[GreenKeys] Why Dual Coil Electromagnets?

[jhhaynes at earthlink.net]

I'm sure it's just a matter of convenience.  Looking at the
fundamentals, you are going to have a nearly-closed magnetic
path in iron, with a little air gap where the armature moves.
The magnetic path may be U-shaped or W-shaped or cylindrical
or whatever, but it is nearly a closed path.  Then for a given
amount of air gap and a given amount of magnetic pull you get
a definite amount of amp-turns for the winding.  Specify the
current and you have determined the number of turns.  If you
are designing a relay to operate on a certain voltage then
the current and voltage determine the resistance and that
determines the wire size.  Otherwise you'd like to use as big
wire as you can, limited by physical space and the cost of
copper.  Now you can distribute those turns just about any way
you like over the closed magnetic path.  If you are making telephone
relays you want to be able to put a lot of them side by side on
a panel, so you make them with a single slender winding.  If
you are making old-fashioned telegraph sounders you don't have that
much of a space constraint so you distribute the turns over two
windings.  Having two windings makes it easy to have a choice of
connecting them in series or parallel for different operating
currents.  But if that is important you can just as well put two
windings on a single spool and connect them in series or parallel.


jhhaynes at earthlink dot net