1.42 WELDING TRANSFORMERS

Figure 1.59 shows a schematic diagram of a welding transformer having thin primary windings with a large number of turns. On the other hand, the secondary has more area of cross-section and less number of turns ensuring less voltage and very high current in the secondary. One end of the secondary is connected to the welding electrode, whereas the other end is connected to the pieces to be welded. If any high current flows, heat is produced due to the contact resistance between the electrode and the pieces to be welded. The generated heat melts a tip of the electrode and the gap between the two pieces is filled.

Figure 1.59 Welding Transformer

Figure 1.60 Volt-ampere Characteristic of a Welding Transformer

The winding used for the welding transformer is highly reactive. Otherwise, a separate reactor may be added in series with the secondary winding.

Figure 1.60 shows the volt-ampere characteristic of a welding transformer.

1.42.1 Reactors Used with Welding Transformers

To control the arc, various reactors are used with welding transformers. Some methods to control the arc are given below:

i. Tapped reactor: With the help of taps on the reactor, the output current is regulated. This has limite number of current settings shown in Figure 1.61.

ii. Moving coil reactor: Figure 1.62 shows a moving coil reactor in which the reactive distance between primary and secondary is adjusted. The current becomes less if the distance between the coils is large.

Figure 1.61 Tapped Reactor

Figure 1.62 Moving Coil Reactor

iii. Moving shunt reactor: Figure 1.63 shows a moving shunt reactor in which the position of the central magnetic shunt can be adjusted. Change of the output current is obtained due to the adjustment of the shunted flux.

iv. Continuously variable reactor: Figure 1.64 shows a continuously variable reactor in which the height of the reactor is continuously varied. Greater reactance is obtained due to greater core insertion and hence the output current is less.

v. Saturable reactor: Figure 1.65 shows a saturable reactor. To adjust the reactance of the reactor, the required DC excitation is obtained from a DC controlled transducer. Reactor approaches saturation if the DC excitation current is more. Therefore, changes of current are obtained due to the change of reactance.

Figure 1.63 Moving Shunt Reactor

Figure 1.64 Continuously Variable Reactor

Figure 1.65 Saturable Reactor

Designing Your Own Transformer

written by: Swagatam • edited by: Lamar Stonecypher • updated: 11/1/2011

Designing a transformer is not easy simply because the criteria involved with these

devices are critical and elaborate. However some meticulously arranged data regarding

the various calculations can make the procedure easier. Learn how to make a transformer

through using simple formulas.

Introduction

We have already studied a lot about transformers in Bright Hub and we know that it’s

simply a device used for either stepping-up or stepping down an applied input AC through

magnetic induction in between its two windings.

Basically a transformer will have the following main components:

Iron core stampings (configured either as U/T or E/I, generally the later is used more

extensively)

Central plastic or ceramic bobbin surrounded by the above iron core stampings

Two windings (electrically isolated and magnetically coupled) using super enameled

copper wire made over the bobbin

Normally the winding which is designated to receive the input supply is termed as the

“Primary” and the winding which in response to this input produces the required induced

voltage as the output is termed as the “secondary” winding.

Designing your own transformer as per a specific application can be interesting, but not

feasible without calculating the various parameters typically involved with them. The

following discussion will take you through a few important steps and formulas and explain

how to make a transformer.

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Calculating the Core Area (CA) of the Transformer

The Core Area is calculated through the formula given below:

CA = 1.152 ×√ (Output Voltage × Output Current)

Calculating Turns per Volt (TPV)

It is done with the following formula:

TPV = 1 / (4.44 × 10-4 × CA × Flux Density × AC frequency)

where the frequency will depend on the particular country’s specifications (either 60 or 50

Hz), the standard value for the flux density of normal steel stampings may be taken as 1

Weber/sq.m, for ordinary steel material the value is 1.3 Weber/sq.m

Primary Winding Calculations

Basically three important parameters needs to be figured out while calculating the primary

winding of a transformer, they are as follows:

Current through the primary winding

Number of turns of the primary winding

Area of the primary winding

Let’s trace out each of the above expressions:

Primary Winding Current = (Secondary Volts × Secondary Current) ÷ (Primary Volts ×

Efficiency), the average value for the efficiency of any transformer nay be presumed to be

0.9 as a standard figure.

Number of Turns = TPV × Primary Volts

Primary Winding Area = Number of Turns / Turns per Sq. cm (from the table A)

Reading Table A is easy – just find out the relevant figures (wire SWG and Turns per

sq.cm.) by tallying them with the closest matching value of your selected primary current.

Secondary Winding Calculations

As explained above, with the help of Table A you should be able to find the SWG of the

wire to be used for the secondary winding and the TPV simply by matching them with the

selected secondary current.

