This article describes different types of losses in a transformer, including Iron Losses, Hysteresis Loss, Eddy Current Loss, Copper Loss, Stray Loss, and Dielectric Loss.
Transformers are used in the power system to transfer electrical energy between different voltage levels efficiently. Like other electrical machines, transformers are not 100 % efficient because various losses occur during operation. The transformer losses are called transformation losses, which impact the performance and efficiency of transformers. It is important to understand these losses in the transformer to optimize the transformer performance and minimize energy wastage.
Types of Losses in a Transformer
Transformer losses are broadly categorized into Core Losses (or Iron Losses) and Copper Losses. Additionally, Stray Losses and Dielectric Losses also contribute to the overall loss in transformers.
1. Iron Losses
Iron losses occur in the magnetic core of the transformer. When the primary winding of the transformer receives alternating voltage, the current in the primary winding sets up the alternating magnetic flux in the transformer core, which causes losses in the core of the transformer. This loss is also called core loss. These losses are constant under specified operating conditions and it has two main components: Hysteresis Loss and Eddy Current Loss.
Hysteresis Loss
The transformer core magnetizes and demagnetizes during each cycle of the alternating current(AC). After demagnetization, the magnetic domains realign in the positive cycle and consume energy. The energy consumed in the process of realignment of the magnetic domain causes hysteresis loss in the transformer, and it depends on the magnetic properties of the core material and the frequency of the AC supply. The formula of hysteresis loss is,
Where:
- η = Hysteresis constant (depends on the core material)
- Bmax = Maximum flux density in wb/m2
- f = Frequency of AC supply( Hz)
- V = Volume of the core (m3)
The core material significantly impacts hysteresis losses. Materials such as silicon steel and amorphous metals have lower hysteresis loops and are commonly used to minimize iron losses.
Eddy Current Loss
In a transformer, the alternating magnetic flux flows in the closed magnetic circuit. The varying magnetic flux induces electromotive force(EMF) and causes current to flow in the core. The magnitude of this current depends on the EMF generated and the resistance of the magnetic core.
The induced EMF causes current to circulate within the core material, and this circulating current is called Eddy Currents. However, these circulating currents do not perform any useful work Instead, they cause energy dissipation in the form of heat, commonly referred to as I²R loss, which is known as Eddy Current Loss in magnetic materials.
The solid piece of the core has a higher resistance and therefore it has higher eddy current loss. To minimize these losses, the core is constructed using thin laminations. These laminations are insulated from each other which restricts the path of eddy current and as a result, reduces the energy loss and improves the efficiency of the transformer.
The formula of the eddy current loss is ;
Where:
- Ke = Eddy current constant. Its magnitude depends on factors such as the properties of the magnetic material, including the core’s volume and resistivity, as well as the thickness of the laminations used.
- t = Thickness of the core lamination (m)
- Bm- maximum value of flux density in wb/m2
- f = Frequency of AC supply (Hz)
- V = Volume of the core (m3)
2. Copper Loss (Ohmic Loss)
Copper loss in the transformer is load-dependent and occurs due to resistance of primary and secondary windings. The loss occurs due to the resistance of the winding, therefore it is also called ohmic loss.
The copper loss in the primary and secondary winding depends on the resistance and the current in the respective winding. Thus, the copper loss of the transformer is the sum of losses in the primary and secondary winding. For example, if the resistance and current in the primary winding are R1 and I1 and, the secondary winding resistance and current are R2 and I2, then the copper loss in the primary is I12R1, and the copper loss in the secondary is I22R2 .
The total copper loss of the transformer is,
From the above formula, it is clear that the copper loss depends on the current and the resistance. Thus higher current and increased resistance results in higher copper loss. The copper loss can be minimized by the use of conductors with low resistance, such as high-purity copper and optimized winding design.
3. Stray Loss
In an ideal transformer, it is assumed that all the flux produced in the winding links to the primary and secondary winding and there is no flux leakage. However, in a practical transformer, the flux produced in the primary does not 100 % link to the windings and some parts of the flux leak from the winding and core. Leakage flux induces circulating currents in the windings or nearby metallic parts, leading to additional I²R losses. Stray magnetic fields can cause localized heating in parts of the core or structural elements which contribute to energy loss.
These losses are relatively small compared to core and copper losses but can still impact overall efficiency.
The stray losses can be minimized by ensuring proper insulation selection, optimized core design, and precise placement of winding & components, and use of high-resistivity materials.
4. Dielectric Loss
Dielectric loss occurs in the transformer due to the movement of charges and dipoles in the insulating materials under an AC voltage. The primary causes of this loss are conduction loss, caused by the minute conductivity of the insulating material, and Polarization Loss which occurs due to the lag in the polarization of the dielectric material compared to the applied alternating electric field. The formula for dielectric loss is,
Where:
- V = Voltage applied across the insulation
- f= Frequency of the AC supply
- C = Capacitance of the insulating material
- tanδ = Dielectric loss angle
The dielectric loss can be minimized by the use of high-quality insulating materials with low dielectric loss angles.
Summary of Types of Transformer Losses
| Type of Loss | Cause | Dependence | Mitigation Methods |
|---|---|---|---|
| Hysteresis Loss | Core magnetization and demagnetization | Frequency, core material | Use silicon steel cores |
| Eddy Current Loss | Induced currents in the core | Core thickness, frequency | Use laminated cores |
| Copper Loss | Resistance in windings | Load current, winding resistance | Use low-resistance conductors |
| Stray Loss | Leakage flux | Design, material conductivity | Optimize design and use non-conductive materials |
| Dielectric Loss | Polarization of insulation | Voltage, frequency, material quality | Use high-quality insulation |
Conclusion
Understanding the types of losses in a transformer—including Iron Losses, Hysteresis Loss, Eddy Current Loss, Copper Loss, Stray Loss, and Dielectric Loss—is essential for improving transformer efficiency and reliability.