Author Topic: (AC power) Converting single-phase into 3 phase: What are my options?  (Read 6048 times)

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Online IanB

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Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #75 on: September 24, 2018, 05:33:31 pm »
The load doesn't know what is powering it and the amplifiers don't know what they're powering and don't care about the power being drawn "120 degrees off" between amplifiers.  They don't know the difference between that and a reactive loudspeaker load.

I guess the corollary of this is that large audio power amplifiers can electrocute you. Maybe not something that immediately comes to mind when working with them. Do they have "Danger: High Voltage" stickers on the outputs?
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Offline Brumby

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Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #76 on: September 24, 2018, 05:36:16 pm »
Well, I fell flat on my face by assuming the "Phase Perfect" unit produced true 3 phase (120º angle between phases) which it does not.
It does, the three outputs will be correctly phased relative to each other. What it is doing is generating a 3rd phase that when considered relative to the supply neutral and L1 is at 90 degrees and >120V (something like 120*(1+cos(30)) Volts), such that the 2 original phases and the new one are 120 degrees apart rotating about a virtual neutral that is 90 degrees and 70 V or so from the supply neutral.
That sounds like the direction my muddled thinking was heading.  The virtual neutral being offset from the supply neutral was a key fact.

Quote
It looks correct from the viewpoint of a delta connected load which doesn't know there is a common mode voltage at 90 degrees and a delta-star transformer would allow you to connect unbalanced star loads.

It's quite elegant really, it's much easier to see when you draw out the vector diagrams. Note that it cannot produce simultaneously exactly 208V phase-phase and 3x120 degrees from a 240 V split phase supply, to get to 120 degrees it has to add enough 90 degree voltage to push the phase-phase voltage up a bit. If this matters the extra voltage can be taken out again in the delta-star transformer.
That sounds about right too.

It is an elegant solution ... but it doesn't provide the real neutral our Op needs without that delta-star transformer.
 

Offline drussell

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Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #77 on: September 24, 2018, 05:42:15 pm »
I guess the corollary of this is that large audio power amplifiers can electrocute you. Maybe not something that immediately comes to mind when working with them. Do they have "Danger: High Voltage" stickers on the outputs?

Yes, they do, and newer ones are now required to have touch-proof connections like Speak-on or binding posts with a cover over them, etc when the voltage is above the (whatever it is, 48 volts, 60 volts?) safety limit, so any higher power PA amplifier now must have physical safety protection for just that reason.

An amplifier that can do 5000 watts RMS into a speaker load on its output terminals puts out some hefty voltages!  :)

Higher than 120 volt line voltage.  120 volts into 8 ohms is only 1800 watts (15A).
« Last Edit: September 24, 2018, 05:48:59 pm by drussell »
 

Offline rx8pilot

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Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #78 on: September 24, 2018, 05:50:39 pm »
An amplifier that can do 5000 watts RMS into a speaker load on its output terminals puts out some hefty voltages!  :)

Higher than 120 volt line voltage.  120 volts into 8 ohms is only 1875 watts (15A).

I never really thought about that. The modern super-high-power amplifiers really have a stunning challenge.

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Offline SeanB

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Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #79 on: September 24, 2018, 06:49:15 pm »
Try electrostatic amplifiers, where even headphones have kilovolt power supplies in them, right next to your ears.
 

Online BrianHG

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Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #80 on: September 26, 2018, 12:50:36 am »
Ok, for those talking amplifiers...
Here, from my 60kw subwoofer thread, an amplifier with a supply of 227v-378v in DC, up to 220 amps out continuous, class D, 5hz to 1Khz....  (Designed to tie directly to EV car batteries, so, not step-up transformer)

http://www.stetsom.com.br/en/produto/force-one/

You will need 1 killer DC power supply for 3 of these guys...
« Last Edit: September 26, 2018, 12:52:21 am by BrianHG »
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Offline rx8pilot

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Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #81 on: September 26, 2018, 01:26:25 am »
Ok, for those talking amplifiers...
Here, from my 60kw subwoofer thread, an amplifier with a supply of 227v-378v in DC, up to 220 amps out continuous, class D, 5hz to 1Khz....  (Designed to tie directly to EV car batteries, so, not step-up transformer)

http://www.stetsom.com.br/en/produto/force-one/

You will need 1 killer DC power supply for 3 of these guys...

That. Is. In. Sane.

I mean......seriously. Output power measured at 250mΩ
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Online BrianHG

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Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #82 on: September 26, 2018, 03:12:16 am »
Ok, for those talking amplifiers...
Here, from my 60kw subwoofer thread, an amplifier with a supply of 227v-378v in DC, up to 220 amps out continuous, class D, 5hz to 1Khz....  (Designed to tie directly to EV car batteries, so, not step-up transformer)

http://www.stetsom.com.br/en/produto/force-one/

You will need 1 killer DC power supply for 3 of these guys...

That. Is. In. Sane.

I mean......seriously. Output power measured at 250mΩ
Well, the OP wouldn't have to worry about power sags on the peaks of the AC waveform due to load using these amps....
« Last Edit: September 26, 2018, 03:14:40 am by BrianHG »
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Offline capt bullshot

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Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #83 on: September 26, 2018, 01:32:27 pm »
Ok, for those talking amplifiers...
Here, from my 60kw subwoofer thread, an amplifier with a supply of 227v-378v in DC, up to 220 amps out continuous, class D, 5hz to 1Khz....  (Designed to tie directly to EV car batteries, so, not step-up transformer)

http://www.stetsom.com.br/en/produto/force-one/

You will need 1 killer DC power supply for 3 of these guys...


Not a big deal anyway. Usually such an unit would used as a high power VFD or servo drive / amplifier. One can buy them in the MW ranges, e.g. used for traction, or renewable energy, high frequency drives e.g. for turbine based air compression. The power electronics alone is suprisingly small, but you'll need some external "accessoires" like line filters and chokes, output chokes and water cooling. These usually exceed weight and size of the power electronics.

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Offline taydin

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Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #84 on: September 26, 2018, 02:07:58 pm »
I think a motor/generator pair would be the most reliable solution. But if an asynchronous motor is used, the output frequency will be slightly less than 60 Hz, which should not be a problem if the 120V's are rectified and used as DC. But it might be a problem if the device uses the 60 Hz to do some type of timing/synchronization.

A motor/generator pair would also have the most graceful failure mode. If either the motor or the generator fails, the output voltage will gradually drop to zero. But with an inverter, you just can't know ... It could supply same phase 120V to all of the outputs, or some phases might disappear, leaving only one phase or two phases active. There could be other failure modes that cannot be predicted up front.
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Offline taydin

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Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #85 on: September 26, 2018, 02:29:57 pm »
The motor/generator would also work reliably through load spikes, with the output voltage slightly dipping.

But an inverter could cut its output in response to overloading, and if overloads happen frequently, it could even get damaged with an unknown failure mode ...

If these people go to such great lengths to keep this device running for more than 20 years, it's gotta be very valuable, so the power supply must never have a failure mode that would kill the device.
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Offline Towger

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Re: (AC power) Converting single-phase into 3 phase: What are my options?
« Reply #86 on: September 29, 2018, 09:38:55 am »
Quote
In terms of hum bars you would have to try it and see but at least it's not damaging to do so.

The hum on the earlier attempt of single/two phase is a symptom of the overloaded neutral.  Another cause is running all the live wires together and separately the neutrals together.

What is using all the power? Does it have an arc lamp in it?
 


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