# Practical Applications Of Transformer Engineering Essay

Abstract- Generating power is possible in few Stationss. The power is generated so has to convey to the assorted parts of the state. Large power should be transmitted on really high electromotive force to cut down the sum of Cu stuff and increase the transmittal efficiency. Hence the energy generated is transformed twice, thrice, or even four times before utilised. Such transmutation of Ac from one electromotive force to another is done by transformer.

DISCOVERY The phenomenon of electromagnetic initiation was discovered by Michael Faraday and Joseph Henry in 1831. The relationship between electromotive force or electromotive force and magnetic flux was formalized in an equation now referred to as Faraday ‘s jurisprudence of initiation. This jurisprudence states that whenever there is a comparative gesture between the spiral and magnet voltage is induced in the spiral. The induced voltage stopping points so long as magnetic flux linked with the spiral changed. The induced voltage is straight relative to the clip rate of alteration of magnetic flux linked with the spiral.

## Where, I¦B is the magnetic flux through the circuit.

Fig.1: Faraday ‘s experiment with initiation between spirals of wire

Transformer

A electric current will flux in the secondary twist and electrical energy will be transferred from the primary circuit through the transformer to the burden.

Fig.2: Structure of Transformer

PRINCIPLE The transformer is based on two rules: foremost, that an electric current can bring forth a induced magnetic field by changing with clip and secondly that a altering magnetic field within a spiral of wire induces a electromotive force across the terminals of the spiral. Changing the current in the primary spiral changes the magnetic flux that is developed. The altering magnetic flux induces a electromotive force in the secondary spiral. The electromotive force induced across the secondary spiral may be calculated from Faraday ‘s jurisprudence of initiation, which states that:

Where VS is the instantaneous electromotive force, NS is the figure of bends in the secondary spiral and I¦ peers the magnetic flux through one bend of the spiral. If the bends of the spiral are oriented perpendicular to the magnetic field lines, the flux is the merchandise of the magnetic flux denseness B and the country A through which it cuts. The country is changeless, being equal to the cross-sectional country of the transformer nucleus, whereas the magnetic field varies with clip harmonizing to the excitement of the primary. Since the same magnetic flux base on ballss through both the primary and secondary spirals in an ideal transformer, the instantaneous electromotive force across the primary twist peers. Taking the ratio of the two equations for VS and VP gives the basic equation for stepping up or stepping down the electromotive force.

Coil Winding

Core Assembly

Core-Coil Assembly

Tank-up

Transformer Tank

Painting and Completing

Fig. 3: Transformer demoing each portion

1. Curator: a ) Check the oil degree in the curator. If the degree is low than the optimal grade indicated on the oil degree gage, it should be topped with proper class of transformer oil holding suited breakdown electromotive force value.

B ) The stringency of the cap/plug of the oil filler pipe, drain stopper or drain valve should be checked. The oil degree gage of the curator should ever be unbroken clean so that the oil degree is seeable from a short distance.

Fig.4: Curator

2. BUCHHOLZ RELAY: a ) the observation spectacless should demo that the buchholz relay is decently filled with oil. If necessary, shed blooding can be done from the two pricks. The drain stopper should be tight and no leaking should be at that place.

B ) The screen on the connexion chamber should be opened to detect whether connexions are decently tight.

3. SHUT OFF VALVE: This should ever be in to the full unfastened place while the transformer is being energized.

4. Breath: a ) The stopper at the terminal of the breathing place pipe is to be removed and breather fitted on to the pipe along with the fly nut.

B ) It is necessary before suiting the breathing place to detect the colour of the silicon oxide gel. If necessary, the breathing place should be opened and the silicon oxide gel decently dried up so that its colour is absolutely blue. degree Celsius ) The chamber at the underside of the breathing place should be filled in with dry transformer oil up to the degree marked.

Fig.5: Showing Tank in oil

5. DIAL TYPE THERMOMETER: If it is provided with dismay and trip contacts, these should be set to proper temperature before stimulating the transformer. For counsel intents, it may be mentioned here that a transformer holding temperature rise of 45/55A°C, the trip contact should be set at ambient temperature plus 45A°C and the dismay contact will be 5A° – 10A° prior to this.

6. Tortuous TEMPERATURE INDICATOR: This will be set in the same manner as the dial type thermometer demuring that the trip contact should be set at ambient temperature plus 55A°C.

