Load-No load Losses of Transformer

Losses are the wastage of energy in the device which will affect the overall efficiency of the device. So the losses of the device should be known to design the component with minimum losses. Transformer is an electrical isolation component which transfers the power from primary to secondary without any physical contact. In power transmission from primary and secondary, there are losses in the transformer which are bases on load of the transformer i.e. No-load losses and Load Losses.

Pt = PNL + PLL


          Pt       = Total Losses in the transformer

          PN L  = No-load losses in the transformer

          PLL   = Load losses in the transformer.


No Load Loss of Transformer   

No- Load losses are the steady losses which will not vary according to the load on the transformer. No-load losses are caused by the magnetizing current needed to energize the core of the transformer. When the winding is energized, the exciting current consists of real component (core loss) and a reactive component (energy stored momentarily in the winding inductance). These losses can be minimized by the perfect design of the component and the No-load losses are categorized as shown below

  • Hysteresis losses in the core lamination
  • Eddy current losses in the core lamination’s
  • Dielectric losses in the transformer components during excitation.
  • I2R losses due to no-load current in the primary and secondary windings.
Transformer Eddy current losses

Transformer Eddy current losses


For distribution transformers no load losses are typically less than 5% of the rated current. Hysteresis losses and eddy current losses contribute over 99% of the no-load losses.

Load Losses

Load losses are caused by the winding impedance and vary according to the loading on the transformer. Transformer load losses can be divided into two parts:

  • Loss used by transformer winding electrical resistance when load currents flow
  • Eddy current losses caused by currents circulating within the winding conductors

They include heat losses (I2R) and eddy currents in the primary and secondary conductors of the transformer. Heat losses, or I2R losses, in the winding materials contribute the largest part of the load losses. They are created by resistance of the conductor to the flow of current or electrons.

How to measurement of Loss

Measurements of transformer loss are usually conducted during open-circuit and short-circuit tests. With the open-circuit test (load = 0) the measured losses are the no-load loss. During the short-circuit test the secondary is shorted and primary voltage adjusted to achieve a full load current flow. Short-circuit tests give the total transformer loss from which the load loss can be calculated by subtracting the no load loss.

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