routercnc_MK3

[Jonathan]

Silyavski,
Thank you for that link - very good reading. Based on Jonathan's comments and your selection of the 40,000uF model then based on Neil's suggestion then you might only be getting 10,000uF (10,000uF x 2 /2).

Sounds like it to me.

Any issues with running like that on your machine? If I use my board as-is then I'll only get 15,000uF.

Sizing the capacitors is quite tricky - there are formulas floating about on forums, but prizes for finding one with some derivation or reference, not just a rule-of-thumb. You can easily calculate the voltage ripple on the capacitors for a known current (V=I/(C*f)), but what current do you use and what voltage ripple is acceptable? We can get a reasonable idea of the input current from the motor power ratings, but even then it's a bit of a guess as to what voltage ripple the stepper driver will tolerate. You could look at it in terms of what voltage (and hence current) ripple the capacitors will tolerate, or define some minimum input voltage from the transformer you'll allow and go from there. I'd probably go with the former...then take an educated guess!

For a while I had my machine running 4 motors from a 500VA transformer with 11000uF capacitance and I noticed an improvement in obtainable feedrates when I moved one motor to another (320VA) transformer and left the original transformer and capacitors with just 3 motors. So maybe 15000uF is fine ... bit subjective though as I didn't take any readings to verify.

As for modifying the board to get the full 60,000uF - the tracks are 'painted over' ; either that or it is a multilayer board because apart from the solder points there are no exposed tracks on the top or bottom. I'd probably have to demount the capacitors and run flying leads to them from the output of the rectifier.

That's a shame. Do you have a working CNC router, or is this your first? If you want I can send you the dxf to make these ... it's very simple:




Or we could do some sort of swap.

Mains was 243V AC and each secondary coil was 26.4V AC

That's normal - they are a bit high as the rating is when loaded.

I need to wire the secondarys in series but cannot find any markings on the cables to show which is the start of the winding. I've read that it does not matter how they are connected (unlike parallel which is vital). Can someone confirm this is the case?

Not the case. If you get them the wrong way round, the voltages will cancel out and you'll get approximately zero. I would guess that the order they are written on the label is indicative of their polarity, so try connecting red to orange and measure the open circuit voltage between black and yellow.