7. MARSHALLING Box: The Windowss of the marshalling box should ever be unbroken clean so that the readings of the oil temperature index and tortuous temperature index can be easy read from outside. Some desiccating agent may be kept inside the marshalling box so that the box is kept ever in dry status. Make non maintain the Dorr of marshalling box unfastened. It must be locked.

8. EXPLOSION VENT: a ) In instance an equaliser pipe connexion is provided, the valve in the pipe should be kept in unfastened place before the transformer is energized. B ) If the detonation blowhole is provided with an air release device, this should be opened one time to let go of any force per unit area generated indoors and so it should be closed. degree Celsius ) The stop of the blowhole should be integral.

9. Bushing: To forestall sparking bushings are used when wires at low electromotive force and transformer ‘s wire at high electromotive force are connected.

Fig.6: Screening Bushings

10. Coolant:

Fig.7: Coolant High temperatures will damage the twist insularity. Small transformers do non bring forth important heat and are cooled by air circulation and radiation of heat. Power transformers rated up to several hundred kVA can be adequately cooled by natural convective air-cooling, sometimes assisted by fans. In larger transformers, portion of the design job is remotion of heat. Some power transformers are immersed in transformer oil that both cools and insulates the twists. The oil is a extremely refined mineral oil that remains stable at transformer operating temperature. Indoor liquid-filled transformers must utilize a non-flammable liquid, or must be located in fire resistant suites. Air-cooled dry transformers are preferred for indoor applications even at capacity evaluations where oil-cooled building would be more economical, because their cost is offset by the decreased edifice building cost.

TYPES OF TRANSFORMER

1. ON THE BASIS OF TRANSFORMATON RATIO: A ) Increase transformers

A “ step-up transformer ” allows a device that requires a high electromotive force power supply to run from a lower electromotive force beginning. The transformer takes in the low electromotive force at a high current and puts out the high electromotive force at a low current. Transformers merely work with jumping current. Using direct current will make a magnetic field in the nucleus but it will non be a changing magnetic field and so no electromotive force will be induced in the secondary coil.A Using a measure up transformer to increase the electromotive force does non give you something for nil. As the electromotive force goes up, the current goes down by the same proportion. The power equation shows that the overall power remains the same.

P=V x IA Power = Voltage x Current

Fig.8: Step up Transformer

Electricity is foremost produced at the power workss. Electricity is so sent to step-up transformers where low-tension electricity is changed to high electromotive force to ease the transportation of power from the power works to the client. Voltage must be increased so that the electric current has the “ push ” it needs to expeditiously go long distances. From the step-up transformer, transmittal lines carry the high electromotive force electric current long distances through thick wires mounted on tall towers that keep the transmittal lines high above the land. Insulators made of porcelain or polymers are used to forestall the electricity from go forthing the transmittal lines.

## B ) Step-down transformers

A “ step-down transformer ” allows a device that requires a low electromotive force power supply to run from a higher electromotive force. The transformer takes in the high electromotive force at a low current and puts out a low electromotive force at a high current. A measure down transformer has less bends of wire on the secondary spiral, which makes a smaller induced electromotive force in the secondary spiral. It is called a measure down transformer because the electromotive force end product is smaller than the electromotive force input. If the secondary spiral has half as many bends of wire so the end product electromotive force will be half the input electromotive force. Decreasing the electromotive force does non diminish the power. As the electromotive force goes down, the current goes up.

Fig.9: Step Down Transformer

2. ON THE BASES OF WINDINGS:

A ) Core type transformer:

Fig.10: Core Transformer

B ) Shell type transformer:

Fig.11: Shell type transformer

3. ON THE BASES OF SERVICE: A ) Power transformer: Power transformers are used in transmittal web for electromotive force evaluations of ( 440kv, 220kv, 110kv, 66Kv ) and are by and large rated above 200MVA. Power transformer by and large operated at full burden. Hence, it is designed such that Cu losingss are minimal. B ) Distribution Transformers: Distribution Transformers are used in ( 33 kilovolt, 11kv, 6.6 kilovolt ) electromotive force degrees in Distribution web and are by and large rated less than 200 MVA. A distribution transformer is ever on-line and operated at tonss less than full burden for most of clip. Hence, it is designed such that nucleus losingss are minimal.

