Transformer losses are the core source of energy loss during power transmission, directly affecting equipment temperature rise, operational efficiency, and service life. According to the working frequency, transformers can be classified as low-frequency and low-harmonic transformers and high-frequency switching power supply transformers. The materials of their cores, magnetic circuit structures, and operating conditions vary greatly, and their loss composition, generation mechanism, distribution characteristics, and variation patterns are completely different. This article separately analyzes the characteristic manifestations, influencing factors, and distinguishing rules of various losses in low-frequency and high-frequency transformers, and combines engineering practice to analyze the key points of loss control.

Regardless of low-frequency or high-frequency transformers, the total losses are uniformly divided into copper loss (winding loss) and iron loss (core magnetic loss) into two major sections. Four core losses are subdivided:

Under low-frequency conditions, only DC copper loss, hysteresis loss, and macroscopic iron core eddy current loss dominate; under high-frequency conditions, skin effect, proximity effect, and micro-echelon residual loss will sharply increase, becoming the main source of loss increment. This is the most critical dividing line between the loss characteristics of the two.

Low-frequency transformers mostly refer to power distribution transformers and power isolation transformers. The working frequency is fixed at 50Hz, and the core uses cold-rolled silicon steel sheet laminated structure. The loss shows a typical characteristic of "iron loss is stable, copper loss varies linearly with load".

      Hysteresis loss is the main body of iron loss: the magnetic hysteresis loop of silicon steel is narrow, and the coercive force is low. Under power frequency, hysteresis loss accounts for more than 70% of the total iron loss. Each excitation cycle alternates, and the magnetic domains in the silicon steel flip repeatedly, generating frictional heat, and the loss size is proportional to the magnetic flux density and frequency. The design value of the magnetic flux density under low frequency is generally 1.4T to 1.7T, and the values of full-load and no-load iron loss are fixed and not affected by the load current. The temperature rise of the transformer under no-load is basically determined by hysteresis loss.
      Eddy current loss is relatively low: the surface of the silicon steel is coated with insulating paint, with a single sheet thickness of 0.23 to 0.35mm. The amplitude of the inter-plate eddy current induced by the alternating magnetic field under low-frequency is very small, accounting for only about 20% of the iron loss; only when the insulation paint of the iron core is damaged, the laminations are loose, or there are multiple grounding short circuits, will the eddy current loss increase abnormally, manifested as excessive no-load loss and local overheating of the iron core.
      There is no high-frequency residual loss: there are no high-frequency harmonic components in power frequency, and the microscopic relaxation loss of magnetic domains can be ignored. The iron loss curve grows steadily and linearly with the magnetic flux density, and there is no sudden increase inflection point.

      Under low-frequency conditions, the additional AC loss can be ignored, and the copper loss is approximately equal to the DC resistance loss of the winding. The current is uniformly distributed throughout the entire cross-section of the conductor, the skin depth is much greater than the conventional enameled wire diameter, and there is no phenomenon of current concentrating on the surface of the conductor. Copper loss is proportional to the square of the load current, and the copper loss is almost zero in the no-load state, reaching the peak value when the load is full. The three-phase low-frequency transformer has balanced three-phase winding resistance values, and the difference in three-phase copper loss is very small; only when the contact of the tap changer is poor or the connection terminal oxidizes, the single-loop DC resistance increases, and local copper loss overheats abnormally.

The no-load loss of low-frequency transformers is constant, and the load loss changes synchronously with the load; under light-load conditions, iron loss dominates the total loss, and under heavy-load conditions, copper loss exceeds iron loss. The loss fluctuates smoothly, without instantaneous sharp peak losses, and the temperature rise is slow and uniform. Fault losses are mostly caused by insulation damage of the iron core or contact defects of the winding connection.

High-frequency transformers are applied in switching power supplies, inverters, and charging station power supplies, with operating frequencies ranging from tens of kilohertz to several megahertz. The magnetic cores use manganese-zinc ferrite, nanocrystals, and amorphous strip materials. The high-frequency alternating magnetic field generates additional losses that do not exist in low-frequency conditions. The losses exhibit the characteristics of "extremely high no-load iron loss, significant increase in additional losses with load, and increasing losses with frequency exponentiation".