The Number of turns for the secondary winding is also calculated as explained for the

primary winding, however considering high loading conditions of this winding, 4 % extra

turns is preferably added to the over all number of turns. Therefore the formula becomes:

Secondary Number of Turns = 1.04 × (TPV × secondary voltage),

Also secondary winding area = Secondary Turns / Turns per sq. cm. (from table A).

Calculating the Core Size of the Steel Laminations or the Stampings

The core size of the steel stampings to be used may be easily found from Table B by

suitably matching the relevant information with Total Winding Area of the transformer. The

Total Winding Area thus needs to be calculated first, it’s as follows:

Total Winding Area = (Primary Winding Area + Total Secondary Winding Area) × Space for

External Insulation.

The third parameter i.e. the space for the insulation/former etc. may be taken

approximately 25 to 35 % of the sum of the first two parameters.

Therefore, the above formula becomes:

Total Winding Area = (Primary Winding Area + Total Secondary Winding Area) × 1.3

Normally, a core having a square central pillar is preferred and

used - other factors involved are also appropriately illustrated in the adjoining figure and

calculated as follows:

Gross Core Area = Core Area from Table B / 0.9 (sq.cm.)

Tongue Width = √Gross Core Area (cm)

After calculating the Tongue Width, it may be used as a reference value and matched

appropriately in Table B to acquire the actual CORE TYPE.

Your quest regarding how to make a transformer gets over when you finally finish

calculating the stack height, using the formula:

Stack Height = Gross Core Area / Tongue Width.

Table A

The table below helps you to select the gauge and turns per sq. cm of copper wire by

matching them with the selected current rating of the winding appropriately.

SWG------- (AMP)------- Turns per Sq.cm.

10----------- 16.6---------- 8.7

11----------- 13.638------- 10.4

12----------- 10.961------- 12.8

13----------- 8.579--------- 16.1

14----------- 6.487--------- 21.5

15----------- 5.254--------- 26.8

16----------- 4.151--------- 35.2

17----------- 3.178--------- 45.4

18----------- 2.335--------- 60.8

19----------- 1.622--------- 87.4

20----------- 1.313--------- 106

21----------- 1.0377-------- 137

22----------- 0.7945-------- 176

23----------- 0.5838--------- 42

24----------- 0.4906--------- 286

25----------- 0.4054--------- 341

26----------- 0.3284--------- 415

27----------- 0.2726--------- 504

28----------- 0.2219--------- 609

29----------- 0.1874--------- 711

30----------- 0.1558--------- 881

31----------- 0.1364--------- 997

32----------- 0.1182--------- 1137

33----------- 0.1013--------- 1308

34----------- 0.0858--------- 1608

35----------- 0.0715--------- 1902

36----------- 0.0586---------- 2286

37----------- 0.0469---------- 2800

38----------- 0.0365---------- 3507

39----------- 0.0274---------- 4838

40----------- 0.0233---------- 5595

41----------- 0.0197---------- 6543

42----------- 0.0162---------- 7755

43----------- 0.0131---------- 9337

44----------- 0.0104--------- 11457

45----------- 0.0079--------- 14392

46----------- 0.0059--------- 20223

47----------- 0.0041--------- 27546

48----------- 0.0026--------- 39706

49----------- 0.0015--------- 62134

50----------- 0.0010--------- 81242

Table B

This Table B enables you to make your own transformer design by comparing the

calculated Winding Area with the relevant required Tongue Width and Lamination Type

number.

Type-------------------Tongue----------Winding

No.---------------------Width-------------Area

17(E/I)--------------------1.270------------1.213

12A(E/12I)---------------1.588-----------1.897

74(E/I)--------------------1.748-----------2.284

23(E/I)--------------------1.905-----------2.723

30(E/I)--------------------2.000-----------3.000

21(E/I)--------------------1.588-----------3.329

31(E/I)--------------------2.223-----------3.703

10(E/I)--------------------1.588-----------4.439

15(E/I)-------------------2.540-----------4.839

33(E/I)--------------------2.800----------5.880

1(E/I)----------------------2.461----------6.555

14(E/I)--------------------2.540----------6.555

11(E/I)---------------------1.905---------7.259

34(U/T)--------------------1/588---------7.259

3(E/I)-----------------------3.175---------7.562

9(U/T)----------------------2.223----------7.865

9A(U/T)----------------------2.223----------7.865

11A(E/I)-----------------------1.905-----------9.072

4A(E/I)-----------------------3.335-----------10.284

2(E/I)-----------------------1.905-----------10.891

16(E/I)---------------------3.810-----------10.891

5(E/I)----------------------3.810-----------12.704

4AX(U/T) ----------------2.383-----------13.039

13(E/I)--------------------3.175-----------14.117

75(U/T)-------------------2.540-----------15.324

4(E/I)----------------------2.540----------15.865

7(E/I)----------------------5.080-----------18.969

6(E/I)----------------------3.810----------19.356

35A(U/T)-----------------3.810----------39.316

8(E/I)---------------------5.080----------49.803

Kapp regulation diagramFrom Encyclopedia Magnetica

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S. Zurek, Kapp regulation diagram, Encyclopedia Magnetica, {accessed 6 Apr 2013}