IDEALTRANSFORMER The idealisations are as follows: 1. Magnetic circuit is additive and has infinite permeableness. The effect is that a vanishingly little current is adequate to set up the given flux. Hysteresis loss is negligible. As all the flux generated confines itself to the Fe, there is no escape flux.

2. Winds do non hold opposition. This means that there are no Cu losingss, nor at that place

is any ohmic bead in the electric circuit.

LOSSES IN TRANSFORMER

An ideal transformer would hold no energy losingss, and would be 100 % efficient. In practical transformers energy is dissipated in the twists, nucleus, and environing constructions. Larger transformers are by and large more efficient, and those rated for electricity distribution normally perform better than 98 % .

All transformers have Cu and nucleus losingss.

1. Copper loss:

Copper loss is power lost in the primaryA and secondary twists of a transformer due to the ohmic opposition of the twists. A A Copper loss, in Watts.

Copper Loss I2PA A RP+ I2SA A RS

Where IPA A A A = A A A primary current

ISA A A A = A A A secondary current

RPA = A A A primary twist opposition

RSA A = A A A secondary twist opposition

2. Core loss:

A ) Hysteresis losingss

Each clip the magnetic field is reversed, a little sum of energy is lost due to hysteresis within the nucleus. For a given nucleus stuff, the loss is relative to the frequence, and is a map of the extremum flux denseness to which it is subjected.

B ) Eddy currents

Ferromagnetic stuffs are besides good music directors, and a solid nucleus made from such a stuff besides constitutes a individual short-circuited bend throughout its full length. Eddy currents hence circulate within the nucleus in a plane normal to the flux, and are responsible for resistive warming of the nucleus stuff. The eddy current loss is a complex map of the square of supply frequence and reverse square of the stuff thickness.

Mechanical losingss

In add-on to magnetostriction, the jumping magnetic field causes fluctuating electromagnetic forces between the primary and secondary twists. These incite quivers within nearby metalwork, adding to the buzzing noise, and devouring a little sum of power.

Stray losingss

Escape induction is by itself mostly lossless, since energy supplied to its magnetic Fieldss is returned to the supply with the following half-cycle. However, any escape flux that intercepts nearby conductive stuffs such as the transformer ‘s support construction will give rise to purl currents and be converted to heat. There are besides radiative losingss due to the hovering magnetic field, but these are normally little.

EFFECIENCY

WHAT Cause Losings

1. Due to the big value for the permeance ( I?r of the order of 1000 as compared to air ) the magnetising current demand decreases dramatically. This can besides be visualized as a dramatic addition in the flux produced for a given value of magnetising current.

2. The magnetic medium is additive for low values of initiation and exhibits impregnation type of non-linearity at higher flux densenesss.

3. The Fe besides has hysteresis type of non-linearity due to which certain sum of power is lost in the Fe ( in the signifier of hysteresis loss ) , as the B-H feature is traversed.

4. Most of the flux lines are confined to press way and therefore the common flux is increased really much and leakage flux is greatly reduced.

5. The flux can be easy ‘directed ‘ as it takes the way through steel which gives great freedom for the interior decorator in physical agreement of the excitement and end product twists.

6. As the medium is made of a carry oning stuff Eddy currents are induced in the same and produce losingss. These are called ‘eddy current losingss ‘ . To minimise the eddy current losingss the steel nucleus is required to be in the signifier of a stack of insulated laminations.

APPLICATION OF TRANSFORMER

1. Instrument transformers

Instrument transformers comprise a big class of current and possible transformers for assorted electromotive force, frequence and physical size scopes. We have broken them up into several different groupings: low electromotive force, which are system electromotive forces under 15kV ; high frequence, runing frequence over 1kHz ; and size scopes from board saddle horse parts up to current transformers with window sizes of 254mm by 610mm. Read through the different types we supply below and utilize our Instrument

Fig. 12: Instrument transformer

2. Potential Transformers:

Used chiefly in a measure down environment to supervise electromotive force. They are designed for connexion line-to-line or line-to-neutral in the same mode as ordinary voltmeters. The secondary electromotive force bears a fixed relation with the primary electromotive force so that any alteration in possible in the primary circuit will be accurately reflected in the metres or other devices connected across the secondary terminuss. Potential transformers can be used with voltmeters for electromotive force measurings or they can be used in combination with current transformers for watt-meter or watthour metre measurings. They are used besides to run protective relays and devices, and for many other applications, Since they are used in a monitoring capacity, they by and large require much greater truth in design.

