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Projects by members => Projects by members => Sebosfato => Topic started by: sebosfato on January 06, 2016, 14:36:30 pm

I will here develop this tutorial for you learn how to use simple math to design a coil and predict its behavior under pulsed conditions.
First of all when working with a transformer core we than should try use the maximum of its ability.
the window is the first thing to think about.. the number of turns you can get into it will depend on the diameter of wire, the required isolation and coil former dimensions..
how to simplify and always get it work?
First calculate how much turns you can do for the core... for that we are goingto divide the core window area by 2 assuming we are able to use half of it with copper and the other half with space and insulation,... than divide this number by the diameter of the wire used squared since we are dealing with area... the result is the maximum number of turns you can do with this wire you have...
the isolation depends on the material you use but the bests are nomex... or poliimide films...
this films are required to isolate the coils layers at worst case condition...
here we find that the coil will occupy a maximum of space in the core...
the power a transformer can delivered is limited to the amount of copper you can wind on it...
the maximum voltage this transformer will be able to get out will depend on the primary turns and the power suppy voltage youhave
so for example we found that we can put 500turns plus the primary few turns in the core... how many turns can we use?
here you must calculate according to the core crossectional area, voltage, frequency and the proprieties of the core
number of turns of the primary for a pulsing dc voltage is N = to V/Bmax/Area/freq/2
above this voltage or bellow this freq the core will saturate
so the maximum pulse on voltage for your maximized transformer is the ratio between this turns of the coil and the primary and the input voltage
during pulse off the colapse voltage will depend on the resistance across the primary (for a simpler control of it) and the current it was charged to, and consequently the secondary coil will receive this voltage in proportion to the turn relation again.. so its a multiplification effect...flyback
If the secondary has a blocking diode like a flyback the pulse on is blocked so no current flow and thereto the collapse will be maximum since the secondary coil didnt developed any contrary magnetic field that would cancel the energy accumulated into the core by the primary
the collapse voltage depends on your switching level of protection...and of course the resistance across the primary... the voltage that will apear across the primary during pulse off it V=I*R this voltage is many times able to burn your switch...
to prevent that the switch must be protected.. TVS and MOV techonlogy is available
to use them you must watch the amp rating and voltage rating i use them in series and parallel to increase the power proprieties
i get 1000v limit
remember you may be measuring the rms current so the peak may be more than twice your reading... watch this that can confuse you... the better way is to measure it on oscilloscope to get peak reading... if possible...
http://www.ieee.li/pdf/essay/practical_magnetic_design.pdf (http://www.ieee.li/pdf/essay/practical_magnetic_design.pdf)
(http://xtronic.org/wpcontent/uploads/2010/05/copperawgmetrictable.jpg)

more info to come
https://www.eeh.ee.ethz.ch/uploads/tx_ethpublications/dalessandro_selfcapacitance_HV_transformers.pdf (https://www.eeh.ee.ethz.ch/uploads/tx_ethpublications/dalessandro_selfcapacitance_HV_transformers.pdf)
http://www.ti.com/lit/ml/slup125/slup125.pdf (http://www.ti.com/lit/ml/slup125/slup125.pdf)

