Re: [CAD_CAM_EDM_DRO] Power Supply and Capacitor Questions Help!!

rogersmach wrote:

> Hello,
>
> I'm in the process of assembling a CNC control consisting of the
>following: 4 axies, 3-40 in/lb DC brush type servo motors that are
>rated 40v/K at 2500 rpm 13.7 amps maximum current. 1 servo motor for
>the 4th axis which I have not obtained yet but will be smaller. I'm
>using Mach2 for a control program. Bob Campbell's Sound Logic
>breakout board for Mach2. I'm using Gecko 320 drives. I'm using a
>power supply from the old control of my bridgeport which has 4 set of
>output taps on the secondary and their voltage values are: 1 set at
>31 volts AC, 1 set at 10 volts AC, 1 set at 4 volts AC and the other
>set read 0.
>

The 31 VAC winding will produce 43 V DC (1.414 X RMS voltage is
the peak, which is what the rectifier captures.) If you put the 10 V
winding
in series with the 31 V, you get 41 V AC, which will produce about 58 V
DC. That sounds pretty good. Of course, the 10 V winding must be
capable of providing sufficient current. This is assuming that the 10 V
and 31 V windings are completely isolated, and not just taps on one
long winding.

> In talking with Mariss at Gecko drive (brilliant man,
>hate to bother him), he suggested that I use about 60 volts DC with
>his drives. Which would reduce the speed with little effect on the
>torque (if I understood him correctly). And I would need a 100 volt
>DC capacitor rated at 45,000 macrofarad or greater. I could not
>locate a cap with this value and had to settle for 3 100 volt
>computer grade DC caps at 15,000 macrofarad. Question: Can I wire
>these 3 capacitors in series to achieve the same Macrofarad value?
>

No, you want to wire them in PARALLEL. Series would give you a
5000 uF cap at 300 V.

> I
>intend on using a 1.2k 5 watt bleed resistor on each. Next Question:
>Would it be possible for me to run the 31 volt taps through one
>bridge rectifier and 10 volts through another bridge rectifier, wire
>them in parallel to get my output up to around 41 volts DC?
>

It is much simpler to wire the AC windings in SERIES, then use one
rectifier. This requires the two outputs to be isolated from each other.

> Is it
>possible to change the configuration of the primary side and get
>higher values?
>

Maybe, depending on how the primary is configured. If the primary
winding was originally a 220 V winding, and you are running it on
120 V, that may be possible to turn it up. It may go too high, however,
for your needs. (twice the 31 V would be too high for the Gecko drives.)

> I intend to use the present supply temporarliy to get
>operational, then acquire a more adequate supply later. At the 31
>volts it appears to be pushing the X axis (servo running directly off
>of bridge at 31 volts DC without capacitor
>

DO NOT DO THIS! This will blow the capacitor in the Gecko, at least,
in VERY short order! You MUST have decent filtering of the DC supply
to limit the ripple current to safe values inside the Gecko. Wire up those
caps and install them.

> and a 2.5:1 reduction)
>about 30 inches/min Id like to get it around 70 or 80 inches /min
>rapid. Any more than that is unsafe on a bridgeport. Any input on my
>inquiries would be appreciated.
>
>

I have run my Bridgeport up to 120 IPM, but never feed that fast for safety
reasons. (My reaction time is JUST so fast.)

Put the capacitors on the power supply, and I think things will go a LOT
better. It is amazing the drives work at all. Stepping up the DC voltage
will help, of course. With 40 V/ KRPM, you will get just about 1000
RPM out of the motors. With 2.5:1 reduction, that will give a leadscrew
speed of 400 RPM. Assuming Bridgeport 5 TPI screws, you should expect
80 IPM. Without the filtering, of course, the motors have much less than
40 V available most of the time.

Jon