Fig. 13: Potential transformer

3. Metering Toroidal Current Transformers:

Traditional, window type current transformers for mensurating 50-400HZ currents of 5A to 15000A with secondaries of 0.1A, 1A and 5A ( particular secondary currents are available ) . Burden: Bacillus 0.1 through 1.8 ( 2.5VA to 50 VA ) with Accuracy category: 0.2 to category 5.0 as per IEC 185 or category 0.3, 0.6 or 1.2 as per ANSI C 57.13. Inside diameters of up to 8.00 ” . Many theoretical accounts are available as U.L. recognized devices. Applications include:

aˆ? UPS systems

aˆ? Transportation switches

aˆ? Motor-generator sets

aˆ? Commercial sub-metering,

aˆ? 3 CT ‘s in one bundle for 3-phase metering

aˆ? Accurate measurement for metering/WATT/VAR

aˆ? Current detection, entering, supervising & amp ; control

aˆ? Control panels and thrusts

aˆ? Standard CT used as mensurating criterion for comparing

aˆ? Winding temperature index ( WTI ) for power transformers

aˆ? Summation current transformers.

Fig.14: . Metering Toroidal Current Transformers

Large Frame Current Transformers For mensurating 50-400HZ currents in coach saloon and other big music director systems. Typical constellation is 400A to 12000A primary current with secondary of 1A or 5A Inside countries every bit little as 3.00 ” Ten 7.00 ” and every bit big as 7.00 ” Ten 27.00 ” and 10.00 ”

X 24.00 ” . All theoretical accounts are available with optional climb home bases for “ bulk-head ” climb. Some theoretical accounts are U.L. recognized devices.

4. Split-Core Current Transformers

This type of current transformer is available to mensurate AC currents from 100A to 600A, at 50 to

400HZ. They are really popular in sub-metering applications where bing systems are being upgraded and it is impractical to insulate the primary music director. It is even possible to put in this type of transformer while the music director is energized, nevertheless it is overriding that certain safety safeguards be followed under such conditions. Rectangular in form, standard split-core theoretical accounts are available with window dimensions up to 4.00 ” Ten 7.50 ” . Even larger, usage designed sizes are available by particular order. Secondary evaluations of 5A, 1A, and 100ma are all common in split-core current transformers. Two theoretical account groups are available, SP and SPS. The former is provided with a

unstained steel screw-clamp set procuring the two nucleus halves ; the latter has a UV resistant nylon set. All ratios are available in either type. Electrical and magnetic public presentation is indistinguishable for the two groups.

Fig.15: Split-Core Current Transformers

5. Miniature Current Transformers

These are constructed utilizing one of the undermentioned methods: Plastic shell, Resin casted, Resin

dipped, Tape insulated, . Typical bends ratio: 4000: 1 to 500: 1 and Accuracy: Class 0.1 to Classify

Applications include:

Fig.15: Split-Core Current Transformers

aˆ? Energy metres for accurate current measuring

aˆ? Current control

aˆ? Current signature of motors

aˆ? Ground mistake feeling

aˆ? Monitoring of procedure parametric quantities

aˆ? AC degree to logic conversation & A ; saloon graph

aˆ? As a transducer in instrumentality

6. Relay Class Protection Current Transformers

This type of CT includes oil-immersed bussing and Resin molded versions. Primary current scope from 5 Amp to 5000 Amp with secondary current 5A, 1A, or 01.A. Typical Burden B 0.1 through

B 4.0 ( 2.5VA to 50 VA & A ; more ) and Accuracy Class As per ANSI C 57.13 and IEC 185. Applications include:

aˆ? Protection relays/Relay panels

aˆ? Earth mistake protection

aˆ? Bussing type, oil-immersed CT in power transformer

aˆ? Control window glasss and switch boards

aˆ? Air/Gas circuit surfs

aˆ? Motor control cells

aˆ? Power control centres

aˆ? Bus saloon protection systems

aˆ? Differential protection systems

Fig.16: Relay Class Protection Current Transformers

7. Medium electromotive force Instrument Transformers

These are used with a system electromotive force 3.3kV to 25kV and BIL 4.5 to 125 full moving ridge crest kilovolt. They are faithfully constructed utilizing vacuity dramatis personae with epoxy resin/polyurethane rosin and are able to defy heavy mistake conditions but are non made for exposure to sunlight.. Single CT ‘s can be built with multiple nucleuss ; for illustration – 1 for measurement and another for relaying are possible. Besides multitap secondaries can be provided ( up to 4 ) . Typical primary current 5 Amp to 3000 Amp and secondary current 5A/1A/01.A. Applications include:

