The properties of an Ideal Transformer

Ideal transformer

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

 i_pn_p - i_sn_s = 0

Ideal transformer connected with source V_P on primary and load impedance Z_L on secondary, where 0 < Z_L < ∞.

The ideal transformer presents certain very counter-intuitive properties.

• The ideal transformer has no winding resistances. The most immediate conclusion is that ohmic losses in the windings are absent, however, it does lead to some other interesting features as pointed later.
• There is no leakage flux. All flux is confined within the core and links both windings. Therefore, it has zero series inductance, although the construction resembles that of an inductor, since the series inductance arises due to flux that links only one of the windings and not both. To use a transformer as an inductor, one of the windings has to be disabled.
  
• The voltage conversion ratio is perfect and independent of the current being drawn, since there is no drop across the series inductance or resistance on either side.
• The core has infinite permeability. This means that the B-H curve is vertical so that no magnetizing current is required to establish the flux. Now that there is no leakage inductance and no magnetizing current is drawn, an ideal transformer does not store any energy.
• No hysterisis is observed since zero magnetizing current is required to establish any flux. The core laminations are infinitesimally thin so that no eddy current can flow and hence there are no ohmic losses in the core.
• The permeability remains infinite for all values of B. In other words, the ideal transformer does not saturate. This means that a DC applied to one of the windings will result in a sustained DC voltage being induced on the other winding. In a real transformer, applying a DC voltage will cause a rising current and hence a changing flux in the core only momentarily till the core saturates or the current gets limited by the series resistance, so that only a pulse is observed on the other winding. In an ideal transformer, the flux will continue rising linearly and I will not need any current to keep this flux rising, so that a sustained DC voltage is induced on the other winding and the source on the primary will not supply any current unless I load the secondary.
  
• The flyback converter in fact takes advantage of the non-ideal behavior of real transformers wherein energy to be transferred from the input to the output is momentarily stored in a flyback “transformer” so that is is often designed with an air gap.

Characteristics of a transformer:

• It is not an energy conversion device.
• It is a phase shifting device offering 180° phase shift to its signals.
• Negative feedback action is accommodated.
• It is a singly excited device.
• It is a two port network.
• It works on the principle of Faraday's laws of Electromagnetic Induction.