Transformer types
From Wikipedia, the free encyclopedia
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Transformer with two windings and iron core. |
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Step-down or step-up transformer. The symbol shows which winding has more turns, but not usually the exact ratio. |
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Transformer with three windings. The dots show the relative configuration of the windings. |
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Transformer with electrostatic screen preventing capacitive coupling between the windings. |
A variety of specialized transformer types have been created for the electrical transformer to fulfill certain niche roles. 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.
Contents
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[edit] Power transformers
[edit] Laminated core
This is the most common type of transformer, widely used in appliances to convert mains voltage to low voltage to power electronics
- Widely available in power ratings from 1.2w to several kilowatts
- Insulated laminations minimize eddy current losses
- Most use a split bobbin, giving a high level of insulation between the windings
- Rectangular core
- Core laminate stampings are usually in EI shape pairs. Other shape pairs are sometimes used.
- Mumetal shields can be fitted to reduce EMI (electromagnetic interference)
- A screen winding is occasionally used between the 2 power windings
- Many such transformers have a thermal cut out built in, many don't
- 4 turns per volt is typical for continuous use
- Occasionally seen in low profile format for use in restricted spaces
- laminated core made with silicon steel with high permeability
[edit] Toroidal
Doughnut shaped toroidal transformers are used to save space compared to EI cores, and sometimes to reduce external magnetic field. These use a ring shaped core, copper windings wrapped round this ring (and thus threaded through the ring during winding), and tape for insulation.
Toroidals compared to EI core transformers:
- Lower external magnetic field
- Smaller for a given power rating
- Higher cost in most cases, as winding requires more complex & slower equipment
- Less robust
- Central fixing is either
- bolt, large metal washers & rubber pads
- bolt & potting resin
- Overtightening the central fixing bolt may short the windings
- greater inrush current at switch-on.
Rectangular toroids are also sometimes encountered. These use a rectangular version of a toroidal core, usually wound as a multilayered single flat steel strip. Windings are placed on 2 of the 4 sides of the core.
[edit] Autotransformer
An autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed voltage is applied across a portion of the winding, and a higher (or lower) voltage is produced across another portion of the same winding. While theoretically separate parts of the winding can be used for input and output, in practice the higher voltage will be connected to the ends of the winding, and the lower voltage from one end to a tap. For example, a transformer with a tap at the center of the winding can be used with 230 volts across the entire winding, and 115 volts between one end and the tap. It can be connected to a 230-volt supply to drive 115-volt equipment, or reversed to drive 230-volt equipment from 115 volts. Since the current in the windings is lower, the transformer is smaller, lighter cheaper and more efficient. For voltage ratios not exceeding about 3:1, an autotransformer is cheaper, lighter, smaller and more efficient than an isolating (two-winding) transformer of the same rating.
In practice, transformer losses mean that autotransformers are not perfectly reversible; one designed for stepping down a voltage will deliver slightly less voltage than required if used to step up. The difference is usually slight enough to allow reversal where the actual voltage level is not critical. This is true of isolated winding transformers too.
[edit] Variac
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.
[edit] Constant voltage transformer (ferro-resonance)
By arranging particular magnetic properties of a transformer core, and installing a resonant tank circuit (a capacitor and an additional winding), a transformer can be arranged to automatically keep the secondary winding voltage constant regardless (within some limits) of any variance in the primary supply without additional circuitry or manual adjustment. CVA transformers run hotter than standard power transformers, for the regulating action is dependent on core saturation, which reduces efficiency somewhat. The output waveform features heavy distortion.
[edit] Stray field transformer
A Stray field transformer has a significant stray field or a (sometimes adjustable) magnetic bypass in its core. It can act as a transformer with inherent current limitation due to its lower tight coupling between the primary and the scondary winding, which is unwanted in much other cases. The output and input currents are low enough to prevent thermal overload under each load condition - even if the secondary is shortened.
Stray field transformers are used for arc welding and high voltage discharge lamps (cold cathode fluorescent lamps, series connected up to 7,5 kV AC working voltage). It acts both as voltage tranformer and magnetic ballast.
[edit] Polyphase transformers
For three-phase power, three separate single-phase transformers can be used, or all three phases can be connected to a single polyphase transformer. The three primary windings are connected together and the three secondary windings are connected together. The most common connections are Y-Delta, Delta-Y, Delta-Delta and Y-Y. 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 Y 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.