aˆ? Metering and Relaying

aˆ? Energy metre panels

aˆ? Medium electromotive force switch cogwheels and control panels

aˆ? Medium electromotive force circuit surfs

aˆ? Motor Control Panels

Fig.17: Average electromotive force Instrument Transformers

8. Personal computer saddle horse 50 to 400Hz Current Transformers

These offer a little footmark for the design applied scientist looking to sensor current on board. They can besides be used for Metering Class ( Burden from B O.1 to B 1.8 with accuracy category from 0.3 to 2.4 as per client demand. ( As per ANSI C 57.13 and IEC 185 ) and for Relay Class Burden from B 1.0 to B 4.0 and relay electromotive force category from C 10 to C 400 or T200 as per client demand. ( As per ANSI C 57.13 and IEC 185 ) Secondary current scope from 0.1 to 5 A. Typical buildings are fictile shell or rosin molded. Applications include:

aˆ? Ground mistake sensing

aˆ? Metering

Personal computer saddle horse 2OkHz to 2OOkHz Current Transformers

These are used for mensurating high frequence primary currents up to 15 Amperes with primary to secondary isolated to 2500 VAC and have optimum public presentation over designated current and frequence scopes. Applications include:

aˆ? Isolated current feed-back signal in switch manner power

supplies

aˆ? Lighting

aˆ? Switch controls

aˆ? Ultra-sound current

aˆ? High declaration echo sounder current

aˆ? Isolated bi-directional current detector with full moving ridge span

Fig.18: Personal computer saddle horse 50 to 400Hz Current Transformers

## 9. Air core transformers: Another sort of particular transformer, seen frequently in radio-frequency circuits, is the air nucleus transformer. ( Figure below ) True to its name, an air nucleus transformer has its twists wrapped around a nonmagnetic signifier, normally a hollow tubing of some stuff. The grade of yoke ( common induction ) between twists in such a transformer is many times less than that of an tantamount iron-core transformer, but the unwanted features of a ferromagnetic nucleus ( eddy current losingss, hysteresis, impregnation, etc. ) are wholly eliminated. It is in high-frequency applications that these effects of Fe nucleuss are most debatable.

Fig.19: Air nucleus transformers

Air nucleus transformers may be wound on cylindrical ( a ) or toroidal ( B ) signifiers. Center tapped primary with secondary ( a ) . Bifilar weaving on toroidal signifier ( B ) . The interior tapped solenoid twist, ( Figure ( a ) above ) , without the over twist, could fit unequal electric resistances when DC isolation is non required. When isolation is required the over twist is added over one terminal of the chief twist. Air nucleus transformers are used at wireless frequences when Fe nucleus losingss are excessively high. Frequently air nucleus transformers are paralleled with a capacitance to tune it to resonance. The over twist is connected between a wireless aerial and land for one such application. The secondary is tuned to resonance with a variable capacitance. The end product may be taken from the pat point for elaboration or sensing. Small millimetre size air nucleus transformers are used in wireless receiving systems. The largest wireless senders may utilize meter sized spirals. Unshielded air nucleus solenoid transformers are mounted at right angles to each other to forestall isolated yoke. Stray yoke is minimized when the transformer is wound on a toroid signifier. ( Figure ( B ) above ) Toroidal air nucleus transformers besides show a higher grade of yoke, peculiarly for bifilar twists. Bifilar twists are wound from a somewhat distorted brace of wires. This implies a 1:1 bends ratio. Three or four wires may be grouped for 1:2 and other built-in ratios. Winds do non hold to be bifilar. This allows arbitrary bends ratios. However, the grade of matching suffers. Toroidal air nucleus transformers are rare except for VHF ( Very High Frequency ) work. Core stuffs other than air such as powdery Fe or ferrite are preferred for lower wireless frequences.