edited 1 Jan 2012 — 1 Feb 2013 reviewed by J. Leicht on 1 Feb 2013

Kapp regulation diagram[1]

fot. S. Zurek, Encyclopedia Magnetica, license: CC-BY-3.0

Kapp regulation diagram - a graphical method of determining the voltage regulation in a

transformer caused by changes in load and power factor. [1]

The output voltage of a mains power transformer when loaded reduces for inductive load (power

factor lagging) and increases for capacitive load (power factor is leading).

The Kapp diagram is helpful in finding the voltage drop or increase (voltage regulation). The main

disadvantage is that the voltage regulation phasors are much smaller than the radii of the main

circles, so the diagram has to be drawn on a very large scale to get sufficiently accurate results.[1]

Drawing algorithm

In order to create the diagram it is necessary to know the equivalent reactance X02 and resistance R02

of the transformer as referred to the secondary side. The following algorithm should be used:

1. Draw phasor OL representing secondary terminal voltage V2 on load2. Draw OX representing the phase of the secondary current at an angle Φ2 to OL such that

cosΦ2 is the power factor of the load3. Draw phasor LM (I2 R02 - voltage drop on resistance referred to the secondary side) parallel

to OX, and then MN (I2 X02 - voltage drop on reactance referred to the secondary side). The resulting NL is the total voltage drop.

4. Transfer the impedance triangle NLM to OO'P which gives O'L = ON = 0V2. Therefore, for given secondary current the locus of N is a circle with centre O and radius 0V2, while the locus of L has the same radius but with the centre O'

5. To find the voltage drop on full load at any power factor the radius OQS should be drawn at at angle Φ to OX. If the impedance triangle is drawn in position UQT then OU = OS. The length of QS represents the voltage drop.

References

Making a welder out of an old transformer.

Seeing the post a while back about the burnt out 3 phase transformer has got me thinking again.

After getting my old Forney welder, I have been playing around with seeing how hard it really is to design your own welder to operate the way you want it too. That has led me to  experiments with a couple old 480V to 120/240 volt transformers I have.

I have decided to post a little article on possible methods to convert an old transformer into a welder. Opinions on my ideas is welcome.

This post will be very long.

The type of transformer you are looking for is one with a large open core. A single phase with two coils on a single donut core (two leg). Or a three phase one with three windings and three legs that are bridged at the top and bottom (three leg).

Those types have a large core that will have a lot of flux leakage when driven by only one leg.

Transformers that have the E-I core with the return path going around both sides of a single winding have very little flux leakage and you will have a problem designing a reasonable current limited transformer out of it.

You get the current limiting action by arranging the windings on the core so that there is a large leakage path between them.  The farther apart you have

the primary and secondary, the more flux leakage. The closer you have them located, the leakage and you will get less of a current limiting action.

The primary produces flux when it energized. That flux tries to take the easiest path to form a loop. Be it steel, air, or water. The easiest path for it to take is steel, when the coil is wound around a steel donut, then the magnetic flux generally prefers to go through that steel donut to form a loop. Path of least resistance. But if you put a shorted coil on the other side of the loop, then it stops the flux from passing through the complete donut. The flux will jump across the middle of the donut between the primary and secondary to complete the flux loop. Longer, and narrower the gap that the flux has to jump across, the more counter EMF the secondary has to generate to force the primary EMF to jump that gap.

A tall but narrow donut with the primary on the right and secondary on the left leg. With the primary and secondary side by side. It will take more amp turns in the secondary to get the primary flux to bypass the secondary.

If you have a  wide but short donut with the primary and secondary on the right and left leg, the gap between the primary and secondary is large. Then the gap between the top and bottom part of the donut  where the flux has to jump is short and wide. It takes a lot less current in the secondary to force the primary flux to jump the gap between the primary and secondary.

And example of a simple transformer for making a welder.A 240/480 to 120/240 single or three phase transformer.

Single phase has....One 240V primary and a 120V secondary on each leg. The primaries are stung in series for 480V or in parallel for 240V. The secondary windings  are in series for 120/240 center tapped. Or can be put in parallel for 120V. Often the secondary windings are split between the legs to even out loading, and reduce leakage. That means that there is two 60V secondary windings on each side that is in series with the opposed one on the other side.

