+Circuit symbols Transformer with two windings and iron core. Transformer with three windings. The dots show the relative configuration of the windings. Transformer with electrostatic screen preventing capacitive coupling between the windings.
Various types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday, and share several key functional parts.
Power transformer
Laminated core
This is the most common type of transformer, widely used in electric power transmission and appliances to convert mains voltage to low voltage to power electronic devices. They are available in power ratings ranging from mW to MW. The insulated laminations minimize eddy current losses in the iron core.
Small appliance and electronic transformers may use a split bobbin, giving a high level of insulation between the windings. The rectangular cores are made up of stampings, often in E-I shape pairs, but other shapes are sometimes used. Shields between primary and secondary may be fitted to reduce EMI (electromagnetic interference), or a screen winding is occasionally used.
Small appliance and electronics transformers may have a thermal cut-out built into the winding, to shut-off power at high temperatures to prevent further overheating.
Toroidal
Donut-shaped toroidal transformers save space compared to E-I cores, and may reduce external magnetic field. These use a ring shaped core, copper windings wrapped around this ring (and thus threaded through the ring during winding), and tape for insulation.
Toroidal transformers have a lower external magnetic field compared to rectangular transformers, and can be smaller for a given power rating. However, they cost more to make, as winding requires more complex and slower equipment.
They can be mounted by a bolt through the center, using washers and rubber pads or by potting in resin. Care must be taken that the bolt does not form part of a short-circuit turn.
Autotransformer
An autotransformer consists of only one winding that is tapped at some point along the winding. Voltage is applied across a terminal of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. The equivalent power rating of the autotransformer is lower than the actual load power rating. It is calculated by: load VA × (|Vin – Vout|)/Vin. For example, an auto transformer that adapts a 1000 VA load rated at 120 volts to a 240 volt supply has an equivalent rating of at least: 1,000 VA (240 V – 120 V) / 240 V = 500 VA. However, the actual rating (shown on the tally plate) must be at least 1000 VA.
For voltage ratios that don't exceed about 3:1, an autotransformer is cheaper, lighter, smaller, and more efficient than an isolating (two-winding) transformer of the same rating.Donald G. Fink & H. Wayne Beatty. (1978). Standard Handbook for Electrical Engineers, 11th Edition. McGraw Hill, pp. 10–44 and 10–45. Large three-phase autotransformers are used in electric power distribution systems, for example, to interconnect 220 kV and 33 kV sub-transmission networks or other high voltage levels.
Variable autotransformer
By exposing part of the winding coils of an autotransformer, and making the secondary connection through a sliding carbon brush, an autotransformer with a near-continuously variable turns ratio can be obtained, allowing for wide voltage adjustment in very small increments.
Induction regulator
The induction regulator is similar in design to a wound-rotor induction motor but it is essentially a transformer whose output voltage is varied by rotating its secondary relative to the primary—i.e., rotating the angular position of the rotor. It can be seen as a power transformer exploiting rotating magnetic fields. The major advantage of the induction regulator is that unlike variacs, they are practical for transformers over 5 kVA. Hence, such regulators find widespread use in high-voltage laboratories. T. J. Gallagher and A. J. Pearmain (1983): "High Voltage – Measurement, Testing and Design",
Polyphase transformer
For polyphase systems, multiple single-phase transformers can be used, or all phases can be connected to a single polyphase transformer. For a three-phase transformer, the three primary windings are connected together and the three secondary windings are connected together."3-Phase Step-Up Transformer", Agile Magnetics, retrieved June 8, 2016. Examples of connections are wye-delta, delta-wye, delta-delta, and wye-wye. A vector group indicates the configuration of the windings and the phase angle difference between them. If a winding is connected to earth (grounded), the earth connection point is usually the center point of a wye winding. If the secondary is a delta winding, the ground may be connected to a center tap on one winding (high-leg delta) or one phase may be grounded (corner grounded delta). A special purpose polyphase transformer is the zigzag transformer. There are many possible configurations that may involve more or fewer than six windings and various tap connections.
Grounding transformer
Grounding or earthing transformers let three wire (delta) polyphase system supplies accommodate phase to neutral loads by providing a return path for current to a neutral. Grounding transformers most commonly incorporate a single winding transformer with a zigzag winding configuration but may also be created with a wye-delta isolated winding transformer connection.
Phase-shifting transformer
This is a specialized type of transformer which can be configured to adjust the phase relationship between input and output. This allows power flow in an electric grid to be controlled, e.g. to steer power flows away from a shorter (but overloaded) link to a longer path with excess capacity.
