What are the losses in transformers? Explain eddy current, hysteresis, leakage flux, etc.

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What are the losses in transformers? Explain eddy current, hysteresis, leakage flux, etc. [Copy link]

The magnetic flux of the primary coil and the secondary coil of the transformer are consistent. But in fact, the transmission of the magnetic field is accompanied by losses, which can be roughly divided into the following situations:

Eddy current loss

Hysteresis loss

Magnetic leakage loss

1. Eddy Current Losses

Faraday said that a changing magnetic field can generate electricity, and Lenz said that this electricity will hinder the change of the magnetic field, which is called back electromotive force (Back EMF).

The magnetic core of the transformer is in a changing magnetic field, and electricity will be generated on it, manifesting as many small eddy currents. This phenomenon is called eddy current effect.

Figure 1- Eddy Current

Eddy currents are useful in some cases, for example, household induction cookers heat food based on the eddy current principle. However, they will cause losses in the transformer.

There are two ideas to solve the eddy current problem:

Decompose the large magnetic core into a large number of small magnetic cores, such as laminated, powder, etc.

Use high resistivity materials, such as ferrite ceramics, silicon steel, etc.

1. Lamination

For the "口"-shaped or "日"-shaped magnetic cores in the transformer, they are not a single column, but a combination of many layers of magnetic cores. There is insulation isolation between the magnetic cores, which makes the eddy current only generated in a very narrow area (the area is perpendicular to the magnetic field). Since the size of the eddy current is proportional to the area, this can greatly reduce the eddy current. In addition, the thinner the magnetic core, the better the effect.

Figure 2 - Transformer core monomer (left) vs. laminated (right), where B represents magnetic field and red represents eddy current

Figure 3-Laminated core of transformer (“日”/“E+I” shape)

2. Ferrite

Ferrite is a ceramic material with iron oxide as its main component. Most ferrites are magnetic materials used to make permanent magnets (hard ferrites) and transformer cores (soft ferrites). The high resistivity of ferrites can reduce eddy current phenomena:

Figure 4 - Transformer core made of ferrite

2. Hysteresis Losses

Hysteresis means that the magnetization of a substance depends not only on the external magnetic field at that time, but also on the magnetization result before. That is, once magnetized, the magnetism will be retained. To demagnetize, a magnetic field in the opposite direction needs to be applied.

We can see the clues from the BH curve:

Symbol B refers to magnetic flux density, which is also translated as magnetic induction intensity in China. I think the former is more accurate.

The symbol H refers to the magnetic field strength (Magnetizing Force), which is the magnetomotive force per unit length (as mentioned above), and can be understood as being proportional to the current passing through the coil;

Figure 5-BH curve

The evolution process is:

The origin of the coordinate axis (a) indicates that the material is not magnetized. Then a positive current is input to the coil (H axis is positive), and magnetism appears on the material, and the magnetic flux is positive (red dotted line).

When the current increases to a certain level, the magnetic flux growth becomes slow and enters the magnetic saturation state (b);

If the current is stopped, the substance can still exhibit magnetism (c);

The magnetism disappears only when a current in the opposite direction is applied (d).

In the transformer, the magnetic core is repeatedly magnetized by alternating current. We hope that the hysteresis of the core is as small as possible, otherwise more energy will be wasted in overcoming the hysteresis.

The ferrites mentioned above are divided into two types: soft and hard. Soft ferrites are more easily demagnetized, which means that the narrower the shape in BH (the smaller the distance between d and g):

Figure 6 - Hard magnet (left) to soft magnet (right)

3. Leakage Magnetic Flux

Although the magnetic core has a high magnetic permeability, there is always some magnetic flux that does not propagate along the core and leaks to the outside, which is called leakage flux.

Figure 7-Leakage Flux

Leakage magnetic flux does not participate in the magnetic coupling energy transfer between the primary coil and the secondary coil. They are like inductors, connected in series in the circuit. As an impedance, the inductor will produce a voltage drop, so the actual voltage on the transformer will be smaller:

Figure 8 - Transformer equivalent circuit (simplified inductor series)

In summary, we have explained the loss phenomenon on the transformer. When the secondary coil is open, a small current will pass through the primary coil. Part of this current is the magnetizing current mentioned above, and the other part is the current caused by the loss (Core Loss Current) described in this article. The sum of the two is called the exciting current.

PowerAnts

Taking 50HZ as an example, the eddy current path of the whole silicon steel block is far less exaggerated than that shown in Figure 2 on the left. The actual penetration depth is about 0.15-0.2mm. The real purpose of stacking multiple pieces is not to compress the current into a single piece, but to reduce the magnetic flux of a single piece, and the induced electromotive force in the piece is proportionally reduced, thus greatly reducing eddy losses.

In addition, because the width does not change, taking the square magnetic circuit as an example, the current path of a single chip is reduced by about half, so the size of the eddy current is not proportional to the thickness of the chip.