On further reflection I've concluded that the only way to get more magnetic flux into a single secondary is to increase the applied
voltage to a single (equivalent) primary coil.
If you reduce the number of primary turns, you'll
increase amp turns, provided you don't change input voltage.
Halving e.g. the number of turns will divide primary inductance
by roughly a factor of 4, increasing the current by the same
factor. So you'll double amp turns.
You certainly don't want to overload the driving bridge,
so the pertinent question is to match the bridges voltage
and current capabilities to the primary coil.
When driving e.g 4 primaries in parallel, it doesn't make
any difference, if you think of the primaries connected in
series and driven by 4 times the voltage, since the voltage
will split up into 4 equal parts as long as the primaries
behave equally.
Assume the primaries are coupled by k=1. The inductance of the
series circuit will then be 16 times as much (square of total turns)
as a single coil. The current will then be proportional to
4/16 (=total voltage/total inductance) of the current of a single
coil.
The current in each of the paralled coils is 1/4 of a that of
a single coil. The current can be seen as shared in this case
by the 4 coils driven in parallel.
If the primaries are uncoupled (k=0), then the series inductance
will be 4 times that of a single coil and the current will be the
same in each coil as in a single coil.
In practice, the situation is a mixture of the extreme cases of
k=1 and k=0. More coupling between the primaries will reduce
the current, so that you'll need to reduce the number of turns
to load the bridges appropriately.
Steve Wards and Jeff Larsons Gigantor http://www.pupman.com/listarchives/2011/Sep/msg00024.html
used a set of 4 paralleled primaries. I believe they found it
more economical to use several less powerful transistors
instead of a single monster bridge to achieve their
power goals.
Udo
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