 Example of Y Y Connection |
[edit] Resonant transformers
A resonant transformer operates at the resonant frequency of one or more of its coils and (usually) an external capacitor. The resonant coil, usually the secondary, acts as an inductor, and is connected in series with a capacitor. When the primary coil is driven by a periodic source of alternating current, such as a square or sawtooth wave at the resonant frequency, each pulse of current helps to build up an oscillation in the secondary coil. Due to resonance, a very high voltage can develop across the secondary, until it is limited by some process such as electrical breakdown. These devices are used to generate high alternating voltages, and the current available can be much larger than that from electrostatic machines such as the Van de Graaff generator or Wimshurst machine.
Examples:
- Tesla coil
- 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
- Flyback transformer of a CRT television set or video monitor.
- 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.
Other applications of resonant transformers are as coupling between stages of a superheterodyne receiver, where the selectivity of the receiver is provided by the tuned transformers of the intermediate-frequency amplifiers.
A voltage-regulating transformer uses a resonant winding and allows part of the core to go into saturation on each half-cycle of the alternating current. This effect stabilizes the output of the regulating transformer, which can be used for equipment that is sensitive to variations of the supply voltage. Saturating transformers provide a simple rugged method to stabilize an AC power supply. However, due to the core saturation losses accompanying this type of operation, efficiency is low.
[edit] Ferrite Core
Ferrite core power transformers are widely used in switched mode power supplies (SMPSUs). The powder core enables high frequency operation, and hence much smaller size to power ratio than laminated iron transformers.
Ferrite transformers are not usable as power transformers at mains frequency.
[edit] planar transformer
Manufacturers etch spiral patterns on a printed circuit board to form the "windings" of a planar transformer. (Manufacturers literally wind pieces of wire on some core or bobbin to form the windings of other kinds of transformers).
Some planar transformers are commercially sold as discrete components -- the transformer is the only thing on that printed circuit board. Other planar transformers are one of many components on one large printed circuit board.
- much thinner than other transformers, for low-profile applications (even when several PCBs are stacked)
- almost all use a ferrite planar core
[edit] Oil cooled transformer
For multi-kilowatt high power work, oil cooled transformers are usually used. These have metal pipes running through the core carrying a thin oil. This oil thermosyphons around its circuit, carrying heat to tubes external to the transformer core for dissipation. These are common in power stations & substations.
[edit] Isolating Transformer
Most transformers isolate, meaning the secondary winding is not connected to the primary. But this isn't true of all transformers.
However the term 'isolating transformer' is normally applied to mains transformers providing isolation rather than voltage transformation. They are simply 1:1 laminated core transformers. Extra voltage tappings are sometimes included, but to earn the name 'isolating transformer' it is expected that they will usually be used at 1:1 ratio.
[edit] Microwave Oven Transformer
The Microwave Oven Transformer (MOT) is a laminated core transformer with a number of specific properties. The widespread availability of free high power MOTs has stimulated interest in their re-use in projects, but their various issues make this less easy than it first appears.
MOTs from consumer microwave ovens have several distinguishing features:
- Power rating from 450w to 1.5kW
- MOTs typically overheat in around 15 minutes with forced air cooling, and faster without
- Very high magnetisation current due to typically 1v per turn
- Core connected direct to seconday winding
- Core has magnetic shunts to control current output
- 2 secondary winds, one of a couple of kV, one low voltage high current for the magnetron heater.
- welded core
- High power to weight ratio
This combination of features leaves them with almost no as-is uses, and significant work is required for their use in projects. Their most popular re-use is in arc welders.
[edit] Instrument transformers
[edit] Current transformers
A current transformer (CT) is a 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 relaying in the electrical power industry where they facilitate the safe measurement of large currents, often in the presence of high voltages. The current transformer safely isolates measurement and control circuitry from the high voltages typically present on the circuit being measured.
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 4000:5 CT would provide an output current of 5 amperes when the primary was passing 4000 amperes. The secondary winding can be single ratio or have several tap points to provide a range of ratios. Care must be taken that the secondary winding is not disconnected from its load while current flows in the primary, as this will 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.
[edit] Voltage transformers
Voltage transformers (VTs) or potential transformers (PTs) are another type of instrument transformer, used for metering and protection in high-voltage circuits. They are designed to present negligible load to the supply being measured and to have a precise voltage ratio to accurately step down high voltages so that metering and protective relay equipment can be operated at a lower potential. Typically the secondary of a voltage transformer is rated for 69 or 120 Volts at rated primary voltage, to match the input ratings of protection relays.
