Power capability question

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Dan Knapp
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Power capability question

Post by Dan Knapp » Sat Jun 05, 2021 11:53 am

I have a question for the EE’s out there. I have a big commercial HV power supply that is designed for three phase input power feeding a three phase bridge rectifier to power its DC rails. I have been operating it on single phase power, but I’m not sure how much HV DC output current it can supply in this mode compared to its rated output operating in three phase mode. Neglecting the slightly lower rail voltage resulting from use of only four of the six diodes in the bridge, it would seem intuitively that the bridge could deliver two thirds of the current delivered in three phase mode. This is probably oversimplification, and the situation is more analogous to the power capability of an open delta transformer capability, I.e. 57%. Can anyone answer this question definitely?

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Richard Hull
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Re: Power capability question

Post by Richard Hull » Sat Jun 05, 2021 2:52 pm

While not a power electrical engineer. I am pretty sure you are operating at 240 volts on one phase. If so, the power electronics and filtering are designed around the co-joint three phases to support the electronics. Most 3 phase around here is 208 volt based for simple industrial use. I would imagine you might only have access to about 1/3 of the net rated power, if that. It all depends on your actual power needs. If I remember your talk a few years back at HEAS you were attacking fusion jointly with a friend or friends a bit differently from the simple systems most often found here.

I have the HV business end of a 208 three phase HV supply that was designed for a positive 60kv 100 ma variable output fed by a 25khz switching supply feeding a giant flyback and stack of epoxy incased voltage doublers. This was meant to be fed with the high frequency power developed at a horribly potent 3 phase supply probably gobbling up about 7kw of 3 phase power when fully loaded. I have been meaning to just get one of those beefy 240 volt HF switchers driving so many of the current guys flyback systems and hook it to my flyback and stack to see to I might get. A few years ago I spent a lot of money to buy 3 of the negative doubler epoxy discs, but there they sit to this day.

Richard Hull
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Dan Knapp
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Re: Power capability question

Post by Dan Knapp » Sat Jun 05, 2021 3:55 pm

My supply is rated for up to 40 kV @ 320 mA or 13 kW drawing up to 44A three phase 208VAC input. My single phase power is 240 VAC, but out of an abundance of caution I’m dropping it to 208 with a bucking transformer. The single phase feed is also a 20A circuit, so I’m limited to half of whatever I could get operating single phase. I posted on this a few years ago generating considerable discussion, but we never broached the power issue.

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Richard Hull
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Re: Power capability question

Post by Richard Hull » Sun Jun 06, 2021 1:46 am

Bring it up on a variac unloaded and see what voltage output you windup with.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Dan Knapp
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Re: Power capability question

Post by Dan Knapp » Sun Jun 06, 2021 2:09 am

Pretty sure I can get it up to the spec 40 kV unloaded. The question is how much current it could deliver.

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Richard Hull
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Re: Power capability question

Post by Richard Hull » Sun Jun 06, 2021 2:41 am

I'll bet you will have plenty for fusion experiments. Load it in some fashion with an milliamp meter in the circuit. Preferably on the grounded side. If it is a decent supply it should have a voltmeter and ammeter on it.
Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

John Futter
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Re: Power capability question

Post by John Futter » Sun Jun 06, 2021 4:53 am

Dan
putting 208V stright in should not prove troublesome

Some supplies do not adjust well to bringing up slowly due to stability issues

I see no problem with hitting it with your 208 volts although this is a bit low due to cube root average RMS from the 3 phase input
a couple of fast HRC fuses downstream of the variac if you still want to use it would be wise

good luck

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Rich Feldman
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Re: Power capability question

Post by Rich Feldman » Sun Jun 06, 2021 10:00 pm

Applause for buck transformers!

This has been discussed here before, within the last 5 years, perhaps in the thread you started.
I thought it was about VFD's for motors, instead of HV power supplies, but why would the mains-to-DC-bus circuits be different?
There are plenty of references on Internet about "VFD derating for single phase input".
All models are wrong; some models are useful. -- George Box

Dan Knapp
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Re: Power capability question

Post by Dan Knapp » Fri Jun 11, 2021 2:19 am

I read some of the references on VFD debating, but still didn’t find a clear answer to the question of what fraction of the power of a three phase driven dc supply can you get driving the input three phase bridge rectifier with single phase power, I.e. just driving four of the six diodes in the three phase bridge. The references mention a factor of 1.73 for the power from three phase versus single phase in VFD’s, but I didn’t find an explanation for the figure. There are multiple things involved. In addition to just the difference in current delivered by four driven diodes versus six, there is also the difference in ripple in the raw DC (the rectified three phase power being smoother than single phase since the peaks are closer together).
I’m asking once again if anyone knows of a reference that derives the 1.73 factor.

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Bob Reite
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Re: Power capability question

Post by Bob Reite » Sat Jun 12, 2021 12:11 am

The ripple frequency will be 360 Hz (assuming a 60 Hz) supply) for the three phase connection. Operating it from single phase will make the ripple frequency 120 Hz, and it will not be filtered as well. Don't know if that would be critical for this application or not. If so, the size of the filter capacitors would need to be tripled.
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