What is an Ideal Transformer? Properties, Working, and Phasor diagram

An ideal transformer is a transformer that has no winding resistance(no copper loss), no core loss, no magnetic leakage, and zero magnetizing currents.

But in existence, there is no such ideal transformer, that is, a transformer with ideal properties is hypothetical.

However, it is important to know about the ideal transformer for a better understanding and an easier explanation of a practical transformer. In an ideal transformer, certain assumptions are made which are close approximations for a práctical transformer.

Properties of Ideal transformer

In an ideal transformer, the windings are purely inductive and there will be no loss in the transformer core. A purely inductive circuit will have zero resistance, which indicates that there will be no voltage drop and no copper loss in the windings. An ideal transformer has 100% efficiency.

The following are the properties of an ideal transformer.

1. No winding resistance – It means the primary and secondary windings have zero resistance. The ideal transformer will have no ohmic power loss and no resistive voltage drop in an ideal transformer.
2. No magnetic leakage – there is no leakage flux and all the flux set up is confined to the core and links both the windings.
3. No iron loss – hysteresis and eddy current losses in the transformer core are zero.
4. Zero magnetizing current – the core has infinite permeability and zero reluctance so zero magnetizing current is required to establish the requisite amount of flux in the core.

Working of Ideal transformer

Consider the below given ideal transformer. An alternating voltage source V₁ is connected to the primary winding of the transformer and the secondary is kept open.

When the voltage V₁ is applied, the transformer draws a very small current. This current merely magnetizes the core and is hence called the magnetizing current I_m. Since the coil is purely inductive, the current I_m lags behind the applied voltage V₁ by 90⁰.

This current when flowing through the primary coil establishes the flux φ in the core. The flux produced is always proportional to the flow of current and hence both are in phase with each other.

This alternating flux φ links the primary and secondary windings magnetically. It produces a self-induced emf E₁ in the primary winding and mutually induced emf E₂ in the secondary winding.

The EMF induced in the primary winding E₁ is equal and opposite to the applied voltage V₁. Thus it is called counter emf or back emf.

The EMF E₂ is induced as a result of flux φ linking with the secondary winding through the core of the transformer. This emf is antiphase with V₁ and its magnitude is proportional to the rate of change of flux and the number of turns in the secondary winding.

Phasor diagram of Ideal transformer

The phasor diagram of the ideal transformer on no-load is drawn as shown below.

Since the flux φ is common to both the windings of a transformer, it is taken as the reference phasor to draw the phasor diagram.

The magnetizing current I_m is drawn on the same phase as φ, as both are in phase with each other. As the magnetizing current I_m lags behind the V₁ by 90⁰, it is drawn at 90⁰ in the anti-clockwise direction.

Also, the induced EMFs E₁ and E₂ in the primary and secondary windings respectively are in phase with each other. It is drawn exactly opposite to that of V₁ phasor.

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