Paul Hovnanian P.E. said:
Possibly worse packing and resulting higher leakage flux. If one wrap is
adjacent to another that is many turns away, the voltage between them
will be higher. This will result in higher voltage stress and perhaps
distributed capacitance.
Each turn going 'the other way' will have a reversed voltage induced in
it w.r.t. its neighboring turns. Do his enough times and the backward
turn voltages will cancel the forwards turn voltages. You'll have a zero
volt winding.
Actually, this might just be an astatic winding.
The simplest way of winding a toroid - going clockwise around the ring, and
stopping when the last turn is up agains the first turn - gives you a single
loop in the plane of the toroid, and an external magentic field. Taking the
"end"of the winding back around the toriod in the oppostie direction to the
progression of the winding, while still wrapping the wire around the toroid
in thre same direction as before, and taking it off the toroid only when you
get back to the "start" cancels this loop in the plane of the toroid, and
the external magnetic field.
There are more complicated astatic winding schemes for a toroid but this one
would work.
The problem with talking about toroids is that you have two different planes
to wory about - the winding around the body of the toroid has to keep on
going around the cross-section of core material in the same sense, to
maximise the inductance, but the progression of the winding around the
toroidal loop in the plane of the toroid itself really should reverse
half-way though the winding process (or in one scheme, a quarter of the way
through the proess and again at three-quarters of the way to completion).
