Re: 5HP rotory elect. phase control

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Dave Lewis by way of Terry Fritz <twftesla-at-qwest-dot-net>" <hvdave-at-earthlink-dot-net>

Steve,

Since the motor runs at constant RPM, a very simple model could be
used.   An inductor in series with a resistor would do the trick.   The
RL values can be obtained by measuring the amplitude and phase angle of
the motor current with respect to the voltage.   I think this a
measurement is necessary since there are so many factors to consider
that it just easier to measure it.

Once you have the current amplitude and phase angle you can calculate
the RL model parameters.  

                    V * tan(a)
      L = ---------------------------------------
           2 * Pi * f * I * sqrt( 1 + tan(a)^2 )


           2 * Pi * f * L
      R = ----------------
               tan(a)

Where:
L in Henerys
V in Volts rms
a is phase angle 0 to 90 deg or 0 to Pi/2 radians, take your pick
Pi is 3.142
f is 60 Hz
I is current amplitude measured in Amp rms
R in Ohms

For a 120V, 60Hz, and 1 Arms current lagging at 30deg I got L=0.159 and
R=104.

Other models are plausible and just as valid since the frequency is
always 60Hz.  It shouldn't matter which model you use as long as the
impedance at 60Hz is correct.  For instance a parrallel RL would work
too.

This is a simple model and will not address other issues such as start
up and loss of sync.  To do so would require a much more sophisticated
model that includes the rotor impedance and slip frequency among other
things.  These models typically require a host of measurements on the
motor to construct and probably not worth it in the end.

Regards
Dave Lewis




Tesla list wrote:
> 
> Original poster: "Steve White by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<slwhite-at-zeus.ia-dot-net>
> 
> Does anyone know of an adequate model for a motor while running? An
> inductor? I have been looking at John Freau's phase shifter circuit and it
> appears to be a bandpass filter with a center frequency of 60 Hz feeding the
> motor. If I could model the motor, I could analyze the filter
> characteristics. I presume that as the inductor (variac) is varied, the
> center frequency of this bandpass filter changes slightly. This would
> explain why you want to see a resonant rise. The presence of the resonant
> rise would indicate that you have implemented a 60 Hz center frequency
> bandpass filter. Since analog filters have a non-linear phase
> characteristic, this would also explain why the phase shifting action takes
> place. As you change the inductance, you slighlty change the center
> frequency of the filter and also changes the phase angle of the voltage. If
> I could model the motor, I think that I could analyze the filter
> characteristics and quantify the component values. I am attempting to remove
> most of the guess-work from selecting the capacitance value for a particular
> motor.
> 
> Steve: Coiling in Iowa
> 
> ----- Original Message -----
> From: "Tesla list" <tesla-at-pupman-dot-com>
> To: <tesla-at-pupman-dot-com>
> Sent: Wednesday, March 27, 2002 11:31 AM
> Subject: Re: 5HP rotory elect. phase control
> 
> > Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <FutureT-at-aol-dot-com>
> >
> > In a message dated 3/27/02 1:16:06 AM Eastern Standard Time,
> tesla-at-pupman-dot-com
> > writes:
> >
> >
> > >
> > > so this encouraged me to finish my 5HP 3600 RPM rotory project.
> > > I was a little worried about being able to make John Freau's phase
> > > control work on such a large motor, but it does work after all.  The
> > > optimal capacitance is around 85 uF using a 10 amp 120 volt variac for
> > > control.
> >
> >
> >
> > Hi Kevin,
> >
> > Nice !   I think the 240 volt operation lets the cap value be much
> > smaller than otherwise.  Perhaps 1/2 or 1/4 the size that it would
> > need to be for 5HP at 120 volts.  Your results also strengthen my
> > suspicion that the cap value requirements depend somewhat on
> > the motor load, not only on the motor HP rating.
> >
> > Cheers,
> > John
> >
> >
> >