Power transformer question? copper wrap count...?

Question

I fully understand the transformer function and design, i also understand the ratio count on primary/secondary cables. I also understand the need for many wraps on a transformer to control current.. (ex a 1:10 Ratio transformer can not be biult with primary having 1 wrap and a secondary having a 10 wrap, it needs to be like 100 pri and 1000sec wraps.). But my question is how do I know how many wraps to use in the ratio conversion. **Again I understand the ratio count and its conversion toward voltage stepping, but dont understand how to know how many wraps i need to use for the primary in the first place to use the ratio.. I assume is a number to control the current flow.

Answer 1

Another factor in selecting the number of turns is the "volts per turn" on the primary winding.  This determines the voltage stress between two adjacent windings.  For a utility distribution transformer, the turn-to-turn voltage might be in the range of 10 to 30 volts.  Let's assume 30 volts per turn.

Assuming a 7200 volt primary, you would need 7200 / 30 = 240 primary "wraps".  The insulation between two windings only needs to support 30 volts.  This is why transformers can often get by with just a costing of enamel or varnish as insulation on the wire.

Answer 2

The number of turns on the primary depends on the magnetisation current that you want (the current with no load on the output). This, in turn, depends on how close to saturation you want to drive the core. Start with a peak of 4,000 gauss for most core materials, and size the primary to give this amount of B field for the frequency and voltage of concern. Use the fomula

Phi = N*Current/Path Length

Edit:

Nice job, godzyx, this is a much more complete answer.

Answer 3

Go to this web site HOWSTUFFWORKS.com they have a full diagram that may hep explain this better for you.

Answer 4

You start with the amount of power you will need to push through the transformer, and the frequency of operation (or frequency range).  That will dictate what kind of core material you will be using.

Then, you go to the manufacturer of the core material and ask them for their formulas for windings.  Depending on the power requirement, current requirement, and frequency -- those formulas will pretty much tell you how many windings you will need in the primary.

Here is an example for powdered toroids and E-cores:

http://www.mag-inc.com/pdf/2006_Powder%2...

http://www.mag-inc.com/powder/2006a_Powd...

(formula and tables at bottom)

Another way is by trial-and-error.

I've wound transformers, both ways.

.

Answer 5

I hate to dive in when so many top contributators have answered.

The actual number of turns depends on the General Transformer Equation:

V_1 = 4.44 f N_1 φ_MAX

where:

V_1 = primary voltage

f = frequency

N_1 = number of turns on primary

φ_MAX = maximum flux of transformer core.

So the primary voltage and frequency are typically fixed for a given location based upon the electric power company.

Therefore the only true variable is the maximum flux that the core material, typically steel, can handle.

Now flux is given by Magnetic Circuit's Ohm's Law

φ = F_m/R_m = NI/R_m

where:

φ = flux in Webers (Wb).

F_m = Magneto-motive force in Ampere-turns = N I.

N = number of turns

I = current in Amps

R_m = reluctance in A-t/Wb

So the number of turns and reluctance are fixed for a given magnetic circuit.  So flux is directly proportional to current, which means that flux increases as current increases, until the core material is saturated or maximum flux is reached.  

Saturation can be thought of as a fully wet sponge.  Add another drop and it flows off.  So current added beyond saturation is wasted.

So once an iron material is selected for the transformer core, you can determine the maximum number of turns on primary side.  Then the number of turns for secondary side can be selected using the transformation ratio for step-up, step-down or isolation.

Actual current flow depends upon physical size of transformer and wire used.  Big transformer supplies big currents 8).

Hope this helps...