Transformer Losses; Eddy current Hysteresis losses

Losses in Transformers: Core Losses & Copper Losses Explained

Transformers are the backbone of electrical power systems, used for stepping up and stepping down voltages in transmission and distribution. Like all electrical devices, they are not 100% efficient. Some part of input power is lost in the form of heat, called transformer losses.

Since transformers are static devices (no moving parts), they don’t have mechanical losses like motors or generators. Instead, they primarily suffer from electrical losses, which can only be minimized, not eliminated.

The two main types of transformer losses are:

1. Core Losses (Iron Losses)

2. Copper Losses (Ohmic Losses)

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1. Core Losses (Iron Losses)

Core losses occur in the magnetic core of the transformer. These losses are independent of load current and remain constant, hence also called No-Load Losses.

Core losses consist of:

• Hysteresis Losses

• Eddy Current Losses

(a) Hysteresis Losses

The transformer core is made of CRGO Silicon Steel (Cold Rolled Grain Oriented Steel), which is ferromagnetic. When alternating magnetic flux passes through the core, magnetic domains inside the material continuously realign with the changing flux.

This constant re-alignment consumes energy, leading to hysteresis loss.

Formula for Hysteresis Loss:

$$W_h = K_h \times f \times (B_m)^{1.6}$$

Where:

• $W_h$ = Hysteresis loss (W)

• $K_h$ = Hysteresis constant

• $f$ = Supply frequency (Hz)

• $B_m$ = Maximum flux density (Tesla)

👉 Hysteresis losses are directly proportional to frequency and flux density (to the power of 1.6).

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(b) Eddy Current Losses

When alternating magnetic flux passes through the transformer, some flux links with conductive core laminations and metallic parts. This induces circulating currents, known as eddy currents, which cause unwanted heating and energy loss.

Formula for Eddy Current Loss:

$$W_e = K_e \times f^2 \times (B_m)^2 \times K_f^2$$

Where:

• $W_e$ = Eddy current loss (W)

• $K_e$ = Eddy current constant

• $f$ = Supply frequency (Hz)

• $B_m$ = Maximum flux density

• $K_f$ = Form constant

👉 Eddy current losses are proportional to the square of both frequency and flux density.

✅ To reduce eddy current loss, transformer cores are made of thin laminated sheets with insulation coating, instead of a solid block of steel.

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2. Copper Losses (Ohmic Losses)

Copper losses occur in the primary and secondary windings of the transformer when current flows. These are load-dependent losses, hence also called Variable Losses.

Formula for Copper Loss:

$$W_{cu} = I^2 R$$

Where:

• $I$ = Load current

• $R$ = Resistance of winding

👉 In addition to $I^2R$ losses, stray load losses occur due to leakage flux linking with nearby metallic parts.

✅ Copper losses increase with load, hence are zero at no-load and maximum at full load.

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🔑 Key Differences Between Core Losses & Copper Losses

Feature	Core Losses	Copper Losses
Nature	Constant (No-load)	Variable (Load-dependent)
Components	Hysteresis & Eddy current	I2R & stray losses
Dependence	Flux density & frequency	Load current & winding resistance
Minimization	CRGO steel, laminations	High-quality copper, low resistance windings

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✅ Practical Insights for Engineers

• Efficiency point: Transformer efficiency is maximum when Core Loss = Copper Loss.

• Design strategy: Distribution transformers are designed for low core loss (as they run continuously at no load), while power transformers are optimized for low copper loss (as they run near full load).

• Maintenance: Checking winding resistance, core heating, and no-load current helps in monitoring transformer health.

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⚡ Conclusion:
Transformer losses can’t be eliminated, but with advanced materials (like CRGO steel and amorphous core) and optimized winding design, they can be minimized. Understanding core and copper losses is essential for improving power system efficiency and reducing operational costs.

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