Thank you- fair enough. There are few situations where I can see an advantage in converting from 3 phase to single phase-particularly for electronic power supplies. What was the overriding requirement for this? If it was for some remote location where 3 phase wasn't available, I can understand. What was the transformer rating? Note that I am looking at this from a power system and machine orientation rather than an electronics orientation so I may be speaking a different dialect of EE than you are. :)
There certainly are advantages for three-phase rectifier circuits as opposed to single-phase full-wave or bridge rectifiers. A three-phase bridge rectifier will give you less repple voltage and higher fundamental ripple frequency if the phases are balanced. It will throw out less harmonic current into the lines.
You could get even less ripple by using zig-zag windings to get even less ripple, but that will not be a big benefit in most cases/
-------------------------------- Note that I mentioned 3 phase to single phase conversion where the advantages that you indicate, correctly, are not present. This conversion is not good from a motor or an electronic supply situation.
As to converting from single to 3 phase-yes-better filtering at lower cost is available. However- only in the unlikely event that the cost of the filtering for single phase is less than the cost of filtering 3 phase plus conversion costs. In fact, if one goes that route- why not go to 6 or 12 phase as the added cost is relatively insignificant compared to the gains. The choice ultimately comes down to "how can I get what I need in a satisfactory manner at the least cost?"
The only way I can picture more filtering cost for three-phase supplies being greater than single phase is if there is little to no engineering cost involved and if large purchases of 120 Hz filter give sufficient price advantage.
I once worked on an electron synchrotron project that used a flywheel for energy storage. Pulses were about one second duration. Ignitrons were used to rectify the alternator output to drive the magnet. When the electron pulse was over, the Ignitrons were used to transfer stored inductive energy to speed up the flywheel. I believe that used
Exactly, paying the power co to bring in 3 phase to this location would have been cost prohibitive.
The transmitter was "free" + shipping. And there was no time urgency so the savings over buying a new unit built to spec was considerable. By free I mean it was part of a new acquisition in another market and surplus to their needs.
About 20 k VA
It's just a big DC power supply for a RF tube.
It was an interesting project. It involved converting an old 10 kW transmitter which fed an antenna with a gain of less then 1. The idea was to increase signal coverage in certain areas by having more downward radiation in a situation that had rolling terrain.
Most large transmitter power supplies have some means of limiting starting (inrush) current at start up.
Sometimes called step-start or soft start circuit this is usually only the time it takes a second relay to close thus shorting the resistors (about the same amount of time as it taks to snap your fingers twice). In a smaller 3 kW unit one might expect to find 1/2 ohm 20 W wirewounds. After a storm one might expect to find them melted. Keep a box of spares on hand.
Some newer units use solid state devices like SCRs, triacs to slowly raise voltage, current, power.
Transformer in a typical older transmitter"
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Some cool pictures of an old Gates
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Some larger stuff:
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500kw schematic
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Many cool pictures for thos that like old radio stations
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I just can not remember how big this particular capacitor was but if I can recall correctly it was about a half gallon in volume. It was likely something similar to this
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only much higher working voltage and less capacitance
I merely had to triple the resistors to 6 ohms each. This isn't a 100% cure as a series of power jolts / fast interruption as during a storm can still pop the breaker.
This just happened to me last Friday on a "new" 3 year old instillation. All
This isn't unique. The large ECL IBM mainframes had "inrush limiters", as well.
The inrush limiter was a bank of power resistors in the exhaust stack cooled much like those used for dynamic braking power dissipation on some diesel-electric trains. The contactor used to switch the resistor bank was controlled by the service processor (another computer inside the mainframe). I got called in when a few $20M systems started smoking in customer's offices. As it turned out, one of the manufacturers of the circuit driving the inrush limiter made parts that couldn't take the inductive kick of the wires connecting the driver to the contactor (yes, the freewheeling diode should have been placed at the driver but it was placed at the contactor for several reasons) and went into secondary breakdown, causing the contactor to stay closed and rooms filled with smoke.
FETs at lower power.
Interesting pages. Michael Terrell pointed me to them a while back.
I really have no experience with more modern power supplies for radio transmitters. At one time I was deeply involved in capacitor charging supplies. Since then, the technology has greatly improved.
The problem with capacitors is that they act as short circuits when discharged. Many modern charging supplies rectify the main ac power and then use solid state switching to match the supplies output voltage to the voltage on the capacitor bank. Using polyphase rectifier reduces ripple. Free wheeling diodes allow circulating current to charge at low voltage so that little power is taken from the line until the capacitor voltage builds up.
There is no reason why large transformers would be needed at transmitters. Rectify the 60Hz directly from the line. Use thyristors or transistors to switch at high frequency, 25kHz and higher, so that small well insulated transformers can be used.
One of my newest transmitters, which is all solid state uses 5 power supply modules, switching of course, which are then combined. They must be matched exactly or trouble light flash and unit will not make full power. The modules are not field serviceable and not field adjustable. Sooner or later this will make a $78,000 + investment so much junk. This same unit used 10 RF amplifiers which combine, also not field serviceable.
Repair of switching supplies is pretty hard compared to linear and the voltages and current involved make it quite deadly to work on.
Now picture you self at the top of a mountain after your snow cat nearly fell off the road a few times. Its midnight and there isn't heat because the transmitter is used to heat the building.and its broke. Now tell me if you would really be in the mood to troubleshoot a switching supply down to component level?
It has been decades now, but I do recall a situation using some Wabash regulating transformers. I did not know how they worked but I do know they drew excessive starting current with random switches. I got a fast acting reed relay for triggering a triac at the voltage peak. That helped a lot.
This was a capacitor charging supply. Low level electronics were on between pulses. Each pulse used the relay to energize the (almost worthless) Wabash transformer. The small relay closed at the peak of the supply voltage and was kept closed until the charging cycle was completed. The only trick was to adjust this fast relay to close at the voltage peak.
This was about 5kVA. If I did not say so, the relay kept the thyristors on all the time once the pulse started. The thyristor was used just to keep the transformer starting transient in check.
I also was working on other large (for us) charging system. One charged two banks of capacitor banks to plus and minus 19kV using a vacuum switch to change the polarity of the charging supply. We probably used phase controlled mercury thyratrons so as to keep the efficiency high. These were the days when solid state thyristors and rectifiers were ridiculously expensive. My memory for the details are hazy.
Another technique I used was to have a very leaky (reactance) transformer feeding rectifiers to charge capacitors. This method had a very low power factor, Power factor correcting capacitors were used on the input end so that the power factor started lagging at the beginning of the charge and became leading when the charge was near completion. This arrangement led to very slow charging toward the end of the charging cycle. Extra voltage capability was put into the transformer to get up to the desired charge fairly quickly. The slow charge could, if left going long enough, could charge the capacitors to almost double the desired voltage. Thus, to be on the safe side, two independent methods were used to stop the charge when the desired voltage was reached.
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