help sizing toroidal PSU to stepper/drives

[irving2008]

Probably I'm wrong with my nema34 steppers but I've seen a lot of different Chinese specs and I do not trust them, and the ones sold in Europe and USA have the same Chinese specs sheet... and they are so optimistic compared to the good brands.
The 3.1Nm has too high inductance for my 42V drives.
I have missed the rotor inertia 560g·cm2 vs. 850g·cm2. High speeds without good acceleration are worthless. Will see them performing in real life... to many calculations for a hobby engineer.

yet we all use them because the inductance effect determines the corner speed of the motor, the speed where torque drops off, and a good mechanical design should never operate above the corner speed, at least not when cutting.

Any recommendation regarding the PSU power? I have read somewhere that 60% of the steppers/drives current is enough for a toroid. Is this true?
And another probably dumb question.. with 25A at 30V AC will I have 25A or 18A at 42V DC? .

Yes, it's generally true and often much less in practice. But you do need enough bulk smoothing capacitance to cope with the occasional peak demand.

25A @ 30V AC will give you 25A @ 42V DC

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[paulus.v]

Yes, it's generally true and often much less in practice. But you do need enough bulk smoothing capacitance to cope with the occasional peak demand.

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* )

[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.