<P> Referring to the two schematic models pictured at right below, an ideal transformer is a theoretical, linear transformer that is lossless and perfectly coupled . Perfect coupling implies infinitely high core magnetic permeability and winding inductances and zero net magnetomotive force . </P> <P> A varying current in the transformer's primary winding creates a varying magnetic flux in the transformer core and a varying magnetic field impinging on the secondary winding . This varying magnetic field at the secondary winding induces a varying EMF or voltage in the secondary winding due to electromagnetic induction . The primary and secondary windings are wrapped around a core of infinitely high magnetic permeability so that all of the magnetic flux passes through both the primary and secondary windings . With a voltage source connected to the primary winding and load impedance connected to the secondary winding, the transformer currents flow in the indicated directions . (See also Polarity .) </P> <P> According to Faraday's law, since the same magnetic flux passes through both the primary and secondary windings in an ideal transformer, a voltage is induced in each winding, according to eq. (1) in the secondary winding case, according to eq. (2) in the primary winding case . The primary EMF is sometimes termed counter EMF . This is in accordance with Lenz's law, which states that induction of EMF always opposes development of any such change in magnetic field . </P> <P> The transformer winding voltage ratio is thus shown to be directly proportional to the winding turns ratio according to eq. (3). common usage having evolved over time from' turn ratio' to' turns ratio' . However, some sources use the inverse definition . </P>

Describe principle constructions of transformers also explain testing of transformers