I asked to a local winder make me an PT 150VAC 0,2A because I want to power a schematic drawing 170VDC 143mA (measured) but when connected the new PT voltage dropped to 154VDC, when told to the local winder said his calculations are correct:
Primary done with wire 0,25mm of diameter equal to 0,0491mm2 multiplied by 3A mm2 gives 0,15a so for 230v are 34va secondary done with wire 0,32 of diameter equal to 0,0804 mm2 x 3A gives 0,24A, so for 150v gives 36va
Primary tourns 1610 :230v = 7 tourns for volt
Secondary tourns are 1085 :7 =155v.
Someone can help to tell me if the local winder calculations are ok and if are ok why drops the voltage?
TIA
Felipe
Primary done with wire 0,25mm of diameter equal to 0,0491mm2 multiplied by 3A mm2 gives 0,15a so for 230v are 34va secondary done with wire 0,32 of diameter equal to 0,0804 mm2 x 3A gives 0,24A, so for 150v gives 36va
Primary tourns 1610 :230v = 7 tourns for volt
Secondary tourns are 1085 :7 =155v.
Someone can help to tell me if the local winder calculations are ok and if are ok why drops the voltage?
TIA
Felipe
What are the resistances of the primary and secondary windings?
What is the VA rating of the transformer?
What is the transformer regulation?
Have you measured the AC input voltage and the AC output voltage?
What are they?
BTW,
the MINIMUM secondary rating for 143mAdc is ~280mAac
and for cool running specify ~500mAac
That equates to 0.5Aac * 150Vac = 75VA for the secondary and ~80VA for the Primary.
What is the VA rating of the transformer?
What is the transformer regulation?
Have you measured the AC input voltage and the AC output voltage?
What are they?
BTW,
the MINIMUM secondary rating for 143mAdc is ~280mAac
and for cool running specify ~500mAac
That equates to 0.5Aac * 150Vac = 75VA for the secondary and ~80VA for the Primary.
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it all starts with the load, what voltage and at what current?
then a traffo is designed with the load in consideration...
170VDC 143mA, if you wanted this load, then the unloaded dc rectified voltage would be much higher, up to 250+vdc....for such a small traffo...
then a traffo is designed with the load in consideration...
170VDC 143mA, if you wanted this load, then the unloaded dc rectified voltage would be much higher, up to 250+vdc....for such a small traffo...
That is a calculation of wire current handling. Where is your calculation of voltage drop? Small transformers have poor voltage regulation.merlin el mago said:
You appear to be aiming at a 30VA (150V, 0.2A AC) transformer, yet you have a DC load of 24W (170V, 0.143A). That is unlikely to work - it will overheat if the DC draw is constant. You need a 40-50VA transformer at least, designed to allow for some voltage drop under load.
Your winder has delivered what you asked for. It wasn't what you should have asked for.
The following is all guessed or approximate. I'm more used to dealing with Switch Mode Transformers.
0.250mm Copper Wire has a resistance of 0.462 Ohms/m
0.315mm Copper Wire has a resistance of 0.291 Ohms/m
Winding resistance will be MLT x NT x RM
MLT= Mean Length of Turn in Meters.
NT = Number of Turns.
RM = Resistance per Meter.
Let's say the centre leg of your transformer is a 2.5cm square so the minimum MLT is 0.1M.
RPri = 0.1 x 1610 x 0.462 = 74R4
RSec = 0.1 x 1085 x 0.291 = 31R6
Transformer Equation
B = Vrms/(4.44 X f X Np X Ae)
B = 230/(4.44 X 50 X 1610 X 625E-6)
B = 1.03T
It does not look like you would be saturating the core.
Otherwise those are some approximate numbers for you to check things with. It does look like your resistive losses might be significant in terms of voltage drops.
Oh, as DF96 mentions your 3A/mm^2 is a current handling fudge. For an SMPS Transformer the number used is 4A/mm^2 but an SMPS transformer is smaller with the thermal impedances being nominally lower. Also the windings are much shorter, fewer turns and layers. Maybe 3A/mm^2 is a bit high...
Calculations for Design Parameters of Transformer | Engineer Experiences
Suggests 2A/mm^2
0.250mm Copper Wire has a resistance of 0.462 Ohms/m
0.315mm Copper Wire has a resistance of 0.291 Ohms/m
Winding resistance will be MLT x NT x RM
MLT= Mean Length of Turn in Meters.
NT = Number of Turns.
RM = Resistance per Meter.
Let's say the centre leg of your transformer is a 2.5cm square so the minimum MLT is 0.1M.
