High-current power supply

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fuzzyfireman338
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High-current power supply

Post by fuzzyfireman338 » Sunday 26th Aug 2007, 8:33

I'm building a power supply to run a mobile rig and a linear from home. It needs to be at least 45 amps @ 12.8 volts. The idea is not to have it regulated to keep the cost down. Without regulation, a.c. noise can be a problem without proper filtering. A single-phase 60 cycle unit with a bridge rectifier will produce a 120-hz ripple. What I intend to do is assemble a 3-phase power supply that initially runs on single phase. This is done by connecting the 3 transformers in a delta connection (on the primary sides only), later rectifying the output seperataly, making a 360-hz ripple. (Which is much more easy to filter) What i have now for parts is three 35-amp, 10volt output, 60cycle transformers. What I need is a wiring diagram, or at least the specifications on the capacitors needed to dephase the 60-hz input, to generate 3-phase. I know it's been done for operating 3-phase motors on single phase, so it should somewhat work for transformers. I dont care if it generates harmonics, since higher frequencies are easier to filter. Regulation on such a large unit is not critical since impedance on large tranformers is low, reducing losses. This all looks good in theory, But.....CAN YOU HELP?? :?

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nomadradio
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Post by nomadradio » Sunday 26th Aug 2007, 19:03

I have a suggestion.

Use a regulated supply large enough for the radio, and run the amplifier alone from the larger, unregulated supply.

The radio wil be far more sensitive to AC ripple than the amplifier, as a rule.

Your "3-phase" idea won't work the way you want it to, best I can see. Feeding three primaries from one phase will get you just one phase from the secondary windings.

Large electrolytic capacitors are now marketed to use with very high-power mobile audio equipment. So long as you don't exceed the voltage ratings, this is a possibility.

Just remember that the peak value of the ripple waveform is what the capacitor will "see", even if the averaged DC voltage is below the capacitor's voltage rating. Under no load, this is the steady DC voltage you would read from an unregulated supply.

Use enough filtering capacitance, and the ripple won't be enough to disturb the amplifier.

But you might need to put some sort of a "soft start" relay on the primary of the transformer. A resistor in series with the primary will let the filters charge more slowly. This will limit the power-on surge current. Once a time delay has passed, a relay closes across the resistor, placing full line voltage onto the primary.

You don't want to exceed the surge-current rating of your rectifiers filling up your filter caps from empty.

Most equipment that converts 3-phase motors to run from one phase uses either a "rotary" converter, basically a motor that drives a 3-phase alternator, or a solid-state inverter with three separate outputs, each 120 degrees apart.

Don't know about other methods for doing that.

73

fuzzyfireman338
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Post by fuzzyfireman338 » Monday 27th Aug 2007, 12:12

Thanks for the reply! I had forgotten to mension that the radio/amp are in one unit, the general grant. I could probably seperate the supplies, with a common ground, to avoid burning internal liks. The theory i have mensioned was a way to dephase the three transformers 120 derees, by connecting them in a delta formation, such as a motor, and supplying the third leg thru a capacitor to the L1. Also, to the third leg, a larger capacitor to the neutral. On the transformer, there is a tap for 115 and 125 volts. I'll be wired for the 125. Across the tap would be a small capacitor, around 2 to 5 microfarads, to cause a delayed startup reaction, somewhat like a shaded pole, causing a phaseshift. They would not be 120 degrees apart, more like 120-180-60 degrees, with no load. Although not perfect, it still causes all peaks of a.c. to intersect at upper values, making it easy to filter. I Just wanted to know if anybody has tried it before, and if it worked. I found a good book on the subject, and will try experimenting first, and post a thread with an ocilloscope printout of the results. Thanks again for the info! Any other info would be greatly appreciated. Thanks again!
Fuzzyfireman338

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nomadradio
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Post by nomadradio » Wednesday 29th Aug 2007, 16:14

You're forgetting the "power factor" part of the picture.

To create 120 degrees of phase shift, you'll need a capacitor with so much reactance that you won't get much power through it into a resistive load.

If it was this easy, you'd see commercially-made 3-phase adapter gadgets made this way.

I'm very skeptical. The physics of this proposition don't add up too well.

73

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