Transformer Losses – Different Types of Losses in a Transformer

Electrical Technology

Types of Energy Losses in an Electrical Transformer

An ideal transformer is characterized by zero energy losses, making it 100% efficient. However, practical transformers in real-life applications experience energy dissipation in the windings, core, and surrounding structures. Larger transformers tend to be more efficient, with distribution transformers typically operating at efficiency levels exceeding 98%.

Experimental transformers utilizing superconducting windings, which employ superconductors (a material with nearly zero losses) can achieve efficiencies of 99.85%, effectively eliminating transformer losses. However, their availability for general consumer and commercial applications may be delayed.

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Power Losses in a Transformer

The different losses in the transformer are as follows

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Copper Losses (Winding Resistance)

The electric current flowing through the windings causes resistive heating of the conductors. At higher frequencies, skin effect and proximity effect create additional winding resistance and losses. The total copper losses in a transformer can be found using the following equation.

I₁²R₁ + I₂²R₂ = I₁²R ₀₁ + I₂²R ₀₂

W_C = I₁²R₁ + I₂²R₂

Core or Iron Losses

There are two types of core or iron losses in an electrical transformer.

Hysteresis Losses

Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For a given core material, the transformer losses are proportional to the frequency, and is a function of the peak flux density to which it is subjected.

We can find Hysteresis losses by this formula.

W_h = Kη B_max^1.6 f v … Watts

Where:

W_h = Hysteresis losses in Watts
Kη = Coefficient of eddy current
B_max = Max. value of flux density in wb/m²
f = Supply frequency in Herts
v = Volume of the magnetic material in m³

Eddy Current Losses

Ferromagnetic materials are also good conductors, and a core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material.

The eddy current loss is a complex function of the square of supply frequency and inverse square of the material thickness. Eddy current losses can be reduced by making the core of a stack of plates electrically insulated from each other, rather than a solid block; all transformers operating at low frequencies using laminated or similar cores.

We can find Eddy currents losses by the following formula.

W_e = P B_max² f² t² … Watts

W_e = K_e B_max² f² t² v … Watts

Where:

W_e = Eddy current losses in Watts
K_e = Coefficient of eddy current
B_max = Maximum value of flux density in wb/m²
f = Supply frequency in Herts
T = Thickness of lamination in meters
v = Volume of the magnetic material in m³

Stray Losses (Leakage Flux)

Leakage inductance is by itself largely lossless, since energy supplied to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive materials such as the transformer’s support structure will give rise to eddy currents and be converted to heat. There are also radiative losses due to the oscillating magnetic field, but these are usually small and negligible.

Dielectric Loss

In the solid insulation or transformer oil i.e. insulation material of the transformer, dielectric loss occurs when the solid insulation gets damaged or the oil gets deteriorated or its quality decreases over the time. Hence, the overall efficiency of the transformer may be affected due to this loss.

Related Post: Why Transformer Rated In kVA, Not in KW?

Other Losses

Magnetostriction Losses

Magnetic flux in a ferromagnetic material, such as the core, causes it to physically expand and contract slightly with each cycle of the magnetic field, an effect known as magnetostriction. This produces the buzzing sound commonly associated with transformers, and can cause losses due to frictional heating.

Mechanical Losses

In addition to magnetostriction, the alternating magnetic field causes fluctuating forces between the primary and secondary windings. These incite vibrations within nearby metalwork, adding to the buzzing noise, and consuming a small amount of power.