RE: Phase adjust (excessive PFC !)

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Original poster: "R.E.Burnett by way of Terry Fritz <twftesla-at-uswest-dot-net>" <R.E.Burnett-at-newcastle.ac.uk>


Hi Guys,

Maybe I can shed some light on what might be happening here:

> 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,