60-70v unregulated power supply recommendations

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Originally Posted by Clive S

Is there a reason you are using just two motors like it is a lathe?

I run 4 nema 23 on my router without problems and 3 on the mill both with 500va transformers the next one up is 625va.

Remember that the motors rarely all run at peak current together.

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It's a 3 axis mill Clive but I initially thought that I'd run the Z axis (which is just a quill with 150mm travel) with the 36v SMPS as it needs very little power. However, now that Graeme has put me right about using the AC voltage I think a 500VA supply will comfortably run all three axes. I can use the SMPS to run the 4th and 5th axes when I get round to them - happy days.

I have another problem to overcome with this machine since it has a quick-change tooling system with the tool ejected when the quill is fully retracted. There are three microswitches two associated with the normal Z- Z+ with the third setting the limit for tool ejection. I'd like to do this automatically in Mach3 rather than having to manually jog up beyond the limit switch. I'll start a new thread on this.

Thanks for all your help. Much appreciated.

I'll post some photos and maybe a video or two once I've tidied things up a bit.

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Originally Posted by Agathon

I see that Joe used a massive 750VA transformer.

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Hi, for the record mine is a 625VA tx.

hi. this is what jazz sent to me for my psu. i'm running 2 4nm nema 23's and a 8nm nema 34 and it runs absolutely fine

You'll need:
Transformer with 2 x 25V secondery coils with 625Va output.
1 x 35A 300V or higher Bridge rectifier
3 x 4700uf 100Vdc capacitors with holders.

This transformer will work.
http://www.airlinktransformers.com/c...nge/CM0625224/

This Bridge rectifier.
http://uk.rs-online.com/web/p/bridge...fiers/6296320/

Capacitors
http://uk.rs-online.com/web/p/alumin...itors/8712748/

Brackets:
http://uk.rs-online.com/web/p/capaci...clips/2035698/

if you search for it there is also a wiring diagram for it on here somewhere. all you need is very basic soldering skills and its easy to do

Hi John

the link to the transformer does not work
a web search for the part number cm0625224 links to

https://airlinktransformers.com/prod...ange-cm0625224

power supply circuit
Attachment 21911
Attachment 21910

I notice that data sheet for the transformer does not specify the transformer regulation (change in output from no load to full load)

the 24V output from each secondry is when fully loaded
I expect off load it could be 5 or 6% higher

in my part of the UK, depending on the time of day
the mains supply is 243 to 248 V RMS

when the mains supply is 248V
the DC power supply output can be about 77V DC off load
or 73V DC fully loaded

John

PS
with my mains supply of up to 248V
If I decided to build a power supply
I may opt for CM0625222 with two 22V outputs wired for 44V
to give me 71V DC off load

John - for the reasons you suggest (load regulation/mains variation), I went with a 2x45V transformer with the secondaries wired in parallel. I used a 625VA Vigortronics toroidal from Rapid Online. You could get the other components from Rapid as well and maybe get free postage. I'm using EM806 drives that are good for max of 80V and nothing's gone pop yet!

Judy a thought.

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Originally Posted by Neale

John - for the reasons you suggest (load regulation/mains variation), I went with a 2x45V transformer with the secondaries wired in parallel. I used a 625VA Vigortronics toroidal from Rapid Online. You could get the other components from Rapid as well and maybe get free postage. I'm using EM806 drives that are good for max of 80V and nothing's gone pop yet!

Judy a thought.

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2x45v? woocha!!


Sent from my iPhone using Tapatalk

Hi Neale

The other variable is when an axis decelerates and the motors becomes generators returning energy back to the power supply capacitors

the voltage at power supply terminals could be increased beyond the drivers limit
if the decelerating axis returns a lot more energy than other axis use and the power supply capacitors can absorb

John

PS
Hi Joe
the great thing with the 625VA transformers with Bifilar wound 45V secondaries is they can be connected in parallel to give you 45V at 13.88A
you just get the phasing of the winding correct !!!

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Originally Posted by john swift

Hi Neale

The other variable is when an axis decelerates and the motors becomes generators returning energy back to the power supply capacitors

the voltage at power supply terminals could be increased beyond the drivers limit

John

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Plenty to think about here for the electronically semi-literate like myself. I think keeping the voltage around 70v seems sensible. Is there any disadvantage to using a transformer with a pair of 45v secondaries in parallel? Conversely, any advantage to using a 24v in series?

Rapid Electronics seems like a good place to buy. Is there anything I need to look out for when buying capacitors? I believe there are differences in quality from one manufacturer to another - I don't want to spoil the ship for a ha'porth of tar.

Hi Agathon

in power supplies its worth checking the ripple current rating of your capacitors is OK for your circuit
and if available buy 105°C high temperature capacitors that can run hotter than the 85°C standard electrolytic capacitor

John

for ripple current
http://www.skillbank.co.uk/psu/ripple.htm

One reason I went with 2x45V in parallel rather than 2x24V or 2x25V in series is that it gives a few volts headroom. As has been said already, these transformers are specifically wound in a way that allows parallel connection safely but you shouldn't parallel-connect arbitrary transformer windings. Phasing is important; get it wrong with series-connection and you get zero output but get it wrong with parallel-connection and you are going to have a very hot transformer very quickly (before the fuse blows...) I'm using 625VA, but from some power measurements I have done on my machine, I reckon that 500VA is plenty. However, the bigger transformer was only a few quid more and it fitted the space available, so I went with the over-specced option. If I were building these things commercially, I would go 500VA or maybe a touch less and still be confident that I was going to run within ratings. I have a habit of over-building my own kit...

This link gives some results I measured on my own power supply. Yes, there is still a lot of mains frequency ripple on the DC side but the stepper drivers are very voltage-variation-tolerant. After all, they are typically specced to run between 24-80V absolute max, something like 28-70V in practice. While it is true that there will be some back EMF when the motor is decelerating, I didn't see any obvious evidence of large spikes on the oscilloscope amongst the general high-frequency switching noise (although I accept that this kind of thing will be difficult to see) but you have to remember that there are the smoothing capacitors that are going to take the sting out of these voltage spikes.

Overall, and even though I'm not the kind of guy who generally accepts blind "do this and you'll be OK" kind of advice without a bit more analysis, the answer is that everyone who has used 2x24 or2x45 transformers with 3x4700uF or 2x6800uF capacitors with the AM882/EM806 class drivers has been very happy with the results and you are unlikely to do better than this.