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 I0, 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 I0 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 I₀ comprises two components:

• Reactive (Magnetizing) Component Im

• In quadrature with the applied voltage V₁

• Generates core flux without power consumption

• Active (Power) Component I_w

• In phase with V₁

• 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 E₁ and E₂ 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 V₁ is drawn equal and opposite to E₁, as their no-load difference is negligible.

• The active component I_w is aligned in phase with V₁.

• The no-load current I₀ is the phasor sum of I_m and I_w.

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