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Rectifier Electrical / Electronic Question


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  #1  
Old 11-21-2018, 05:54:46 PM
Vanman Vanman is offline
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Default Rectifier Electrical / Electronic Question

Suppose I have a 60 volt, 50 ampere, single phase AC source, and I want to supply a 30 volt, 100 ampere DC load.

The source and the load need not be isolated.

Center tapped transformer secondary windings in conjunction with two diodes are frequently employed so as to achieve full wave rectification without the need for a four diode full wave rectifier.

I wish to use a single winding transformer with a center tapped 60 volt winding, and two diodes, connected across the source.

The question is: What is the required ampacity of the transformer winding?

I believe it would be 50 amperes. Is this correct?
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  #2  
Old 11-21-2018, 06:08:22 PM
Thaumaturge Thaumaturge is offline
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Default Re: Rectifier Electrical / Electronic Question

Roughly... rule of thumb is 20% overhead on desired rating. Critical design spec for transformers however isn't amps, but VA, or basically continuous heat load on windings without release of magic smoke.

For the kind of load you are looking for though, I'd go for a switching supply.
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If you could live with 220VAC input TWO of these in paralell would cover it for roughly $200 https://m.banggood.com/AC-200V-250V-...yABEgJShvD_BwE
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Last edited by Thaumaturge; 11-21-2018 at 06:31:00 PM. Reason: Fat fingers, touch keyboard.
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Old 11-21-2018, 06:26:51 PM
Bob Willman Bob Willman is offline
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Default Re: Rectifier Electrical / Electronic Question

Is your input voltage RMS AC or a peak voltage? What your diagram shows is not a standard transformer which would have 2 windings, a primary and a secondary. If you are going to draw 100 A from the winding, the wires in the winding must be capable of 100 A. The diodes will also need to be rated for 100 A. Without some capacitors after the diodes the output will be pulsed DC not a true DC voltage. I'm not sure your circuit will do what you want it to.

Bob
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Old 11-21-2018, 07:48:20 PM
Vanman Vanman is offline
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Default Re: Rectifier Electrical / Electronic Question

The AC voltage of the supply is RMS. The circuit is just going to be charging a big battery, sometimes with a load on it. So it doesn't need to be clean or smooth or even regulated DC.
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Old 11-21-2018, 08:41:45 PM
armandh armandh is offline
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Default Re: Rectifier Electrical / Electronic Question

do it in Watts

100x30=3000 watts

3000/117 about 26 amps
3000/60 is yes 50 amps

RMS volts not peak
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Old 11-21-2018, 10:14:34 PM
DMeed DMeed is offline
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Default Re: Rectifier Electrical / Electronic Question

Think about when the diodes are conducting. Each diode is conducting for 1/2 the AC waveform and off for the other 1/2 the AC waveform. So each diode is acting alone. For 100 amps output, you will need 100 amps through each diode and from the transformer windings as well. You are getting the 100A*30V or 3000 watts alternately from each half of the transformer. So the transformer needs to be rated for 6000 watts.

Are the 60V CT transformer windings fed from a separate 60v AC source or are they are the sole source of current for feeding the diodes. It might make a difference if the current through the diode can come from the 60V AC source separately from the transformer winding (and the windings are just splitting the centre. Might work with a 50 amp winding in that case (?someone else will have to chime in on that one?) There was a thread showing how to feed a single phase split 120/120v circuit from a 240v Generator with a transformer rated 1/2 the generator wattage. In any case the diodes will have to be 100 amp anyway.

Thinking further - with diodes in there steering the current each 1/2 cycle I don't see how it would balance the current over both windings.

Last edited by DMeed; 11-21-2018 at 11:01:49 PM.
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Old 11-21-2018, 11:10:14 PM
Railroads Railroads is offline
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Default Re: Rectifier Electrical / Electronic Question

Keith, What do you need 100 amps at 60v for?

Is this some exciter unit or something?

Robert
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Old 11-21-2018, 11:33:30 PM
G.M.Johnson G.M.Johnson is offline
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Default Re: Rectifier Electrical / Electronic Question

If it doesn't have to be exactly 30 volts a 36 volt fork truck battery charger will do the trick. You can find them in 208/240 single phase input versions. If you are stuck with exactly 60v input then scratch that idea.
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Old 11-22-2018, 04:10:35 AM
pegasuspinto pegasuspinto is offline
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Default Re: Rectifier Electrical / Electronic Question

Your supply as described will produce about 42 volts DC. Just keep that in mind!
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Old 11-22-2018, 08:32:05 AM
armandh armandh is offline
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Default Re: Rectifier Electrical / Electronic Question

Quote:
Originally Posted by DMeed View Post
Think about when the diodes are conducting. Each diode is conducting for 1/2 the AC waveform and off for the other 1/2 the AC waveform. So each diode is acting alone. For 100 amps output, you will need 100 amps through each diode and from the transformer windings as well. You are getting the 100A*30V or 3000 watts alternately from each half of the transformer. So the transformer needs to be rated for 6000 watts..
I'm not sure that is correct

if the 60 Volt winding is producing 3 KW, [50 A]
each half can produce 1.5 KW. [50A at half the voltage]
summed together after the diodes still 3 KW [half the voltage at 100 A]

the half wave, the diodes are alternately not conducting, does not reduce the total power transferred [IMHO]


the kind of counter EMF kick one can get here it might not be a bad idea to put a MOVs or neon bulbs across the diodes
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Last edited by armandh; 11-22-2018 at 08:52:35 AM.
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