An ideal Transformer

Ideal Transformer Construction

An ideal Transformer is an imaginary Transformer. An ideal transformer is one which has the following Characteristics −

1. The primary and secondary windings have negligible (or zero) resistance.
2. No leakage flux, i.e., whole of the flux is confined to the magnetic circuit.
3. The magnetic core has infinite permeability, thus negligible mmf is require to establish flux in the core.
4. There are no losses due winding resistances, hysteresis and eddy currents. Hence, the efficiency is 100 %.

Working of Ideal Transformer

Ideal Transformer on No-Load

Consider an ideal transformer on no-load, i.e., its secondary winding is open circuited (see the figure). Thus, the primary winding is a coil of pure inductance.

A very small magnetising current Im is drawn to create the flux in the core, which lags the applied voltage by 90° when an alternating voltage V1 is applied to the primary winding. An alternating flux m that is proportional to and in phase with the magnetising current Im is created. The primary and secondary windings are magnetically connected by this alternating flux (m), which also induces EMF E1 in the primary winding and EMF E2 in the secondary winding.

The EMF induced in the primary winding E₁ is equal to and in opposition to the applied voltage V₁ (according to Lenz’s law). The EMFs E₁ and E₂ lag behind the flux (ϕ_m) by 90°,. Although their magnitudes depend upon the number of turns in the primary and the secondary windings. From the phasor diagram of the ideal transformer on no-load, it is clear that the flux is common to both the windings, hence it can be taken as the reference phasor. Also, the EMFs E₁ and E₂ are in phase with each other, but E₁ is equal to V₁ and 180° out of phase with it.

Ideal Transformer On-Load

When load is connected across the terminals of secondary winding of the ideal transformer,. The transformer is said to be loaded and a load current flows through the secondary winding and the load.

Consider an inductive load of impedance Z_L is connected across the secondary winding of the ideal transformer (see the figure). Then, the secondary EMF E₂ will cause a current I₂ to flow through the secondary winding and the load, which is given by,

I2=E2ZL=V2ZLI2=E2ZL=V2ZL

Since, for an ideal Transformer, the EMF E₂ is equal to Secondary terminal voltage V₂.

Here, the load is Inductive, therefore, the current I₂ will lag behind the E₂ or V₂ by an angle ϕ₂. Also, the no-load current I₀ being neglected because the Transformer is ideal one.

The Secondary winding’s current (I2) creates a mmf (N2I2). That generates a flux (2) that is in the opposite direction of the main flux (m). As a result, the core’s total flux deviates from its initial value, but it shouldn’t deviate from its original value. Therefore, the primary current must create an MMF. That can balance out the demagnetizing effect of the secondary mmf N2I2 in order to keep the flux in the core at its initial value. In order to achieve this, the primary current I1 must flow.

N1I1=N2I2N1I1=N2I2

⇒I1=N2N1×I2=KI2⇒I1=N2N1×I2=KI2

As a result, the primary winding needs to draw enough current to counteract the Demagnetizing effects of the Secondary current and maintain a constant main flux in the core. In order to maintain a constant mutual flux (m),. When the secondary current (I2) increases, the primary current (I1) also increases in the same way.

It is clear from the phasor diagram of the ideal Transformer on-load. That the Secondary current I₂ lags behind the Secondary Terminal voltage V₂ by an angle of ϕ₂.