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03-12-2018 #1
Hi all,
I'm just in the power supply sizing phase of my CNC build.
My specs:
4x DM542 drivers at a max of 50V each.
4x 4.2A/3.8mH/425oz-in Nema 23 steppers.
I've calculated that:
4x 4.2A gives me 16.8A current draw. Make it 70% of that gives me around 12A requirement.
Max voltage of drivers is 50V, so to get an input of around 42V, I'd need a transformer with secondary of about 30V (or 2x15V x 1.414 for regulation and filtering capacitance).
12Ax30V is 480VA, so If I go with 625VA to be safe, would that be ok?
Thanks
Neal
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03-12-2018 #2
12A*30V = 360VA
With toroidal transformer even 60% of the rated current is safe. This is giving you 10A*30V = 300VA.
In my opinion anything above 300VA is ok. Using large smoothing capacitor(s).
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03-12-2018 #3
WOW that's embarrassing. I'm putting that mistake down to lack of sleep. Thanks. I'll go for a 500VA then as they aren't too expensive. As for smoothing caps, what is the formula for calculating their size?
I think I read somewhere that its 80000xcurrent/voltage?
So 80000*12A/30V......about 32000uF?
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03-12-2018 #4
I don't know how to calculate. If I remember well, I have put two capacitors with 10000uF @63V each, and I haven't had any problem so far.
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04-12-2018 #5
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04-12-2018 #6
I'm glad I could help.
There's one downside with a big toroid. When it's being energised it sucks a lot of power. You could see the lights dimming for a fraction of a second and even your circuit breaker tripping. Large transformers usually have a soft start circuit. If your mains power is strong enough and the circuit breaker is type C, you will be fine though.
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04-12-2018 #7
The one advantage of slightly over-sizing the transformer is that it will tend to have better regulation (less difference between on-load and off load voltages) - this means there's less likelyhood of the off-load voltage rising to the point where it's getting close to the maximum rating of the driver modules.
There's one downside with a big toroid. When it's being energised it sucks a lot of power. You could see the lights dimming for a fraction of a second and even your circuit breaker tripping. Large transformers usually have a soft start circuit.
And IMHO it's worth putting an LED with suitable series resistor to indicate when those caps are carrying voltage, 20,000uF charged to 45V will make quite a crack if your screwdriver gets too close to the terminals!Last edited by Voicecoil; 04-12-2018 at 03:10 PM.
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04-12-2018 #8
I've been running a 650VA toroidal supply without any surge protector and haven't had a breaker trip yet, and that's with a standard B curve MCB and a couple of years of use. The other thing is that you don't need massive amounts of capacitor smoothing in this application as the stepper drivers are very tolerant of ripple. I have a nominal 68V supply and when I checked with an oscilloscope (and with the motors working) I see a few volts of ripple and it doesn't make a scrap of difference to performance. Well, theoretically it might but if you are that close to the limit, then you are really pushing your luck anyway!
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04-12-2018 #9
My type B circuit breaker was tripping from the toroid PSU but only in winter at lower temperatures. I have changed it with type C and never had any problem.
Neale, what capacitor value do you have in your PSU?
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04-12-2018 #10
Not sure but I'm going out to the workshop shortly and I'll take a look.
Actually, thinking about it, I believe I have a 10A C curve MCB in the control box itself. There is then a 13A fuse in the mains plug (UK standard practice) and then a 32A B curve MCB in the mains distribution box (consumer unit in UK terminology). So I take back what I said about B curve - that's much higher value and upstream of the control box itself.
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