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Rectifier Electrical / Electronic Question


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  #11  
Old 11-22-2018, 09:05:58 AM
Thaumaturge Thaumaturge is offline
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Default Re: Rectifier Electrical / Electronic Question

Concur on transformer rating of 3KVA.
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  #12  
Old 11-22-2018, 01:41:42 PM
DMeed DMeed is offline
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Default Re: Rectifier Electrical / Electronic Question

Quote:
Originally Posted by armandh View Post
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]
In an AC system the top of the coil is alternately +30v in relation to the centre tap and -30v in relation to the centre tap. (NO I'm not worrying about peak voltage, just the rms).

So on the +ve half of the wave form I'm looking at 100 amps leaving the top of the coil, travelling through the diode, out through the load and back to the centre tap of the coil. Now where does that 100 amps go? It is +ve in relation to the bottom of the coil, so it wants to complete the circuit by travelling through the lower half of the coil to the -ve side of the AC. I just can't see how it is going to split and go 1/2 toward the -ve side of the coil and 1/2 toward the +ve side. Willing to be wrong, but not seeing it yet.
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  #13  
Old 11-22-2018, 01:49:09 PM
Vanman Vanman is offline
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Default Re: Rectifier Electrical / Electronic Question

Thank you all for the replies.

This hypothetical transformer is just two 30 volt windings in series. The supply is 60 volts AC from a generator.

In thinking about it furfher, it appears that the transformer still functions as an autotransformer, just as it would if it were supplying 30 volts AC @100 amps. In that scenario, without the diodes, each 30 volt winding carries 50 amps. One serves as the primary, the other as the secondary, so the transformer KVA required would be 1-1/2, ie 30 volts x 50 amps.

The same is true with the diodes back in the circuit, except that the windings switch roles every half cycle. The winding supplying the conducting diode is the secondary, and adds 50 amps to the 50 amps from the supply. The other winding is the primary, taking 50 amps from the supply. Next half cycle the windings reverse roles. The 100 amps on the common lead splits, half going in one winding, the other half to the other winding.

If there were a capacitor and NO load on the DC side, it is true that the DC voltage would indeed climb to ~42 volts. But any load would drag this down. With no capacitor and just a resistive load, and the rms DC voltage would still be 30. Charging a battery would fall somewhere between the two. A DC reactor in series with the battery and rectifier would smooth out the current, making less heating in the transformer and generator.

Diodes themselves are a peculiar thing as their forward voltage drop is practically constant, regardless of current. As such their maximum peak current is many times greater than their continuous current. The average current is what must not exceed the continuous current rating. So a 50 amp diode would serve perfectly well here. Half the time it passes 100 amperes, but the other half is zero. The average is thus 50 amperes.
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  #14  
Old 11-22-2018, 02:30:01 PM
armandh armandh is offline
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Default Re: Rectifier Electrical / Electronic Question

Quote:
Originally Posted by DMeed View Post
Willing to be wrong, but not seeing it yet.
"once more into the breach" with pictures

picture it as two separate supplies each supplying 1.5KW 30VDC +

each feeding a load. you are still transferring 3KW of power it does not change

it is the same total area under the curve, see attached
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Old 11-22-2018, 03:40:03 PM
pegasuspinto pegasuspinto is offline
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Default Re: Rectifier Electrical / Electronic Question

3kva is the heat rating of the transformer. When making 3kva of output, it can dissipate the resulting heat without failing. There should be a separate amp rating on the transformer. If you exceed that rating, you risk just blowing the wire apart like a fuse. Your drawing up to 100A from each winding, even if only for a split second.

What the heck battery are you dumping 3kw of charge into? Surplus submarine batteries? That would be pretty severe for any single string of automotive size battery.
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Old 11-22-2018, 05:07:48 PM
Vanman Vanman is offline
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Default Re: Rectifier Electrical / Electronic Question

Haha, no, not car batteries. A submarine battery would be sweet, but too big and heavy for this application. The battery will be an electric forklift battery. Way bigger than a car battery, but not even as big as a single submarine battery cell.

Here are a couple of diagrams depicting current flow during each half cycle of the supply, of course ignoring peak vs RMS etc etc.

In the upper drawing, the upper winding is the primary, absorbing 1-1/2 kva, and the lower winding is the secondary, delivering the 1-1/2 kva.

In the lower drawing, the lower winding is the primary absorbing 1-1/2 kva, and the upper winding is the secondary, delivering the 1-1/2 kva.
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  #17  
Old 11-22-2018, 06:18:17 PM
pegasuspinto pegasuspinto is offline
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Default Re: Rectifier Electrical / Electronic Question

30v*100A is 3000 VA. 60v*50A is 3000VA. The Supply and load is 3kva.
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  #18  
Old 11-22-2018, 06:49:12 PM
Vanman Vanman is offline
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Default Re: Rectifier Electrical / Electronic Question

Quote:
Originally Posted by pegasuspinto View Post
30v*100A is 3000 VA. 60v*50A is 3000VA. The Supply and load is 3kva.
Yes, but since an autotransformer is used instead of an isolating transformer, a portion of the power is delivered directly to the load, with no tranformation required.

In this case, since the voltage ratio is 2 :1, half the power is transformed, and the other half is delivered directly from the supply (after dropping 1/2 of it's voltage in the primary).

In the more typical example below, a 120 volt, 50 amp load, 6 kva, is supplied from a 240 volt, 25 amp source via a 3 kva autotransformer. The upper winding is the primary and is in series with the line and the load, dropping 120 volts at 25 amps. The secondary is in paralell with the load, and adds 25 amps at 120 volts to the 25 amps from the supply.
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  #19  
Old 11-22-2018, 07:52:22 PM
Rich Mc Rich Mc is offline
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Default Re: Rectifier Electrical / Electronic Question

If the 60 volt winding was two 30 volt windings in series and the windings were changed and paralleled then one could then deliver 100 amps DC thru a full wave bridge rectifier, else I would agree with Dmeek that a 6Kva transformer is needed.
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  #20  
Old 11-22-2018, 09:40:20 PM
Vanman Vanman is offline
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Default Re: Rectifier Electrical / Electronic Question

A full wave bridge has the disadvantage of dropping twice as much voltage, about 4 volts instead of 2. You'll find most things designed to deliver low voltage DC use two diodes with a center tapped transformer secondary.

The 60 volt supply is just a 120 volt, two wire generator running at ~1/2 speed. So the fact that the circuit halves the voltage makes it perfect for me.

Because of this, the frequency will be 30 cycles. Therefore I need a transformer with twice as much iron, ie designed for twice as much voltage. A 60 cycle transformer designed with two 60 volt windings (that I will run on 30 volts, 30 cycles), each capable of 50 amps, would be perfect. If it existed lol. Will likely end up winding my own.
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