Any type of electrical equipment will experience losses during long-term operation, and power transformers are no exception. The losses in power transformers are mainly divided into copper loss and iron loss.
Copper Loss
Definition and Principle:
Copper plays an important role in transformers. The windings of transformers are usually made of copper conductors, and the so-called "copper loss" refers to the loss generated by these copper conductors. Copper loss is also known as load loss, which is a variable loss that changes with the load. For example, when a step-down transformer operates under load., Current flows through the conductors, and resistance in the conductors causes power loss. According to Joule’s law, resistance generates heat when current flows through it, and the greater the current, the higher the power loss. Therefore, resistance loss is proportional to the square of the current and is unrelated to voltage. Since it varies with current, copper loss (or load loss) is a variable loss and is a major source of loss during transformer operation.
Influencing Factors:
Current magnitude: Copper loss is proportional to the square of the current, so current magnitude is a key influencing factor.
Winding resistance: The resistance of the windings directly affects copper loss. Higher resistance leads to higher copper loss.
Number of coil layers: More layers increase the path length for current in the winding, which increases resistance and thus copper loss.
Switching frequency: The impact of switching frequency on copper loss is directly related to the transformer's distributed parameters and load characteristics. When both present inductive characteristics, copper loss decreases as switching frequency increases; when both present capacitive characteristics, copper loss increases as switching frequency increases.
Temperature influence: Load loss is also affected by the transformer's temperature. Additionally, the leakage flux caused by load current induces eddy current loss within the windings and stray losses in nearby metal parts.
Calculation Methods:
There are two formulas:
Based on rated current and resistance:
where I is the rated current of the transformer, Rc is the resistance of the copper conductor, and Δt is the operating time of the transformer.
Based on rated current and total copper resistance:
where I is the rated current of the transformer, and R is the total copper resistance, calculated as:
R = (R1 + R2) / 2
where R1 is the copper resistance on the primary side, and R2 is the copper resistance on the secondary side.
Methods to Reduce Copper Loss:
Increase the cross-sectional area of the transformer windings: This reduces conductor resistance and effectively lowers copper loss.
Use high-quality conductor materials: Using materials such as copper or aluminum foil helps reduce winding resistance.
Reduce light-load operating time of the transformer: Limiting the proportion of light-load operation time helps reduce copper loss.
Iron Loss
Definition and Principle:
Unlike copper loss, iron loss is unrelated to the windings or current magnitude. As the name suggests, iron loss is associated with the transformer core and is also known as no-load loss, as it occurs regardless of whether the transformer is fully loaded or running without a load. Therefore, it is considered a fixed loss. However, under load, the power loss may decrease as the electric field intensity decreases.
Classification:
Hysteresis loss: The operating principle of a transformer is based on electromagnetic induction to achieve voltage and current conversion. The magnetic flux inside the transformer flows through the iron core, which has magnetic resistance, similar to how conductors resist electric current, and generates heat. This type of loss is known as hysteresis loss.
Eddy current loss: When the primary winding of the transformer is energized, the magnetic flux generated by the coil flows through the iron core. Since the core itself is also a conductor, an electromotive force is induced on the plane perpendicular to the magnetic lines, forming a closed loop of current within the cross-section of the iron core, known as an eddy current.
The loss caused by this eddy current is called eddy current loss. To reduce this, the core is made of thin laminated sheets, as thinner sheets have greater resistance and thus smaller currents.
Influencing Factors:
Operating voltage and frequency: Iron loss is related to the transformer's operating voltage and frequency, as these affect the magnetic field strength and hysteresis in the core.
Core material: The hysteresis characteristics of the core material affect the amount of iron loss. Poor choice of materials will increase hysteresis loss.
Manufacturing process: The manufacturing process also affects iron loss. For example, the lamination method of the core and insulation treatment will impact the overall loss level.
Calculation Methods:
Based on rated current, hysteresis, and resistance loss:
where I is the rated current, Rm is the hysteresis loss of the core, and Ra is the resistance loss of the core.
Based on constants, magnetic flux density, and operating frequency:
where P_iron is the iron loss, Kf is a constant, Bm is the magnetic flux density, and f is the transformer's operating frequency.
Methods to Reduce Iron Loss:
Use high-quality core materials: Selecting core materials with low hysteresis loss can effectively reduce iron loss.
Optimize manufacturing processes: Improving lamination methods, insulation treatments, and other manufacturing techniques can reduce iron loss.
Rational design: During the design phase, optimizing the structure and selecting appropriate parameters helps lower iron loss.