Power
To design a transformer is important to think about how much voltage drop there will be on it such that the load receive the proper power.
how do we do that?
well it has to do with choosing the wire gauge...
Normally a power dissipation of 5% can be allowed on the secondary and 5% on the primary so geting from worst case situation is possible to determine how much resistance can be allowed on the coils and so according to window size and crossection area of the core the number of turns must be calculated first... than a estimation of the lenght of wire needed for that turns.. than you go for a table and find the wire that fits that resistance for the estimated lenght of wire required for the calculated turns... than check if the wire will actually fit the transformer window... and your done..
the thing is the voltage drops must be calculated such that the primary has the right number of turns to provide the right power.
for example we need a 9v300ma output under 30ohm load
how much will be the open circuit voltage of the transformer?
well it depends...
but if you take as an example a reversed engineering case... my transformer from the probe
it has 3 ohms on the secondary and so a current of 300ma will create a voltage drop of 0,9v under maximum load... so the power dissipated in the secondary is 0,27w and the load 2,7w
this case 10% is allowed under load at the secondary... and probably other 10% at the primary...
this is a restricted current output transformer example... its made in this way to get such a voltage drop under worst case condition...
so we would need to design the secondary to have 9,9v at first glance to offer 9v 300ma to the 30ma load...
but there is something missing... the primary voltage drop!
we must account for the voltage drop in the primary at loaded condition to know the right number of turns that will compensate for the resistance of the wire...
is easy
you can consider basically the primary under load... assuming 3,3w at 230v it would result 14ma to get 14ma to dissipate 0,27 w is required a resistance of 1377ohms
the secondary turns than must account for the voltage drop of the primary before the value can be used.. so in our case... vdrop is 19,6v so the resulting voltage useable for induction was 23019,6=210,4v so
remembering the secondary must receive 9,9 v to give 9 under the rated load, you need to use 1: 210,4/9,9 turn ratio... 21,25
Now we only need to find the right core for this watts..

tables

wire types

There are some different types of isolation used for transformers...
there are at least 5 things to be considered, volume, dielectric strenght, max temperature, dielectric constant, compatibility with oil, price, specific resistance... availability, time....
The one that is most important is the availability, if you have few to choose, get the higher streghnt that fits your pocket
theres
nomex
poliproplilene
mylar
paper
oil
encapsulation
The air problem is only solved by encapsulating while outgassing or with oil but making the vaccumm to take the ar out of the coils (outgassing)... those are the best way to go... oil willgive from 10kv/mmm to 40kv/mm or higher isolation depending on the oil used... kitchen canola oil proved to be very helpful in the case you cant get proper mineral or even beter silicon oil.. oil conducts heat well and help maintain the coil stability.. it also has a greater dielectric constant than air.
Encapsulating resin can give higher voltage ratings but may be poorer in heat dissipation.. but is more pratic to use... the temperature of operation may be below 180 degree..
Polyester would melt at 130 also so is little dangerous...
the best options are nomex or paper and oil or combinations of paper plastic layers sold already for rolling motors and transformers
if the wire isolation survive 180 degree the isolation between layers must do the same
there are diferences in the comom resin and the resin for encapsulating also...

Why impregnation is needed in highvoltage?
Oil is the most used in the high voltage lines as it provides good heat dissipation and stability... it has from 15 to 70kv/mm strenght,
to Use oil is not simply to fill the transformer cavity with it...
oil must get into the coils to work because the all point is to get air out of the coil spaces...
air at high voltage ionizes and create a situation where the coils burn the insulators and than arcs
the air has a low strenght of 3kv/mm so as you reach this electric fields the air develop corona discharges around the coil destroying it
to get the oil to fill a process is made with vaccuum where the coil is dip into oil inside a vaccum chamber such that the air is forced to go out and that the oil is forced to go in when atmosferic pressure rises again...
the ideal is to apply vaccum first than dip the coil than bring positive pressure...
the same must be done when doing resine encapsulament
theres some different types of oils and resin for encapsulation
there is oils that are called parafinic that cannot be used bellow 0degress celcius
the ideal is to go for a liquid with good proprieties of isolation and low viscosity
i tried canola oil... and worked very nice... for reducing the viscosity i heated in mary bath ... during vaccum... the process last many hours (12) if you use paper as insulator between the coils because paper holds increadible amounts of air...is increadible how much air come out of it... i did a spĂral generator with copper foil and kraft and coils with 30 layers using kitchen cooking paper... you may need to add enought layers of paper to get the thickness you want.. better have a micrometer on hands..
the funiest part is cutting the isolators
i found that it can be used but is better to use nomex or some other high temperature isolator... or even polyester sandwiched with paper..
oil expands so dont encapsulate it without letting a breath for pressure to not rise inside the coil and it not explode..
oil can cause severe fire! be carefull specialy with high voltage...
there are epoxy resins specially developed for transformers where they prevent fire and conduct somewhat better the heat
i believe the lowest viscosity will be the ideal for filling...
the coils must have spaces for the air to come out and oil to get in to be able to do it faster and better. .

