Two Transformers

[bsparky]

I hope this far from the CT Trans. post because I don't want to get caught up in that one. All you guys are sharp.....Getting back to my Capacitance post awile back, I finally got back to the club and measured the current that the skeet machines were drawing' 44 amps. The transformers were suppose to be 40 amps at 12 volts, now I can't believe that 44 amps for about 5 seconds would cause the supplies to fall to 7 volts if they are truely rated for 40 amps. Now my question is this if I were tie two power supplies after the bridge rectifiers together would this be safe and would this take care of my problem. I know its not very elegante but the transformers are bought. The others we would take care of properly. Any insight.... Thanks Bill

[upsmaster]

hi
as a rule D.C will not share the load equaly...you can try to force it... may not be easy..A.C. shares better.
joe

[gerty]

Have you checked the primary voltage ?
It's not fed by 300' of 16 ga extension cord is it? Also, have you checked the voltage on the secondary before the bridge ? I agree with you, 4a over for 5 secs shouldn't cause such a drop...

[Chris Smith]

When coupling two large amperage AC transformers together in the AC mode, try diode protection between the two, that or a very large Resistor [wattage] of one ohm or less to stop any imbalances between transformers. <p>Impedance mis-matches can cause the transformers to over heat.<p>IF however you rectify both, and then deliver the DC amperages together, and just use a simple [large] diode to isolate them.

[bsparky]

How do you wire the diode for isolation. Do you come off one of the bridge rect. then thru the diode then connect the supplies together? Just one diode on one supply? Bill

[Chris Smith]

There are many configurations. <p>You take A Center tap approach, which equals one lead from each transformer in phase, and in a Y configuration you can Join the two leads together via one diode each assuming your center tapping the TWO for a higher current, and not a higher voltage.<p> It helps take up the slack in impedances, current timing mismatches, etc, as long as you can use the DC component, or your going to fully rectify it any way? <p>Like a center tapped transformer, but to make the joint you form it with two diodes in the shape of a Y with both leading OUT [or in] so that they must mutually conduct through a isolating diode to get to the sink. <p>Also a single Large wattage resistor can help to couple two transformers in the AC mode, [center tap or no center tap] because any imbalances will feed into this resistor and tend to heat it up rather than the Windings per see. Like a fuse/ weak link, BUT Not perfect? <p>OR you can make two bridge rectifiers, one for each transformer and simply join their out puts, but if there is a large mismatch in current, then join them through ONE large wattage, low resistance, resistor on ONE of the higher current output transformers to even out your feed. <p>Two supposedly identical transformers, are not automatically identical.

[bsparky]

Thanks Chris I will try it with the full bridge trigger the launcher with each power supply alone to see which one has the most current then raise or lower the resistance to match the current. Another question to get 45 - 50 amps DC after bridge rectificaton how does that translate into KVA which is the way transformers are rated.

[bsparky]

Yes Gerty I check the AC when it was firing there was no change on the AC side at least in voltage did not check current. Should I check current if the voltage did not drop at all?

[gerty]

If the voltage didn't drop on the primary it would indicate a sufficent supply . How about the voltage on the secondary, before the bridge, if that didn't drop low I would suspect a bad bridge.
What are you using for a bridge, is it rated high enough, is it getting warm/hot ,it shouldn't..
Don't overlook a loose connection, at 44 amps it'll drop the voltage as you describe..

[Chris Smith]

12 volts out at 50 amps is 600 watts. <p>To do a KVA you need to measure the input draw of each transformer, and add them up to do the math in Watts or KVA.

[bsparky]

Gerty didn't check secondary AC , bridges are rated for 50 amps all connections are soldered execpt at the bridge were I used crimp connections and no they are not getting hot. Chris I quess what I'm asking is if I want 12vdc at 50amps what rating for the transformer should I ask for. say starting this from scratch and how would you figuer this out. There must be some plaace on the web that would explain this. Just haven't found it yet. Thanks guys.

[rshayes]

The next step is to determine where the voltage drop occurs. The secondary voltage of the transformer may be dropping under load or the bridge rectifier may be a problem.<p>The current rating on rectifiers is based on the current it can carry before burning out. This is often where the voltage drop is above .6 to .8 volts you normally expect from a silicon junction diode. At maximum rated current the voltage drop could be 1 to 2 volts, depending on the amount of heatsink available. A bridge has two diodes conducting at the same time, so the drop could approach 3 or 4 volts. Under heavy load, the transformer may drop an additional volt or two. Measuring these transient drops might be awkward. It is a little too slow for viewing on an oscilloscope and a little fast for accurate reading on a digital voltmeter. Even an analog meter might not respond fast enough.

[Chris Smith]

What I would use for “rough math” is that if you want absolutely 600 watts out or point 6 kva, and your using TWO transformers to produce this, That’s point 3 each in theory. <p>I would add in a safety factor and use two separate “point .5 kva” transformers to be on the safe side, giving you about 35% to 40% over kill as a minimum. If you anticipate more draw some where down the line use bigger transformers.<p>Also the input KVA rating is a consumption Value of the transformer with load, before it drops in voltage, and it may not represent the actual out put values that you want? Heat is also another factor.<p>There is always a loss when “Transforming” electricity so to be on the safe side and not come up short on the out put, I would go for TWO ,....point 6 or point 75 kva transformers in case they are poorly constructed. This way the heat is down, the strain is down, and the safety factors are up.