The transformer winding high-voltage connection points are typically labelled as H1, H2 (sometimes H0 if it is internally grounded) and X1, X2, and sometimes an X3 tap may be present. Sometimes a second isolated winding (Y1, Y2, Y3) may also be available on the same voltage transformer. The high side (primary) may be connected phase to ground or phase to phase. The low side (secondary) is usually phase to ground.
The terminal identifications (H1, X1, Y1, etc.) are often referred to as polarity. This applies to current transformers as well. At any instant terminals with the same suffix numeral have the same polarity and phase. Correct identification of terminals and wiring is essential for proper operation of metering and protection relays.
While VTs were formerly used for all voltages greater than 240V primary, modern meters eliminate the need VTs for most secondary service voltages. VTs are typically used in circuits where the system voltage level is above 600 V. Modern meters eliminate the need of VT's since the voltage remains constant and it is measured in the incoming supply.
[edit] Pulse transformers
A pulse transformer is a transformer that is optimised for transmitting rectangular electrical pulses (that is, pulses with fast rise and fall times and a relatively constant amplitude). Small versions called signal types are used in digital logic and telecommunications circuits, often for matching logic drivers to transmission lines. Medium-sized power versions are used in power-control circuits such as camera flash controllers. Larger power versions are used in the electrical power distribution industry to interface low-voltage control circuitry to the high-voltage gates of power semiconductors. Special high voltage pulse transformers are also used to generate high power pulses for radar, particle accelerators, or other high energy pulsed power applications.
To minimise distortion of the pulse shape, a pulse transformer needs to have low values of leakage inductance and distributed capacitance, and a high open-circuit inductance. In power-type pulse transformers, a low coupling capacitance (between the primary and secondary) is important to protect the circuitry on the primary side from high-powered transients created by the load. For the same reason, high insulation resistance and high breakdown voltage are required. A good transient response is necessary to maintain the rectangular pulse shape at the secondary, because a pulse with slow edges would create switching losses in the power semiconductors.
The product of the peak pulse voltage and the duration of the pulse (or more accurately, the voltage-time integral) is often used to characterise pulse transformers. Generally speaking, the larger this product, the larger and more expensive the transformer.
Pulse transformers by definition have a duty cycle of less than 1, whatever energy stored in the coil during the pulse must be "dumped" out before the pulse is fired again.
[edit] RF Transformers
There are several types of transformer used in rf work. Steel laminations are not suitable for rf.
[edit] Air core transformers
These are used for high frequency work. The lack of a core means very low inductance. Such transformers may be nothing more than a few turns of wire soldered onto a PCB.
[edit] Ferrite core transformers
Widely used in IF stages in radio receivers. These are mostly tuned transformers, containing a threaded ferrite slug that is screwed in or out to adjust IF tuning. The transformers are usually canned for stability and to reduce interference.
[edit] Transmission line transformers
For radio frequency use, transformers are sometimes made from configurations of transmission line, sometimes bifilar or coaxial cable, wound around ferrite or other types of core. This style of transformer gives an extremely wide bandwidth but only a limited number of ratios (such as 1:9, 1:4 or 1:2) can be achieved with this technique.
The core material increases the inductance dramatically, thereby raising its Q factor. The cores of such transformers help improve performance at the lower frequency end of the band. RF transformers sometimes used a third coil (called a tickler winding) to inject feedback into an earlier (detector) stage in antique regenerative radio receivers.
[edit] Baluns
Baluns are transformers designed specifically to connect between balanced and unbalanced circuits. These are sometimes made from configurations of transmission line and sometimes bifilar or coaxial cable and are similar to transmission line transformers in construction and operation.
[edit] Audio transformers
Audio transformers are usually the factor which limit sound quality when used; electronic circuits with wide frequency response and low distortion are relatively simple to design.
Transformers are also used in DI boxes to convert impedance from high-impedance instruments (for example, bass guitars) to enable them to be connected to a microphone input on the mixing console.
A particularly critical component is the output transformer of an audio power 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.
For good low-frequency response a relatively large iron 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 designers discovered how to design amplifiers without them.