Three phase with three legs that are bridged at the top and bottom. Each leg has one primary and one secondary. wye or delta on primary and secondary

If you are lucky, there is enough room between the existing coils to wind your custom windings.

If it is tightly packed, then you may have to remove all, or part of, the windings on one side to make room for your custom windings..

Pick a coil/side/leg for your primary. If you have a transformer that has a mixture of damaged windings and good ones, then pick a good one. It should have a 208,240 277, or 480V primary and one or more secondary windings on that leg of the transformer.

Remove the damaged windings. Then start experimenting.

Example. (from the experiments I did with one of my transformers. 15KVA 240/480 to 120/240 single phase) Your transformer has a 240V primary on each

leg. Hook  120V to one primary. Run a single turn or a couple turns around the core and measure the voltage across it. If you have three turns and you get 3.6V AC then your transformer is running at 1.2V per turn. Since designed operating voltage for the transformer is twice what you have the primary hooked too, then designed operating level will be 2.4V per turn.

When testing, you want to leave the transformer hooked to the lowest exciting voltage you have, that will make the testing safer, and the currents developed for that core the lowest possible.

Lets say that you have the above stated situation. 120V hooked to a 240V winding which yields you a 1.2V per turn. On a single phase transformer.

That tells you that the primary on each leg has 100 turns.Some will have a secondary by it's self on each leg that has 50 turns.Some single phase transformers will have each secondary split between both legs, so you have two 25 turn secondary windings on each leg.

When you have the one primary excited with 120V you will have 120V on the other primary and 60V or two 30V on the secondary windings on each leg.

Now to find out the current characteristics of your transformer.

There is two factors that limit current in the setup. The conductor resistance. And the flux leakage. Conductor resistance makes the windings hot, so we want the windings big enough that that isn't a factor.

Lets say that your transformer has an available current of 2040Amp turns into a shorted leg when the other leg is excited by 120V.

If you had a single turn large enough to handle that current on that leg then you would have close to 2040A on that turn when it's shorted, or 1.2V when it's open.

(If you made that single turn with a 4 foot piece of 10g wire, the current would be limited by the wires resistance. 1.2V across 4 feet of wire is close to 300 amps. The wire will get hot very quickly. The wire will be giving in, not the flux in the transformer. The voltage across the leg doesn't drop, the wire is just forced to drop the voltage across it's length.)

Lets continue with that train of though.

If you had two turns of wire adequate for the current, you would get 2.4V when open, and 1020A when closed. That would be in about the right range for spot welding, but not for arc welding. So lets continue on.

10 turns would yield 12V at 204A

20 turns would yield 24V at 102A close but not quite.

30 turns would yield 36V at 68A getting real close to the right voltage.

40 turns yield 48V which does quite well at 51A with the 1/16 6013 rod.

50 turns yields 60V that also does quite well at around 41A

Considering that the two low voltage windings I have are 25 turns each. I

can have one winding plus 15 more turns of wire with it to get 40 turns, or use both in series to get 50 turns.

So, my transformer without major modification can serve as a 40 or 50a welder with the addition one 15 turns of wire on one side. You could use the other unused 100 turn primary on that side for a 120V OC 20A welder. Now if I could only find 1/32 welding rod!  :-)

For the secondary windings left over on the side that you are driving as the primary.  since they are in the same coil as the driven primary, there will be no real current limiting. So you could use them as a CV output to drive a mig gun, If you hook them in parallel for 30V. Considering that my transformer is rated at 15KW you should have 120A continuous duty on hand to drive that mig gun.

That is what you could do with a very simple winding arrangement.

Now, lets get to the complex winding arrangements for an arc welder.

Lets go for a base 50V OC on all outputs.

Lets hook up the two secondary windings on the output (current limited) side in parallel so that we have 30V OC with a short circuit output of 81A that will allow us to make use of what is already there.

Take that as are base winding. The winding can handle up to 120A

continuously (two 60A windings in parallel)

For are 50A tap, run are base winding in series with 16

additional turn new winding. That yields 49V at close to 50A

For the 40A tap, we can't just use a 25 turn tap in series with the base winding because that will yield 60V OC. Remember we want a nominal 50V OC. Here is where we get into the complex windings.

To do that, we have to have the additional 25 turn winding in series with a buck winding on the primary driven side.  A buck winding on the primary side is close coupled with the driven winding so it will reduce the output voltage but it won't reduce the current available at the output since there won't be any current limiting action.

So, you will have a winding with 25 turns on the current limited side plus 8 reverse turns on the driven side. That will yield you 40A at 50V OC. (you get the output current from 50 turns on the current limited side, and you use 8 buck turns to get the voltage down to spec) Now, it won't be exactly that because with the buck winding, you are not directly shorting the current limited side. So you may have to take a winding may have to take a turn off of each side until current comes up to spec for that tap.

