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Transformer on No Load Condition

Edwiin

05/28/2025

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:

Topics

Machines Transformer Electric transformer

Edwiin

Hello，I'm Wdwiin. A decade of hands-on experience in electrical engineering, specializing in high-voltage systems, smart grids, and renewable energy technologies. Passionate about technical exchange and knowledge sharing, committed to interpreting industry trends with professional insights to empower peers. Connection creates value—let’s explore the boundless possibilities of the electrical world together!

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