Core Loss and Copper Loss in Transformer

Copper loss

When someone in electric industry says copper, many people may think of copper wire. In fact, he copper in the copper loss of the transformer also refers to copper wire. Most transformer windings are made of copper wires. Generally, the number of turns is large, so the resistance will not be ignored, and the resistance means that there is power loss.

Definition & Principle

Copper loss—also called load loss—refers to the resistive dissipation in these windings. Since it varies with load current, it is considered a variable loss.

When a transformer operates under load, there will be resistance when current flows through the windings, resulting in resistance loss, that is I²R losses (Joule heating). According to Joule’s Law, the resistance flowing through the current will produce Joule heat, and the greater the current, the greater the power loss. Therefore, the resistance loss is proportional to the square of the current ΔP = I²R, and is independent of voltage. It is precisely because it changes with the current size, so the copper loss ( load loss ) is variable loss, it is also the main loss in the operation of the transformer.

Influencing Factors

• Current magnitude: Copper loss is proportional to the square of the current (I²). So the current is the key factor affecting the copper loss.
• Winding resistance: Higher resistance (due to material or design) increases loss.
• Number of winding layers: More layers extend current path length in the winding, the resistance will increase and result in copper loss.
• Switching frequency:

When the load characteristics and the distribution parameters are inductive, the copper loss decreases with the increase of the switching frequency.

When it exhibits capacitive characteristics, the copper loss increases with the increase of switching frequency.

• Temperature effects:

Load loss is also affected by the operating temperature. Load current can result leakage flux, which produces eddy current loss in the windings and stray loss in metal part outside the winding.

Calculation Methods

There are two calculation formulas:

Formula based on rated current and resistance:
Copper loss (kW) = I² × Rc × Δt
I = Rated current of the transformer
Rc = Resistance of copper wire
Δt = Operating time of the transformer

Formula based on rated current and total copper resistance:
Copper loss(kW) = I² × R
I = Rated current of the transformer
R = Total copper resistance of the transformer

The total copper resistance R of the transformer can be calculated by the following formula:
R = (R1 + R2) / 2
R1 = Primary-side copper resistance of the transformer
R2 = Secondary-side copper resistance of the transformer

Methods to Reduce Copper Loss:

• Increasing the winding cross-sectional area of the transformer, it will reduce conductor resistance, thereby effectively lowering transformer copper loss.
• Using high-quality conductor materials, like copper foil or aluminum foil to reduce winding resistance.
• Reducing transformer light-load operation time, we can limit the proportion of light-load operation time to help reduce transformer copper loss.

Iron Loss (Core Loss)

Definition and Principle

Unlike copper loss, the iron loss or core loss of a transformer is independent of windings, current and other factors. As the name suggests, it is related to the iron core and is generated within it. Iron loss is also called “no-load loss” because it exists under both full-load and no-load state, making it a fixed loss in transformers. However, during operation, the power loss will decrease with the electric field strength reducing.

Classification

Transformer iron loss is divided into hysteresis loss and eddy current loss.

Hysteresis Loss

Transformers operate on electromagnetic induction to change voltage and current. When magnetic flux flows through the iron core, the core’s magnetic reluctance (similar to electrical resistance in conductors) causes energy dissipation as heat. This loss is called hysteresis loss.

Eddy Current Loss
When the primary winding is energized, the generated magnetic flux flows through the core, because the iron core is conductive, it induces a voltage perpendicular to the magnetic field, creating circulating currents (eddy currents). It is precisely because the iron core will produce eddy current, so it is made into a thin piece, because the thinner the resistance, the smaller the current.

Influencing Factors

• Operating Voltage & Frequency
  Iron loss is related to the transformer’s operating voltage and frequency, because these factors will affect the core’s magnetic field strength and hysteresis in the core.
• Core Material
  The hysteresis properties of the core material will affect the size of the iron loss. If the core material is not well selected, the hysteresis loss will increase.
• Manufacturing Process
  The manufacturing process of the transformer also has a certain influence on the iron loss. For example, the lamination method and insulation treatment of the iron core will affect the iron loss.

Calculation Methods

Formula Based on Rated Current & Core Loss Components
Iron Loss (kVA) = I² × (Rₘ + Rₐ)

I = Rated current

Rₘ = Core hysteresis loss component

Rₐ = Core resistive loss component

Empirical Formula Based on Flux Density & Frequency
Pₜ = Kₜ × Bₘ² × f

Pₜ = Total iron loss

Kₜ = Material constant

Bₘ = Peak flux density (T)

f = Operating frequency (Hz)

Reduction Methods

• Select High-Grade Core Materials
  Select low-hysteresis-loss materials like grain-oriented silicon steel).
• Process Optimization
  Improve core lamination method and insulation treatments.
• Design Optimization
  Optimize structural parameters during the design phase to minimize flux density variations.

Relationship Between Transformer Core Loss, Copper Loss, and #Bushings

1. Fundamental Relationship

#Tansformer Bushings serve as insulated conduits for HV/LV conductors but indirectly influence power losses:

• No direct loss generation: Bushings themselves don’t contribute to core/copper losses.
• Loss amplification: Poor bushing conditions can exacerbate existing losses.

Loss Flow

• Primary Path: Core loss (hysteresis/eddy) → Magnetic circuit
• Secondary Path: Copper loss (I²R) → Windings + bushing contacts

2. Impact Analysis

(1) Core Loss (Iron Loss) Connection

Factor	Mechanism	Bushing    Solution
Magnetic   Flux	Steel-core   bushings distort main flux → Eddy loss	Use   non-magnetic (Al/Composite) bushings
Insulation   Aging	Partial   discharges near bushings → Local core heating	Monitor   #tanδ and PD levels

Flux Distortion

• Normal: Uniform flux in core (low loss)
• With Magnetic Bushing: Flux leakage → Increased eddy current loops

(2) Copper Loss Connection

Factor	Mechanism	Bushing    Solution
Contact   Resistance	Loose/dirty   bushing-winding joints → I²R loss	Apply   non-loose contacts +clean bushings regularly
Current   Capacity	Undersized   bushings → Overheating → ↑Winding temperature	Match   bushing Iₙ to transformer rating

Thermal Profile

Hotspots: Bushing joints → Elevated winding resistance → Higher copper loss

3. Optimization Strategies

Design Phase:

Select Dry-type bushing to reduce capacitive heating, we have produced dry-type bushing for many years, including porcelain bushing and polymer bushing.

Non-magnetic stems to prevent flux interference.

Operational Phase:

• Infrared inspections: Detect hot joints (ΔT > 15°C indicates risk).
• Loss segregation tests: Isolate bushing-related losses during no-load/load tests.

4. Summary

Loss    Type	Bushing    Influence	Mitigation
Core   Loss	Flux   distortion + localized heating	Non-magnetic   materials + PD monitoring
Copper   Loss	Contact   resistance + thermal coupling	Precision   joints + current derating

Efficiency Gain: Proper bushing design/maintenance can reduce total losses by 0.5–2% (typical for 36kV+ transformers).

Discover more transformer bushing technology on our website:www.heweipower.com