Different types of Transformer Losses – Copper, Iron, Stray, Dielectric losses

In every power system, the amount of useful energy would be always less than the total amount of energy applied to the system. Because some amount of energy always transforms into unwanted forms. No any practical equipment’s could be an ideal system with a zero power loss. Such a difference in the required output to the input can be called as power loss. The power loss in a system is a serious matter to be considered as the efficiency of the system is purely depending on the amount of loss it can be reduced.

In an electrical transformer, the power losses are occurred in the windings and cores of transformers due to heating and magnetic losses caused by eddy current loss, hysteresis loss, stray loss, dielectric loss, and other unwanted effects.

The major power loss in transformers are caused by,

1. copper loss
2. Iron Losses or Core Loss

Power losses like stray, dielectric, etc… have no considerable percentage in the total power loss of a transformer. Hence, it is a negligible quantity.

Copper Loss

Copper loss occurs due to the ohmic resistance in both primary and secondary winding.

Calculating the copper loss in the primary and secondary winding,

Total Copper Loss, P_cu = I₁²R₁ + I₂²R₂

The values of Resistance R₁ of the primary winding and R₂ of the secondary winding are constant.

I₁ –  primary current, I₂– Secondary current

Then it is clear from the above equation that the copper loss varies with the amount of current (square of the current) through the windings.

Iron Losses or Core Loss

Iron loss occurs in the transformer core due to the alternating magnetic flux. It consists of eddy current loss and hysteresis loss.

Iron loss = Hysteresis loss + Eddy Current loss

Referring the below equations of hysteresis and eddy current loss,

Both the Eddy current loss and hysteresis loss depend upon the magnetic properties of the core material, f-frequency of the AC supply and the B_m– maximum flux density.

Hysteresis Loss

Hysteresis loss causes due to the reversal of magnetization for changing the orientation of magnetic domains during each half cycles of the alternating current.

By Steinmetz formula hysteresis loss, W_h =K_h f B_m^1.6 w/m³

K_h – hysteresis constant

Choosing proper core material which has a low hysteresis coefficient and high permeability can minimize the hysteresis loss.

Eddy Current Loss

It is the result of induced current in the core due to the electromagnetic induction in ferromagnetic material, which flows as loops of electrical current within the conductor.

Eddy Current loss, W_e = K_e f ² B_m² t ² w/m³

K_e – co-efficient of eddy current

t – Thickness of the sheet

Eddy current loss can be reduced by using an iron core made of thin laminated sheets and material with high electrical resistivity. It reduces the induced EMF and the amount of current flow.

Stray Loss

Stray losses are due to the leakage flux in the transformer. This leakage flux induces eddy current that appears all over in metal parts which are under the magnetic leakage field called as the stray loss.

Stray loss can be reduced by minimizing the leakage flux.

Dielectric Loss

It occurs in the insulating material of the transformer that is in the oil of the transformer. It occurs when the insulating materials and the oil losses its quality. The transformer oils are subject to change its parameters such as dielectric strength, tan𝛿, moisture, chemical parameters (dissolved impurities – dissolution of copper), physical parameters, etc…

Periodic testing of the oil and maintaining the insulation quality is an effective way to reduce dielectric loss.

Additional points

• Iron loss (Hysteresis loss, eddy current loss) and dielectric loss are no-load losses which are independent of the transformer load.
• Whereas the copper loss and stray loss varies with the load current.
• Iron loss is a constant loss and copper loss is a variable loss.
• Copper loss is determined by the short circuit test and Iron loss is determined by the open circuit test.