help sizing toroidal PSU to stepper/drives

Hello all,

I already have the stepper and drives and I'm not sure about the PSU current and voltage.

My four steppers (chinese) have the following specs (if real): nema34, 66mm length, 4A, 0.6Ω, 1.6mH and the drives (chosen to fit my budget) are based on the THB6064 chip: 4.5 A absolute; 4.0 A operating range; 50V absolute maximum; 42-45V? operating.

The toroidal transformer will be manufactured to my specs.
How about two sec. windings at 33V and a total of 630 or 750VA?

Why two windings... I will most probably upgrade the drives to 80V in the future and it will be nice to be able to reuse the toroid.
I also want to build a B/C head or a rotary axis soon and to be powered from the same PSU.

Any help will be greatly appreciated

I bought mine here;
Chassis mounting toroidal transformers

Two 33v windings in series will give you 66 * 1.414 = 93v DC which is too much.

A 625VA 2x24 would be better, this would give approx 34v dc in parallel, 68v dc in series
Standard Range Toroidal Transformers: CM0625224: 625VA 230v to 2x24v

Why Nema34 and not 23s?

33v secondary will give 43 to 48v on supply variation which doesn't leave much room for regenerative dumping on a 50v drive. I'd go for 30v secondary.

Don't see how you'd reuse the toroid on an 80v drive. 2*33*1.414-1=92v +/- 5%, you need 2 x 27v secondaries for maximizing an 80v drive.

When I'll need to reuse the toroid for 80V I'll take out some wire from the secondary.

Quote:

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Originally Posted by irving2008

Why Nema34 and not 23s?

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Not sure. I think it will give me more torque at high speeds (1.6mH nema34 vs. 3.5mH nema23). Plus the robust 14mm shaft.

Quote:

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Originally Posted by irving2008

33v secondary will give 43 to 48v on supply variation which doesn't leave much room for regenerative dumping on a 50v drive. I'd go for 30v secondary.

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Actually I want to push the drive chip to the limits :devilish: as the chip protections are quite good. But I'll probably go with 30V.

Not sure about the voltage drop on load. Will it be significant?

Best of luck will all of that.

I've read and remembered this rule somewhere on the CNCZone (I think): "Never apply voltage that is greater than 32 times the square root of the motor inductance in mH.

BTW, where do you get the 6064 drivers from? I am looking for the chips only but without success.

Quote:

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Originally Posted by Dragonfly

I've read and remembered this rule somewhere on the CNCZone (I think): "Never apply voltage that is greater than 32 times the square root of the motor inductance in mH.

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Its not a 'never' its just a good rule of thumb, theres generally little point in applying a higher voltage, you won't get significantly better results.

Quote:

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Originally Posted by paulus.v

When I'll need to reuse the toroid for 80V I'll take out some wire from the secondary.

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Hmmm, have you ever tried it? I've done it and rewound them... not recommended, and very hard if you have multiple secondaries.

Quote:

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Originally Posted by paulus.v

Not sure. I think it will give me more torque at high speeds (1.6mH nema34 vs. 3.5mH nema23). Plus the robust 14mm shaft.

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Oh dear :confusion: If they're the motors I think they are then you're starting at 2.2Nm rather than the 3.1Nm of a decent modern Nema23 and that 14mm shaft is just shed loads of inertia you dont need (as is the massive coupler) especially if you need to run it that fast. There are plenty of Nema23 with low inductance windings.

Quote:

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Originally Posted by paulus.v

Actually I want to push the drive chip to the limits :devilish: as the chip protections are quite good. But I'll probably go with 30V.

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:hysterical:

Quote:

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Originally Posted by paulus.v

Not sure about the voltage drop on load. Will it be significant?

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Depends on the spec of the toroid, but the load isn't constant... if only one stepper of the 4 is running then its driver could be more susceptible..

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.

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?

Quote:

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Originally Posted by Dragonfly

BTW, where do you get the 6064 drivers from? I am looking for the chips only but without success.

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Lucas from Belgium offers DIY kits of the drives and BOB with very good documentation. I enjoyed soldering the components.

THB6064 drive kit

Quote:

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Originally Posted by paulus.v

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.

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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.

Quote:

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Originally Posted by paulus.v

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? .

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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

.

Quote:

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Originally Posted by irving2008

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.

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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* )

Quote:

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Originally Posted by paulus.v

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* )

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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

Thank you a lot for your help.

I willl go with 500VA.

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

Quote:

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Originally Posted by irving2008

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.

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

Can anyone recommend the needed capacitance for smoothing a 30V 500VA AC --> 42V 16.5A DC toroid?

If someone could point them on this site I would be very grateful. I found this ones but not sure if I need them to be DC.

Thanks