other necessary parts of the transformer are conservator tank which is used to provide the necessary oil storage such that the pressure of the oil under heavy loads settles down. When the oil in the transformer subject to the heat, naturally, oil expand and contract. Under this, oil is subjected to heavy pressure so without a conservator tank, there will be a chance of bursting the transformer.

The bushings provide the insulation to the output terminals to be taken from the windings of the transformer. These can be porcelain or condenser type bushing and based on the level of operating voltage these are selected.Because of simple, durable and rugged construction, transformers require a little maintenance. Because of no moving parts, the efficiency of the transformer is very high which may vary from 95% to 98%.

The insulated stranded conductors are used as windings for carrying high currents. This insulation avoids turns to contact with other turns.