Transformer on No Load Condition

No-Load Operation of Transformer

When a transformer operates under no-load conditions, its secondary winding is open-circuited, eliminating load on the secondary side and resulting in zero secondary current. The primary winding carries a small no-load current

I_{0}

, comprising 2 to 10% of the rated current. This current supplies iron losses (hysteresis and eddy current losses) in the core and minimal copper losses in the primary winding.

The lag angle of

I_{0}

is determined by transformer losses, with the power factor remaining very low—ranging from 0.1 to 0.15.

No-Load Current Components and Phasor Diagram
Components of No-Load Current

The no-load current

I 0

comprises two components:

Reactive (Magnetizing) Component $Im$
- In quadrature with the applied voltage $V 1$
- Generates core flux without power consumption
Active (Power) Component $I w$
- In phase with $V 1$
- Supplies iron losses and minor primary copper losses

Phasor Diagram Construction Steps

The magnetizing component $I m$ is in phase with the magnetic flux ϕ, as it generates the magnetizing flux.
Induced EMFs $E 1$ and $E 2$ in primary/secondary windings lag the flux ϕ by 90°.
Primary copper losses are negligible, and secondary current $I2 = 0$ , eliminating secondary losses.
The no-load current $I0$ lags $V1$ by angle ϕ₀ (no-load power factor angle), as depicted in the phasor diagram.
The applied voltage $V 1$ is drawn equal and opposite to $E 1$ , as their no-load difference is negligible.
The active component $I w$ is aligned in phase with $V$ ₁.
The no-load current $I 0$ is the phasor sum of $I m$ and $I w$ .

From the phasor diagram drawn above, the following conclusions are made: