help sizing toroidal PSU to stepper/drives

[irving2008]

With my maximum of 24A (6 motors at 4A) should I go with 500VA or 630VA and what capacitance do I need for smoothing?

On the webpage of the toroid manufacturer the calculations are slightly different, and I'm a bit confused, probably I should ask them.

Full wave bridge (FWB) With capacitor:
VAC = 0.8 x ( VDC +2 )
IAC = 1.8 x IDC
WATTS = VDC x IDC (output)
VA = 1.4 x ( WATTS + 2 IDC* )

Those calcs are for steady state load and make assumptions about power factor etc. to limit core heating and losses. If you go that route you'll end up with a massively overspec'd, physically large and overpriced transformer.

Steppers are nothing like that, the duty cycle is nothing like steady state. I don't know why you have 6 motors but I'd be surprised if you need more than 60% of the total load and most of the peakiness is handled by the capacitors.

So use:
VAC = (VDC + 1.5)*0.7
IAC = IDC
VA = IAC * VAC, then chose next largest

for 2 parallel secondary windings each is rated at IAC/2

The 1.5 is voltage drop across rectifier, can be 1, 1.5 or 2 depending on product.

So 24A @ 60% = 15A approx. = IAC (or 2 windings @ 7.5A each)
VAC = (42+1.5)*0.7=30V
VA = 30*15=450 so use 500 or 625VA

[paulus.v]

Thank you a lot for your help.

I willl go with 500VA.

Can you recommend the capacitors specs that I will be needing?

Steppers are nothing like that, the duty cycle is nothing like steady state. I don't know why you have 6 motors but I'd be surprised if you need more than 60% of the total load and most of the peakiness is handled by the capacitors.

I intend to build a lightweight 2 axis head for 3D foam milling and a rotary axis for wood.