<P> Z L ′ (\ displaystyle Z'_ (\ text (L))) is the apparent load impedance of the primary . </P> <P> 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 proportional to its number of windings . In the secondary winding the voltage is determined by Faraday's equation 1 (see box at right). In the primary winding the voltage is determined by equation 2 . 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>

What is the purpose of the primary windings of a transformer