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Offline vipulan12Topic starter

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the use of transformers
« on: June 29, 2013, 08:32:44 PM »
hey, I was wondering about the current the flows out of a transformer

transformers converts voltages to a higher or lower value but in that process energy or current  is lost when converted to a higher voltage, if that is the case is  current generated when the voltage is stepped down?

also if the voltage in a DC current were to increase would that also mean more current is being drawn from the battery(ohms law) and therefore kills the battery faster?also if that were the case couldn't the battery potentially heat up?

thanks

Offline waltr

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Re: the use of transformers
« Reply #1 on: June 30, 2013, 08:15:54 AM »
Your question as is can not be answered. You seem to have some misconceptions of how transformer work.

Quote
transformers converts voltages to a higher or lower value but in that process energy or current  is lost when converted to a higher voltage, if that is the case is  current generated when the voltage is stepped down?
They store energy as a magnetic field in the core (iron for 50-60Hz) on applying a current from the primary.
When the current in the primary is off (AC mains zero crossing) the magnetic field in the core collapses and this induces a current to flow in the secondary winding.
This means that a transformer does not work with DC since it is the changing magnetic field that transfers energy to the secondary.

The turns ratio of the primary to secondary and the primary Voltage sets the secondary Voltage.

The ratio can be a step down or step up of the output Voltage. If the transform were "IDEAL" then all the energy input will be output. However, there are 'losses' in the transformer (winding resistance and eddy current in the core) that drops the efficiency (less than 100%) and produces heat. This is the same regardless of whether the transformer is a step up or down.

Quote
also if the voltage in a DC current were to increase would that also mean more current is being drawn from the battery(ohms law) and therefore kills the battery faster?also if that were the case couldn't the battery potentially heat up?
Now you sound to be talking about some type of power supply that uses a Battery.
As I stated above the transformer does not work with DC current. Now there are DC to DC converters that use a transformer but they also have a circuit to switch the DC On-Off into the transformer and thus create/collapse the magnetic field in the core to transfer energy.

Lets just treat this DC-DC converter as a 'black box' that we ca change the output voltage of.
Lets also assume the DC-DC convert efficiency is 100% to start.

If the Load resistance is constant and you increase the Voltage applied then the current increases as per Ohm's Law and so does the power (P = I * E). If this increased Voltage is from the above DC-DC converter with the same input Voltage (battery) then the power (and current) drawn from the battery also increases and the battery will be discharged sooner.
Battery time to discharge = battery capacity (Amp-Hr) / current draw (Amps)

Will the Battery heat up? That depends on other factors such as the Current draw verse the battery's size, chemistry and construction. For any current draw battery temperature will increase but will it get above its rated operating temperature?

For this check the Battery's Specs and data sheet. There should be a spec that states the degrees of temperature rise per Amps drawn.

Offline Pogertt

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Re: the use of transformers
« Reply #2 on: June 30, 2013, 06:42:16 PM »
Quote
When the current in the primary is off (AC mains zero crossing) the magnetic field in the core collapses and this induces a current to flow in the secondary winding.


From wikipedia.org:

A transformer is a static electrical device that transfers energy by inductive coupling between its winding circuits. A varying current in the primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic flux through the secondary winding. This varying magnetic flux induces a varying electromotive force (emf) or voltage in the secondary winding.



http://en.wikipedia.org/wiki/Transformer

I argue that as primary voltage and current are varying through all 360 degrees of an AC cycle, that the voltage induced in the secondary is in relation to that changing excitation.




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Offline jwatte

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Re: the use of transformers
« Reply #3 on: July 01, 2013, 09:57:48 AM »
I agree -- it's not just the "collapsing" of the field at the zero crossing that induces the voltage in the secondary winding; it's the act of changing the field.
Everything else waltr said was legit, though, so you should still pay attention to his post :-)

Offline Pogertt

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Re: the use of transformers
« Reply #4 on: July 01, 2013, 11:07:15 AM »
Another hair to split:

"This means that a transformer does not work with DC since it is the changing magnetic field that transfers energy to the secondary."

What about breaker point controlled automotive ignition systems?

Points close, current flows into the coil.
Coil becomes saturated, maximum current flows in coil.
Points open, and current is interrupted and magnetic field collapses.
Spark plug ignites air / fuel mixture.
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Offline jwatte

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Re: the use of transformers
« Reply #5 on: July 01, 2013, 02:37:49 PM »
It's mentioned in the thread above that opening/closing a voltage source is one way to turn DC into AC. Specifically, closed == full current; open == no current; thus, current alternates. (And because of rise/fall times, the "==" only becomes true over time.)

