st599
Well-Known Member
No, the current drawn on the primary depends upon the current drawn on the secondary.
Where to find an 240V to 110V step down transformer in EU?
- Thread starter jacquesdietwaio
- Start date
brianrunyard
Well-Known Member
What arc welders do you know of that require 10kW?
Agreed they are high current, but they are low voltage.
Agreed they are high current, but they are low voltage.
Norman_E
Well-Known Member
That surprises me. I thought current drawn was largely a function if the total resistance of the primary winding in accordance with Ohms law. For example if the resistance of the winding is 20 Ohms, and the voltage 240, then the current is 12 amps. This would be accurate for a DC circuit, but AC calculations are effected by the power factor and impedence, however I still think that the above analysis is not far out.
EDIT. Any electrical engineers out there, to give us chapter and verse on this?
EDIT. Any electrical engineers out there, to give us chapter and verse on this?
jacquesdietwaio
New Member
To everyone that replied,
Thank-you very much for your input.
Reading about everyones opinions and experiences have really helped me a lot to decide what to get.
Kind regards, and hope you all have a pleasant week further.
Jacques
Thank-you very much for your input.
Reading about everyones opinions and experiences have really helped me a lot to decide what to get.
Kind regards, and hope you all have a pleasant week further.
Jacques
john_morris_uk
Well-Known Member
[ QUOTE ]
Surely as soon as you connect a transformer you have put the primary winding in to the mains circuit, and that is what draws the current. I do not know if the current drawn by the primary winding is any different when the secondary winding is connected, or not, but I suspect that any difference is small in relation to the total.
My own 3Kva 110 volt power supply is a large and heavy wire wound transformer, not some kind of electronic step down device.
[/ QUOTE ]Doesn't work like that. The primary will draw current according to what you take out of the secondary side. If you are drawing nothing out of the transformer, it will draw very little current at all on the primary (mains) side. The tiny current that it does draw will be used in heating up the wire in the primary, losses in the core of the tranformer, and possibly some noise generation (sometimes you can hear transformers 'buzzing'. The power designation of the transformer is the designed maximum power that you can put through it, and nothing out = nothing in. The laws of physics can't be denied.
Surely as soon as you connect a transformer you have put the primary winding in to the mains circuit, and that is what draws the current. I do not know if the current drawn by the primary winding is any different when the secondary winding is connected, or not, but I suspect that any difference is small in relation to the total.
My own 3Kva 110 volt power supply is a large and heavy wire wound transformer, not some kind of electronic step down device.
[/ QUOTE ]Doesn't work like that. The primary will draw current according to what you take out of the secondary side. If you are drawing nothing out of the transformer, it will draw very little current at all on the primary (mains) side. The tiny current that it does draw will be used in heating up the wire in the primary, losses in the core of the tranformer, and possibly some noise generation (sometimes you can hear transformers 'buzzing'. The power designation of the transformer is the designed maximum power that you can put through it, and nothing out = nothing in. The laws of physics can't be denied.
john_morris_uk
Well-Known Member
[ QUOTE ]
That surprises me. I thought current drawn was largely a function if the total resistance of the primary winding in accordance with Ohms law. For example if the resistance of the winding is 20 Ohms, and the voltage 240, then the current is 12 amps. This would be accurate for a DC circuit, but AC calculations are effected by the power factor and impedence, however I still think that the above analysis is not far out.
EDIT. Any electrical engineers out there, to give us chapter and verse on this?
[/ QUOTE ]Its not simple to explain. You need to get into the weeds of magnetic field theory etc. The DC resistance of the primary has very lttle to do with it. In simple terms the constanty varying magnetic field produces a very poor power factor (and hence no current in phase) when there is no load on the secondary.
That surprises me. I thought current drawn was largely a function if the total resistance of the primary winding in accordance with Ohms law. For example if the resistance of the winding is 20 Ohms, and the voltage 240, then the current is 12 amps. This would be accurate for a DC circuit, but AC calculations are effected by the power factor and impedence, however I still think that the above analysis is not far out.
EDIT. Any electrical engineers out there, to give us chapter and verse on this?
[/ QUOTE ]Its not simple to explain. You need to get into the weeds of magnetic field theory etc. The DC resistance of the primary has very lttle to do with it. In simple terms the constanty varying magnetic field produces a very poor power factor (and hence no current in phase) when there is no load on the secondary.
Bergman
Well-Known Member
I'll have a go
Think about AC voltage applied to primary. With secondary open circuit.
As voltage rises from 0 current flows through the primary winding.
This causes a magnetic field to be created
As the voltage then starts to fall on the next half cycle the magnetic field starts to collapse.
This collapse creates a voltage opposed to the falling voltage
This limits the current flow so little or no power is dissipated in the transformer.
If there is a load on the secondary the magnetic field creates a voltage in the secondary which causes a flow of current.
This current creates a magnetic field which opposes the field created in the primary and thus reduces the opposing voltage allowing more current to flow in the primary.
You find (not that it matters in the least in this application) that the voltage is reversed between primary and secondary ie a +ve going phase in the primary produces a -ve going voltage in the secondary.
And all this happens 50 times a second.
Sorry its a bit difficult to get a clear picture of how this all works, but trust me
It does!
The pedantic will call me to account for not mentioning the phase relationship between voltage and current but I think that may complicate the explanation.
Just as a sanity check - think how hot the xformer would get if the primary alone were dissipating 10kW
We'll save copper losses, iron losses and hysteresis for next time.
Think about AC voltage applied to primary. With secondary open circuit.
As voltage rises from 0 current flows through the primary winding.
This causes a magnetic field to be created
As the voltage then starts to fall on the next half cycle the magnetic field starts to collapse.
This collapse creates a voltage opposed to the falling voltage
This limits the current flow so little or no power is dissipated in the transformer.
If there is a load on the secondary the magnetic field creates a voltage in the secondary which causes a flow of current.
This current creates a magnetic field which opposes the field created in the primary and thus reduces the opposing voltage allowing more current to flow in the primary.
You find (not that it matters in the least in this application) that the voltage is reversed between primary and secondary ie a +ve going phase in the primary produces a -ve going voltage in the secondary.
And all this happens 50 times a second.
Sorry its a bit difficult to get a clear picture of how this all works, but trust me
It does!
The pedantic will call me to account for not mentioning the phase relationship between voltage and current but I think that may complicate the explanation.
Just as a sanity check - think how hot the xformer would get if the primary alone were dissipating 10kW
We'll save copper losses, iron losses and hysteresis for next time.
VicMallows
Well-Known Member
Excellent explanation .... better than most text books I've come across.
Vic
Vic
Cliveshep
Well-Known Member
I'd change the tools as there are loads of cheap 220v tools all over. A 10kva trannie is a massive bit of kit and most are site transformers with multiple tappings of 16 and 32a c/w breakers. You need a special beast to get 10kva out of a single tapping so a site trannie probably isn't for you. Even if you want to run a welder they are cheap enough in 220 (240v) over here, far cheaper than a trannie would be. A 180A stick welder on ebay would cost you around £80 or less for a fan-cooled turbo welder, we use them at work from ebay - work just fine.