Classification of Transformer losses
A transformer is a static device, hence mechanical losses (like windage or friction losses) are absent in it. There are mainly two types of losses in the transformer. They are
1. Core losses or Iron losses
- Hysteresis loss
- Eddy current loss
2. Copper losses
Apart from these two types, there are other losses are present in the transformer namely, stray load loss and dielectric loss. These are small and hence usually be neglected.
Iron or Core Losses in Transformer
This type of loss occurs in the magnetic core of the transformer. It is the sum of hysteresis loss and eddy current loss.
1. Hysteresis loss (Ph)
Hysteresis loss occurs due to the reversal of magnetic field due to the applied alternating current in the transformer core. It depends mainly on the material of the core and this loss can be reduced by using high-grade core material. Cold rolled grain oriented (CRGO) silicon steel is commonly used as transformer core to reduce Hysteresis loss. The Hysteresis loss is represented as,
where
kh = constant depending upon volume and quality of core material
Bm = maximum flux density in the core
f = frequency of alternating flux or supply
x = Steinmetz constant
Steinmetz constant value varies from 1.5 to 2.5.
2. Eddy current loss (Pe)
Eddy current loss occurs due to the flow of eddy currents in the core material. To reduce the eddy losses thin laminations which were insulated from each other are used. Eddy current loss is represented as
where
ke = constant depending upon volume and resistivity of core material and thickness of laminations.
Bm = maximum flux density in the core
f = frequency of alternating flux or supply
Therefore, the total core or iron loss can be written as
Pi = Ph + Pe
Copper losses or Ohmic losses
The copper losses occur in the windings of the transformer due to their internal resistance. If R is the total resistance of the windings of the transformer and I is the current drawn, then the copper loss is I2R (W). As the copper loss depends upon the current which changes with the change in load, making the copper loss a variable loss. Hence copper losses are variable and increase with an increase in load current.
Stray load losses
Stray load loss is an eddy current loss in which the eddy currents are caused by leakage fields. The leakage fields present in transformer induce eddy currents in conductors, tanks, channels, bolts etc. and these eddy currents give rise to stray load losses.
Dielectric losses
This loss occurs due to the insulating materials used in transformer namely, the oil in the transformer and coil insulations used for the primary and secondary windings of the transformer.
