Re: LARGE DC MOTOR speed control question

One thing you might want to take into account is the lousy power
factor on the 120 VAC line with this large of a capacitor. All the
current drawn would occur very near the top of each line half cycle
and could easily exceed 100 Amps per pulse.

My understanding is electric meters "charge" you for a poor power
factor load, which in this case would look like a 100 Amp draw
instead of a 17 Amp one.

A 10% ripple voltage is more reasonable from a power factor point of
view. Use this equation for calculating capacitor size for 10% ripple
at rated current:

C = ( I * 80,000 ) / V

Where C is in microfarads, I is the current you want and V is your
power supply voltage. For your case the result would be 8,000 uF.
Use 200 VDC rated capacitors minimum.

Mariss

--- In CAD_CAM_EDM_DRO@egroups.com, Jon Elson <jmelson@a...> wrote:
>
>
> STAN MCDONALD wrote:
>
> > Someone stated putting a cap across the output of the rectifiers
to
> > increase the voltage closer to the 180 that the motor needs
> > because when fed with 120 volts it comes to 130 rectified. Since
my
> > motor is a 3 hp dc 180 volt arm and 90/180 field and i have 180
> > arm and 180 field, and the motor draws 17 amps (name plate). What
> > value of cap do I need and can I out the cap on the input of the
> > rectifiers instead.
>
> Roughly 140,000 uF will give about 1 Volt ripple. You could
probably
> tolerate
> several times that, so probably as little as 5000 uF will work fine.
> If you put the capacitors on the input, you will blow breakers or
the
> caps.
> They will try to remove all AC from the line, thereby drawing huge
> current.
>
> >
> > When I measured 240 AC rectified I got a reading of 280 and
clearly
> > this is way more then the 180 that the motor asks for and the
> > motor will run but she gets warm enough you can fry eggs on it.
>
> Right, you need to rectify 120 V AC to get approx. 170 V DC.
Rectifying
> 240 V AC
> will give about 340 V DC. With the large current draw, you might
use a
> 240/120
> step-down transformer before the rectifier.
>
> Jon