Machines and power

What is a transformer? How it works, parts and equations

A transformer is a static electrical machine that passes AC power from one circuit to another through a shared magnetic field, raising or lowering the voltage on the way without changing the frequency. Two windings on an iron core do the work, and the ratio of their turns sets the ratio of their voltages.

By The electrician.wiki editors · Updated Oct 14, 2026 · How we check

In short

It works by mutual induction: alternating current in the primary winding sets up an alternating flux in the core, and that flux induces a voltage in the secondary. Steady DC produces no output.
V₂ ÷ V₁ = N₂ ÷ N₁ = I₁ ÷ I₂. A 230 V to 12 V transformer has a turns ratio of 19.2 : 1, and its secondary carries 19.2 times the primary current.
The EMF equation E = 4.44 f N Φm sets the turns: a 9 cm² core at 1.2 T and 50 Hz needs 4.17 turns per volt, so 959 turns for 230 V.
Losses are small: our 100 kVA example is 98.0 % efficient at full load and peaks at 98.7 % at 38 % load, where copper loss equals iron loss.
Transformers are rated in kVA, not kW, because their heating follows current and voltage whatever the power factor of the load.
Faraday showed the principle in 1831; Zipernowsky, Bláthy and Déri built the closed-core transformer in Budapest in 1885; William Stanley ran the first transformer-based AC system in 1886.

What is a transformer?

A transformer is an electrical machine with no moving parts that transfers AC power between two or more circuits by electromagnetic induction. It changes voltage and current in inverse proportion and leaves the frequency alone: 230 V at 50 Hz can become 12 V at 50 Hz, or 11 kV at 50 Hz can become 433 V at 50 Hz. Every power station, substation, street, phone charger and doorbell depends on one.

The basic transformer has three parts: a primary winding connected to the supply, a secondary winding connected to the load, and a core of laminated steel that carries the magnetic flux from one to the other. The two windings are not electrically connected. Energy crosses the gap only through the magnetic field, which is also why a transformer can isolate a circuit from the mains.

A single-phase core-type transformer. The primary has more turns than the secondary, so this one steps down: in the worked example, 959 turns for 230 V and 50 turns for 12 V. Our drawing.

How a transformer works: the working principle

A transformer works on mutual induction. An alternating voltage across the primary drives an alternating current through it, and that current magnetises the core. Because the current reverses 50 or 60 times a second, so does the flux. By Faraday’s law, a changing flux linking a coil induces a voltage in it, so the same flux induces a voltage in every turn of both windings.

The supply drives a small magnetising current through the primary, enough to set up the flux Φ in the core.
The flux alternates with the supply and links both windings, because the steel guides it round the closed core.
Each turn of each winding has the same voltage per turn induced in it. The secondary voltage is that figure times its number of turns.
When a load is connected, secondary current flows. Its magnetising effect opposes the flux, so the primary draws extra current to cancel it: the primary current rises with the load automatically.

That last step is what makes a transformer self-regulating. Its ampere-turns balance: N₁ × I₁ ≈ N₂ × I₂. With no load, the primary draws only its magnetising current, a few percent of rated current or less in a power transformer. Under load, power flows through the core in proportion to what the secondary takes. Nothing in it moves or switches.

The same principle explains why a transformer will not work on steady DC. A constant current makes a constant flux, a constant flux induces nothing, and the primary winding is left as a low resistance across the supply. It draws a large current until its fuse or its insulation gives way. The simulator shows both cases side by side: the transformer on AC and the transformer on DC, where the output dies away as soon as the current stops changing.

