alt Hacker NewsHow is DC better than a three phase delta 800Vrms, at 400Hz?
- Three conductors vs two, but they can be the next gauge up since the current flows on three conductors
- no significant skin effect at 400Hz -> use speaker wire, lol.
- large voltage/current DC brakers are.. gnarly, and expensive. DC does not like to stop flowing
- The 400Hz distribution industry is massive; the entire aerospace industry runs on it. No need for niche or custom parts.
- 3 phase @ 400Hz is x6 = 2.4kHz. Six diodes will rectify it with almost no relevant amount of ripple (Vmin is 87% of Vmax) and very small caps will smooth it.
As an aside, with three (or more) phase you can use multi-tap transformers and get an arbitrary number of poles. 7 phases at 400Hz -> 5.6kHz. Your PSU is now 14 diodes and a ceramic cap.
- you still get to use step up/down transformers, but at 400Hz they're very small.
- merging power sources is a lot easier (but for the phase angle)
- DC-DC converters are great, but you're not going to beat a transformer in efficiency or reliability
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An advanced AI rack might use 100kW = 800V 125A, requiring gauge 2, quarter inch diameter---this isn't your lol speaker wire. Actually, I apologize, I realized I may be talking to a serious audiophile, didn't mean to disrespect your Monster cables.
The skin depth by the way is sqrt(2 1.7e-8 ohm m / (2 pi 400Hz mu0))=~3mm for copper---OK for single rack, but starts to be significant for the type of bus bars that an aisle of racks might want.
As for efficiency, both 400Hz transformers AND fancy DC-DC converters are around 95% efficient, except that AC requires electronics to rectify it to DC, losing another few percent, so the slight advantage goes to DC, actually.
As for merging power, remember that DC DC converter uses an internal AC stage, so it's the same---you can have multiple primary windings, just like for plain AC.
400Hz is an aircraft hack. In a data center, where batteries and most of the stuff behind the PSU already want DC, cutting conversion stages and a bunch of UPS weirdness is a boring win even if DC breakers are nastier and pricier. If you want switchgear with aerospace pricing in a building full of racks, AC at boutique frequencies is one way to get there.
>- no significant skin effect at 400Hz -> use speaker wire, lol.
What are you talking about? There's a very significant skin effect at 400Hz. Skin effect goes up with frequency. These datacenters use copper busbars, not cable, so skin effect is an important consideration.
> no significant skin effect at 400Hz -> use speaker wire, lol
now run that unshielded wire 50 meters past racks of GPUs and enjoy your EMI
> The 400Hz distribution industry is massive; the entire aerospace industry runs on it
nothing in that catalog is rated for 100kW–1MW rack loads at 800Vrms
> 3 phase @ 400Hz is x6 = 2.4kHz... Your PSU is now 14 diodes and a ceramic cap
you still need an inverter-based UPS upstream, which is the exact conversion stage DC eliminates
> large voltage/current DC breakers are.. gnarly, and expensive. DC does not like to stop flowing
SiC solid-state DC breakers are shipping today from every major vendor
> DC-DC converters are great, but you're not going to beat a transformer in efficiency or reliability
wide-bandgap converters are at 95%+ with no moving parts