Now lets go for the 60A tap.

Lets take the base winding and add a 9 turn new winding to the current limited side.(9 turns of the new 15 turn winding for the 50A tap) And run that to a 7 turn boost winding on the primary driven side. That will get you 60A at 50V. (34 turns on the current limited side plus 7 boost

turns)

The highest we could wind this transformer for is 70A and still maintain 100% duty cycle, which would be 29 current limited turns plus 12 boost turns. (base winding plus 4 with 12 boost turns)

Now if we went with interment ant duty we could easily go 80A which is the base winding plus 12 boost windings. In actuality, that setup would be closer to 100A or higher because the current limited winding will be driven beyond shorted. It will actually be driven reverse polarity. (0V - boost voltage)

So, just experiment and move the winding around till you find the current you want.

You can tailor it to get about any OCV you want. If you want a digging arc or a rubbery arc, you can tailor it to fit your desires.

To get 100A out, at 50 OCV you will want to run it on a 50A 120V breaker.

If you drive the primary at full rated 240V you will have to arrange the secondary  and buck boost windings to compensate for a higher amp turns that is available on the current limited side, plus the additional volt turns.

"Now wait a second" you say "how does all that help me find out how much current my transformer will put out?????????"

Well. Put some test windings on your transformer and Either run you selected

transformer primary off of 120V or even 60Vor 30V from another transformer. Have a current meter on the test windings and short them out for a second to see what you get. Rearrange the test windings and try again.

When you get the current down to a relatively low level at 30 or 60 V then you can step up to the next drive voltage level and check what affect that has. Rearrange the windings, and step up another voltage level.

I would not suggest that you power the selected primary to full rated voltage and then short a test winding on it with no prior knowledge of the transformer in question. You may be in for a little bit more current than you bargained on. Plus a lot of melted wires to boot.

One way of bringing up the power level more gradually is use a large variac to slowly bring up the voltage level on a shorted test winding and see where the current levels off. You will be able to see if the current is climbing way faster than you anticipated with the voltage, and if you will need to change your winding layout before bringing it to full voltage.

If you have a transformer that you have taken one of the coils off one of the legs, then you will have a lot more freedom to arrange the windings.

If you have a three phase transformer with two bare legs then that opens up a world of possibilities. When you short a winding out on one leg, then the flux will shift to the other leg. You could use one winding on one leg as an output winding and have a winding on the other that is connected to SCR's to

vary the current. When the SCR's are full on, then it forces all the current to the output winding, and when they are full off, then none of the current is forced to the output. (it bypasses the output winding through the open SCR winding)Don't forget the secondary windings on the drive leg. They can be run in series with the primary to reduce the volts turn drive level at a specified supply voltage!!!

Now it is getting late, and I am getting tired, so I am going to wrap this up and post it to see what you people think.

CURSO DE SOLDADURA SMAW (11)

20:15    No comments

Capitulo 11: MAQUINAS DE SOLDAR (transformador).

Aparato eléctrico que transforma la corriente eléctrica bajando la tensión de la red de alimentación a una tensión e intensidad adecuada para soldar. Dicha CA de baja tensión (65 a 75 voltios en vacío) y de intensidad regular. Permite obtener la fuente de calor necesaria para la soldadura.

El transformador consta de un núcleo que está compuesto por láminas de acero al silicio y de dos bobinas de alambre; el de alta tensión, llamado PRIMARIO y el de baja tensión llamado SECUNDARIO.

La corriente que proviene de la línea circula por el primario.

Los transformadores se construyen para diferentes tensiones, a fin de facilitar su conexión, en todas las redes de alimentación.

La transformación eléctrica se explica de la forma siguiente: "La corriente eléctrica que circula por el primario genera un campo de lineas de fuerza magnética en el núcleo, dicho campo actuando sobre la bobina secundaria, produce en este, una corriente de baja tensión y alta intensidad, la cuál se aprovecha para soldar.

CARACTERÍSTICAS

La regulación de la intensidad se hace comúnmente por dos sistemas:

1- Regulación por bobina desplazante: Consiste en alejar el primario y el secundario 

    entre sí.

Observación: Esta sistema es recomendable por su regulación gradual.

2- Regulación por clavija: Funciona aumentando o disminuyendo el número de espiras.

Los transformadores se conocen también como MAQUINAS ESTATICAS por no tener piezas móviles.