      Hysteresis loss increases rapidly with frequency: The hysteresis loop of ferrite is narrow, but at high frequencies, the magnetic domain flipping speed cannot keep up with the alternating magnetic field, and the unit-period hysteresis loss continuously increases; at the same time, the magnetic flux density of the high-frequency transformer is only 0.2T to 0.4T, and the magnetic flux density is reduced to suppress hysteresis heating.
      Micro eddy current loss becomes the core increment: Ferrite is a powder sintered material, and there are a large number of tiny conductive particles inside. The high-frequency magnetic field will induce microscopic eddy currents in the particles, and the loss increases with the square of the frequency. For every increase of one frequency, the eddy current loss increases fourfold. This is the primary cause of excessive iron loss in high-frequency transformers.
      Residual relaxation loss cannot be ignored: At megahertz frequencies, the magnetic domain flipping has a relaxation lag effect, generating additional residual losses. The higher the frequency, the higher the proportion of this loss. Moreover, the high-frequency magnetic core loss is sensitive to temperature, and above 80°C, a "loss increase - temperature rise" positive feedback occurs, easily causing magnetic core thermal breakdown.

At high frequencies, the skin effect and proximity effect completely change the current distribution, and copper loss no longer follows the law of direct current resistance:
      Skin effect: High-frequency current flows only in the thin layer on the surface of the conductor, and the effective conductive cross-sectional area of the conductor is significantly reduced, and the equivalent alternating resistance is several times that of direct current resistance. The thicker the wire diameter, the more severe the deterioration of loss.
      Proximity effect: The primary and secondary windings are arranged closely in multiple layers, and the alternating magnetic field of the adjacent conductors couples with each other. A circulating current is generated inside the conductor, and the additional loss of multiple layers of windings can reach 3 to 10 times the direct current copper loss.
In engineering, it is necessary to use Litz wires and thin copper foil windings to disperse the current and suppress additional copper loss.

Even with no-load conditions in high-frequency transformers, the iron loss brought by high-frequency excitation is still considerable; the harmonic peaks generated by the switching transistor's on-off still instantaneously produce pulse-like peak losses, and the temperature rise speed is much faster than that of low-frequency transformers. The total loss is extremely sensitive to frequency, temperature rise, and winding winding process. Minor process defects can lead to a doubling of losses.
Four. 

1.Differences in loss dominant types: The bottleneck of low-frequency loss is in the DC copper loss and macroscopic eddy current of silicon steel sheets; the bottleneck of high-frequency loss is in the micro eddy current of the magnetic core, the skin effect of the winding, and the proximity additional loss.
2.Frequency sensitivity differences: Low-frequency loss is proportional to the first power of frequency; high-frequency iron loss, additional copper loss are proportional to the square of frequency and even higher powers.
3.Optimization schemes distinction: For low-frequency transformers, the optimization focus is on selecting high-grade low-loss silicon steel sheets and reducing the DC resistance of the winding; for high-frequency transformers, it is necessary to match low-loss ferrite magnetic cores, layer-wound Litz wires, reducing the working magnetic flux density, and increasing the insulation gap of the winding to weaken magnetic coupling.
4.Differences in fault determination: Low-frequency loss abnormalities are mostly caused by iron core grounding short circuits and poor contact of the tap switch; high-frequency loss over-limit is mostly caused by poor winding process, improper selection of magnetic core, and excessive switching harmonics.

Due to different operating frequencies, there are essential differences in the energy loss mechanisms of magnetic cores and windings of low-frequency and high-frequency transformers. The loss laws of low-frequency transformers are simple and stable, and the control focus is on the insulation of the magnetic core and the DC circuit resistance; the loss of high-frequency transformers is the result of multiple additional losses, and the loss is constrained by frequency, winding process, and magnetic core temperature, and loss control is the core difficulty in power equipment design. Accurately distinguishing the loss characteristics of the two can be targeted for magnetic core selection, winding structure design, and fault detection, effectively reducing transformer heating, improving power conversion efficiency, and extending equipment operation life.