>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,
This is as I have measured.
>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.
Once the motor is running, can the high-resistance winding be
disconnected?
>
>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.
>
None of the centrifugal blower that I've seen switch the high-resistance
winding 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.
Agreed
>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.
>
Tnx, Jeff
>
>> >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.
>
>
>_______________________________________________
>Amps mailing list
>Amps@contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
>
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
end
|