[Prev][Next][Index][Thread]

Re: Differential Expansion

Hi John,

	You bring up a good point that is not often mentioned.  Plastics and
especially paper/cardboard coil forms can expand and contract substantially
with temperature and humidity.  I always try to wind the wire very "tight"
so that it is under as much tension as possible.  Then I always (know that
I know better) coat it will many thick coats of polyurethane to insure that
windings will not come loose over time.  Operation and rides in the car are
very hard on secondaries and they really need to be very solid in order to
hold up over time.  My original research coil is still in heavy use but it
has needed re coating a number of times to fix abrasions and loose windings
due to expansion and contraction with humidity and temperature.  Some of my
small coils had their windings simply fall to the floor with a little touch
after the form had contracted and let all the wire loose.  If I were to
make another major secondary, I would strongly consider PVC insulated wire
that would stand up to hard use far better than fragile magnet wire....

Cheers,

	Terry


At 02:42 PM 10/22/1999 -0700, you wrote:
>Greetings All,
>
>If anyone is planning on winding a secondary coil larger than 6" you
>might want to consider the following.   I wound a nice tight 12" x 36"
>secondary coil using PVC pipe and #20 magnet wire.  When I took the coil
>off the winding fixture the temperature that evening was in the mid
>80's.  I stored the coil in an upright position figuring I would start
>the coating process in the morning.  The next morning the temperature
>had cooled off to about the low 70's.  I noticed my coil had gaps in the
>windings with some of the coils actually overlapping.  I was a victim of
>differential expansion.  PVC has a larger linear expansion rate per
>degree (roughly a factor of ten) than copper.  Although the delta T was
>not significant the results were still dramatic.  I was able to sort out
>the crossed windings and retention the coil to my satisfaction within a
>couple of hours by hand (uhg).   I believe this situation could have
>been avoided by either winding the coil during a lower than average
>temperature or by coating the coil immediately after winding.
>
>Bigger is better,
>-John Pepper
>