Construction Of Automatic Phase Changer

TRANSFORMER
A transformer consists of two coils (often called windings) linked by aniron core as shown below there is no electrical connecting between the cals instead they are liked by a magnetic field created in the core.
 Transformers are used to convert electricity from one voltage another with minimal loss of power.
    They only work with ac (alternating current) because they require a changing magnetic field to be created in their core ransformer can increase voltage (step-up) as well as reduce voltage (step-down) alternating current flowing in the primary (input) coil creates a continually changing magnetic field in the iron core. This field also passes through the secondary (out put) coil and the changing strength of the magnetic field induced alternating voltage in the secondary coil if the secondary coil is connected to a load the induce voltage will make an induced current flow. The correct term for the induced voltage is induced electromotive force which is usually abbreviated to induced e.m.f the iron care is laminated to prevent eddy currents flowing in the core these current produced by the alternating magnetic field inducing a small in the core just like that induced in the secondary coil eddy current waste power by needlessly heating up the core but they are reduced to a negtiggible  amount by laminating the  iron because  this  increases the electrical  resistance of the core without affecting  its magnetic properties.
Transformers have two great advantages over other methods of changing voltage.
1.    They provide total electrical isolation between the input output, so they cam be safety used to reduce the high voltage of the mains supply.
2.    Almost not power is water in a transformer. They have a high efficiency (power cut / power in) of 95% or more mains transformer are the most common type. They are designed to reduce the ac mains supply voltage. (230 -240v in the UK or 115 -120v in some countries) to a safer low voltage. The standard mains  supply voltages are officially 115v and 230v , but 120v and 240v and 240v are the values usually  quoted and  the difference is of no significance  in most cases.to allow for the two supply voltages mains transformers usually have two separate primary coils {windings} labelled o-12v. The two coils are connected in series for 240v {figure2a} and in parallel for 120v {figure 2b}.
They must be in the correct way round as shown in the correct diagrams because the coils must be connected in the corret sense {direction}.
Most main trasformers have two seperat secondary coils {e.g labelled 0-9v,0-9v} which may be used separate to give two independent supplies, or connected in series to create a centre- tapped coil or one coil with double the voltage.
Some mains transformers have a centre – tap half way through the secondary coil and they are labelled 9-0-9v for example.
They can be used to produce full wave rectified oc with just two diocles, unlike a standard secondary coil which requires four diocles to produce. Full –wave rectified dc