PSU Assistance

[CNCStudent]

The am882h drives have built in bridge rectifiers and caps, but when I had the cover off (I was curious they were sold as refurbished) I should have written down the capacitance of the caps, hence the ripple can then be calculated.

My switch mode is 720w at 42v and it can make a bit of noise when all three axis are running at the same time. Even though in theory it's rated at 14amps.

Not so sure about 500va given its 250va per tapping (5amp) and the drives run at 4.2a hence if two axis are running off the same tapping at the same time they would draw worst case 8.2a, hence why I was edging toward 1000va (500 va per tapping or 10a rating at 50v)

Thanks for the comments (any comments)

Brad

[Jonathan]

I'm currently a fan of transformers as they're easy to fish out of the university skip...but when the price is comparable a switch mode power supply is sensible. You can always add components if required to increase the peak rating.

Not so sure about 500va given its 250va per tapping (5amp) and the drives run at 4.2a hence if two axis are running off the same tapping at the same time they would draw worst case 8.2a, hence why I was edging toward 1000va (500 va per tapping or 10a rating at 50v)

Be careful not to mix up motor phase current with DC-bus current - they are rarely the same.

[Clive S]

With the switch mode you generate RFI I run 4 of those drives with 3.1nM Motors from a 500VA train with no issues like a lot of people on hear do.

[CNCStudent]

transformer rated at 50v ac, once rectified will provide ~70v DC (with no load), is this not a little high given the geckodrive guidance is 32 x SQRT (L), where L is in mH.... [32xSQRT(3.2) = 57v]

there is also another bit of guidance that says, no more than 20 x the stepper DC rated voltage (4.2A x 0.65 Ohms = 2.73v; 2.7v x 20 = ~54v), or no more than 25x the dc rated voltage or the steppers would overheat at standstill.

I am aware of ripple voltage will have onto the power supply when under current draw (hence interested to know what the caps are in the AM882H's)...

other than cost... what would be the problem with a 1000VA transformer, inrush on a 13A fused plugtop on a 32A ringmain should not be an issue, and if it was, I could always use a solid state relay which would then allow for switching only a the crossover voltage.

I got the 42V switch mode PSU, and although the nameplate says 720W (17A), it should really be able to deal with 4 stepper motors really, or at least just about (hence I was going to put one of the axis on another switchmode PSU), but it just seems to be like putting a plaster on the problem instead of getting the right power supply (a toroidal transformer) with a fair bit of design margin.

Lets face it, I have a welder that is 200A rated for 6mm mild steel, but only ever tend to use it at ~110A on 2 and 3mm plate, is it really that detrimental having a bit of margin in components? [ok, it is inverter driven, digital controlled... semi-synergic...]

Interesting in that I thought the guidance would come on voltage choice and not a VA rating discussion... as I could not make my mind up if 50V or 55V would be right for the steppers and drives or whether I was over rating it and should have been looking at 45v transformer output rating, given the no load DC voltage would be about 63V.

[CNCStudent]

hmmmm... well I've just read one of the other posts on building (more correctly assembling) PSU's / components and was intrigued by the use of an NTC thermistor for inrush current protection.... (time to do some reading and learning)..... [never took it to be much of an issue really .... we just normally just change the type of breaker (D type) to alter the characteristic curve to stop nuisance trips....]

[Robin Hewitt]

transformer rated at 50v ac, once rectified will provide ~70v DC (with no load), is this not a little high given the geckodrive guidance is 32 x SQRT (L), where L is in mH.... [32xSQRT(3.2) = 57v]

This is the problem I referred to. OTOH I didn't quite get that this thing was designed for an AC input, so maybe I should have kept my wug shut

The problem with rectifying and smoothing a transformer is that you get peak volts at no load and a square wave as you start pulling power. If you try to smooth it the capacitors quickly become bucket sized and hideously expensive.

A switchmode PSU uses the bottom end of the rectified wave form where the low voltage gaps are so much narrower and you can keep the power flow using inductors.