http://www.daycounter.com/LabBook/MutualInductance.phtml (http://www.daycounter.com/LabBook/MutualInductance.phtml)

current limit, reduce coupling, voltage drop

load

Internal impedance
impedance match
impedance of a coil
impedance of a sistem of coils

hh

dot

layers

parasites

more parasites

res

Vic
It works by charging the chokes and leting their field to collapse by making a secondary coil open the diode switch during pulse off..doubling the frequency of the applied voltage allowing to restrict the amps and allow voltage to go over and does the work in a dead short condition...
The relation of turns L2>Chokes allow the system to charge and collapse..
both chokes are in subtracting configuration with respect to the secondary...

The wave form you input in the primary is basically the same as the output in a well coupled transformer
if you apply 12v pulse on a 1:1 transformer it will develop 1 amp at the secondary if a 12ohm load is connnected to it in short terms disconsidering losses... this 1 amp will be constant given its within the power handling of the transformer
the primary however will have a current that increases from 1 amp and on to keep the voltage constant at the output...
the current theoretically increase as a linear ramp for a inductor with lowe resistance to inductance ratio
in the case of the transformer as the load current will depend on the load impedance and translate it to the source with the transformation factor squared proportion.. also as load resistance change the primary current is affected too... for example a charging capacitor as load will not provide a linear ramp of current at the primary because at the start the capacitor seems like a short than it looks as open circuit as it reach the output voltage of the secondary

yes is to be scared hahaha

Principal parameters of cores are
Bmax (Maximum flux density)
Crossection Area
Window (space for the coils and insulation)
Al (relates thepermeability of the core to the geometry, determines the inductance a coil will have for a number of turns of wire squared)
Gap
Magnetic path lenght
to determine the Al of a core make a coil with n turns measure the inductance and divide the measured value by n squared and you get the value of Al good to use for your measurement. Gap should be lowest or zero ... for toroids spread the turns over all the toroid as the gap is distributed in it... the more turns more precision.. but not too many turns are required... 10 or so is ok depending on core size... Al is normaly measured in nanoHenries or microhenries... the nucleus i have are 3000nH per turn squared
To find the Bmax of the core design a numbe of turns for a predetermined lower bmax say 0,100T and vary the voltage to see where the core saturates... getting the voltage where it saturated back to the equation will equate for the correct bmax of the core...
Common knowledge say Bmax of ferrite is 0,3T and silicon iron laminates 1,5T
Window will tell you the turns you can get into the core... for high voltage insulation a winding factor of 50% is assumed for some space is required for insulation... from this you must aproximate from the 50% remain space to the maximum number of turns you can get for a particular wire diameter... assume the wire to be square so you dont get fooled by empty space among round wires... so the window remain in mm2 divided by the wire diameter also square so in mm2... result is the number of turns.
When working with a core the lenth of the wire becomes less important than the turn numbers to get similar voltages is simple as getting the same number of turns regardless of the difference in diameter of the layer of the coil. Because the core closes the Bfluxaround the all coil.

http://scholar.lib.vt.edu/theses/available/etd07132006114120/unrestricted/Draft_after_ETD_Review.pdf (http://scholar.lib.vt.edu/theses/available/etd07132006114120/unrestricted/Draft_after_ETD_Review.pdf)

http://www.elect.mrt.ac.lk/EE201_resonance_mutual.pdf (http://www.elect.mrt.ac.lk/EE201_resonance_mutual.pdf)