## 10. Tesla Coil: One noteworthy illustration of an air-core transformer is the Tesla Coil, named after the Serbian electrical mastermind Nikola Tesla, who was besides the discoverer of the revolving magnetic field AC motor, polyphase AC power systems, and many elements of wireless engineering. The Tesla Coil is a resonating, high-frequency step-up transformer used to bring forth highly high electromotive forces. One of Tesla ‘s dreams was to use his spiral engineering to administer electric power without the demand for wires, merely airing it in the signifier of wireless moving ridges which could be received and conducted to tonss by agencies of aerial. The basic schematic for a Tesla Coil is shown in Figure below.

Fig.20: Tesla spiral

Tesla Coil: A few heavy primary bends, many secondary bends.

The capacitance, in concurrence with the transformer ‘s primary twist, forms a armored combat vehicle circuit. The secondary twist is wound in close propinquity to the primary, normally around the same nonmagnetic signifier. Several options exist for “ exciting ” the primary circuit, the simplest being a high-voltage, low-frequency AC beginning and flicker spread: ( Figure below )

System flat diagram of Tesla spiral with spark spread thrust. The intent of the high-voltage, low-frequency AC power beginning is to “ bear down ” the primary armored combat vehicle circuit. When the flicker spread fires, its low electric resistance Acts of the Apostless to finish the capacitor/primary spiral armored combat vehicle circuit, leting it to hover at its resonating frequence. The “ RFC ” inductances are “ Radio Frequency Chokes, ” which act as high electric resistances to forestall the AC beginning from interfering with the hovering armored combat vehicle circuit. The secondary side of the Tesla spiral transformer is besides a armored combat vehicle circuit, trusting on the parasitic ( stray ) electrical capacity bing between the discharge terminus and Earth land to complement the secondary twist ‘s induction. For optimal operation, this secondary armored combat vehicle circuit is tuned to the same resonating frequence as the primary circuit, with energy exchanged non merely between capacitances and inductances during resonating oscillation, but besides back-and-forth between primary and secondary twists. Tesla Coils find application chiefly as freshness devices, demoing up in high school scientific discipline carnivals, cellar workshops, and the occasional low budget science-fiction film. It should be noted that Tesla spirals can be highly unsafe devices. Nathan birnbaums caused by radio-frequency ( “ RF ” ) current, like all electrical Burnss, can be really deep, unlike tegument Burnss caused by contact with hot objects or fires. Although the high-frequency discharge of a Tesla spiral has the funny belongings of being beyond the “ daze perceptual experience ” frequence of the human nervous system, this does non intend Tesla spirals can non ache or even kill you! I strongly advise seeking the aid of an experient Tesla spiral experimenter if you would ship on constructing one yourself.

## 11. Linear Variable Differential Transformer: A additive variable differential transformer ( LVDT ) has an AC driven primary lesion between two secondary ‘s on a cylindrical air nucleus signifier. A movable ferromagnetic bullet converts supplanting to a variable electromotive force by altering the yoke between the goaded primary and secondary twists. The LVDT is a supplanting or distance measuring transducer. Unit of measurements are available for mensurating supplanting over a distance of a fraction of a millimetre to a half a metre. LVDT ‘s are rugged and soil resistant compared to linear optical encoders.

Fig.21: LVDT

The excitement electromotive force is in the scope of 0.5 to 10 VAC at a frequence of 1 to 200 KHz. A ferrite nucleus is suited at these frequences. It is extended outside the organic structure by an non-magnetic rod. As the nucleus is moved toward the top twist, the electromotive force across this spiral increases due to increased yoke, while the electromotive force on the bottom spiral lessenings. If the nucleus is moved toward the underside twist, the electromotive force on this spiral increases as the electromotive force decreases across the top spiral. Theoretically, a centered bullet outputs equal electromotive forces across both spirals. In pattern escape induction prevents the nothing from dropping all the manner to 0 V. With a centered bullet, the series-opposing wired secondary ‘s natural giving up V13 = 0. Traveling the bullet up additions V13. Note that it is in-phase with with V1, the top twist, and 180o out of stage with V3, underside twist. Traveling the bullet down from the halfway place additions V13. However, it is 180o out of stage with with V1, the top twist, and in-phase with V3, underside twist. Traveling the bullet from top to bottom shows a lower limit at the centre point, with an 180o stage reversal in go throughing the centre.

Recognition