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What is the effect of having opposite polarity in primary and secondary coil?. Would this result in the current in both windings propagating in the same direction? If so, would the cemf from the secondary to the primary result in a negative voltage across the primary (change of polarity) and the primary then be a series assisting voltage source?

If this isn't correct could you please explain what happens.

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    \$\begingroup\$ Since the transformer is passing an AC voltage from primary to secondary, winding polarity does not matter. In a flyback power supply, the polarity matters. \$\endgroup\$ – Sparky256 Apr 16 '18 at 5:44
  • \$\begingroup\$ I actually meant polarity. Got a bit confused. Sorry about that. I have reworded the question. \$\endgroup\$ – user1897830 Apr 16 '18 at 6:55
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There is no significance of the apparent winding direction of primary and secondary in a transformer, apart from the polarity. There is no additional performance difference. Consider that you can reverse the winding direction simply by calling 'the other end' of your winding 'the start'. Changing the name of something doesn't change the way it behaves.

When you're building something for which the polarity of the transfer between primary and secondary is important, within an oscillator, feedback system, or for a flyback, you control this by making sure you know which wires are 'starts' and 'finishes' of the coils.

Just a few people, who usually hang out on the tin foil hat and free energy forums as well, do pay attention to the 'helicity' of windings, and ascribe magical properties to this aspect of Tesla coils and wireless power transfer setups. You can safely ignore them.

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What is the effect of having opposite polarity in primary and secondary coil?. Would this result in the current in both windings propagating in the same direction?

Consider how a transformer works in its simplest form: -

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Source

With both primary and secondary wound together and in same direction, current exiting the secondary is in the opposite direction to that load current entering the primary. This means both load-related fluxes cancel in the core leaving only the primary magnetization current.

For this simple exercise in transformer action you can ignore the mag current.

If I wound the secondary spiralling in the opposite direction to the primary the output voltage would be reversed and the current would be reversed but still, the fluxes cancel due to the opposite way the secondary was re-wound.

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