What are No-Load Losses( Excitation Losses)?
For an ideal transformer, the input power is equal to output power, however, in reality, this is not true. The output power of the transformer is always less than the input power. The energy can neither be created, nor it can be destroyed; the same principle is applicable for the energy balancing of the transformer as well.
The power drawn at the primary is equal to the losses in the transformer plus the power delivered at the secondary side of the transformer. The losses in the transformer can be broadly categorized into two categories;
1. Iron losses or core losses, dielectric loss, and stray eddy current loss
2. Copper loss and Stray losses
Iron losses or core losses:
What is hysteresis loss in the transformer?
The hysteresis loss in the transformer core can be calculated using below given hysteresis loss formula.
Hysteresis Loss Formula
Eddy Current Loss
What is Eddy Current loss in the transformer?
The flux that passes through the core induces a voltage of different magnitude at various places on the surface of the core and the other conducting parts of the transformer. Because of potential difference at various places on the core surface, the current set up in the core. These current set up in the core is called eddy currents.
Because of eddy currents, the heat loss equal to I2R loss occurs in the core.
The eddy current loss depends on the square of the current flowing in the upper part of the core and the resistance of the core material. The solid sheet of the iron block has less resistance because of the larger cross-section area. The magnitude of the voltage induced in a solid block is more and as a result, the eddy current loss is more in the solid iron block.
Why is the iron core of a transformer Laminated?
To minimize the eddy current losses, the iron core is formed using several thin electrically insulated sheets called lamination.
The area of the eddy current path gets reduced and the losses decrease because of the decrease of induced voltage and increase of resistance. The thin laminated sheet has higher resistance. The flow of eddy current in laminated sheets is as given below.
The current flowing in the sheet is equal to;
I= Potential difference of the EMF induced in laminated core/Resistance of the sheet.
In addition to the dependency of eddy current on the stamping resistance, other electrical parameters like primary voltage and frequency also affect the eddy current loss.
The eddy current loss formula of the transformer is given below.
Eddy Current Loss Formula
Effect of Voltage and Frequency Variation on Transformer
Let us understand how the hysteresis and eddy current loss get affected by changes in frequency and voltage. We will take the four cases for the study of no-load losses of transformer.
Effect on no-load losses – When the frequency is increased/decreased keeping voltage constant
The hysteresis loss is proportional to the frequency. The hysteresis loss should increase with an increase in the frequency, however, the hysteresis loss remains almost unchanged. The reason is that the flux density in the core gets decreased in the same proportion of the increased frequency.
In a similar way, the hysteresis loss should decrease with a decrease in frequency, however, the hysteresis loss remains almost unchanged because of increased flux density in the core.
The eddy current loss is proportional to the square of the flux density and frequency. With an increase in the frequency, the eddy current remains unchanged because the product of B2m f2 remains unchanged as flux is proportional to the ratio of V/f. With the decrease in frequency, the flux density in the core gets increased in the same proportion of frequency decrease and thus the eddy current loss remains unchanged.
Effect on no-load losses- When the voltage is increased/decreased keeping the frequency constant
The hysteresis loss is directly proportional to the voltage and flux density. The hysteresis loss increase with an increase in voltage. The magnetic flux density is also proportional to the voltage. Thus the hysteresis loss is proportional to the square of voltage if the frequency is kept constant.
Hysteresis loss, Wh ∝ V2
Eddy current loss, We ∝ V3
Effect on no-load losses – When the frequency is increased/decreased and voltage is also increased/decreased in the same proportion
If the frequency is increased and voltage is also increased in the same proportion then the flux density in the core remains unchanged and, in this case, the hysteresis loss will increase proportionally to the increase of frequency, The eddy current loss will increase in the square proportion of the increased frequency.
Effect on no-load losses – When the frequency is increased/decreased and voltage is also increased/decreased in different proportion
In this situation, the eddy current and hysteresis loss will increase or decrease because of the following reasons.
- Increase/decrease of frequency
- Increase/decrease of flux density