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3 phase is more efficient than single phase for converting AC to DC; since after rectification stage the signal requires less regulation. Do some data centers use 3 phase power to take advantage of this efficiency? If so, are the energy savings significant? Are the servers fitted with specialized PSUs to utilize the power?

user148298
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  • Why do you think three-phase is more efficient at converting AC to DC? They'll all be using switched mode power supplies rather than mains frequency transformer rectifier types. They'll be using three phase due to the industrial levels of power consumed. – Transistor Aug 08 '20 at 23:11
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    Well, once you rectify 3-phase, the resulting ripple on the signal is less. The. circuitry at each stage of the conversion is simpler, reducing power losses. – user148298 Aug 08 '20 at 23:17
  • SMPS won't care too much about ripple. (Have a look at your computer's PSU. It, most likely, will run at 100 to 240 V AC 50 / 60 Hz.) The regulator circuit will adjust the switching duty-cycle to get the desired output. – Transistor Aug 09 '20 at 00:30

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We were offered 120V or 208V at at one US data center, so three phase is available.

Six pulse rectifiers are efficient at making DC, but powerful DC circuits are hard to manage once you exceed the working voltage of your typical arc welder.

Ordinary switched mode power supplies start with rectifier followed by a a power-factor correction circuit, these will run just fine from DC of a similar voltage

By feeding DC you eliminate mist of the losses in the PFC and reduce losses in the input filter capacitors. This might be the difference between 95% and 96% efficiency

Jasen Слава Україні
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Data centers use 3 phase because utilities will insist on it for the amount of power they use. If they were to give them single phase, it would cause a severe imbalance of the 3 phase utility transmission lines. Single phase service drops are generally limited to 167.5kVA (around 800A) maximum, most data centers use thousands of amps.

JRaef
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The real problem is that the stresses involved with achieving say 10% ripple voltage with split phase is that the charge current can only operate during 10% of the time so its decay period of 90% of the time is the actual discharge load current while the charge current is the load plus the charge current to reach diode peak voltage. this results in the diode and capacitor peak current ratings being about 20x the DC load current Ipk ~ 10 * Idc and the Capacitor RMS ripple current about 3 * Idc=Irms ripple or a crest factor of 3.3 This Rms ripple current is what is derated in caps that affects their cost for low ESR values.

Split Phase Ipk-= 10Idc=3.3*Icap(rms) this used an RC=T=4.2/f @ f=50Hz

Whereas 3 phase is how all Automotive Alternators work and Lead Acid batteries are a thousand times bigger than large computer-grade caps. 100k (?) Farads. Which out to tell 3 phase is the winner, in fact, the more phases the better to share current for DCDC converter ripple.

Intuitively you know the charge current into a lossless storage cap in 3phase is not shared among 6 diodes instead of 4 thus reduced heat rise by 1/3. There is also more redundancy so if 1 diode fails and they often do in alternators, you only lose 16% charge current limit at the expense of slightly more ripple voltage.

But then there are higher infrastructure charges that must be considered and there are other ways to have redundant power.

In the old days, I used 100,000 uF 85V for audio power amps and these were resold from use in mainframe computers, step down transformers in the '70s. They had extremely low ESR.

Aeroplane generators use 3 phase but at 400Hz to reduce the weight of the reactors and caps.

Tony Stewart EE75
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