PDF | Transformers may be classified according to dielectric insulation material as follows: • Oil-filled transformers• Dry type transformers. identify the common types of transformers from their schematic diagrams. size overcurrent protection for dry-type transformers operating at V or less. TRANSFORMER TYPES. Instrument transformers. Instrument transformers comprise a large category of current and potential transformers for various voltage.
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latively common point of view on most of those transformers types. Keywords: classification, distribution transformers, power transformers, reactors, transfor-. There are various types of transformer used in the electrical power system for different purposes, like generation, distribution and transmission and utilization of . This article discusses different types of transformers and their applications. step- up, step-down, air core, iron core, power transformer, indoor& outdoor.
Both the coils have high mutual inductance. A mutual electro-motive force is induced in the transformer from the alternating flux that is set up in the laminated core, due to the coil that is connected to a source of alternating voltage.
Most of the alternating flux developed by this coil is linked with the other coil and thus produces the mutual induced electro-motive force. The alternating current supply is given to the first coil and hence it can be called as the primary winding.
The energy is drawn out from the second coil and thus can be called as the secondary winding. In short, a transformer carries the operations shown below: Transfer of electric power from one circuit to another.
Transfer with the principle of electromagnetic induction. The two electrical circuits are linked by mutual induction. The two coils are insulated from each other and from the steel core. The device will also need some suitable container for the assembled core and windings, a medium with which the core and its windings from its container can be insulated. In order to insulate and to bring out the terminals of the winding from the tank, apt bushings that are made from either porcelain or capacitor type must be used.
In all transformers that are used commercially, the core is made out of transformer sheet steel laminations assembled to provide a continuous magnetic path with minimum of air-gap included. The steel should have high permeability and low hysteresis loss. For this to happen, the steel should be made of high silicon content and must also be heat treated.
By effectively laminating the core, the eddy-current losses can be reduced. The lamination can be done with the help of a light coat of core plate varnish or lay an oxide layer on the surface. For a frequency of 50 Hertz, the thickness of the lamination varies from 0.
Types of Transformers Types by Design The types of transformers differ in the manner in which the primary and secondary coils are provided around the laminated steel core. According to the design, transformers can be classified into two: 1. The coils used for this transformer are form-wound and are of cylindrical type.
Such a type of transformer can be applicable for small sized and large sized transformers. In the small sized type, the core will be rectangular in shape and the coils used are cylindrical. The figure below shows the large sized type. You can see that the round or cylindrical coils are wound in such a way as to fit over a cruciform core section. In the case of circular cylindrical coils, they have a fair advantage of having good mechanical strength.
The cylindrical coils will have different layers and each layer will be insulated from the other with the help of materials like paper, cloth, micarta board and so on. The general arrangement of the core-type transformer with respect to the core is shown below. Both low-voltage LV and high voltage HV windings are shown. 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. An autotransformer has one winding that is tapped at some point along the winding. Voltage is applied across a portion of the winding, and a higher or lower voltage is produced across another portion of the same winding.
The equivalent power rating of the autotransfomer is lower than the actual load power rating. It is calculated by: For voltage ratios that don't exceed about 3: 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.
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. 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.
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. 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 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.
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. 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. 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. In this case, it is short-circuit inductance which is actually acting as a current limiting parameter. The output and input currents are low enough to prevent thermal overload under all load conditions—even if the secondary is shorted.
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. It acts then 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.
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. Often only secondary winding has a resonant capacitor or stray capacitance and acts as a serial resonant tank circuit.
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. 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 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. Manufacturers 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 one of many components on a printed circuit board. A planar transformer can be thinner than other transformers, which is useful for low-profile applications or when several printed circuit boards are stacked. 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, and an oil-to-water heat exchanger may also be used. For example, until in South Africa. Cast-resin power transformers encase the windings in epoxy resin. These transformers simplify installation since they are dry, without cooling oil, and so require no fire-proof vault for indoor installations.
The epoxy protects the windings from dust and corrosive atmospheres. However, because the molds for casting the coils are only available in fixed sizes, the design of the transformers is less flexible, which may make them more costly if customized features voltage, turns ratio, taps are required. An isolation transformer links two circuits magnetically, but provides no metallic conductive path between the circuits.
