On a sunny day (Fri, 27 Dec 2013 12:28:36 -0600) it happened Tim Wescott wrote in :
Normally, you will need a way to equalize current For real high power the way would be an inductor _before_ the capacitor. This limits the peak current, but lowers the output voltage, With 3 bridges and 3 inductors it could work. From this in your case it could be simpler to use 4 high power diodes, optionally with the inductor. But that is old design, those inductors are huge, Look here:
Is the capacitor wired properly? If it's backwards, it will draw high current at the voltage you blow the fuse. I saw it quite often, when assembly would put one in backwards
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The nasty thing with that configuration is that the inductor will also carry DC, requiring a quite big choke with an air gap (size comparable to the output transformer in audio applications). Putting the inductance prior to the rectifier and you do not have to handle the DC current and no air gap needed.
You could get a much better answer if you told us what you want to do. Do you really need more than 35 Amps? How much more? What voltage do you need? The Variac is way to small for any more than a few amps. A variac for 35 amps is likely to cost more than $1000 and weigh 50 kilos A Variac is a very different animal to a rheostat and should not be confused with one.
Does each bridge circuit operate correctly on its own???
Some responses from you would help solving the overall problem.
Bridge rectifiers can be purchased in enormous sizes meant for powering heavy machinery. There's no need to gang them up. Check any online parts catalog.
Micro Commercial MB354W-BP Fairchild GBPC3504W
400V 35A continuous at 55C. The surge current charts shows they can survive a 60Hz 4 cycle inrush current of 200A. That inrush current rating is at least as high as normal-grade electrolytic caps allow.
Thank you everyone for your replies. My replies to some of the posts:
I did check each one of the bridges and the capacitors for faulty devices, incorrect polarity, short circuits and open. Didn't find anything there. Also, the three bridges are identical.
I would doubt that inrush current is an issue here. Each time that I ran this experiment, I was careful enough to increase the variac output voltage very slowly. (Ofcourse, connecting an uncharged capacitor bank to full output voltage of the variac would surely trip the circuit breakers on the utility mains line.)
Not that it matters, but we use a 230V/50Hz mains supply with the installation capable of handling 15A of current. (The variac fuse would blow somewhere around 10V AC output with no load connected to the output.)
I am not sure, but I get a feeling that some of us reading this post have "registered" this as "bridge blowing-up". No, it is the variac fuse that blows-up. Infact, there was no heating on any of the bridge (or capacitors for that matter).
I didn't know about the balancing resistor scheme. As already indicated, it may not be the solution for this problem, but, thanks for the idea, simple way of controlling current hogging.
I guess, monitoring the variac current with a clamp meter (or may be even an oscilloscope may be a good idea).
Variacs are usually auto transformers and auto transformers work best when the output is +/-30 % from the input (70 - 130 %) voltage. Trying to get out only 5 % of the input voltage may cause some problems to the autotransformer. If there are some advanced protection mechanism on the variac, this might be triggered at such low setting, even if it might OK around the input voltage.
A no load full wave rectified 230 Vac will generate about 325 Vdc and your capacitor bank will be charged to that potential at the top of each half cycle. As a back of the envelope calculation, assuming the voltage is allowed to drop to 300 Vdc during 8 ms, until the diodes starts to conduct again during the next half cycle, with 6.8 mF the load can consume more than 20 Adc or 6.2 kWavg, quite lot for a single phase feed.
If only possible, I would use a three phase 230/400 V feed and with standard 6 pulse rectifier, 480 Vdc(avg) will be generated with 4.2 % rms ripple _without_ any filtering capacitors. Reducing the ripple, quite small capacitors would be sufficient. If the 480 Vdc is slightly too much/too little, a small autotransformer will handle that change.
Get three 12 V car headlights and connect each in series from a rectifier to the variac output and increase the output voltage slowly and monitor the illumination levels of each lamp. The illumination level should be quite similar and they definitively will even out the current in different bridges.
Use an RMS-reading meter to see what the fuse is "measuring".
Best regards, Spehro Pefhany
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On a sunny day (Sun, 29 Dec 2013 13:32:27 +0200) it happened snipped-for-privacy@downunder.com wrote in :
Exactly, and auto transformers have no mains isolation. It could just be he has the + or - of the bridge output somewhere connected to [test]equipment that is is on mains ground via a 3 core cable. A scope will do.
That would be a very basic mistake, and should always be checked before even applying power.
To understand why auto transformers are attractive with small up/down voltage conversion (say +/- 10 % to +/- 30 %) , one should remember that in an ordinary transformer, all the power is transferred through the iron core, while in an auto transformer, only the power related to the voltage _difference_ goes through the iron core, in this case only
10 % to 30 %.
You can think about the auto transformer as an ordinary transformer with the primary connected to the input voltage and secondary with 10 to 30 % of the input voltage. connect the secondary in series with the input voltage and you get 10-30 % boost (in phase) or 10-30 % drop (connect in antiphase). Only a small amount of the power goes through the iron core.
Trying to run an auto transformer (variac) at only 5 % of input, practically all power flows through the iron core of the transformer, might cause saturation and similar problems.
To the OP I think you mentioned that the application is to charge a battery. Is this true?
Also, the others are trying to tell you that the reason the variac is blowing the fuse may be that one of the diodes is now defective. Just because the fuse is in the variac and the diodes don't get hot does not the diodes are good.
Do you know how to check the diodes with an Ohm meter?
Huh? In *any* transformer, all the primary current goes through the primary, and all the secondary current through the secondary. The secondary amp turns oppose the primary amp turns, resulting (in a perfect transformer having no leakage inductance) in zero net amp turns hence zero flux. Connecting windings in series makes no difference.
Where does the rest go?
You cannot saturate a transformer core with secondary amp turns. Even a shorted secondary won't do it. Saturation occurs where the magnetizing inductance (inductance of primary with open secondary) allows enough current to flow to saturate the core. That's why big current transformers, such as use in distribution equipment buzz like hell if the secondary is O/C. That's a good danger warning.
COTS transformers are generally designed to run as close to saturation as possible, at rated voltage and frequency, to economize on iron.
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