VENTAJAS

El uso del transformador se ha generalizado por:

-4-    Bajo costo de adquisición

-5-    Mayor duración y menor gasto de mantenimiento

-6-    Mayor rendimiento y menor consumo en vacío

-7-    Menor influencia del soplo magnético

DESVENTAJAS

Entre sus desventajas se pueden mencionar:

-8-    Limitación en el uso de algunos tipos de electrodos

-9-    Dificultad para establecer y mantener el arco

MANTENIMIENTO

Debe mantenerse el equipo libre de polvo y humedad

PRECAUCIÓN

Toda acción de limpieza debe efectuarse con la máquina desconectada

Al instalarla debe elegirse un lugar seco fijando en la máquina, una conexión a tierra.

ACTIVIDAD

27- Nombre las dos formas para regular el amperaje en las máquinas estáticas

28- ¿Cuáles son los principales componentes de una máquina estática y a que se debe su nombre.

TRANSFORMER DESIGN, CONSTRUCTION & THEORY

Presented here is an overview of transformers for hobbyists that want to expand their knowledge of this essential electrical component.

Of the 3 basic passive electrical components, R, L, C, the transformer is simply a special case of the inductor with either taps or multiple

windings that are coupled.It will operate over the full range of frequencies and may be either iron or air cored; most of the following discussions will concentrate on low

frequency, iron cored transformers.

The basic transformer to be discussed is the voltage (or power transformer), which operates at mains frequency and is step up or down.

There are also special cases, such as current transformers, pulse transformers and audio (output and driver) types that cater for a wider

band of frequency.3 phase transformers are simply a special case of the single-phase

transformer with the core arranged to give a balanced magnetic circuit.

Transformers consist of a core and windings; the core is usually an iron alloy to suit the application and the windings consist of coils of insulated

copper or aluminium wire.

There is no absolutely correct design for any given transformer; as in all matters of good engineering there are many design and cost

compromises to be made.Having selected a core and decided on a flux density, the T/V figure is calculated and then it is a matter of seeing if the turns will fit on the

bobbin available for the selected core.

THE CORE

All transformers follow the basic transformer equation and work by reason of the fact that the magnetic flux, produced by the primary

applied voltage is constantly changing. This is basic magnetic theory. It appears confusing as it is taught in many systems of units. I will

concentrate on the MKS system where the units are Metres. DO NOT USE mm!!!

This is a practical system where the unit of flux density is the TESLA. Normal transformers operate at levels around 1T depending on the

materials used.

Transformer equation:

N = E / 4.44 B F Ae Where:

N = turnsE = applied voltage in volts

4.44 is a constant for sine wavesB = desired flux density in Tesla

F = frequency in HzAe = transformer centre limb area in M²

Stalloy or silicon iron, which is widely used as a core material is normally operated at a flux density of 1T. Unisil, a grain orientated material,

which is a little more expensive can be run at 1.5 T. Special alloys for Mil and aerospace applications can be run up to 2T where a compact design

is a requirement.Running at a higher flux density allows the turns/ volt to be reduced and the cost of copper and copper losses; however this increases the losses in the iron and increases the magnetising (idling) current. Design curves

are available showing watts loss per Kg at various flux densities.Traditional design theory would make copper and iron losses equal at

normal load levels (say 80% of full load)

An important fact of life with iron cores is Saturation. When we increase the voltage across an iron cored coil the current will initially increase in a

linear manner. When we reach a certain level the current will start to increase much more rapidly than the voltage. This is the “Knee point”

where a lot of transformers are designed to operate.Note that Stalloy has a fairly rapid turn into saturation, Unisil much less

so. Hence lower distortion when used in the output transformer of a Hi-Fi amplifier.

To determine the operating level of an unknown core it is simply a matter if winding a know number of turns onto the core, and then

plotting a V/A curve. At the point at where it starts to turn into saturation will be the operating level, and it is then easy to read off the Turns/Volt

from the plot.

Other core losses are determined by the circulation of eddy currents in the laminations. Thinner lams and lower frequencies give lower losses,

but increase the cost of the lams. A solid core would have very high losses, hence the lams are lightly oxidised to give insulation between

them. These losses are frequency dependant.Most laminations available these days are “lossless” types.

This means that they are “E” & “I” shapes stamped out of sheet so as to leave no waste. Many other forms will be encountered in vintage

equipment.Transformers should normally be laminated to leave as small a gap between lams as possible, interleaving and tapping them together

during assembly. The exception to this is transformers carrying DC, as in Class A valve output stages, where the standing DC current could cause saturation and a small air gap, usually made from thin paper is inserted

between the “E” & “I” lams that are not now interleaved.

Clamping bolts, when inserted through the lams should have an insulation washer between the lam face and the nuts. If this is not done

there can be high currents circulating through the bolt.

