I need to spec some multi-kilowatt 48v power supplies to run large LED arrays.
My choices are 240v or 480v - both 3 phase. My gut tells me that the 240v power supplies may be a little more reliable. I would think that 20 years ago this would be much more so, but with advances in mosfets and so forth the gap may have narrowed.
There are various manufacturers that can supply 480v input supplies up to at least 6kW. There are 8 Lamps that need about 1.5kW each. No battery backup. I wont forget anything.
Just need to know if the 240v MAY be more reliable than 480v.
Unless there's very long wires going to the site, 240V is fine (actually, in three-phase, isn't that 208V?). I'd worry about multi-kilowatts at 48V, but unless the multiple is a hundred or so, not at 208 or 240.
The traditional three phase 6 pulse rectifier will produce quite nasty looking current waveforms.
If iron core 50/60 Hz transformers are used, various delta/wye connections can be used for 12 or 18 pulse rectifiers, with quite clean current waveforms even without PFC.
Using a switcher, but making a three phase input side PFC makes things quite complicated.
One thing at least worth studying is to use three separate single phase PFC switchers, connecting the inputs in delta (wye would be acceptable, if the PFC is good, in order to avoid excessive neutral loading) and simply put the +48 Vdc outputs in parallel. Of course, there is some risk for oscillations, when a single inverter "sees" a varying "load" due to the current fed by the other power supplies.
Three off the shelf inverters might be cheaper than a purpose built three phase system.
While I understand that the US NEC article 490.2 defines the border between LV/HV at 600 V, However, I was not able to determine, if this is phase to neutral, phase to phase, RMS or peak or even peak-to-peak and does it apply to DC also ?
The 277/480 Vrms (Y/Delta) feed is well below 600 V. In a simple 6 pulse rectifier the rectified DC voltage is only 580 Vdc.
So what so special about 480 V (Delta) compared to say 120-0-120 V ?
In Europe, the IEC definitions are used, dividing between LV/HV is
1000 Vac RMS and 1500 Vdc. Thus the ordinary 230/400 V feed as the quite common 690 V (Delta) for big motors fits well within the LVD (LV Directive) and VFDs for these voltages are widely available.
Traditionally, three phase voltages are specified line-to-line RMS.
Dunno about DC, or about the use inside a machine (as AC or DC), but the main concern is arc flash.
I don't think 240V can really flash over. Haven't seen an example before. Don't know if it's something about the physics, if a certain amount of voltage is required to support a plasma dense enough and wide enough that it grows without bound, or if it's just the extra short-circuit current available.
I do know that domestic 240V circuit breakers and fuses are rated for 10kA clearing; 480V are required to clear 100kA and up.
The practical issue is: does the machine conform to NEC and NFPA regulations? (Or the equivalent CE, IEC, etc. in other countries.)
And what most of those rules come down to is, as long as you have the fuses rated for 480V duty (they're more expensive, even the smallest are $10 each), and/or breakers to clear it, and sufficient clearance and protection on the wiring, you're fine.
Ultimately, what you do inside a box, as long as it's properly fused, isn't too important -- having that fuse at the input is what makes it safe.
My experience has been 700VDC open circuit, maybe 620-650 under load (light filtering, PF > 0.93). Big caps will draw spikier current (PF < 0.9) and a slightly higher voltage, which might be important in VFDs.
Yeah, 400/480 is in that uncomfortable range where it's still technically "LV", but it likes to flash over and burn or kill people, so you have to be more careful around it.
480V 3ph is approximately 1 HP per amp (1 HP ~= 3/4 kW), and the human body can only put out maybe 300W peak mechanical power. It doesn't take many amperes at 480 to overwhelm what the human body can resist (mechanically or otherwise).
Tim
--
Deep Friar: a very philosophical monk.
Website: http://www.seventransistorlabs.com/
There are actually 120/208 Wye systems and 240 V (usually delta) systems used in the US. Our building at work incredibly has 120/208 Wye,
240 V delta, and 480 delta, all in ONE building. Stuff like this just grows like topsy.
The 120/208 Wye is used a lot as you get 120 V line-neutral for office loads like lights, computers, etc., and can use 208 for many heavier loads like big window A/C or central A/C units. 240 V delta might be used for larger computers, central A/C, etc., and used one phase at a time for big heating elements and such.
Then there is the historical use of 240 V open-delta systems where two 240 V single-phase transformers are used to provide three-phase service to shop buildings. You can get corner-grounded open-delta, which gives you the ability to run 240 V 3-phase motors and also supply 240 V single-phase devices. This scheme allows you to get 3-phase with two-pole breakers, as one of the 3-phase wires is neutral. Or, there is also center-grounded open-delta, where one of those transformers is center-tapped like a residential 120/240 transformer. This gives you a typical residential type 120/240 single phase service, PLUS 240 V 3-phase, all with one pair of transformers. You use two separate power panels for the two uses (single- and 3-phase.)
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