On Wed, 4 Jan 2006 10:14:13 -0600, "Robert Swinney"
wrote:

Don sez:
" Sure it does. With the idler spinning, a voltage is generated in
the third leg that is in quadrature to line voltage, even if there are
no capacitors anywhere. Transformer action can not produce a
quadrature voltage so it must be (and is) generated by the rotating
rotor field -- which always is in quadrature with the stator field."

I'm not sure what you mean, Don. You said "Transformer action can not
produce a quadrature voltage so it must be (and is) generated by the
rotating rotor field -- which always is in quadrature with the stator
field".

Firstly, I don't understand why the issue must be complicated by bringing
the rotor field into the picture.

Because it's there, producing phase shifts and emf's that cannot be
produced by a network of similar topology having only R's and L's.

It is well known the stator field and
rotor field are more or less locked into rotation at the same speed, but it
is incongruous to speculate the rotor field is solely responsible for the
stator field's third leg voltage. Remember we are essentially talking about
a single phase motor here with an open coil connected to the center point of
the line-fed main winding. I respectfully submit the third leg voltage is
not in quatrature with line voltage. The only way for that to be a true
statement would be in the special case of a precise amount of capacitance
connected from one line side to the end of the 3rd leg coil; an amount of
capacitance (start cap if you will) necessary to achieve an exact 90 degree
phase shift between line voltage and the 3rd. leg.

The rotor field is always in space quadrature from the stator field.
This is well-established in about any textbook on the subject. That
being the case, the emf it induces in the third leg is necessarily
in quadrature with the emf impressed by the line (and countered by the
stator field) in the other two windings.

Now consider a Y-connected motor. Rotate the Y 30 degrees clockwise so
the right hand leg is horizontal, with line connected across the
lefthand legs. Note that the vertical components of the excited coil
windings add while the horizontal components cancel. Therefore,
there is no emf induced in the horizontal third winding by direct
transformer action. Emf induced in the third winding is therefore
solely due to the rotor field -- and since that field is in quadrature
with the stator field, the emf in the third winding must be in
quadrature with the excitation voltage. QED.

The terminal voltage on a loaded third winding will vary from exact
quadrature due to I Z drops, which have opposite sense in the excited
windings from those in a loaded third leg -- look at the directions of
current flow. . But if you connect a scope from third terminal to
neutral (center of the Y) in an unloaded idler, it would show an emf
in quadrature with line voltage.

Credit to Jerry Martes for showing me this aspect of induction idlers.