Toroidal transformer cores can be run at higher flux densities and have the advantage of a much lower stray field, useful in audio applications

THE WINDINGS

These are usually of enamelled copper wire but various other insulations are available. The insulation should be as thin as possible consistent

with a suitable electrical withstand level. Windings often had a thin layer of paper between layers of winding. With present wire insulation this is not necessary every layer, and is often replaced with Type 56 Polyester

tape every few layers, again depending on application.The normal wire current density is typically 3A per mm² and this figure is

often shown in wire tables. Note that wire sizes, in SWG, AWG or mm are based on bare wire, not

coated diameter.The current density figure is based on a max internal temperature for the transformer of 120°C; the limit for normal materials, other design

factors may dictate different values.Wire tables will also give the resistance/ Mtr of wire. This is used to

calculate the winding resistance from the mean length of turn. Note that copper resistivity increases by 0.3% per °C rise.

The transformer equation gives the primary turns; secondaries can then be calculated by the turns ratio. If very exact ratios are required it may

be necessary to increase the primary turns so as to get an integral number of turns on the secondary of importance.

If an exact voltage is required it is often necessary to add compensating turns to a secondary to allow for the volt drops (calculated from mean length of turn and resistance per mtr) in both primary and secondary. It can also be necessary to allow for the increase of resistance due to

temperature rise as well.Further compensation is sometimes necessary to allow for the imperfect coupling between primary and secondary which appears as a parasitic

loss.

Primaries and secondaries are normally wound on top of each other for good coupling; where additional safety separation is required, they may be wound side by side with a centre insulated barrier (some transformer

kits with pre-wound primaries are like this).This gives inferior coupling and additional compensating turns will be

needed to compensate for this.For very close coupling the primary and secondary will be wound

together with Bi-filar wire. Where insulation levels make this impossible, the primary and secondary are wound in several sections, one on top of the other. For balance this would usually be one more primary section

than secondary section.

Wire is commonly insulated with synthetic enamels in either 1 or 2 coats. Most modern enamels can be vaporised with solder and are

known as solderable types.Where high levels of insulation are required in a small space (such as a switch mode transformer), then triple insulated wire can be used. This is certified for supply to low voltage windings without additional insulation.

An earthed copper screen is often placed between the primary and secondary; this is for both safety and as an interference screen against

noise impulses.

Windings are not normally carried to the outside of the bobbin on multi-layer windings due to the danger of a turn slipping down the side of the

bobbin and “seeing” a higher voltage, possibly causing breakdown. Paper margin tapes, a few mm wide are often used here.

With EHT transformers this margin is often tapered inwards (wider) as the outer layers are added and the winding voltage to earth increases.

Copper is the important part of the winding so thin insulation is used, as there is only a relatively low voltage between turns and layers. This is

usually capable of withstanding a minimum of 120ºC.

The windings are often vacuum impregnated to seal against moisture and prevent chafing movement under operating or fault conditions. It is possible to spray varnish them while building, or they can be paraffin

wax impregnated in a container of molten wax.For voltages above 6KV it is usual to seal the transformer in a can of

specially refined mineral oil.

Tapping points on windings can be “brought out” of the body of the winding by looping the wire out and back again. Lead-out wires are often soldered on to the winding at an appropriate point. These must be well

insulated.

The forgoing should allow a reasonably competent person to design and build their own transformers when size and cost are not the driving

factors as in commercial applications. It should always be remembered that these devices are connected to the mains and can easily kill or start serious fires; all possible safety precautions should be taken when using

them.

© Ed Dinning 2009

Please help,I want to design a welding machine of 50A,100A and 150A output.?my input supply is of 220v 30Anow give the idea about wire size of primary and secondary and core size.

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Welding transformers are somewhat different to normal "voltage" transformers... They are Leakage transformers, so the design specs are likely quite different. Believe this is what provides the current limiting, and stop the welder blowing a fuse while the arc starts/if it shorted out, and helps stop all kind of other nasty stuff happening

You'll have to do a LOT more research on the matter, and on your own head be it. here a few pointers

believe arc welding machines normally output about 55V. The design of arc welder is to dump the power into an arc. Once struck the resistance of the arc is likely to be far higher than that of an "decently" sized coils. As such much more the heating power will end in the arc. remember P=R*I^2 . If the arc has 100* the resistance of the coils, the coils will only produce 1/101 the heat.

