[Amps] Induction Motors was: Muffin Fans and hot exhaust

[2]

>
>----- 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. 

I have seen air-compressor motors with a centrifugal switch/start 
winding, but I have never seen a centrifugal blower motor with such a 
switch.  Air-compressor motors appear to draw c. 5-times the rated watts 
until the centrifugal switch opens - so I doubt that the start winding 
has high R.  

> 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.
>
done that

>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. 

Agreed. which is why I call the higher-R coil the run direction winding. 

>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.
>
Thanks for the R&D, Jeff. 

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
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