Why do transformers only work with AC and not DC?

Transformers operate on the principle of electromagnetic induction, which requires a *changing* magnetic flux to induce an electromotive force (EMF) in the secondary coil. Alternating current (AC) continuously changes its magnitude and direction, thereby producing a continuously changing magnetic field and, consequently, a changing magnetic flux. Direct current (DC), on the other hand, produces a constant magnetic field. A constant magnetic flux does not induce any EMF, so a transformer connected to a DC source will not produce any output voltage in the secondary coil. In fact, connecting a transformer to DC can even damage the primary coil due to its low resistance.

What is the purpose of the laminated soft iron core in a transformer?

The laminated soft iron core serves two crucial purposes. Firstly, soft iron is a ferromagnetic material with high magnetic permeability, meaning it can easily concentrate and channel magnetic flux. This ensures that almost all the magnetic flux produced by the primary coil links with the secondary coil, maximizing mutual induction. Secondly, the core is laminated (made of thin sheets insulated from each other) to significantly reduce eddy current losses. Eddy currents are induced circulating currents within the core itself, which generate heat. Laminating the core increases the electrical resistance path for these currents, thereby reducing their magnitude and minimizing energy dissipation as heat.

What is meant by the 'transformation ratio' of a transformer?

The transformation ratio (or turns ratio), denoted by $k$, is a fundamental characteristic of a transformer. It is defined as the ratio of the number of turns in the secondary coil ($N_s$) to the number of turns in the primary coil ($N_p$). Mathematically, $k = N_s / N_p$. This ratio directly determines how the voltage is transformed: if $k > 1$, it's a step-up transformer (voltage increases); if $k < 1$, it's a step-down transformer (voltage decreases). For an ideal transformer, it also relates the voltages: $V_s / V_p = k$ and currents: $I_s / I_p = 1/k$.

What are the main types of energy losses in a real transformer?

Real transformers experience four primary types of energy losses, which reduce their efficiency. These include: 1) Copper losses, due to the resistance of the primary and secondary windings ($I^2R$ heating). 2) Eddy current losses, caused by induced circulating currents in the core, generating heat. 3) Hysteresis losses, resulting from the energy expended in repeatedly magnetizing and demagnetizing the core material. 4) Flux leakage, where some magnetic flux produced by the primary coil fails to link with the secondary coil. Engineers design transformers to minimize these losses through material selection and construction techniques.

How is the efficiency of a transformer calculated, and what does it signify?

The efficiency ($eta$) of a transformer is a measure of how effectively it converts input electrical power into output electrical power. It is calculated as the ratio of the output power to the input power, usually expressed as a percentage: $eta = ( ext{Output Power} / ext{Input Power}) imes 100%$. For an ideal transformer, efficiency is 100%, meaning no energy is lost. In real transformers, efficiency is always less than 100% due to various energy losses (copper, eddy current, hysteresis, flux leakage). A higher efficiency indicates less energy waste and better performance, which is crucial for large power transformers that handle significant amounts of energy.

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Electromagnetic Induction

Transformer

Q: What are the main types of energy losses in a real transformer?

Real transformers experience four primary types of energy losses, which reduce their efficiency. These include: 1) Copper losses, due to the resistance of the primary and secondary windings ($I^2R$ heating). 2) Eddy current losses, caused by induced circulating currents in the core, generating heat. 3) Hysteresis losses, resulting from the energy expended in repeatedly magnetizing and demagnetizing the core material. 4) Flux leakage, where some magnetic flux produced by the primary coil fails to link with the secondary coil. Engineers design transformers to minimize these losses through material selection and construction techniques.

Q: How is the efficiency of a transformer calculated, and what does it signify?

The efficiency ($eta$) of a transformer is a measure of how effectively it converts input electrical power into output electrical power. It is calculated as the ratio of the output power to the input power, usually expressed as a percentage: $eta = ( ext{Output Power} / ext{Input Power}) imes 100%$. For an ideal transformer, efficiency is 100%, meaning no energy is lost. In real transformers, efficiency is always less than 100% due to various energy losses (copper, eddy current, hysteresis, flux leakage). A higher efficiency indicates less energy waste and better performance, which is crucial for large power transformers that handle significant amounts of energy.

Physics

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Version 1Updated 22 Mar 2026

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A transformer is a static electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. It is primarily used to change the voltage level of an alternating current (AC) supply without changing its frequency. This transformation is achieved by varying the number of turns in its primary and secondary coils, which are wound around a common laminated…

Quick Summary

A transformer is a static device that changes the voltage level of an AC supply using the principle of mutual induction. It consists of a primary coil and a secondary coil wound around a laminated soft iron core.

When AC flows through the primary, it creates a changing magnetic flux that links with the secondary, inducing an EMF. The ratio of output voltage to input voltage is equal to the ratio of the number of turns in the secondary coil to the primary coil, known as the transformation ratio ( $k = N_s/N_p$ ).

If $k > 1$ , it's a step-up transformer; if $k < 1$ , it's a step-down transformer. For an ideal transformer, input power equals output power ( $V_p I_p = V_s I_s$ ), implying that current is inversely proportional to voltage transformation.

Real transformers suffer from losses like copper loss ( $I^2R$ ), eddy current loss (minimized by lamination), hysteresis loss (minimized by soft iron core), and flux leakage. Efficiency is the ratio of output power to input power, typically high for power transformers.

Transformers are essential for efficient power transmission and voltage regulation in electronic devices.

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Key Concepts

Turns Ratio and Voltage Transformation

The turns ratio, $k = N_s/N_p$ , is the primary factor determining voltage transformation. If you know the…

Power Conservation and Current Transformation

In an ideal transformer, power is conserved, meaning input power equals output power ( $P_p = P_s$ ). Since $P…

Minimizing Eddy Current Losses

Eddy currents are induced in the transformer core due to changing magnetic flux, leading to energy…

Principle — Mutual Induction.

Works with — AC only, NOT DC.

Ideal Transformer Equations

- $\frac{V_s}{V_p} = \frac{N_s}{N_p} = k$ (Transformation Ratio) - $\frac{I_s}{I_p} = \frac{N_p}{N_s} = \frac{1}{k}$ - $V_p I_p = V_s I_s$ (Power Conservation)

Types

- Step-up: $N_s > N_p \implies V_s > V_p, I_s < I_p$ - Step-down: $N_s < N_p \implies V_s < V_p, I_s > I_p$

Losses (Real Transformer)

1. Copper Loss: $I^2R$ in windings (minimize with thick copper). 2. Eddy Current Loss: In core (minimize with laminated core). 3. Hysteresis Loss: In core (minimize with soft iron core). 4. Flux Leakage: Magnetic flux not linking (minimize with proper winding).