Parts of a transformer

A small control transformer is little more than a core and two windings. A distribution or power transformer adds the parts that cool it, protect it and connect it to the network:

Part	What it does	What to know
Core	Carries the magnetic flux between the windings	Thin sheets of grain-oriented silicon steel (CRGO), insulated from each other to stop eddy currents. Amorphous-metal cores cut the no-load loss further; high-frequency transformers use ferrite.
Primary winding	Takes power from the supply	Copper or aluminium conductor, enamelled or paper-wrapped. On a step-down unit it has the most turns of thinner wire.
Secondary winding	Delivers power to the load	Fewer turns of heavier conductor on a step-down unit, because it carries more current. In concentric windings the low-voltage winding usually sits next to the core.
Insulation	Keeps turns, layers, windings and core apart	Enamel, paper, pressboard and oil. Its temperature limit sets the transformer’s rating.
Tank and oil	Cools and insulates	Mineral oil or natural ester carries heat to the radiators. Dry-type and cast-resin units use air and are chosen indoors.
Radiators and fans	Shed the heat	Cooling classes such as ONAN (oil natural, air natural) and ONAF (fans added) are on the nameplate; fans raise the rating.
Conservator and breather	Let the oil expand without meeting damp air	The silica-gel breather dries incoming air and turns colour when spent.
Buchholz relay	Detects internal faults	Fitted in the pipe to the conservator; gas from an internal fault trips an alarm, then the breaker.
Bushings	Bring the connections through the tank	Porcelain or polymer insulators sized for the winding voltage.
Tap changer	Adjusts the turns ratio	Off-circuit taps, commonly in 2.5 % steps, on distribution units; on-load tap changers on power transformers.
Nameplate	States the ratings	kVA, voltages, %Z, vector group (such as Dyn11), cooling class, frequency and weight.

The core comes in two classic forms. In a core-type transformer, the drawing above, the windings surround the core limbs. In a shell-type transformer the core surrounds the windings, with the flux splitting into two outer limbs. Both forms date from the first practical transformers of 1885 and both are built today.

Transformer equations: turns ratio and EMF

Two equations describe almost everything a transformer does. The turns ratio links the voltages and currents of the windings, and the EMF equation links the voltage of a winding to its turns, the frequency and the flux in the core.

The turns ratio

Every turn has the same voltage induced in it, and the ampere-turns balance, so for an ideal transformer:

\frac{V_{2}}{V_{1}} = \frac{N_{2}}{N_{1}} = \frac{I_{1}}{I_{2}}

The turns ratio is usually written a = N₁ ÷ N₂. In the worked example below, 959 turns on the primary and 50 on the secondary give a = 19.18, so 230 V on the primary gives 11.99 V on the secondary. A 60 VA load draws 5 A from the secondary and only 0.261 A from the primary. The voltage goes down by the ratio and the current goes up by it, so the volt-amperes are the same on both sides.

The ratio also scales impedance, by its square. The 2.4 Ω that the 60 VA load presents at 12 V looks like 883 Ω from the primary: Z₁ = a² × Z₂. Audio output transformers use the same effect to match a loudspeaker to an amplifier.

The EMF equation

If the flux in the core is a sine wave with peak value Φm, Faraday’s law gives the RMS voltage induced in a winding of N turns at frequency f:

E = 4.44 \cdot f \cdot N \cdot Φ_{m} = 4.44 \cdot f \cdot N \cdot B_{m} \cdot A

The constant is 2π ÷ √2 = 4.4429. The second form uses the peak flux density Bm in tesla and the net core area A in m². Rearranged, it gives the turns a designer needs for each volt:

\frac{N}{E} = \frac{1}{4.44 \cdot f \cdot B_{m} \cdot A}

Worked example. A 230 V, 50 Hz transformer on a core with 9 cm² of steel, worked at 1.2 T, needs 1 ÷ (4.44 × 50 × 1.2 × 0.0009) = 4.168 turns per volt. That is 959 turns for the 230 V primary and 50 turns for a 12 V secondary. The peak flux is 1.08 mWb, and putting it back into the EMF equation returns 230.1 V. In practice designers add a few percent to the secondary turns to make up for the voltage the windings lose under load. The same core on a 60 Hz supply needs fewer turns, 3.473 per volt, or 417 turns for 120 V, because a faster change of flux induces more voltage per turn.

Two consequences follow. A transformer designed for 60 Hz runs with about 20 % more flux on 50 Hz, which can push its core into saturation and overheat it. And for a given voltage, more core area means fewer turns. The example runs at 0.24 V per turn; a large power transformer, with a core hundreds of times bigger, runs at tens of volts per turn.

