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#60035 - 12/21/05 10:54 AM Transformers: mind-boggling, mysterious beasts
Spark Master Flash Offline
Member

Registered: 07/28/03
Posts: 145
Loc: Las Vegas, Nevada, USA
Transformers are strange and mysterious. Maybe somebody can drop some science on me.

Knowing ohm's law, I need to get my head together on this whole transformer thing. I know how transformers work, so you don't have to deal with that. I'm concerned with transformers as related to ohm's law. I have various questions, but where to start? Let me just start with one set of questions first.

If I have a transformer that's 120v in and 12v out, then I put a 3 amp load on the 12 volt side, it seems that I would have 36 watts and 4 ohms of resistance.

But wait a second - 120 volts is coming in to power things. Do I calculate my amps (3) based on 120 volts? If so, that would mean that I'm working with 360 watts, and 40 ohms of resistance. Is my amp load different on the 120 volt side, or does something in my resistance and my amperage change somehow between the 120v and the 12v sides? Does the 120v side "know" or "feel" the amperage load of the 12v side?

How do I deal with the discrepancy between the amperage on the output side of the transformer at 12 volts, and the input voltage, which changes all the numbers?

I understand that the resistance doesn't change, so this messes with my mind.

This is my first set of questions.

Upcoming: questions related to variac-controlled isolation transformer calculations regarding ohm's law.

Thanks for your help!
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"When in doubt, short it out"

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#60036 - 12/21/05 11:06 AM Re: Transformers: mind-boggling, mysterious beasts
ghost307 Offline
Member

Registered: 05/09/05
Posts: 884
Loc: Chicago Illinois USA
Watts in equals watts out. The amperage changes to make that work. If you get 36 watts out, you had to put 36 watts in. 36 watts on the 12V side is 3 amps; 36 watts on the 120V side is 0.3 amps.
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#60037 - 12/21/05 11:53 AM Re: Transformers: mind-boggling, mysterious beasts
BobH Offline
Member

Registered: 10/16/04
Posts: 134
Loc: Newfane, N.Y USA
Keep primary and secondary calcs separate and it will make sense. Power will remain the same. W.

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#60038 - 12/21/05 11:53 AM Re: Transformers: mind-boggling, mysterious beasts
jwhite Offline
Member

Registered: 12/01/05
Posts: 67
Loc: Trenton, NC, USA
watts in = watts out X power factor. that is listed on the transformer. usually around 98 to 99 percent.

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#60039 - 12/21/05 12:06 PM Re: Transformers: mind-boggling, mysterious beasts
Spark Master Flash Offline
Member

Registered: 07/28/03
Posts: 145
Loc: Las Vegas, Nevada, USA
So...the watts in equals the watts out (thanks ghost & bobH)...the resistance of the wire is NOT the same, but increases with an increase in "heat" (thanks to gfretwell).

Wait a second...I thought the resistance remains the same, but now I'm hearing that the resistance changes with the heat? Does that mean that the resistance increases with an increase in the voltage, or the heat itself?
_________________________
"When in doubt, short it out"

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#60040 - 12/21/05 12:53 PM Re: Transformers: mind-boggling, mysterious beasts
jwhite Offline
Member

Registered: 12/01/05
Posts: 67
Loc: Trenton, NC, USA
spark master, your question will not get answered any faster because you post several new questions.

the resistance of a transformer before you apply power will be near zero.

when the transformer is powered the magnignetic fields cause impeadance. due to the magnetic flux created by the windings.

impedance works the same as resistance when doing calculations.

try asking another specific question. we may be able to help, or you may need to study this in a school.

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#60041 - 12/21/05 03:08 PM Re: Transformers: mind-boggling, mysterious beasts
Spark Master Flash Offline
Member

Registered: 07/28/03
Posts: 145
Loc: Las Vegas, Nevada, USA
Okay, White, I feel you. I feel your pain. I'm white too.

Here's a specific question. If I'm cutting foam on my hot wire cutter using 5 volts AC (and 3 amps) that's coming out of my 12 volt transformer which is controlled by a 120 volt variac set at 50 volts AC output, what exactly makes that different or safer than controlling my voltage with a variac alone set at 5 volts?

I guess the physical separation of the primary and the secondary doesn't mean there won't be some big amp load if somebody inadvertently contacts the cutting wire.

They say to use a transformer after the variac because it's safer than going off the variac's single wire toroid style transformer for some reason. I believe I can change the size of the fuse in the variac to a smaller amperage rating, so I wonder what it is that makes it safer to run the 2nd transformer after the variac.

And White, read this: "...magnignetic fields cause impeadance..." Where did you learn that? Be nice.

Thanks.
_________________________
"When in doubt, short it out"

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#60042 - 12/21/05 03:41 PM Re: Transformers: mind-boggling, mysterious beasts
Spock Offline
Member

Registered: 06/02/05
Posts: 12
Loc: Brisbane, QLD, Australia
fraid i have to agree with jwhite, sounds like you don't know how a transformer works. Mebbe some reading on that first at the local library would help.

Resistance varies with temperature, yes. Every material has a thermal co-efficient, changes it's dimensions as it heats, also changes the excitation energy available to the atoms hence changing the resistance, usually in a relatively linear way.

Magnetic field cause impedance, particularly if there is a variation in the field (changing current flow or core variation)based on lenz's laws of magnetics. This is derived from the inductance just as a resistive element has impedance derived from resistance.

Capacitors exhibit impedance properties also while i'm at it, charge on the plate will eventually reach saturation in a dc situation causing an open circuit and rising impedance based on capacitance.

Having said all that, i did work on a site wher half a dozen engineers took three days to work out why one of their number flew across a subyard after removing a 9VDC powered loop test unit from the secondary of an 3.3kV to 415 trannie.

Spock

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#60043 - 12/21/05 04:12 PM Re: Transformers: mind-boggling, mysterious beasts
Spark Master Flash Offline
Member

Registered: 07/28/03
Posts: 145
Loc: Las Vegas, Nevada, USA
I know how transformers work, one coil around an iron core creates a magnetic field in the iron core and the other coil is electrically energized by the magnetized iron core, and the ratio of the number of coils determines if the voltage is stepping up or down.

I've got some devices with super small trannies,like smaller than an inch wide, small wire, and I wonder what their ratings might be.

Thanks.
_________________________
"When in doubt, short it out"

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#60044 - 12/21/05 04:26 PM Re: Transformers: mind-boggling, mysterious beasts
jwhite Offline
Member

Registered: 12/01/05
Posts: 67
Loc: Trenton, NC, USA
spark master, if you know so much about transformers, then why are you asking questions?

I have tried to answer you... do you want help or not?

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