ElectronicsTransformers

Everything you wanted to know about Transformers!

Other Transformer Information

• [http://www.geofex.com/Article_Folders/xformer_des/xformer.htm]
• [http://www3.telus.net/chemelec/]

"The Following information was archived from http://home.att.net/~allencoilpage/xformproj.htm on April 1st 2008"

This is a basic starting guide. See my links page for sites with the heavy details. The examples are for transformers with 2 secondarys connected in series.

1. Measure the transformer core. There are two main considerations for core size.

 Winding window. For E I or EE cores there will be a certain amount of space for the windings. Since home made windings usually take more space than factory windings, space may become critical on small cores. One way around this is my stretch core shown above. This problem will show up in the discussion about materials below.
 The other main consideration is the core cross section area measured in square inches. This is only the leg of the core that actually goes through the primary windings. A small neon transformer may have a center leg only 1.22" X 1.75" = 2.18 square inches.

2. Calculate primary turns. Small NST example: core area 1.25" X 1.75" = 2.18 sq. in. N = (E X 10^8) / (4.44 X F X A X B) Where F = frequency A = area in sq. inches. B = magnetic flux assume 60,000 E = voltage N = (K X E) / A Where K = 6.50 when f = 60 hz or K = 7.507 when f = 50 hz 6.50 X 120Volt = 780 780 / 2.18 Sq. inch = 357 turns approx. turns on original - 300 - 325

X Ray transformer example: core area 3.2" X 3.2" = 10.24 sq. in. 6.50 X 120 = 780 780/10.24 = 76.17 turns approx turns on original primary - 76

Microwave oven core example: core area 1.55" x 2.55" = 3.952" 780 / 3.952 = 197.36 turns approx turns on original primary - 130

 No wonder small transformers always have so many primary turns. Bigger cores require fewer primary turns.
 Manufacturers may not always use the full # of turns. On some NST's and MOT's there have been fewer turns than calculated. On the Xray transformer they used the full required turns. For full output one should try to get as close as possible.

3. Calculate secondary turns.

 Small NST example:  350 primary turns

Turns Per Volt = primary voltage / primary turns Nst 120 / 350 = .342 TPV XRay 120 / 76 = 1.57 TPV

Say the desired output voltage is 15Kv or 7500 volts per secondary. Therefore, turns secondary = output volts / tpv NST 7500 volts / .34 TPV = 22058 turns XRay 7500 volts / 1.57 TPV = 4777 turns

Here are some more formulas to help calculate voltage and current.

E = Voltage I = Current(amps) S = secondary P = primary T = turns Voltage and Current EP = ES X IS / IP IP = ES X IS / EP IS = EP X IP / IS ES = EP X IP / IS

Voltage and Turns EP = ES X TP / TS TP = EP X TS / ES ES = EP X TS / TP TS = ES X TP / EP

Current and Turns IP = IS X TS / TP TP = IS X TS / IP IS = IP TP / TS TS = IP X TP / IS

Materials:

Core Size:

 Bigger is better:

1. Increased core cross-sectional-area allows less turns to be used for

   the same voltages at both windings. (More volts/turn.)  This reduces
   resistive losses. Makes it much easier to wind.

2. Increased winding window allows thicker wire to be used. Also

   reducing resistive losses.

Wire size: Primary

 Primary wire should be magnet wire. Other types can be used, hovever there are considerations. The insulation should be as thin as possible. The insulation should be able to handle the temperature. Wire wound on the core will get hotter than normal. Insulation should be able to handle some substantial voltage stress.

Magnet wire fits the bill on all. The drawback is that larger sizes are not as easy to find and costly. Choosing the size is of course dictated by the current. There are plenty of charts around. Usually 12 AWG magnet wire can handle up to 20 amps for short runs. If using PVC coated wire, bear in mind that the wire will run hotter. So don't rely on temp rise charts. Secondary

 For me, #34 AWG is about the smallest wire I can work with. This is fine for smaller transformers. A typical 15Kv 30 ma. NST would have 38 to 40 AWG wire. So using 34 - 32 AWG should make a very robust secondary.
 Larger transformers up to 5000 watt transformers can use up to 28 AWG.

Wire chart for common secondary wire. 40 Awg - 44 mA 38 Awg - 74 mA 36 Awg - 117 mA 34 Awg - 187 mA 32 Awg - 304 mA 30 Awg - 477 mA 28 Awg - 761 mA

Interlayer insulation:

 Artist Craft paper works great for transformers in oil. Don't use cheap craft paper for mailing packages. Artist paper is low in acid and made out of finer, better quality pulp.
 If the transformer is to be used dry then better precautions are needed.
 Polyetheleyne is an good, cheap material. A draw back is a low melting point. I have tried this but I didn't really like it. The main reason is that I want the inerlayer insilu to have some stiffness to support the new windings. Paper is thin but gives good support. In the stretch core I used adding machine tape, soaked in polyurathane. Then as the windings are being put on I brushed on poly. It sounds like a big mess but it was not to bad.

Mylar , teflon They works fine. May degrade in oil.

Example - Max voltage for each secondary is 7.5Kv for a 15000 volt transformer. Heavy build magnet wire insulation is rated from 4 - 7KV. On my Neon transformers the insulation is only ~.002" thick.

 If you need to conserve winding space use thinner insulation in the inner layers( ~003") and thicker insulation(~.007) on the outer layers where the voltage is higher. Any voltage spikes fron a Tesla coil will zap the outer layers first. Extra cautions here will help. On the XRay transformer about 10 outer layers were space wound, getting gradually wider spacing on each layer. On my X-Pig transformer there was plenty of room for the windings so I used 2 layers(.014") of craft paper on the outer 10 layers.

Real Important: Leave enough empty space on the ends of each layer to prevent flashover. Don't under estimate this problem. Leave "At least" , .25" up to .75" on heavy duty transformers.

More Formulas

Reactance

 Reactance in a circuit is the opposition to an alternating current caused by inductance and capacitance, equal to the difference between capacitive and inductive reactance. Expressed in Ohms.

Inductive Reactance X(L) Inductive reactance is that element of reactance in a circuit caused by self-inductance.

X(L) = 2 X 3.1416 X Frequency X Inductance(Henrys) X(L) = 6.28 X 60 X .1(100 millihenry) X(L) = 3768 X .0001 = 37.68 ohms

Capacitative ReactanceX(C) Capacitative reactance is that element of reactance in a circuit caused by capacitance.

X(C) = 1 / (2 X 3.1416 X Frequency X Capacitance) X(C) = 1 / (3768 X .00001) 10 uf X(C) = 26.53 ohms

Voltage drop calculations - Inductance negligible V = Drop in circuit voltage R = Resitance per foot of conductor(Ohms/ft.) I = Current in coindutor(amperes) L = length of conductor(feet) D = Cross sectional area(Circulir mils) K = Restivity of conductor

     K = 12 for circuits loaded with more that 50% capacity
     K = 11 for circuits loaded with less than 50% capacity
     K = 18 for aluminum conductors

     V = (2K X L I) / D