----- Original Message -----
From: "2" <2@vc.net>
To: "Jeffrey Madore" <K1LE@arrl.net>; "AMPS" <amps@contesting.com>
Sent: Tuesday, July 23, 2002 12:42 PM
Subject: Re: [Amps] Induction Motors was: Muffin Fans and hot exhaust
>
>
> >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?
Good question, Rich. I was going to say yes, but I didn't want to answer so
quickly without taking a look at the type of motor that is often seen on
blowers (with attached 1-4uf capacitor).
On most single phase induction motors, the start winding (high resistance)
is electrically disconnected by a centrifugal switch, current actuated
switch, or other, when the motor comes up to speed. An alternating single
phase field will not start rotation but will maintain it once started. The
phase relationship between the start winding and the run winding is what
determines direction of rotation, and thus gives the ability to reverse the
motor, provided all 4 winding connections are wired out.
Interesting thing to try with a small (~1/2 hp) induction motor, is to
disconnect the start winding, energize the run winding, and quickly give the
shaft a spin. It will start and run in either direction, depending on which
way you spin it.
Another interesting thing to try is to jumper the capacitor on an unloaded
capacitor start motor. Since the start winding is quite resistive, the motor
will still start but there is a noticeable reduction in starting torque, as
there is less phase shift between the start and run windings without the
capacitor.
Getting back to the fractional hp motors often seen on blowers, as I
mentioned, I haven't spent much time working with these. So, I just put one
on the bench to take a closer look. This was a 1amp motor connected to a
Torin blower, with a 4 uf oil capacitor attached.
This type of motor appears more as a poly phase induction motor that
requires the capacitor continually to produce a phase shift for both
starting and running. When I disconnected the capacitor, the motor would not
start. Given a spin, the motor started but ran roughly. Without the
capacitor, the "no load" running current was high. When I loaded the motor,
the running current then became excessive. This is much like one would
expect with a 3 phase motor running on single phase.
When I ohmed out the leads, there appeared to be three windings of equal
resistance, as one would expect with a 3 phase motor. The windings were
delta connected with the "un-fed" leg of the delta broken, and the capacitor
inserted in series. I will in time, have to do some more experimenting and
dissecting, as I am now intrigued.
73, Rich,
Jeff - K1LE - CT ><>
> >
> >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.
> >
> >
> >_______________________________________________
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> >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
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