Ideal transformer vs real transformer

An ideal transformer is the textbook model: windings with no resistance, a core with no losses that needs no current to magnetise it, and every line of flux linking both windings. It obeys the turns ratio exactly and is 100 % efficient. It is a useful model because real power transformers come close: their efficiencies are usually 97 % to over 99 %.

A real transformer departs from the ideal in four ways, which the equivalent circuit captures:

Winding resistance (R₁, R₂): the copper heats with the square of the current. This is the copper or load loss.
Leakage flux (X₁, X₂): some flux links only one winding. It acts as a series reactance, causes most of the voltage drop under load and limits fault current; the nameplate %Z describes it.
Magnetising current (Xm): the core needs a small current to set up its flux, drawn whenever the transformer is energised.
Core loss (Rc): hysteresis and eddy currents heat the steel at a near-constant rate, load or no load. This is the iron or no-load loss.

The approximate equivalent circuit referred to the primary. The magnetising branch (Rc and Xm) draws the no-load current; the series branch (R and X) carries the load current and causes the copper loss and the voltage drop. Our drawing.

The series reactance is why the secondary voltage sags under load. For the example transformer below, with 4.5 % impedance and a per-unit resistance of 0.0176, the reactance works out at 0.0414 per unit. The full-load voltage regulation is then:

ε = r \cdot cos φ + x \cdot sin φ + \frac{{x \cdot cos φ - r \cdot sin φ}^{2}}{2}

That is 1.85 % at unity power factor and 3.92 % at 0.8 lagging: a 433 V secondary drops by about 16.97 V at full load and power factor 0.8 lagging. At 0.8 leading the regulation is -0.98 %, a rise, which is why over-corrected power factor pushes voltages up.

Transformer losses and efficiency

A transformer has two kinds of loss, and they behave differently. Iron (core) loss is fixed: it depends on the voltage and frequency and is the same from no load to full load. Copper (load) loss follows the square of the current, so at half load it is a quarter of its full-load value. Efficiency is output ÷ (output + losses):

η = \frac{x \cdot S \cdot PF}{x \cdot S \cdot PF + P_{i} + x^{2} \cdot P_{cu}}

Here x is the load as a fraction of rating, S the rating, P_i the iron loss and P_cu the full-load copper loss. Take a 100 kVA transformer whose test report gives 260 W of no-load loss and 1,760 W of load loss at rated current:

100 kVA, iron loss 260 W, full-load copper loss 1,760 W. Computed by our transformer engine.
Load	Copper loss	Total loss	Efficiency, PF 1	Efficiency, PF 0.8
25 %	110 W	370 W	98.54 %	98.18 %
50 %	440 W	700 W	98.62 %	98.28 %
75 %	990 W	1,250 W	98.36 %	97.96 %
100 %	1,760 W	2,020 W	98.02 %	97.54 %
125 %	2,750 W	3,010 W	97.65 %	97.08 %

Efficiency peaks where the copper loss equals the iron loss, at x = √(P_i ÷ P_cu) = 38.4 % of rating here, and the peak is 98.67 % at unity power factor. Distribution transformers are deliberately designed this way: they spend most of the day lightly loaded, so a low iron loss matters more than a low copper loss.

Efficiency against load for the 100 kVA example, computed at 121 points. Below about 20 % load the fixed iron loss dominates and efficiency falls away quickly.

All-day efficiency makes the point with energy instead of power. Over a day of 4 h at 100 %, 8 h at 50 %, 12 h at 15 % load (power factor 0.9), the example delivers 882 kWh and loses 17.3 kWh, an all-day efficiency of 98.08 %. The iron loss is 13 % of the losses at full load but 36 % of the day’s lost energy, because it never stops. That is why utilities buy transformers on their capitalised losses, why amorphous-metal cores exist, and why the US Department of Energy and India’s IS 1180 set minimum efficiency levels for distribution transformers.

Step-up and step-down transformers

A step-down transformer has fewer turns on its secondary than on its primary and lowers the voltage. A step-up transformer has more and raises it. They are built the same way, and many can be used either way round within their ratings. Neither changes the power: what the voltage gains, the current loses. Watch it happen in the step-down and step-up circuits in the simulator.