Offline waltr

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Re: the use of transformers
« Reply #6 on: July 01, 2013, 05:04:16 PM »
I didn't do the best explaining the AC only issue with X-formers. The additional explanations for AC sine waves were good.
Thanks

Offline johnwarfin

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Re: the use of transformers
« Reply #7 on: July 01, 2013, 05:17:01 PM »
is  current generated when the voltage is stepped down?

also if the voltage in a DC current were to increase would that also mean more current is being drawn from the battery(ohms law) and therefore kills the battery faster?

also if that were the case couldn't the battery potentially heat up?

for batteries a "dc coverter" is use to step up or down, not a transformer.

when voltage is stepped down generally more current is available. if stepped up, less current.

if a step-up converter is used more current is drawn and battery life is less compared to no converter.

all batteries heat up when more current is drawn. some like small lipo can be destroyed if too much.  really big ones like car battery heat up but usually not noticeable.

Offline Pogertt

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Re: the use of transformers
« Reply #8 on: July 01, 2013, 06:41:37 PM »
It's mentioned in the thread above that opening/closing a voltage source is one way to turn DC into AC. Specifically, closed == full current; open == no current; thus, current alternates. (And because of rise/fall times, the "==" only becomes true over time.)


Once again back to wikipedia.org

http://en.wikipedia.org/wiki/Alternating_current

What you seem to call "thus, current alternates" I would more likely label as pulsating.
As the magnetic flux rises and falls in one direction only.

For the current to alternate in the secondary, the magnetic flux would have to be reversed in the primary winding to produce reversed current flow in the secondary.
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Offline jwatte

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Re: the use of transformers
« Reply #9 on: July 01, 2013, 08:05:59 PM »
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For the current to alternate in the secondary

I see -- "alternate" as in "changes polarity" instead of "changes value over time." (I learned electronics in an obscure Nordic language thirty years ago, and am not actually a native English speaker...)

Transformers can only transfer energy when the current through them changes value over time. Actual alternating polarity is not necessary.


Offline Pogertt

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Re: the use of transformers
« Reply #10 on: July 02, 2013, 10:22:43 AM »
Quote
For the current to alternate in the secondary


I see -- "alternate" as in "changes polarity" instead of "changes value over time." (I learned electronics in an obscure Nordic language thirty years ago, and am not actually a native English speaker...)

Transformers can only transfer energy when the current through them changes value over time. Actual alternating polarity is not necessary.


To produce alternating polarity in the secondary of a transformer, alternating polarity in the primary is required.

Merely increasing or decreasing current in the primary and not reversing the direction of current flow will not reverse current flow in the secondary.


http://en.wikipedia.org/wiki/Dot_convention


"The instantaneous polarities of the voltages across each inductor with respect to the dotted terminals are the same"
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Offline jwatte

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Re: the use of transformers
« Reply #11 on: July 02, 2013, 01:48:43 PM »
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To produce alternating polarity in the secondary of a transformer, alternating polarity in the primary is required.

Obviously. But the questinon was about the voltage/current conversion function of a transformer, which does not require polarity reversal at all. I think we agree on this.

Offline waltr

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Re: the use of transformers
« Reply #12 on: July 02, 2013, 05:32:26 PM »
Yep, jwatt is again correct.

Also, the OP's original question had to with powering for Batteries, a DC source. I and jwatt know of DC-DC power supply topologies that use transformers. These fall into the general term as: SMPS (Switch Mode Power Supply) and there are many type and topologies that do and do not use transformer but they all use a Switch DC source and at least one Inductor (a Transformer can be modeled as three inductors for electrical simulation and calculations).

Some examples
http://en.wikipedia.org/wiki/Flyback_converter
http://en.wikipedia.org/wiki/Single-ended_primary-inductor_converter
http://www.innovatia.com/Design_Center/Cuk_and_SEPIC_Transformer.htm
http://www.eetimes.com/document.asp?doc_id=1273311
http://www.ti.com/lit/an/slyt309/slyt309.pdf

Do note the diagrams and discussions about the CURRENT in the transformer as this is key to understanding how they work.

Offline vipulan12Topic starter

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Re: the use of transformers
« Reply #13 on: July 03, 2013, 02:45:37 PM »
so let's say i was planning to buy a DC to DC conveter how could i reduce the temperature of the battery(power supply)

would adding more batteries in parallel reduce the heat overall?

Offline johnwarfin

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Re: the use of transformers
« Reply #14 on: July 03, 2013, 04:55:43 PM »
yes. and using higher capacity batteries or ones with higher "c rating" runs cooler.

Offline jwatte

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Re: the use of transformers
« Reply #15 on: July 04, 2013, 01:11:41 PM »
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so let's say i was planning to buy a DC to DC conveter how could i reduce the temperature of the battery(power supply)

The heating of the battery comes from the internal resistance (related to source impedance) of the battery. It comes from the formula "power equals resistance times current squared."
From this, you can also tell that the DC DC converter you use has nothing to do with the heating -- it's directly a function of the particular battery (which controls internal resistance) and how much current you draw (which controls current squared.)

If your battery pack has a listed source impedance or internal resistance, and you know your current draw, you can calculate the power burned pretty easily.
If you don't have these numbers handy, go for battery packs with high "C" ratings (meaning they are designed for large amounts of current draw, meaning they likely have low internal resistance.)

 


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