RPri = 0.1 x 1610 x 0.462 = 74R4
RSec = 0.1 x 1085 x 0.291 = 31R6
Transformer Equation
B = Vrms/(4.44 X f X Np X Ae)
B = 230/(4.44 X 50 X 1610 X 625E-6)
B = 1.03T
It does not look like you would be saturating the core.
Otherwise those are some approximate numbers for you to check things with. It does look like your resistive losses might be significant in terms of voltage drops.
Oh, as DF96 mentions your 3A/mm^2 is a current handling fudge. For an SMPS Transformer the number used is 4A/mm^2 but an SMPS transformer is smaller with the thermal impedances being nominally lower. Also the windings are much shorter, fewer turns and layers. Maybe 3A/mm^2 is a bit high...
Calculations for Design Parameters of Transformer | Engineer Experiences
Suggests 2A/mm^2
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use the primary current and the secondary current to calculate the heat generated in the windings.
30VA is too small.
The transformer regulation of a very small transformer can be anywhere from 10% to 30%.
Lets assume 15% for a 30VA toroid.
Output voltage = secondary Turns/primary Turns * Vpri / regulation =
You have:
pri T= 1610
sec T = 1085
Vin = 232Vac
regulation = 15%
Therefore Vout = 232*1085/1610/1.15 = 136Vac if you feed into a resistive load at the rated VA.
But you are overloading the transformer so the heat generated inside goes up and the output voltage goes down.
As DF has said, you need to specify correctly or give the full loading information to the transformer manufacturer and ask them to design and manufacture.
And pay for his time and expertise.
Hi AndrewT
I know a little but I do not know a lot and my, lack of, experience as previously mentioned is in Switch Mode Transformers.
I think this is just going to get very confusing for Merlin. I could be wrong but he is calculating VA ratings per winding based on projected voltage and currents in those windings. I think that might be wrong.
For an SMPS the magnetics come with a PTrans figure which suggests the maximum achievable power throughput for a fully wound core/bobbin combination taking into account the Topology that is being used to drive the transformer.
It seems to me that when you talk about a VA rating in respect of a mains frequency iron cored transformer you are referring to a similar achievable power throughput for a fully wound core/bobbin combination.
If that is the case then talking about a VA rating for a particular winding which appears to be what Merlin is doing is looking at things from the wrong direction. The VA rating is applied to the whole transformer.
I will be bold, slap me if I am wrong, but would presume that VA is ultimately based on the thermal resistance of a fully wound transformer assuming, big assumption because everything is variable, equal power losses in primary and secondary windings and in the core itself. A third for each, or perhaps half core and half windings, and the acceptable temperature rise for the end device.
I guess my missing bit would be what you call 'Regulation'.
Things are generally regulated in an SMPS and the nature of the magnetics or rather their windings is such that IR voltage losses are minimal having designed to cope with the I^2R power losses.
I guess that down at 50Hz IR, regulation voltage losses, become more important and have to be considered in parallel with the I^2R power losses. It is highly likely that I am grasping for straws in the dark or otherwise completely wrong...
http://www.milesplatts.co.uk/contentfiles/MilesPlatts-3788.pdf
Search for "VA Ratings" in the pdf.
I know a little but I do not know a lot and my, lack of, experience as previously mentioned is in Switch Mode Transformers.
I think this is just going to get very confusing for Merlin. I could be wrong but he is calculating VA ratings per winding based on projected voltage and currents in those windings. I think that might be wrong.
For an SMPS the magnetics come with a PTrans figure which suggests the maximum achievable power throughput for a fully wound core/bobbin combination taking into account the Topology that is being used to drive the transformer.
It seems to me that when you talk about a VA rating in respect of a mains frequency iron cored transformer you are referring to a similar achievable power throughput for a fully wound core/bobbin combination.
If that is the case then talking about a VA rating for a particular winding which appears to be what Merlin is doing is looking at things from the wrong direction. The VA rating is applied to the whole transformer.
I will be bold, slap me if I am wrong, but would presume that VA is ultimately based on the thermal resistance of a fully wound transformer assuming, big assumption because everything is variable, equal power losses in primary and secondary windings and in the core itself. A third for each, or perhaps half core and half windings, and the acceptable temperature rise for the end device.
I guess my missing bit would be what you call 'Regulation'.
Things are generally regulated in an SMPS and the nature of the magnetics or rather their windings is such that IR voltage losses are minimal having designed to cope with the I^2R power losses.
I guess that down at 50Hz IR, regulation voltage losses, become more important and have to be considered in parallel with the I^2R power losses. It is highly likely that I am grasping for straws in the dark or otherwise completely wrong...
http://www.milesplatts.co.uk/contentfiles/MilesPlatts-3788.pdf
Search for "VA Ratings" in the pdf.
The words appear to agree??
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