Hi,
This may sound stupid to others but I have been thinking of this for the p[ast 3 days.
How do I compute a transformers VA value?
Example:
1. If I have a 48-0-48/10A transformer, how do I express this in VA?
2. If I have a 48-0-48/1000VA transformer, is there a way to know it's current rating?
Is there a formula for this?
Thanks in advance,
Jojo
This may sound stupid to others but I have been thinking of this for the p[ast 3 days.
How do I compute a transformers VA value?
Example:
1. If I have a 48-0-48/10A transformer, how do I express this in VA?
2. If I have a 48-0-48/1000VA transformer, is there a way to know it's current rating?
Is there a formula for this?
Thanks in advance,
Jojo
Re: Thanks!
VA =P (Watts) when the power factor is unity. That is, when the voltage and current are in phase.
For most things we attempt in DIY the power factor will be unity.
There are other losses, primarily to do with rectification, which cause our useable wattage to be less than the rated wattage or VA.
Cheers,
JojoD818 said:
VA =P (Watts) when the power factor is unity. That is, when the voltage and current are in phase.
For most things we attempt in DIY the power factor will be unity.
There are other losses, primarily to do with rectification, which cause our useable wattage to be less than the rated wattage or VA.
Cheers,
Wheew!
Trigon,
Thanks! Now things are getting clear.
I ask this questions because here in the Philippines I can only order transformers using their secondary voltage ratings and current rating. However, almost all projects I find in the net (especially amps) give the transformers' ratings in VA. Now I know how to convert them ratings.
Jojo
Trigon,
Thanks! Now things are getting clear.
I ask this questions because here in the Philippines I can only order transformers using their secondary voltage ratings and current rating. However, almost all projects I find in the net (especially amps) give the transformers' ratings in VA. Now I know how to convert them ratings.
Jojo
Re: Thanks!
A heavy power supply will reduce the VA rating to half. A "normal" power supply reduce less but this is continuously power. If we talk music power I think you could answer yes to your question.
A rectifier bridge and caps creates reactive power which warms the transformer.
EDIT: dhaen was faster.....
Yes, but only if the load is purely resistive!JojoD818 said:
A heavy power supply will reduce the VA rating to half. A "normal" power supply reduce less but this is continuously power. If we talk music power I think you could answer yes to your question.
A rectifier bridge and caps creates reactive power which warms the transformer.
EDIT: dhaen was faster.....
Re: Thanks!
Hi,
1000VA is 1000W only when your transformer is loaded with resistive load. Current and voltage are then in phase (fi is zero, cosinus fi is 1), and you can get 1KW from transformer.
If you have bridges and caps (complex load), then you can get smaller power (Watts) from your transformer.
Regards
P.S. Circlotron, what is next Q&A ?
Edit; I'm slooow.
JojoD818 said:
Hi,
1000VA is 1000W only when your transformer is loaded with resistive load. Current and voltage are then in phase (fi is zero, cosinus fi is 1), and you can get 1KW from transformer.
If you have bridges and caps (complex load), then you can get smaller power (Watts) from your transformer.
Regards
P.S. Circlotron, what is next Q&A ?
Edit; I'm slooow.
Hey guys thanks for the info.
My test transformer is a 44-0-44/4A. I used 4 6A/200V diodes in bridge mode and 6,800uF per rail. I left it plugged without any load, when I came back it was very warm, not hot but my fingers can't stand the temp. Why?
If it heats up without load, what more if I load it?
Jojo
My test transformer is a 44-0-44/4A. I used 4 6A/200V diodes in bridge mode and 6,800uF per rail. I left it plugged without any load, when I came back it was very warm, not hot but my fingers can't stand the temp. Why?
If it heats up without load, what more if I load it?
Jojo
I did check the orientation of the diodes and it's correctly connected. The caps are 6,800uf/63V per rail. I get +/-61.55 using a DMM. The diodes and caps are not even warm. Only the transformer is warm.
I did load it last night with a mono test amp and worked for about 8 hours. It is still working but when I turn the volume up, the transformer really gets warm.
Can this be normal? Maybe I should buy a bigger one.
I did load it last night with a mono test amp and worked for about 8 hours. It is still working but when I turn the volume up, the transformer really gets warm.
Can this be normal? Maybe I should buy a bigger one.
JojoD818,
To determine whether there is a problem, you must estimate the following:
What is the temperature rise unloaded?
What is the temperature rise loaded?
What will the ambient temperature be where it will be installed?
A way of determining whether it is overrunning is to check the secondary voltage drop when loaded. The manufacturer should supply a figure for "regulation".
Cheers,
To determine whether there is a problem, you must estimate the following:
What is the temperature rise unloaded?