An example application would be in the power supply for medical equipment, when it is necessary to prevent any leakage from the AC power system into devices connected to a patient. Special purpose isolation transformers may include shielding to prevent coupling of electromagnetic noise between circuits, or may have reinforced insulation to withstand thousands of volts of potential difference between primary and secondary circuits.
A solid-state transformer is actually a power converter that performs the same function as a conventional transformer, sometimes with added functionality. Most contain a smaller high-frequency transformer. It can consist of an AC-to-AC converter, or a rectifier powering an inverter. Instrument transformers are typically used to operate instruments from high voltage lines or high current circuits, safely isolating measurement and control circuitry from the high voltages or currents.
The primary winding of the transformer is connected to the high voltage or high current circuit, and the meter or relay is connected to the secondary circuit. Instrument transformers may also be used as an isolation transformer so that secondary quantities may be used without affecting the primary circuitry.
Terminal identifications either alphanumeric such as H 1 , X 1 , Y 1 , etc. This applies to both types of instrument transformers. Correct identification of terminals and wiring is essential for proper operation of metering and protective relay instrumentation. A current transformer CT is a series connected measurement device designed to provide a current in its secondary coil proportional to the current flowing in its primary.
Current transformers are commonly used in metering and protective relays in the electrical power industry. Current transformers are often constructed by passing a single primary turn either an insulated cable or an uninsulated bus bar through a well-insulated toroidal core wrapped with many turns of wire. The CT is typically described by its current ratio from primary to secondary. For example, a Standard secondary current ratings are 5 amperes or 1 ampere, compatible with standard measuring instruments.
The secondary winding can be single ratio or have several tap points to provide a range of ratios. Care must be taken to make sure the secondary winding is not disconnected from its low-impedance load while current flows in the primary, as this may produce a dangerously high voltage across the open secondary and may permanently affect the accuracy of the transformer. Specially constructed wideband CTs are also used, usually with an oscilloscope , to measure high frequency waveforms or pulsed currents within pulsed power systems.
One type provides a voltage output that is proportional to the measured current. Another, called a Rogowski coil , requires an external integrator in order to provide a proportional output. A current clamp uses a current transformer with a split core that can be easily wrapped around a conductor in a circuit. This is a common method used in portable current measuring instruments but permanent installations use more economical types of current transformer. Voltage transformers VT , also called potential transformers PT , are a parallel connected type of instrument transformer, used for metering and protection in high-voltage circuits or phasor phase shift isolation.
They are designed to present negligible load to the supply being measured and to have an accurate voltage ratio to enable accurate metering. Audio transformer[ edit ] Two speaker-level audio transformers in a tube amplifier are seen on the left. The power supply toroidal transformer is on right Five audio transformers for various line level purposes. The two black boxes on the left contain transformers for splitting signals, balancing unbalanced signals , or isolating two different AC ground systems to eliminate buzz and hum.
The two cylindrical metal cases fit into octal sockets ; each one contains a line transformer, the first is rated at ohms, the second is rated at 15, ohms. On the far right is a DI unit ; its transformer with yellow insulation changes a high impedance unbalanced input to a low impedance balanced output.
Audio transformers are those specifically designed for use in audio circuits to carry audio signal. They can be used to block radio frequency interference or the DC component of an audio signal, to split or combine audio signals, or to provide impedance matching between high impedance and low impedance circuits, such as between a high impedance tube valve amplifier output and a low impedance loudspeaker , or between a high impedance instrument output and the low impedance input of a mixing console.
Audio transformers that operate with loudspeaker voltages and current are larger than those that operate at microphone or line level, which carry much less power.
Bridge transformers connect 2-wire and 4-wire communication circuits. Being magnetic devices, audio transformers are susceptible to external magnetic fields such as those generated by AC current-carrying conductors. Audio transformers were originally designed to connect different telephone systems to one another while keeping their respective power supplies isolated, and are still commonly used to interconnect professional audio systems or system components, to eliminate buzz and hum.
