Appreciate your response. Thank you. I will just need to rewind the primary of microwave oven transformer which I had damaged while removing the secondary. The existing secondary was aluminum 17AWG. I was thinking of going with copper of the same gauge. The previous had 2cm air gap on the primary from the core. How important is that ?

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Hi,

You're welcome :-)

I guess you mean the distance between the primary winding and secondary winding or do you mean the distance between the primary coil and the metal of the core itself?

If this is the distance between primary coil and secondary coil then that distance has two main effects. The first is isolation between the primary and secondary wires to keep them physically apart and that makes the transformer safer for humans to use. The second is the leakage inductance, which aids in filtering the DC when using a rectifier on the secondary but also could act to reduce voltage somewhat, so it's a tradeoff.

The formula for the minimum number of turns comes from this transformer equation which is easy to remember:

Bmax=E*1e8/(4.44*FAN)

where

Bmax is the max flux density in Gauss,

E is the AC rms voltage,

F is the frequency in Hertz,

A is the core area in square centimeters,

N is the number of turns.

Since this is the actual B and core material can only go so high and there must be some slack, we might go with Bmax=15000 Gauss rather than the 20000 Gauss that the laminations are probably good up to. We might even go less than that, like 10000 Gauss but of course the lower we go the more turns we need.

A is in square centimeters and the conversion factor for inches to cm is 2.54^2 so we end up with this formula:

Bmax=1.55e7*E/(4.44*FAN)

and here A is in square inches. Lumping the constants, we end up with:

Bmax=3.5e6*E/(FAN)

which is the simplest form.

At 60Hz this comes to:

Bmax=5.8e4*E/(AN)

and at 50Hz this comes to:

Bmax=7e4*E/(AN)

So here we have the formula with A in square inches and at 50Hz.

The design goal should be Bmax=15000 tops.