What is the temperature rise loaded?
What will the ambient temperature be where it will be installed?
A way of determining whether it is overrunning is to check the secondary voltage drop when loaded. The manufacturer should supply a figure for "regulation".
Cheers,
temp rise
dhaen,
It seems odd but the transformers temp unloaded and loaded seems the same.
I mean, I plug it to the wall socket unloaded and leave it for an hour and it gets warm. Then I connect the amp and play it for an hour (moderate volume) and the transformer temp feels just a little warmer. I will find a way to measure it's temp unloaded/loaded to really see if somethings wrong.
BTW, at zero volume i get +/-62V, at loud volumes it drops to +/-55V! I know that the power delivered to my speakers also drops but is there anything I can do about it?
1. Add caps.
2. Change from 4 diodes to one metal bridge diode.
3. Buy new, higher amp transformer. (I hope not
)
Jojo
dhaen,
It seems odd but the transformers temp unloaded and loaded seems the same.
I mean, I plug it to the wall socket unloaded and leave it for an hour and it gets warm. Then I connect the amp and play it for an hour (moderate volume) and the transformer temp feels just a little warmer. I will find a way to measure it's temp unloaded/loaded to really see if somethings wrong.BTW, at zero volume i get +/-62V, at loud volumes it drops to +/-55V! I know that the power delivered to my speakers also drops but is there anything I can do about it?
1. Add caps.
2. Change from 4 diodes to one metal bridge diode.
3. Buy new, higher amp transformer. (I hope not
)Jojo
JojoD818,
If your transformer gets hot without a load, there are only 4 possibilities I can think of:
1. The transformer was built for 60Hz only, and you are using it on 50Hz. - Unlikely: Most toroids are for both.
2. It's incorrectly wired.
3 It's faulty or badly designed.
4 You have run a cable or bolt through the hole in the centre, and it is acting as a "shorted turn". The centre bolt must be fixed on one side only, and not be allowed to touch anything else conductive.
The transformer will have a regulation figure in the region of 10% to 20%. Depends on the size and quality. Check this against the voltage drop you experience "on-load".
Cheers,
If your transformer gets hot without a load, there are only 4 possibilities I can think of:
1. The transformer was built for 60Hz only, and you are using it on 50Hz. - Unlikely: Most toroids are for both.
2. It's incorrectly wired.
3 It's faulty or badly designed.
4 You have run a cable or bolt through the hole in the centre, and it is acting as a "shorted turn". The centre bolt must be fixed on one side only, and not be allowed to touch anything else conductive.
The transformer will have a regulation figure in the region of 10% to 20%. Depends on the size and quality. Check this against the voltage drop you experience "on-load".
Cheers,
Voltage measurement
In our country the standard mains voltage is 220VAC/60Hz. However, I can only measure 208-210VAC at noon and about 215VAC at night. Maybe that is why I don't get the transfrmer to output enough voltage?
What do you mean by 10%-20% regulation?
Example, theoretically, 44-0-44 X 1.4142 = +/-62.22V rail voltage. Does 10% regulation mean that if under load, 62.22VDC-10%=55.998V? If so then my measurements are correct.
Maybe the transformer is just old. I forgot to tell you that it is about 10 years old. Do transformers react to old age?
In our country the standard mains voltage is 220VAC/60Hz. However, I can only measure 208-210VAC at noon and about 215VAC at night. Maybe that is why I don't get the transfrmer to output enough voltage?
What do you mean by 10%-20% regulation?
Example, theoretically, 44-0-44 X 1.4142 = +/-62.22V rail voltage. Does 10% regulation mean that if under load, 62.22VDC-10%=55.998V? If so then my measurements are correct.
Maybe the transformer is just old. I forgot to tell you that it is about 10 years old. Do transformers react to old age?
I think you might be looking at the transforemr from the wrong direction though, as most transformers are specified with voltages under load. And from your readings under load yoou would get a nominal 40 - 0 - 40 transformer (which is a fairly standard spec unit). However, under no load condidtions, the voltage will rise by some ammount (due to the regulation as dharn said), in you case around 10%.
However, I would in general say that if a transformer ran as hot under no load as it did under load that there was a problem with it. BUT, I would also say that if this was the case you wouldn't be able to run it for eight hours under heavy loading, so for now I would not worry too much and just make sure you put it in a well wevtilated case, as far away from your other ocmponents as possible.
However, I would in general say that if a transformer ran as hot under no load as it did under load that there was a problem with it. BUT, I would also say that if this was the case you wouldn't be able to run it for eight hours under heavy loading, so for now I would not worry too much and just make sure you put it in a well wevtilated case, as far away from your other ocmponents as possible.
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