Re: [CAD_CAM_EDM_DRO] Re: What size capacitor should I use?

cnc_4_me wrote:

>Attn CNC power supply people...Please DO NOT put a resistor between
>your bridge rectifer and your capacitor...Because of the time
>constant increase (T=RxC)x5, you will render the recovery of
>capacitor voltage to be to long in time to be of any use to PWM
>drives...
>
>T = time in seconds
>R = resistance in ohms
>C = capacitance in farads
>X 5 = 6 time constants for a full charge
>
>example... Power supply has 30,000Uf and you add a .5 ohm resistor.
>C = 30,000Uf is 10,000 to the minus 6 farad.
>R = .5 ohm
>T = C x R
>T = .03farad x .5 ohms = .015 seconds for 1 time constant.
>It takes 5 time constants to charge a capacitor...So..
>5 x .015 seconds = .075 seconds for capacitor to recover...
>.075 seconds = 75mS = 75000uS
>
>We are PWMing around 22khz or 22,000 cps..At this frequency each
>chopped wave can be as small as 1/22,000 = .0000454 seconds...Or..
>.0454mS....Or 45.4uS
>
>

Does your 30,000 uF capacitor really discharge in 45 uS? I'd like to
see that.
Even the big spot welders can't dump an aluminum electrolytic that fast.

>With resistor in circuit it takes 75mS to recover and we need a fully
>charged cap in .045mS....Adding a .5 ohm resistor makes the system
>
>75mS/.0454mS = 1670 times to slow.
>
>

At 60 Hz, the power line only peaks every 8.33 ms (full wave rectifier).
So, the caps are only (re)charged every 183 PWM cycles with no series
resistance (but, of course, there always is some). In fact, that's the
WHOLE
PURPOSE of the filter caps, is to act as an energy reserve between power
line cycles. Depending on the power supply load, 0.5 Ohms might be too
much, or too little. About the worst case would be a little worse than if
the whole system was DC. So, a 10 A load would drop 5 V across the
resistor.
Since the line voltage isn't present continuously, the cap bank voltage
might dip
7 or 8 volts at a 10 A load. That may be too much, or may be acceptable,
depending on the voltage reserve between the normal voltage and the worst
case demand of the servo amps or stepper drivers.

Jon