Single-phase examples at full rating, ideal transformer. Current = VA ÷ volts on each side.
Transformer	Primary	Secondary	Turns ratio	Rating	I₁	I₂
Halogen or LED lighting transformer	230 V	12 V	19.17 : 1	60 VA	0.261 A	5 A
HVAC control transformer	120 V	24 V	5 : 1	40 VA	0.333 A	1.67 A
Doorbell transformer	120 V	16 V	7.5 : 1	10 VA	0.0833 A	0.625 A
US pole-mounted distribution transformer	7,200 V	240 V	30 : 1	25 kVA	3.47 A	104 A
Step-up for a 480 V machine on a 240 V supply	240 V	480 V	1 : 2	10 kVA	41.7 A	20.8 A

Three-phase transformers add one twist. In the common delta–star (Dyn11) connection, the delta primary winding sees the line voltage and the star secondary winding sees line voltage ÷ √3. An 11 kV to 433 V distribution transformer has a line-voltage ratio of 25.4 but a winding turns ratio of 44 : 1. At 100 kVA it carries 5.25 A on the 11 kV side and 133 A on the 433 V side. Try other ratings in the calculator below.

Transformers in the power grid

Electricity travels from power station to socket through at least three transformers. Generators produce power at a few tens of kilovolts, step-up transformers raise it for transmission, and substations and street transformers bring it back down in stages:

Step 1Generatorabout 11–25 kVAt the power station

Step 2 · transformerGenerator step-upup to 132–765 kVRaises the voltage for long-distance transmission

Step 3Transmission lines115–765 kVHundreds of kilometres with low current and low loss

Step 4 · transformerSubstationdown to 33–69 kV, then 11–35 kVSteps down for sub-transmission and distribution

Step 5 · transformerDistribution transformer11 kV → 433 V · 7.2 kV → 240 VOn a pole, a pad or a plinth near your street

Step 6Your supply230/400 V or 120/240 VIndia, UK and Europe; North America

From power station to socket. Voltages are typical ranges; each network chooses its own levels.

The reason for all this stepping up and down is the I²R loss in the lines. For the same power, current falls in proportion to the voltage and the loss falls with the square of it. Send 100 MW over 50 km of line with 0.1 Ω per km in each conductor (5 Ω):

100 MW three-phase at unity power factor over 5 Ω per conductor. Loss = 3 I²R.
Line voltage	Current	Line loss	Share of the power sent
11 kV	5,250 A	413 MW	413.2 %
33 kV	1,750 A	45.9 MW	45.9 %
132 kV	437 A	2.87 MW	2.9 %
400 kV	144 A	0.313 MW	0.3 %

At 11 kV the arithmetic is impossible: the line would need to carry 5,250 A and would lose more power than it sends. At 400 kV the same line carries 144 A and loses 0.31 %. Only the transformer makes that trade practical, which is why AC, not DC, won the first electrical networks.

The distribution transformer on your street

The last step is the one people see. In India it is the “EB transformer” (named after the state electricity boards): an oil-filled three-phase unit on a plinth or a pole, typically 11 kV to 433 V, built to IS 1180 in ratings such as 25, 63, 100 and 200 kVA. In North America it is a single-phase pole-top or pad-mounted unit, often 7.2 kV to 120/240 V at 25 to 50 kVA, serving a handful of houses. In the UK it is usually a ground-mounted 11 kV to 400/230 V unit in a substation enclosure. If it fails or is overloaded, every customer it feeds loses supply, so report a humming, leaking or sparking unit to your utility rather than going near it.

