Transformer no-load losses in detail: definition, causes and calculations

What is no-load loss of transformer

The no-load loss of a transformer refers to the active power consumed by the transformer when the secondary winding of the transformer is open-circuited and the primary winding applies the rated frequency and rated voltage. That is, in the absence of load, the transformer itself when the operation of the loss. No-load loss is mainly composed of iron loss and no-load current loss.

Iron loss (core loss)

when the transformer core is in the alternating electromagnetic field, the core material will be due to the alternating magnetic flux eddy current and hysteresis effect, resulting in energy loss. This part of the loss is related to the design of the transformer, voltage level and material properties, usually unavoidable.

No-load current loss

when the transformer is in the no-load state, although there is no load, but due to the transformer coil is still current flow, this part of the current generated by the copper loss is called no-load current loss. It is related to the transformer coil DC resistance and current size.

Although no-load losses usually account for a small percentage of total losses, they can still affect the overall efficiency of the transformer, especially during long-term operation.

Reasons for excessive transformer no-load losses

Transformer no-load loss is too large usually indicates that there are problems in the design or operation, the main reasons include:

Poor quality of core material

The material of the core directly affects the hysteresis and eddy current losses. If the core material used such as silicon steel sheet quality is poor, hysteresis loss and eddy current loss will increase, resulting in no-load loss rise.

Iron core design is unreasonable

Iron core cross-sectional area, shape, processing technology and other factors will affect the magnetic flux distribution of the transformer. If the design is not appropriate, the iron core is easily saturated, which in turn increases the loss.

Voltage fluctuation is too large

No-load loss is related to the voltage applied to the primary side of the transformer. When the voltage fluctuation is large, the magnetic flux density in the iron core will change, resulting in increased iron losses. If the actual voltage is higher than the rated voltage, the loss will increase exponentially.

Iron core processing quality problems

Iron core in the processing process, if there is inaccurate cutting, bad insulation between layers or poor contact of iron chips, will increase the hysteresis loss and eddy current loss.

Transformer aging

Transformer with the increase of operation time, the insulation performance and magnetic permeability of the iron core may decline, resulting in increased no-load loss.

How to calculate the transformer no-load loss

The no-load loss of the transformer is mainly composed of iron loss and no-load current loss, of which iron loss is the main part.

The formula for calculating no-load loss is as follows.

no-load loss = k × P × G

Among them:

k: no-load loss process coefficient, depending on the design and manufacturing process of the transformer;

P: unit loss, usually determined by the material properties;

G: iron core weight, depending on the volume and material of the transformer.

Detailed calculation of iron loss:

Iron loss is mainly composed of hysteresis loss and eddy current loss, the formula is:

Among them:

kn and kw: constant;

f: the frequency of the transformer external voltage;

Bm : maximum flux density in the iron core;

n: Shteinmetz constant, usually 2.5 to 3.5.

This means that the no-load loss is closely related to the size of the applied voltage, frequency and the performance of the core material. Maintaining voltage stability and selecting high quality core materials during operation is the key to controlling no-load losses.