In article ,
Tom Veatch wrote:

On Tue, 20 Jan 2009 15:24:09 -0500, Andrew Erickson
wrote:

They do indeed reverse 120 times per second (60 full cycles per second),
but so does the field coil, so the relative magnetic polarity of the one
with respect to the other stays the same. ...

That's the way I understand it and I won't quibble on whether it's 60
or 120 time/sec. Kinda depends on the definition of "reversing". So
"reversing polarity" of the brushes - which I interpret to mean
switching leads - won't have any effect on rotation direction of a
universal or any other type of AC motor. That would be no different
than reversing the plug in the wall socket.

On the other hand, breaking the motor open, and doing some internal
rewiring might accomplish the mission.

Switching the leads to the brushes generally does require breaking the
motor open and rewiring it internally a bit. It's not the same as
reversing the plug in the wall socket (which, as you correctly observe,
would have no effect on the direction of rotation). Swapping the input
leads changes both the field (stator) and the rotor, and so the rotation
stays the same. Swapping the brush leads changes only the rotor, but
leaves the field unchanged, so the relationship between them is reversed.

Here's the a link to a quick schematic sketch of this operation on a
series wound motor that might make things a bit clearer:

http://users.gmavt.net/drewe/pix/misc/Motor.png

--
Andrew Erickson

"He is no fool who gives what he cannot keep to gain that which he cannot
lose." -- Jim Elliot