Types of transformer

Transformers are grouped by what they do, how they are built and how they are cooled. The common types:

Type	What makes it different	Where you find it
Step-down	Fewer secondary turns: lowers voltage	Distribution, control circuits, adapters
Step-up	More secondary turns: raises voltage	Generator step-up, microwave ovens, test sets
Power transformer	Large, often with on-load tap changers	Transmission substations
Distribution transformer	Designed for low iron loss at light load	Poles, pads and plinths near consumers
Single-phase and three-phase	One set of windings, or three on a common core; three-phase units have a vector group such as Dyn11	Homes; industry and utility networks
Isolation transformer	Usually 1 : 1; separates the load from the supply and earth	Shaver sockets, test benches, medical equipment
Autotransformer	One tapped winding shared by both sides; smaller and cheaper, but no isolation	Variacs, motor starters, buck-boost units, grid interconnections
Current transformer (CT)	Steps a large current down to 5 A or 1 A for meters and relays	Metering and protection panels
Voltage transformer (VT or PT)	Steps high voltage down to about 110 or 120 V for meters	Medium- and high-voltage switchgear
Control transformer	Built to tolerate coil inrush; 24 or 120 V secondaries	HVAC units, machine control panels
High-frequency (ferrite)	Runs at tens to hundreds of kilohertz, so the core is tiny	Phone chargers, LED drivers, switch-mode supplies
Dry-type and oil-immersed	Air- or resin-cooled, or cooled by oil in a tank	Indoors and in buildings; outdoors and utilities

High-frequency transformers are the reason a 65 W laptop charger fits in a pocket. The EMF equation says the turns per volt fall in proportion to the frequency, so at 100 kHz instead of 50 Hz the core can be a tiny fraction of the size. The charger rectifies the mains to DC, switches it at high frequency through a small ferrite transformer, then rectifies the output again.

Transformers you meet every day

The transformer in an AC unit or furnace

Every central air conditioner, heat pump and furnace has a small control transformer, usually rated 40 VA, with a primary for 120, 208 or 240 V and a 24 V secondary. It powers the thermostat, the contactor coil in the outdoor unit and the gas valve. At 40 VA it can supply 1.67 A. These are Class 2 power sources, and the NEC limits a Class 2 source between 20 and 30 V AC to 100 VA (Chapter 9, Table 11(A)), which is 4.17 A at 24 V.

When a system goes dead with no display on the thermostat, the transformer and its fuse are the first things to check. Many furnace control boards protect the 24 V circuit with a small automotive-style fuse, and a shorted thermostat cable or a chafed wire at the outdoor unit blows it. A healthy 24 V transformer often reads a few volts high with no load connected, because small transformers regulate poorly. Replace a failed one with the same VA and voltages, and find the short first.

Doorbell and low-voltage lighting transformers

A wired doorbell runs from a transformer of about 16 V and 10 VA, good for 0.625 A; video doorbells often need more VA than the original. Low-voltage landscape and halogen lighting uses 12 V transformers of 60 VA and up. Electronic “transformers” for LED lamps are usually switch-mode drivers, so check that a dimmer and the LED lamps are listed as compatible with them.

Transformer symbols

US schematics draw a transformer as two facing windings; lines between them mean an iron core. IEC drawings, especially single-line diagrams, use two overlapping circles, one per winding. Dots at one end of each winding mark the terminals whose voltages rise together (the polarity).

Symbol	US	IEC 60617
Transformer
Iron-core transformer

Both are free to download as SVG, with the rest of the set, from our electrical symbols chart.

Transformer ratings: why kVA, and how many amps

A transformer’s limit is heat. Copper loss depends on the current and iron loss on the voltage; neither depends on the power factor of the load, so the rating is in volt-amperes, VA or kVA. A 100 kVA unit can deliver 100 kW to a heater but only 80 kW to a load at power factor 0.8, at the same full-load current. The power factor article explains why. Full-load current is the rating divided by the voltage, and by √3 for three-phase:

I_{FL} = \frac{S \cdot 1,000}{V}

I_{FL} = \frac{S \cdot 1,000}{\sqrt{3} \cdot V_{LL}}

Single-phase full-load amps.
kVA	240 V	7.2 kV
10	41.7 A	1.39 A
15	62.5 A	2.08 A
25	104 A	3.47 A
37.5	156 A	5.21 A
50	208 A	6.94 A
75	313 A	10.4 A
100	417 A	13.9 A
167	696 A	23.2 A

Three-phase full-load amps, line current.
kVA	208 V	433 V	480 V
15	41.6 A	20 A	18 A
30	83.3 A	40 A	36.1 A
45	125 A	60 A	54.1 A
75	208 A	100 A	90.2 A
112.5	312 A	150 A	135 A
150	416 A	200 A	180 A
225	625 A	300 A	271 A
300	833 A	400 A	361 A
500	1,390 A	667 A	601 A
750	2,080 A	1,000 A	902 A
1,000	2,780 A	1,330 A	1,200 A

The calculator below is set up for the 11 kV to 433 V distribution transformer from the step-down section. Change the rating, the voltages or the connection, or switch to sizing mode to pick a standard size for a load. The full transformer calculator page adds fault current from %Z and full-load tables.

