An **ideal transformer** is one which has no losses (no iron loss and no copper loss) and no leakage flux i.e. all the flux produced by the primary winding is linking with the secondary winding.

In actual practice, **it is impossible to make such a transformer**but to understand the concepts of transformer it is better to start with an *ideal transformer* and then extend to a practical transformer. In this article I am discussing the **ideal transformer on load**.

Consider an ideal transformer whose secondary is open as shown in the figure. When it is connected to AC supply voltage V_{1} a current *I*_{m} flows through its primary winding.

Since the resistance of the primary coil is zero (i.e. it is purely inductive) the current *I*_{m} lags behind the applied voltage V_{1} by 90^{o}. The losses are zero (as assumed) and no load is applied to the transformer, therefore, the magnitude of current *I*_{m} will be very small.

This current is called magnetizing current *I*_{m} as it produces flux φ in the core. Because flux is produced by *I*_{m} hence it is in phase with *I*_{m}.

This alternating flux links with both secondary and primary windings. When this flux links with secondary winding it produces mutually induced EMF E_{2} in the secondary winding. When this flux links with primary winding it produces self-induced EMF E_{1} in the primary winding.

Both the EMFs (E_{1} and E_{2}) oppose the cause (applied voltage V_{1}) producing it (as per Lenz’s law). Hence these are shown in opposite direction in the phasor diagram.

V_{1} = primary applied voltage

E_{1} = self-induced EMF in the primary winding

E_{1} = V_{1} (because it is an ideal transformer with no resistance and reactance on primary side)

E_{2} = mutually induce EMF on the secondary side

V_{2} = secondary terminal voltage

V_{2 }= E_{2} (because at no load *I*_{2} is zero)

*I*_{1} = *I*_{m}, magnetizing current on primary side*I*_{2} = 0 (no load)

# Ideal Transformer on Load

When a load is applied to the secondary side of an ideal transformer, a finite value of secondary current (*I*_{2}) starts flowing. The magnitude and phase of secondary current, *I*_{2} w.r.t. secondary terminal voltage, V_{2} depends upon the nature of the load.

If the load is resistive, *I*_{2} is in phase with V_{2}.

If the load is inductive (R-L) type, *I*_{2} lags V_{2} by some angle φ_{2}.

If the load is capacitive (R-C) type, *I*_{2} leads V_{2} by some angle φ_{2}.

When *I*_{2} flows, it sets up its own MMF (N_{2}*I*_{2}) and hence creates its own flux φ_{2}.

However, φ_{2} opposes the main flux φ_{m} setup by magnetizing current (as per Lenz’s law).

This momentarily weakens the main flux φ_{m} and therefore primary back EMF E_{1} tends to decrease. Due to the reduction in E_{1}, the difference between V_{1} and E_{1} increases and the additional primary current *I*_{2}’ starts flowing. The *I*_{2}’ component of primary current is called load component of the primary current.

The current component *I*_{2}’, sets up an MMF N_{1}*I*_{2}’ to counter the effect of secondary produced MMF N_{2}*I*_{2}

i.e N_{1}*I*_{2}’ = N_{2}*I*_{2}

or *I*_{2}’ = N_{2}*I*_{2} / N_{1} = K*I*_{2}

and it is 180^{o} out of phase with *I*_{2}. The net primary current *I*_{1} is phasor sum of *I*_{2}’ and *I*_{m} (because *I*_{w} is zero in an ideal transformer).

**Thus due to load on the secondary side, the primary current of the transformer increases to supply the additional power to the load.
Since, winding resistance of an ideal transformer is zero, therefore, its voltage regulation will be zero and its efficiency will be 100%.**

## Phasor Diagrams of Transformer on Load

## Transformer | All Posts

- Single Phase Transformer Working Principle
- Ideal Transformer
- Construction of Three Phase Transformer
- Types of Transformers
- Equivalent Resistance and Reactance of Transformer
- Equivalent Circuit of Single Phase Transformer
- Power Loss in a Transformer
- Open Circuit Test of Single Phase Transformer
- Short Circuit Test on Single Phase Transformer
- Transformer Efficiency
- Regulation of Transformer
- Autotransformer
- Instrument Transformers
- Polarity of Transformer Windings
- Significance of Vector Group of Transformer
- Buchholz Relay Construction | Working
- Why current transformer secondary should not be opened
- Dielectric Strength Test of Transformer Oil
- Transformer Moisture Removal Process

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