Variable-frequency transformer
A variable-frequency transformer is a specialized three-phase power transformer which allows the phase relationship between the input and output windings to be continuously adjusted by rotating one half. They are used to interconnect electrical grids with the same nominal frequency but without synchronous phase coordination.
Leakage or stray field transformer
A leakage transformer, also called a stray-field transformer, has a significantly higher leakage inductance than other transformers, sometimes increased by a magnetic bypass or shunt in its core between primary and secondary, which is sometimes adjustable with a set screw. This provides a transformer with an inherent current limitation due to the loose coupling between its primary and the secondary windings. The adjustable short-circuit inductance acts as a current limiting parameter.
The output and input currents are kept low enough to preclude thermal overload under any load conditions — even if the secondary is shorted.
Uses Leakage transformers are used for arc welding and high voltage discharge lamps (neon lights and cold-cathode fluorescent lamps, which are series connected up to 7.5 kV AC). It acts both as a voltage transformer and as a magnetic ballast.
Other applications are short-circuit-proof extra-low voltage transformers for toys or doorbell installations.
Resonant transformer
A resonant transformer is a transformer in which one or both windings has a capacitor across it and functions as a tuned circuit. Used at radio frequencies, resonant transformers can function as high Q factor bandpass filters. The transformer windings have either air or ferrite cores and the bandwidth can be adjusted by varying the coupling (mutual inductance). One common form is the IF (intermediate frequency) transformer, used in superheterodyne radio receivers. They are also used in radio transmitters.
Resonant transformers are also used in electronic ballasts for gas discharge lamps, and high voltage power supplies. They are also used in some types of switching power supplies.Philips PM3311 Oscilloscope service manual Section 6.2.16 [describing the switch mode power supply],"The unregulated d.c. voltage is applied in the form of pulses to a resonant transformer T1602..." Here the short-circuit inductance value is an important parameter that determines the resonance frequency of the resonant transformer. Often only secondary winding has a resonant capacitor (or stray capacitance) and acts as a serial resonant tank circuit. When the short-circuit inductance of the secondary side of the transformer is Lsc and the resonant capacitor (or stray capacitance) of the secondary side is Cr, The resonance frequency ωs of 1' is as follows
The transformer is driven by a pulse or square wave for efficiency, generated by an electronic oscillator circuit. Each pulse serves to drive resonant sinusoidal oscillations in the tuned winding, and due to resonance a high voltage can be developed across the secondary.
Applications: Intermediate frequency (IF) transformer in superheterodyne radio receiver Tank transformers in radio transmitters Tesla coil Power inverter Oudin coil (or Oudin resonator; named after its inventor Paul Oudin) D'Arsonval apparatus Ignition coil or induction coil used in the ignition system of a petrol engine Electrical breakdown and insulation testing of high voltage equipment and cables. In the latter case, the transformer's secondary is resonated with the cable's capacitance.
Constant voltage transformer
By arranging particular magnetic properties of a transformer core, and installing a ferro-resonant tank circuit (a capacitor and an additional winding), a transformer can be arranged to automatically keep the secondary winding voltage relatively constant for varying primary supply without additional circuitry or manual adjustment. Ferro-resonant transformers run hotter than standard power transformers, because regulating action depends on core saturation, which reduces efficiency. The output waveform is heavily distorted unless careful measures are taken to prevent this. Saturating transformers provide a simple rugged method to stabilize an AC power supply.
Ferrite core Ferrite core power transformers are widely used in switched-mode power supplies (SMPSs). The powder core enables high-frequency operation, and hence much smaller size-to-power ratio than laminated-iron transformers.
Ferrite transformers are not used as power transformers at mains frequency since laminated iron cores cost less than an equivalent ferrite core.
Planar transformer
Manufacturers either use flat copper sheets or etch spiral patterns on a printed circuit board to form the "windings" of a planar transformer, replacing the turns of wire used to make other types. Some planar transformers are commercially sold as discrete components, other planar transformers are etched directly into the main printed circuit board and only need a ferrite core to be attached over the PCB. A planar transformer can be thinner than other transformers, which is useful for low-profile applications or when several printed circuit boards are stacked. Almost all planar transformers use a ferrite planar core.
Liquid-cooled transformer Large transformers used in power distribution or electrical substations have their core and coils immersed in oil, which cools and insulates. Oil circulates through ducts in the coil and around the coil and core assembly, moved by convection. The oil is cooled by the outside of the tank in small ratings, and by an air-cooled radiator in larger ratings. Where a higher rating is required, or where the transformer is in a building or underground, oil pumps circulate the oil, fans may force air over the radiators, or an oil-to-water heat exchanger may also be used.ANSI IEEE Standard C57.12.00 General Requirements for Liquid-Immersed Distribution, Power and Regulating Transformers, 2000