Result

5.249 A primary · 133.3 A secondary

A 100 kVA three-phase transformer from 11,000 V to 433 V carries 5.249 A on the primary and 133.3 A on the secondary at full load.

Primary full-load current: 5.249 A
Secondary full-load current: 133.3 A
Turns ratio (N1 : N2): 44 : 1
Line voltage ratio: 25.4 : 1

Overcurrent protection for the primary and secondary is set by NEC 450.3, and secondary conductor protection by 240.21(C). This calculator gives full-load current only; it does not size the protection.

Show the work

1. Primary full-load current
$I_{1} = \frac{S}{\sqrt{3} \cdot V_{1}} = \frac{100,000 VA}{\sqrt{3} \cdot 11,000 V} = 5.249 A$
2. Secondary full-load current
$I_{2} = \frac{S}{\sqrt{3} \cdot V_{2}} = \frac{100,000 VA}{\sqrt{3} \cdot 433 V} = 133.3 A$
3. Turns ratio (delta primary, wye secondary)
$\frac{N_{1}}{N_{2}} = \frac{V_{1}}{\frac{V_{2}}{\sqrt{3}}} = \frac{11,000 V}{250 V} = 44$
The delta primary winding sees 11,000 V; the wye secondary winding sees 433 V ÷ √3 = 250 V. The line-to-line voltage ratio is 25.4 : 1, but a turns-ratio test reads 44 : 1.

References: NEC 450.3 (2023): transformer overcurrent protection (not calculated here)

Full-load current from the nameplate kVA at rated voltage; three-phase voltages are line-to-line.
The real primary current is slightly higher, by the transformer’s losses and magnetizing current.

Testing a transformer, and staying safe

Before you touch a transformer

Isolate both the primary and the secondary supply, lock off, and prove dead with a tester you have just checked on a known live source. A transformer can be back-fed from its secondary, and a capacitor or generator downstream can keep it live. Utility transformers are the network operator’s equipment: only they work on them.

Check the supply first. On a dead control circuit, measure the primary voltage before blaming the transformer.
Measure the output. With the primary energised and the load disconnected, read the secondary voltage. Zero with a good primary means an open winding or a blown internal fuse.
Measure winding resistance, dead. With both sides disconnected, a small transformer’s primary reads from a few ohms to a few hundred; the secondary reads far less. An open circuit reads infinite.
Check the insulation. Use an insulation resistance tester, not a multimeter, between the windings and from each winding to the core or frame.
Check the ratio. On larger units a turns-ratio test set confirms the ratio against the nameplate and tap position; for a delta–star unit, compare with the winding ratio, not the line ratio.

One rule for current transformers overrides the rest: never open-circuit the secondary of a CT while its primary carries current. With no secondary current to balance it, the primary current drives the core into saturation and the open secondary can develop a dangerous voltage. Short the secondary terminals with the shorting block before removing a meter or relay.

Who invented the transformer?

No single person invented the transformer. Its principle, mutual induction, was demonstrated by Michael Faraday on 29 August 1831 with two coils wound on an iron ring: switching a battery on and off in one coil produced a pulse of current in the other. He published the work in the Royal Society’s Philosophical Transactions in 1832. Induction coils for physics and telegraphy followed, but they used open iron cores and worked on interrupted DC.

In 1882 Lucien Gaulard and John Dixon Gibbs patented a “secondary generator” for distributing AC to lamps, still with an open core. In 1884–85, three engineers at the Ganz Works in Budapest, Károly Zipernowsky, Ottó Bláthy and Miksa Déri, concluded that open cores could not regulate voltage and designed transformers with closed iron cores, in both the core and shell forms still built today. They also connected the transformers in parallel across the supply rather than in series, which is how every distribution network works. Bláthy coined the word “transformer”.

