How to generate transformer no-load current

Transformer is a very important electrical equipment in power system. Its main function is to transform the voltage and current in AC circuit, so as to facilitate the transmission and distribution of electric energy. In the actual operation of the transformer, even if the secondary side (secondary side) is not loaded, that is, it is in no-load state, there will still be a certain amount of current flowing in the primary side (primary side), which is called "no-load current". In this paper, the generation principle of transformer no-load current and its influencing factors will be expounded in detail.

First, the definition of no-load current

When the secondary winding of the transformer is open, that is, no load is connected, if the rated voltage is applied to the primary side, there will still be a small current flowing through the primary side, which is called no-load current. No-load current is usually 2% ~ 5% of rated current, although the value is small, it plays a key role in transformer operation.

Second, the causes of no-load current

1. The need to establish the main magnetic flux

The working principle of transformer is based on the law of electromagnetic induction, and its core is to realize energy transfer through alternating magnetic flux in the iron core. When the primary winding is connected to the AC power supply, in order to establish the alternating main magnetic flux (φ) in the iron core, there must be an excitation current to excite the iron core. This part of the current used to establish the main magnetic flux is the main component of no-load current.

2. Hysteresis and eddy current loss of iron core

Iron core materials will be affected by hysteresis effect and eddy current effect in alternating magnetic field, resulting in energy loss. In order to overcome these losses, the primary side needs to provide additional current components to maintain the change of magnetic flux in the core, which also constitutes a part of no-load current.

3. The influence of nonlinear characteristics

Iron core material has nonlinear magnetization characteristics, that is, at high magnetic flux density, magnetic saturation will occur. When working in the nonlinear region of magnetization curve, harmonic current will be generated, which will distort the waveform of no-load current. Usually, the no-load current waveform is a spire wave, which contains more third harmonic components.

Third, the composition of no-load current

Physically speaking, no-load current can be divided into two parts:

-Reactive component: used to establish and maintain the main magnetic flux. There is a big phase difference between this part of current and the power supply voltage, which belongs to reactive power.
-Active component: used to compensate the hysteresis loss and eddy current loss in the iron core. This part consumes active power, but its value is relatively small.

Therefore, no-load current is mainly excitation current, which consists of reactive excitation component and a small amount of active loss component.

Four, the factors affecting the no-load current

1. Iron core material properties: High-quality silicon steel sheet has high permeability and low loss, which is helpful to reduce no-load current.
2. Structural design of iron core: lamination mode and joint treatment affect the continuity of magnetic circuit, and then affect the no-load current.
3. Power supply voltage and frequency: the higher the voltage, the greater the magnetic flux to be established and the greater the no-load current; The change of frequency will also affect the magnetic flux density and iron loss.
4. Manufacturing technology: such as winding turns and assembly accuracy will also have a certain impact on no-load current.

V. Significance of no-load current

Although the value of no-load current is small, it is of great significance in the design and operation of transformers:

-Reflects the efficiency and core performance of the transformer;
-It is one of the important parameters for evaluating transformer loss;
-It also has certain reference value in parallel operation of transformers and protection setting.

VI. Conclusion

To sum up, the generation of transformer no-load current is determined by many factors, such as establishing main magnetic flux and core loss. It is of great significance for transformer design, operation and energy saving to understand its generation mechanism and related influencing factors. With the development of new materials and new processes, the no-load current will be further reduced, thus improving the overall efficiency and economy of transformers.