[edit] Speaker transformers (public address)
In the same way that transformers are used to create high voltage power transmission circuits that minimize transmission losses, speaker transformers allow many individual loudspeakers to be powered from a single audio circuit operated at higher-than normal speaker voltages. This application is common in public address (aka Tannoy) applications. Such circuits are commonly referred to as 100v line, or sometimes 70v line or 35v line. The term constant voltage or 70 volt speaker circuit is also used, although the audio waveform is a changing voltage.
At the audio amplifier, a large audio transformer may be used to step-up the low impedance, low-voltage output of the amplifier to the designed line voltage of the speaker circuit. Then, a smaller transformer at each speaker returns the voltage and impedance to ordinary speaker levels. The speaker transformers commonly have multiple primary taps, allowing the volume at each speaker to be adjusted in a number of discrete steps.
Use of a specified voltage speaker circuit simplifies handling the issue of the impedance presented to the amplifier output. Maintaining a suitable impedance would be very much more complex if the speakers were arranged in series-parallel), as would adjustment of relative output levels, which is also simplified by the 100v line system. Use of 100v line also much reduces the required wire size. Connecting all speakers in parallel and would not solve these problems either, and would result in the need for very thick cables[citation needed].
The use of higher transmission voltage and impedance also means that power lost in the connecting wire is minimized, even with the use of small-gauge conductors (and leads to the term constant voltage as the line voltage doesn't change much as additional speakers are added to the system). In addition, the ability to adjust, locally, the volume of each speaker (without the complexity and power loss of an L pad) is a useful feature.
[edit] Output transformer (valve)
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.
[edit] Small Signal transformers
Moving coil phonograph cartridges produce a very small voltage. In order for this to be amplified with a reasonable signal-noise ratio, a transformer is usually used 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 popular today, as they can cause hum pickup, and transistorized buffers are now cheaper.
[edit] 'Interstage' and coupling transformers
A use for interstage transformers is in the case of push-pull amplifiers where an inverted signal is required. Here two secondary windings wired in opposite polarities may be used to drive the output devices. These phase splitting transformers are not much used today.
[edit] Cast resin transformers
Cast-resin power transformers have been widely used for a long time. These transformers have the advantage of easy installation and improved fire behaviour in case of class. This indoor type transformer is totally dry, without cooling oil.
[edit] Homemade & Obsolete Transformers
[edit] Transformer kits
Transformers may be wound at home using commercial transformer kits, which contain laminations & bobbin. Or ready made transformers may be disassembled and rewound. These approaches are occasionally used by home constructors, but are usually avoided where possible due to the number of hours required to hand wind a transformer.
Firm clamping of laminations and varnish help to avoid buzz.
[edit] 100% homemade
It is perfectly possible to make the transformer laminations by hand too. Such transformers are encountered at times in 3rd world countries, using laminations cut from scrap sheet steel, paper slips between the laminations, and string to tie the whole thing together. The result works, but is usually noisy due to poor clamping of laminations.
[edit] Hedgehog
Hedgehog transformers are occasionally encountered in homemade 1920s radios. They are homemade audio interstage coupling transformers.
Enamelled copper wire is wound round the central half of the length of a bundle of insulated iron wire (eg florists' wire), to make the windings. 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.
These were sometimes used when the cost of a ready made transformer could not be justified. Inductance tends to be on the low side, with consequent loss of bass. With the speakers of the day this was no bad thing.
[edit] Variocouplers
Variocouplers are rf transformers with 2 windings and variable coupling between the windings. They were standard equipment in 1920s radio sets.
Pancake coil variocouplers were common in 1920s radios for variable rf coupling. The 2 planar coils were arranged to swing away from each other and for the angle between them to increase to 90 degrees, thus giving wide variation in coupling. No core was used. These were mostly used to control reaction. The pancake structure was a means to minimize stray capacitance.
In another design of variocoupler, 2 coils were wound on a 2 circular bands, and housed one inside the other, with provision for rotating the inner coil. Coupling varies as one coil is rotated between 0 and 90 degrees from the other. These had higher stray capacitance than the pancake type.
[edit] Not Transformers
Finally there are some items often mistaken for transformers, but which are not transformers.
Wallwarts: small power supplies with integral mains plug. These contain a transformer plus other parts. Most use a laminated iron transformer, but an increasing number now contain a small SMPSU. These are smaller and much lighter.
Halogen lighting transformers: Toroidal transformers are sometimes used for this task, but most halogen 'transformers' are SMPSUs.