In the United States, William Stanley, working for George Westinghouse, built his own transformers and on 20 March 1886 used them to light shops and offices on Main Street in Great Barrington, Massachusetts. A step-up transformer raised his generator’s 500 V to 3,000 V for the line, and step-down transformers in the buildings brought it to 100 V for the lamps. The IEEE recognised the system as a Milestone in Electrical Engineering in 2004. Within a few years transformers had made long-distance AC transmission possible, and they remain the most efficient machines in the power system.

For the resistance side of the equations, see how Ohm’s law links volts, amps and ohms; to put a transformer’s full-load current into a circuit, continue with the wire size calculator.

Questions people ask

What is the function of a transformer?

To change the voltage of an AC supply, up or down, while passing the power through with very little loss. The same action isolates one circuit from another, scales currents down for meters (current transformers) and matches impedances in audio and radio circuits.

Does a transformer work on DC?

No. A transformer needs a changing flux to induce a voltage, and steady DC makes a steady flux. Worse, a primary winding connected to DC has only its small resistance to limit the current, so it draws a large current, overheats and can burn out. Switch-mode power supplies get around this by chopping DC into high-frequency AC before their transformer.

Why is a transformer rated in kVA and not kW?

Its losses depend on the current (copper loss) and the voltage (iron loss), not on the power factor of what it feeds. A kVA rating holds for any load; a kW rating would only be true at one power factor. At power factor 0.8, a 100 kVA transformer delivers 80 kW.

What is the EMF equation of a transformer?

E = 4.44 f N Φm, where E is the RMS voltage induced in a winding, f the frequency in hertz, N the number of turns and Φm the peak flux in webers. The 4.44 is 2π ÷ √2, which converts the peak rate of change of a sinusoidal flux to an RMS voltage.

Can a transformer increase power?

No. A step-up transformer raises the voltage and lowers the current in the same proportion, so the power out is the power in minus the losses. In our 100 kVA example the losses at full load are 2,020 W, about 2.0 % of the rating.

Why is the core of a transformer laminated?

The changing flux induces voltages in the core steel itself, which drive eddy currents that heat it. Building the core from thin sheets insulated from each other breaks up those current paths. Eddy-current loss falls with the square of the sheet thickness, which is why power transformer laminations are only a few tenths of a millimetre thick.

What is the transformer for an AC unit?

Air conditioners, furnaces and heat pumps have a small control transformer, usually 40 VA, that turns 120, 208 or 240 V into 24 V for the thermostat, contactor coil and gas valve. At full load it supplies 1.67 A.

Who invented the transformer?

No single person. Michael Faraday demonstrated mutual induction with an iron ring and two coils in 1831. The first practical closed-core transformers were built at the Ganz Works in Budapest in 1885 by Károly Zipernowsky, Ottó Bláthy and Miksa Déri, and Ottó Bláthy coined the word. William Stanley used transformers to light Main Street in Great Barrington, Massachusetts, on 20 March 1886.

Sources

Experimental Researches in Electricity (M. Faraday), Philosophical Transactions of the Royal Society 122, the 1831 induction ring · The Royal Society, 1832
Miksa Déri (1854–1938) and the ZBD transformer · Hungarian Intellectual Property Office
Milestones: Alternating Current Electrification, 1886 (Great Barrington, Massachusetts) · IEEE / Engineering and Technology History Wiki
The History of the Transformer · Edison Tech Center
IEC 60076-1:2011, Power transformers – Part 1: General · IEC, 2011
Energy Conservation Program: Energy Conservation Standards for Distribution Transformers, final rule · U.S. Department of Energy, 2024
Transformers and rotating machinery: Indian Standards including IS 1180 (Part 1):2014, distribution transformers up to 2,500 kVA and 33 kV · Bureau of Indian Standards
NFPA 70, National Electrical Code: Article 450 (transformers) and Chapter 9, Table 11(A) (Class 2 power sources) · NFPA

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