mains frequency//secondary current

Jonathan Bright · 24th January 2009, 08:10 AM

Here in Australia the mains power is around 240 volts at 50 c/s.

I picked up some secondhand PS transformers that were salvaged from equipment made in the US of A and designed for 60c/s. They have multi-tapped primaries (115v,120v and 125vs) and secondaries of 36 volts @ 3 amps. Now they seem ideal for my purposes and I hope to run the primaries in series and use the secondaries to power some "double mono" power amps. But there is a question about the current that is now available in their adopted country. I have been getting two opposing pieces of advice.

The first opinion says that as they were designed for 60c/s I will need to DERATE the secondary current 20% if they are run on Aussie mains of 50c/s.

The second opinion states the opposite and suggests I now have access to 20% MORE current.

Both sets of advice are coming from well qualified people. Anyone come up with a definitive answer please?

Thanks, Jonathan

Jonathan Bright · 24th January 2009, 10:05 AM

Pleeeeeese. Someone must know.
Jonathan

AndrewT · 24th January 2009, 10:42 AM

you will certainly not get more current (VA) by going down from 60Hz to 50Hz.

If the transformer is designed to only run on 60Hz then you will have to de-rate it for 50Hz use. But you are never likely to run the transformer at near maximum rating after start up.

One clue to whether it's suitable for 50Hz use is to check running temperature when the output is open circuit.
If it seems to get too hot then maybe it really is a 60Hz transformer.

I think that adding primary windings will help reduce that heat, but I suspect you will not need it.

Can you safely measure the no load primary current? I guess it should be <=10mA. If it's a lot less then 50Hz is OK.

Steerpike · 24th January 2009, 11:04 AM

The seriesed primaries will be good for 2x125=250V at 60Hz, so 240V input is already giving a bit of leeway.

One other thing to note: if you are running the primaries in series, you should ensure both transformers are loaded equally - either by wiring the secondaries in series too, or in parallel.

MondyT · 24th January 2009, 11:53 AM

Hi Jonathan

If the transformer is designed for standard 60Hz mains input then you are going to need to reduce the input voltage by a factor of 50/60=0.833 at 50Hz or you will more than likely saturate the core. If you have a 250v tap for 60Hz, then the maximum 50Hz voltage you can apply is 208v. Manufacturers do not normally give much leeway except say a 10% over voltage capability as copper costs money!

The loading on the secondary does not have any bearing on this as the magnetising current for most part remains the same despite loading and therefore the flux density in the core remains the pretty much same from no load to full load

Cheers
Ray

Lumba Ogir · 24th January 2009, 09:39 PM

Ray,
nice explanation.

Jonathan Bright · 25th January 2009, 04:15 AM

Thanks guys. Appreciated. Jonathan

24th January 2009, 08:10 AM	#1
Jonathan Bright diyAudio Member Join Date: Feb 2004 Location: Queensland	mains frequency//secondary current Here in Australia the mains power is around 240 volts at 50 c/s. I picked up some secondhand PS transformers that were salvaged from equipment made in the US of A and designed for 60c/s. They have multi-tapped primaries (115v,120v and 125vs) and secondaries of 36 volts @ 3 amps. Now they seem ideal for my purposes and I hope to run the primaries in series and use the secondaries to power some "double mono" power amps. But there is a question about the current that is now available in their adopted country. I have been getting two opposing pieces of advice. The first opinion says that as they were designed for 60c/s I will need to DERATE the secondary current 20% if they are run on Aussie mains of 50c/s. The second opinion states the opposite and suggests I now have access to 20% MORE current. Both sets of advice are coming from well qualified people. Anyone come up with a definitive answer please? Thanks, Jonathan __________________ "It was the Spring time of the year when aunt calls to aunt like mastodons across the frozen waste." P.G. Wodehouse.

24th January 2009, 10:05 AM	#2
Jonathan Bright diyAudio Member Join Date: Feb 2004 Location: Queensland	Pleeeeeese. Someone must know. Jonathan __________________ "It was the Spring time of the year when aunt calls to aunt like mastodons across the frozen waste." P.G. Wodehouse.

24th January 2009, 10:42 AM	#3
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	you will certainly not get more current (VA) by going down from 60Hz to 50Hz. If the transformer is designed to only run on 60Hz then you will have to de-rate it for 50Hz use. But you are never likely to run the transformer at near maximum rating after start up. One clue to whether it's suitable for 50Hz use is to check running temperature when the output is open circuit. If it seems to get too hot then maybe it really is a 60Hz transformer. I think that adding primary windings will help reduce that heat, but I suspect you will not need it. Can you safely measure the no load primary current? I guess it should be <=10mA. If it's a lot less then 50Hz is OK. __________________ regards Andrew T.

25th January 2009, 04:15 AM	#7
Jonathan Bright diyAudio Member Join Date: Feb 2004 Location: Queensland	Thanks guys. Appreciated. Jonathan __________________ "It was the Spring time of the year when aunt calls to aunt like mastodons across the frozen waste." P.G. Wodehouse.

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24th January 2009, 11:04 AM	#4
Steerpike diyAudio Member Join Date: Jan 2006 Location: Johannesburg, South Africa	The seriesed primaries will be good for 2x125=250V at 60Hz, so 240V input is already giving a bit of leeway. One other thing to note: if you are running the primaries in series, you should ensure both transformers are loaded equally - either by wiring the secondaries in series too, or in parallel.

24th January 2009, 11:53 AM	#5
MondyT diyAudio Member Join Date: Dec 2008	Hi Jonathan If the transformer is designed for standard 60Hz mains input then you are going to need to reduce the input voltage by a factor of 50/60=0.833 at 50Hz or you will more than likely saturate the core. If you have a 250v tap for 60Hz, then the maximum 50Hz voltage you can apply is 208v. Manufacturers do not normally give much leeway except say a 10% over voltage capability as copper costs money! The loading on the secondary does not have any bearing on this as the magnetising current for most part remains the same despite loading and therefore the flux density in the core remains the pretty much same from no load to full load Cheers Ray

24th January 2009, 09:39 PM	#6
Lumba Ogir Banned Join Date: Apr 2005 Location: Stockholm	Ray, nice explanation.

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