Transformers in power plants are an essential device to deliver power to the electrical grid. An ideal system (transformer or any electrical device) would have no energy loss and be 100% efficient. Such devices are hypothetical and used for evaluating real machine performance. Real transformers in the industry still perform very well and most perform with 98% efficiency. To measure transformer performance, one must know two type of energy losses:

  • No-load losses
  • Load losses

No-Load losses

No-load losses or core losses are caused by induced current in the transformer winding which in turn magnetizes the iron core. Regardless of how much electrical power is flowing through the transformer, core losses occur whenever the transformer is energized. This loss of energy is constant as it depends on the material properties of the laminated steel core. Core/no-load losses can be characterized further  in two category in the transformer:

  • Eddy current loss
  • Hysteresis loss

Eddy current loss

When alternative current is induced to the primary winding of the transformer, it creates a varying magnetic flux.This generates current to the secondary winding as well as in the iron core. This current is called Eddy current. Eddy currents have a circular motion within the core and are responsible for resistive heating (heat loss).

Eddy current power losses can be expressed as:

Pe=t2Vf2KeBm2 [W]

t : Thickness of lamination in meters
V: Volume of magnetic material [m3]
f: Frequency of reversal of magnetic field [Hz]
Ke  :  Eddy current Constant
Bm : Maximum value of flux density [wb/m2]


To minimize Eddy current losses

The best way to minimize Eddy current losses is having the core built from thin laminated sheets of soft iron and insulated from one another. Eddy current losses account for 20% to 50% of the total no-load losses.



Hysteresis loss

Hysteresis losses are defined as energy losses due to magnetic field alteration to the magnetic domains alignment in the core lamination. The repeating core magnetization can be expressed by the frictional movement of the domains alignment which causes heat loss. Hysteresis loss can be expressed as:

Ph=ηBm1.6fV [W]

η: hysteresis or Steinmetz’s constant in J/m3
Bm : Maximum value of flux density [wb/m2]
f: Frequency of reversal of magnetic field [Hz]
V: Volume of magnetic material [m3]



To minimize Hysteresis losses

Having the core material processed by laser treatment or grain orientation helps to minimize hysteresis losses. Soft iron is often used in the industry as its magnetic domains changes rapidly at a lower energy loss. Hysteresis losses account for 50% to 80% of the total no-load losses.


Load losses

Load loss, copper losses or also referred to as winding losses, occur when current is flowing through the primary and secondary winding causing resistive heating of the conducting material. This energy loss is directly proportional to the amount of current provided to the transformer, hence the term “load losses”. This energy loss is expressed by:

PL=IpRp+IsRs [W]

Ip: Primary current [A]
Rp: Primary winding resistance [Ω]
Is: Secondary current [A]
Rs: Secondary winding resistance [Ω]


To minimize load losses

During the transformer design process, increasing the cross sectional area of the conductor or by reducing the winding length reduces load losses.




Eddy Current Loss