About the Transformer (Ideal & Real)
A transformer transfers electrical energy between two or more circuits via mutual inductance through a shared magnetic core. For an ideal transformer the turns ratio governs voltage and current: V2/V1 = N2/N1 = I1/I2. This allows electrical power to be transmitted at high voltage (low current, low resistive loss) over long distances and then stepped down to safe levels for consumers—the key technology enabling modern AC power grids.
This simulation animates core magnetic flux and the induced EMFs in primary and secondary windings. You can add realistic losses—leakage inductance (flux not shared between windings), winding resistance (copper loss), and core losses (hysteresis and eddy-current heating)—and observe how overall efficiency drops under these conditions.
Frequently Asked Questions
How does a transformer step up or step down voltage?
The alternating current in the primary winding creates a time-varying flux in the core. By Faraday's law each turn of both windings links this flux, inducing EMF per turn of dΦ/dt. The primary has N1 turns and the secondary N2, giving V2/V1 = N2/N1. A step-up transformer with N1 = 100 and N2 = 10,000 multiplies voltage by 100 and divides current by 100, keeping power constant.
Why do transformers only work with AC, not DC?
Faraday's law requires a changing flux (dΦ/dt ≠ 0) to induce an EMF. A DC current in the primary creates a constant, steady flux, so dΦ/dt = 0 and no secondary voltage is induced. AC alternates at 50 or 60 Hz, continuously changing the flux and maintaining induction. Specialist power electronics can convert DC to a high-frequency AC, drive a transformer, then rectify the output (switched-mode power supplies).
What are the main losses in a real transformer?
Copper (I²R) losses in the winding resistance increase with load current. Core losses comprise eddy-current losses (proportional to f²B² and reduced by lamination) and hysteresis losses (proportional to fBⁿ, where n ≈ 1.6–2 for silicon steel). Large power transformers achieve efficiencies of 98–99.75%; distribution transformers are typically 97–99%. The losses appear as heat, requiring oil cooling in large units.
What is leakage inductance and why does it matter?
Leakage inductance arises because not all the flux produced by the primary links the secondary—some flux "leaks" into the surrounding air. The leakage inductances appear as series inductances in each winding and cause voltage drops under load, reducing output voltage regulation. Tightly wound, interleaved winding designs minimise leakage inductance, which is critical in high-frequency switching transformers where it can cause large voltage spikes.
What is an ideal transformer and how closely does reality match it?
An ideal transformer assumes zero winding resistance, zero leakage flux, infinite core permeability, and no core losses. Real large power transformers approach this closely: efficiency above 99%, regulation better than 2%, and leakage inductance below 5%. Small audio or signal transformers deviate more, with significant winding resistance and limited frequency response.
How are transformers used in the National Grid?
In the UK National Grid, power stations generate at 11–25 kV, which is stepped up to 275 or 400 kV for transmission. Step-down transformers at grid supply points reduce this to 132 kV, then 33 kV, then 11 kV at primary substations, and finally to 230 V (single phase) or 400 V (three phase) for consumers. Each voltage transformation reduces current and hence I²R losses in cables.
What is the difference between a step-up and an autotransformer?
A conventional two-winding transformer has electrically isolated primary and secondary windings. An autotransformer uses a single winding with a tapping point, so primary and secondary share part of the winding and are electrically connected. Autotransformers are lighter and more efficient for small voltage ratios (e.g., 230 V to 115 V) but provide no galvanic isolation, which can be a safety concern.
Why do transformers hum?
The hum at 100 or 120 Hz (twice the supply frequency) comes from magnetostriction: the core material changes length slightly as the magnetic flux alternates, causing the laminations to vibrate. Loose laminations amplify the sound. Large power transformers are filled with insulating oil and fitted in sealed tanks partly to attenuate this noise. Modern amorphous-metal cores have lower magnetostriction than silicon steel.
What is a switched-mode power supply (SMPS) transformer?
SMPS units rectify the mains to DC, then switch it at 20 kHz–1 MHz to drive a small high-frequency transformer before rectifying and filtering the output. Because transformer core size scales inversely with frequency, an SMPS transformer operating at 100 kHz can be 1000 times smaller than a mains-frequency equivalent. This is why modern laptop chargers are compact and lightweight.
What is the VA (volt-ampere) rating of a transformer?
The VA rating is the product of the rated secondary voltage and current, representing the maximum apparent power the transformer can continuously deliver. It differs from watts (real power) because reactive loads draw current without consuming real power. A transformer rated 1000 VA at 0.8 power factor can supply 800 W of real power. Thermal limits (winding and core heating) set the VA rating.