why the resistances and reactances of the windings are represented as sereis connection whereas the same parameters of core is represented as parallel connection?

Basically the resistance indicates the flow of current and reactance opposition to the magnetic field. Incase of windings, the current responsible for the copper losses is also producing the working inductance. Since the current is same the two parameters are connected in series.

Now in case of core, the current responsible for the core losses and the current required to set up the magnetic field is different. Therefore, the two parametrs are connected in parallel.

Equivalent Circuit

Equivalent circuit of any electrical circuit is basically the electrical equivalent of the device. All the components of the device has to be replaced by their electrical equivalent and arranged properly. Look into the figure below:








One can easily understand that the resistances and inductive reactances in series represents the windings, respectively. The parallel branch of resistance and inductive reactance represents the core parameters. The question arises here why the resistances and reactances of the windings are represented as sereis connection whereas the same parameters of core is represented as parallel connection.

Transformer Core

The core of the transformer houses the primary as well as secondary windings. The main function of the core is to provide a low reluctive path for the magnetic field. The core is made up of silicon steel laminations. The permeability of the core is kept very high so that all the magnetic lines of forces (magnetic flux) may link through the secondary winding.

Transformer

Transforemer is a static electrical device which transfers electrical power from one circuit to another without having any connection. It works on the principle of Faraday's law of electromagnetic induction. Transfer of energy took place due to mutual inductance between the two circuits(windungs).

The figure shown here depicts the opeartion.

There are a few things related to the transforemer, most of the engineers don't know.

Ques: Why transforemer is rated in kVA whereas all other electrical equipments like motors, mixers, drill machines etc are rated in KW.

Ans: Tansformer is an energy(or power) transferring device, its not a load. Whereas all others are load (i.e. suppose to consume electrical power and convert it into useful work). The transformer is supposed to transfer electrical power from primary winding to secondary winding without consuming any power. Therfore the power factor during the load condition can not be defined. Actually the power factor on load of a transformer is considered to be same as that of its load. So in absence of self power factor, the rating of transformer is defined in kVA and not kW (= KVA*power factor)