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4 times as many turns in the primary as the secondary will give you 57 volts with no secondary current. The primary current will be 40 amps when the secondary current is 160 amps. Most 30 amp circuit breakers trip in about one second supplying 40 amps = your welder will be marginal at 150 amps. You are likely ok anywhere from 3 to 5 turns ratio. Pick the wire size that let's you fill about half of the square centimeters available in the transformer core for the primary and half for the secondary. Unless your transformer weighs 5 or 10 kilograms, the core will saturate at about 40 amps input, so that prevents the output from going much over 160 amps, even briefly. I don't know how commercial welders limit the current to 100 amps or 50 amps. Variable flux gap, perhaps? Likely some of the newer ones use switching type power supply technology which uses only tiny transformers and inductors. If you are going to build the welder, a used transformer with the old secondary removed will be easiest, except you have to guess how many turns the primary has. Possibly you can wind 11 turns of any size enameled wire, then energize the transformer. If you measure 11 volts you have one volt per turn and need 55 turns to get 55 volts. I think less than one volt per turn per volt is typical. Neil

Equations

There are two approaches used in designing transformers. One uses the long formulas, and the other

uses the Wa product. The Wa product is simply the cores window area multiplied by the cores area.

Some say it simplifies the design, especially in C-core (cut core) construction. Most manufacturers

of C-cores have the Wa product added into the tables used in their selection. The designer takes the

area used by a coil and finds a C-core with a similar window area. The Wa product is then divided

by the window area to find the area of the core. Either way will bring the same result.

For a transformer designed for use with a sine wave, the universal voltage formula is:Ref:[4][5][6][7][8]

thus,

where,

E is the sinusoidal rms or root mean square voltage of the winding, f is the frequency in hertz, N is the number of turns of wire on the winding, a is the cross-sectional area of the core in square centimeters or inches, B is the peak magnetic flux density in Teslas or Webers per square meter (MKS meas. sys.),

gausses per square centimeter, or lines (maxwells) per square inch (cgs meas. sys.). P is the power in volt amperes or watts, W is the window area in square centimeters or inches and, J is the current density. Note: 10 kilogauss = 1 Tesla.

This gives way to the following other transformer equations for cores in square centimeters (cgs

meas. sys.):

The derivation of the above formula is actually quite simple. The maximum induced voltage, , is

the result of N times the time-varying flux:

If using RMS voltage values and E equal the rms value of voltage then:

and

Since the flux is created by a sinusoidal voltage, it too varies sinusoidally:

where = area of the core

Taking the derivative we have:

Substituting into the above equation and using and the fact that we are only concerned

with the maximum value yields

[edit] Imperial measurement system

The formulas for the imperial (inch) system are still being used in the United States by many

transformer manufacturers. Most steel EI laminations used in the US are measured in inches. The

flux is still measured in gauss or Teslas, but the core area is measured in square inches. 28.638 is

the conversion factor from 6.45 x 4.44 (see note 1) the 6.45 factor is simply the square of 2.54 cm in

1 Inch. The formulas for sine wave operation are below. For square wave operation, see Note (3):

To determine the power (P) capability of the core, the core stack in inches (D), and the window-

area (Wa) product, the formulas are:

where,

P is the power in volt amperes or watts, T is the volts per turn, E is the RMS voltage, S is the current density in circular mils per ampere (Generally 750 to 1500 cir mils), W is the window area in square inches, C is the core width in square inches, D is the depth of the stack in inches and, Wa is the product of the window area in square inches multiplied by the core area in

square inches. This is especially useful for determining C-cores, but can also be used with EI types. The window area is simply the windows height multiplied by its width.

[edit] Simpler formulae

A shorter formula for the core area (a) and the turns per volt (T) can be derived from the long

voltage formula by multiplying, rearranging, and dividing out. This is used if one wants to design a

transformer using a sine wave, at a fixed flux density, and frequency. Below is the short formulas

for core areas in square inches having a flux density of 12 kilogauss at 60 Hz (see note 2):

And for 12 kilogauss at 50 Hz:

[edit] Equation notes

Note 1: The factor of 4.44 is derived from the first part of the voltage formula. It is from 4 multiplied by the form factor (F) which is 1.11, thus 4 multiplied by 1.11 = 4.44. The number 1.11 is derived from dividing the rms value of a sine wave by its average value, where F = rms / average = 1.11.

Note 2: A value of 12 kilogauss per square inch (77,400 lines per sq. in.) is used for the short formulas above as it will work with most steel types used (M-2 to M-27), including unknown steel from scrap transformer laminations in TV sets, radios, and power supplies. The very lowest classes of steel (M-50) would probably not work as it should be run at or around 10 kilogauss or under.

Note 3: All formulas shown are for sine wave operation only. Square wave operation does not use the form factor (F) of 1.11. For using square waves, substitute 4 for 4.44, and 25.8 for 28.638.

Note 4: None of the above equations show the stacking factor (Sf). Each core or lamination will have its own stacking factor. It is selected by the size of the core or lamination, and

the material it is made from. At design time, this is simply added to the string to be multiplied. Example; E = 4.44 f N a B Sf