On Tue, 20 Jan 2009 15:24:09 -0500, Andrew Erickson
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:
"He is no fool who gives what he cannot keep to gain that which he cannot
What I intended was to swap the wires to the two brushes
(this is easy to describe, there are other rewirings that would
accomplish the same thing). Because the brushes activate
the ROTOR magnetization, the swap would reverse the rotor
poles without changing the other magnetic part, the STATOR.
That accomplishes the reversal of a brush-type (universal) motor.
Think of the (temporary N) pole on the rotor, pulled clockwise;
if you make it instead a (temporary S) pole, the attraction
becomes repulsion and the motor goes counterclockwise.
Swapping the wall socket polarity would reverse both rotor
and stator fields, so has no effect on rotation direction.
You most certainly CAN reverse a single phase motor. On some motors
the connections are brought out to make it simple, on others you would
need to dig a bit.
My lathe has a 1 HP single phase cap start motor that can be started
in either direction.
Almost any single phase induction motor will run backwards if spun
fast enough backwards before power is applied
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