Re: Phase adjust (excessive PFC !)

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

Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 12/29/00 9:24:57 AM Eastern Standard Time, 
tesla-at-pupman-dot-com writes:

Brian, Richie, all,

Thanks for the info on these phase shifter issues.  I had not thought
about the over compensating case, except from the reso-rise point
of view.  It seems that a lot more concern will have to be given when
using these larger motors with the phase shifter.  BTW, I've seen
that fast braking effect in my 3 phase homemade converter for my
lathe, when I turn it off in the wrong way.  Richie's suggestion for
using a switch that disconnects the motor when the power is shut
off seems like a good idea to stop the fast braking effect.

What really surprised me is that Brian never found more than a
five degree phase shift, no matter what size capacitor was used.
I don't understand why such a limited phase shift would occur.  
Unless maybe the cap size is quite critical for the larger motors,
but I don't see why that should be.

John Freau
-

> > Original poster: "Basura, Brian"
>  > 
>  > Running with 337.5-405uf caused the motor voltage to rise dramatically
>  > .... Another thing which happened, and was repeatable, was the motor no
>  > longer coasted after the power was removed (it came to a stop like 
someone
>  > had put on the brakes). This was so pronounced that the highest capacitor
>  > caused the set screws to shear off due to the motor stopping so fast...
>  > 
>  > Regards,
>  > Brian B.
>  
>  Capacitors are normally connected in parallel across induction motors to
>  acheive some degree of power factor correction.  However,  one must be
>  careful when sizing the PFC capacitor, particularly with large motors or
>  when the load is capable of storing considerable mechanical energy.
>  
>  The induction motor can be represented by an inductor and resistor 
>  connected in series. The addition of a PFC capacitor across the motor
>  forms a LCR circuit which resonates at a particular frequency.  When the
>  motor is connected to the mains line,  the low impedance of the line
>  shunts the resonant circuit,  so there is no problem.  However,  when the
>  motor//PFC combination is disconnected from the line a high Q resonant
>  circuit is formed.
>  
>  When the power to the motor is turned off,  the residual field of the 
>  rotor causes the motor to _generate_ an alternating voltage at its
>  terminals which is proportional to the speed.  If the motor is slowing
>  down, then it's output frequency gradually falls.
>  
>  If the PFC capacitor is less than critical size,  the resonant frequency
>  is above 60Hz so there is not a problem as the motor slows down.
>  However, if the motor is over-corrected with a PFC cap which is larger
>  than necessary,  then the resonant frequency is below 60Hz.  As the motor
>  slows down,  a resonant condition results in high voltage and current in
>  the motor//capacitor loop,  accompanied by high braking torque.  The
>  potential for damage is greatest in the case of large motors because they
>  can store more energy,  and they use thicker wire making a high Q
>  circuit !
>  
>  For this reason, induction motors should always be under-compensated,  and
>  never critical, or over-compensated.  This ensures that a resonant
>  condition cannot occur as the motor coasts after power down.
>  
>  This behaviour is explained in more detail here:
>  
>  http://home.clear-dot-net.nz/pages/lmphotonics/pwrfact.htm
>  
>  Look under the section on STATIC CORRECTION.
>  
>  It suggests a method to alleviate the resonant condition by using a
>  contactor to break the PFC cap//motor loop after powering down.  This may
>  be a suitable solution for the phase adjuster ???
>  
>  My appologies for the long post,  but I have not seen this behaviour
>  mentioned on the list before,  and it seems appropriate now.
>  
>                           Cheers,
>  
>                           -Richie Burnett,
>