Usually the run winding consists of larger wire than the start winding. Thus
the X/R ratio is high and current is lagging voltage.
The start winding is wound with smaller wire, thus it has a comparatively
high R/X ratio and current is then more in phase with voltage - or it could
be said that the current in the start winding leads the current in the run
winding. This is where the phase shift is developed.
The windings are then physically displaced on the stator so that the phase
shift results in magnetic rotational force.
Adding a capacitor in series with the start winding causes the current in
that winding to lead the current in the run winding to a greater degree, and
thus an overall greater phase shift will be seen between the two winding
currents, and greater starting torque.
Since the start winding is wound with smaller wire (high R/X) it heats up
very fast and is usually switched out when the motor comes up to speed.
I haven't operated on the 4 wire unswitched capacitor motor that you speak
of, Rich, (common to small blowers) but my guess is that the capacitor is in
place to provide current limiting to the start winding, thus eliminating the
need for switching. I'll have to take one apart.
As far as speed control on induction motors, line frequency control is the
only way to maintain torque. And, voltage needs to be adjusted with
frequency (volts/hertz) to keep the current within nameplate. With small
motors, crude speed (and torque) reduction can be had by adding a capacitor
in series with the line. Voltage adjustment alone will work to a degree, but
torque falls off exponentially with voltage.
Jeff - K1LE - CT ><>
> >There has to be some kind of start winding in any single phase motor.
> >It may be a shorted turn in a shaded pole motor ( small motor) or an high
> >value inductive winding for larger ones (which may stay on at all times
or
> >disconnected after motor starts to turn). But for high torque situations
> >you use a capacitor in series with a lower inductance start winding to
get
> >the motor started. The winding is used to determine the direction of
> >rotation. Other wise the motor may start to rotate in either direction if
> >at all.
>
> Semi-agreed. In my mind, on a 4-wire AC motor, an unswitched winding
> that is always in series with a 1 - 8 uF capacitor is a run-winding.
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