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transformer math

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Jamesj

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I have been trying the math to figure out my unknown transformer specs. I have a few details on the part but the numbers just arent working. I have an input of 120v and 161 turns, the secondary is 88 turns. 161 devided by 88 give me a 1.8295 to 1 ratio...65.59 volts. Problem is I only get 34 or so from the transformer. Where I robbed it from, it ran to dual 35v 40 amp fuses (side by side()paralell?) So I know 65v didnt happen. So What am I doing wrong?
Im trying to attach a pic so you can see and maybe help.
Thanks,
J
 

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I suggest you try looking at how it's wired, there appear to be four secondary windings, these are wired in series/parallel - giving 44 turns, NOT 88 turns.
 
Thats what I was thinking but nothing on the net that mentioned that so I wanted to be sure. Any idea how to figure an amperage estimate? My guess is around 36.5 most likley way to high. The apc ups I robbed it from says it has 120v 60hz 10 amp input. (That is the entire ups unit.) but I took the 10 amps and multiplied by the 3.65 step ratio ratio. I took into consideration the 40 amp fuses, so I didnt try to second guess the input amps.
I am concerned with the primarys 17 gauge winding though, I dont think 17 could handle 10 amps, but being wound in a coil changes things from what I think I understand.
 
I have realized the lm317, and just about any other regulator will not work with the amperage (I think I have) to make it an adjustable power supply unit. It should work well mith my next cnc machine... if I ever get around to building it.
 
The product of the primary side volts * amps gives you the power specification of what the transformer will handle...You know these values, approximately, from the fuse rating.
This same VA magnitude also holds for the secondary side.
Measure the total secondary voltage and you can calculate the approximate value for the secondary amps ....
It looks like there may be some sort of division of the secondary amps between the two sets of parallel windings, but the total secondary V*A quantity cannot exceed the value that was calculated for the primary side.

.... It may be advisable to figure a margin of safety on the input amps rating ... say divide by two.... Also check transformer for heat buildup.
 
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After reading the manual for the third time... it says the ups unit is 1250va it also says the transformer is a step-up. but there were two in the unit a small one and this large one. This large one ran to the batteries, through the fuses on one of the center legs. the others ran to the board in various lacations. the 120v side ran to the board also. So I think Im trying to use it backwards. I have read that thats ok,,, but is it?


well looking at the drawing again, they were using it backwards... or both ways... hmmm
 
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After reading the manual for the third time... it says the ups unit is 1250va it also says the transformer is a step-up. but there were two in the unit a small one and this large one. This large one ran to the batteries, through the fuses on one of the center legs. the others ran to the board in various lacations. the 120v side ran to the board also. So I think Im trying to use it backwards. I have read that thats ok,,, but is it?


well looking at the drawing again, they were using it backwards... or both ways... hmmm

1250 VA agrees with the fuse rating that you have quoted ...120 V * 10 amps.The data sheet shows that the # of secondary turns, 44, is less than the # of primary turns, 161. ...You would think that this particular transformer would be employed as a step-down transformer ... not completely sure about this.

Generally, transformers can be used as either step-up or step-down, as long as you keep to the VA specification for both the primary and secondary.
 
Well these are the specs they sent me...

Description Volts Amps Resistance

Winding # 1 Primary 120 7.50 680m ±50m ohms
Winding # 2 Secondary 16 (Tab1 to Red) 27.5 18.0m ohms max.
Winding # 3 Secondary 16 (Tab2 to Red) 27.5 18.0m ohms max.

Other Notes: Secondary resistance include leads.


So if i run it at 32v am I getting 55 amps? or will it stay 27.5?
 
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Im learning!
Well trying....
Ok, after looking at different things is this correct?
how I wire the secondaries is what changes things.
Since each has 16v 27.5 amps:
I wire...

Paralell will be16v@55 amps
series will be 32v@27.5 amp
or I can have 2 seperate at 16v 27.5 amps.

I hope thats right... even more fun when I actually figure it out! lol
J
 
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