Graeme wrote:

I have a motor which runs on 16v AC. It has a double wound field coil,
and is more correctly a universal motor, as it will also run on DC, but
that is irrelevant, I think.

There are four terminals. Power runs from a transformer to A, which is
one side of the field coil.

From there, current runs through the field coil to B, and from B to C,
which is one brush. The other brush is D, from where current runs back
to the transformer.

To reverse the motor, wires are swapped, so that power coming out of the
field coil (B) goes to brush D (not C), and back to the transformer via
C, not D.

What I cannot work out is how to use a switch, or switches, to achieve
remote reversing of the motor. Ideally, I would have two push buttons,
one for each direction, but any switch would suffice. The switch(es)
need to be able to supply the power, and swap the way the brush caps are
connected.

Any thoughts on the switches I need, and how to wire them would be
appreciated. The motor only needs to run for about twenty seconds, in
either direction.


You need a DPDT switch (Double Pole, Double Throw) to allow (C) (D) to
be connected to either (B) or the transformer...

(fixed width font, ASCII art)



Transformer(1) --------------------- (A)

----------------- (B)
|
|
|
+-----o
| \o-------- (C)
Transformer(2) --- | -+- o
| |
| |
| +--o
| \o-------- (D)
+-----o


^ DPDT Switch



The DPDT switch could be either a physical switch, or implemented using
the contacts on a DPDT relay (then arrange push buttons to provide power
to energise the relay / motor).

You could use DPST push buttons, and two relays to achieve exactly what
you want....

Use one pole of each push button, in parallel, to energise the coil of
relay (1), the SPST contacts of which provide the switched power from
Transformer(1) to the field coil (A). This allows either push button to
provide the power to the motor.

Use the second pole of ONE of the push buttons, to entergise the coil of
relay (2), which uses the DPDT contact arrangement shown above.

Using this, either button energises the motor, so the motor will spin,
but one button will invert the wiring of (C)/(D) thus reversing the motor.