Such transformers typically have a ratio between the primary and the secondary. These can also be used for splitting signals, balancing unbalanced signals , or feeding a balanced signal to unbalanced equipment. Transformers are also used in DI boxes to convert high-impedance instrument signals e. A particularly critical component is the output transformer of a valve amplifier. Valve circuits for quality reproduction have long been produced with no other inter-stage audio transformers, but an output transformer is needed to couple the relatively high impedance up to a few hundred ohms depending upon configuration of the output valve s to the low impedance of a loudspeaker.
The valves can deliver a low current at a high voltage; the speakers require high current at low voltage. Most solid-state power amplifiers need no output transformer at all.
Audio transformers affect the sound quality because they are non-linear. They add harmonic distortion to the original signal, especially odd-order harmonics, with an emphasis on third-order harmonics. When the incoming signal amplitude is very low there is not enough level to energize the magnetic core see coercivity and magnetic hysteresis.
When the incoming signal amplitude is very high the transformer saturates and adds harmonics from soft clipping. For good low-frequency response a relatively large magnetic core is required; high power handling increases the required core size. Good high-frequency response requires carefully designed and implemented windings without excessive leakage inductance or stray capacitance.
All this makes for an expensive component. Early transistor audio power amplifiers often had output transformers, but they were eliminated as advances in semiconductors allowed the design of amplifiers with sufficiently low output impedance to drive a loudspeaker directly.
Loudspeaker transformer[ edit ] Loudspeaker transformer in old radio In the same way that transformers create high voltage power transmission circuits that minimize transmission losses, loudspeaker transformers can power many individual loudspeakers from a single audio circuit operated at higher than normal loudspeaker voltages.
This application is common in public address applications. Such circuits are commonly referred to as constant-voltage speaker systems. Such systems are also known by the nominal voltage of the loudspeaker line, such as , and volt speaker systems the voltage corresponding to the power rating of a speaker or amplifier. A transformer steps up the output of the system's amplifier to the distribution voltage. At the distant loudspeaker locations, a step-down transformer matches the speaker to the rated voltage of the line, so the speaker produces rated nominal output when the line is at nominal voltage.
Loudspeaker transformers commonly have multiple primary taps to adjust the volume at each speaker in steps. Output transformer[ edit ] Valve tube amplifiers almost always use an output transformer to match the high load impedance requirement of the valves several kilohms to a low impedance speaker Small-signal transformer[ edit ] Moving coil phonograph cartridges produce a very small voltage.
For this to be amplified with a reasonable signal-noise ratio usually requires a transformer to convert the voltage to the range of the more common moving-magnet cartridges. Microphones may also be matched to their load with a small transformer, which is mumetal shielded to minimise noise pickup. These transformers are less widely used today, as transistorized buffers are now cheaper.
Interstage and coupling transformer[ edit ] In a push—pull amplifier , an inverted signal is required and can be obtained from a transformer with a center-tapped winding, used to drive two active devices in opposite phase. These phase splitting transformers are not much used today. This section does not cite any sources. Please help improve this section by adding citations to reliable sources.
Unsourced material may be challenged and removed. January Transactor[ edit ] A transactor is a combination of a transformer and a reactor.
A transactor has an iron core with an air-gap, which limits the coupling between windings. They are homemade audio interstage coupling transformers. Enameled copper wire is wound round the central half of the length of a bundle of insulated iron wire e. The ends of the iron wires are then bent around the electrical winding to complete the magnetic circuit, and the whole is wrapped with tape or string to hold it together. Variometer and variocoupler[ edit ] Variometer used in s radio receiver A variometer is a type of continuously variable air-core RF inductor with two windings.
The two coils are connected in series. When the two coils are collinear, with their magnetic fields pointed in the same direction, the two magnetic fields add, and the inductance is maximum. If the inner coil is rotated so its axis is at an angle to the outer coil, the magnetic fields do not add and the inductance is less. If the inner coil is rotated so it is collinear with the outer coil but their magnetic fields point in opposite directions, the fields cancel each other out and the inductance is very small or zero.