Power conditioner from a 5KVA power transformer? - diyAudio
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Old 24th August 2014, 02:55 PM   #1
AllenB is offline AllenB  Australia
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Default Power conditioner from a 5KVA power transformer?

I came across this by accident when I was given a wrong address, and here it was sitting on their verandah by the front door. I saved them the trouble of disposing of it, as it turns out.. and I tossed it on the back of my bike.

It weighs 50Kg (110lb). The primary is intended to see 415V +/- 5% or 10% (single phase). Secondary is 220V at 22.7A. The mains here is typically 240-245V (with domestic outlets rated at 10A).

If I use this it would be supplying a valve preamp and maybe four or six power channels. These would probably include around 100W of rails typically 250V-500V across, and mostly choke input and shunt regulated.. as well as 100W of heaters from higher Z sources.

I'm not sure whether I'd be wasting my time with this. I'm intending to use balanced rails, decoupling and filtering regardless, where I can. Also whether I'd be better winding the secondary up to 1:1 or even using this in reverse.
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Old 24th August 2014, 03:05 PM   #2
AndrewT is offline AndrewT  Scotland
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It looks like some of the windings have multiple tappings.

I suspect it is the 415Vac primary that is multi-tapped.
This is to get the most out of the transformer and yet not suffer overheating.
One sets the primary tap to closest to the mains supply voltage.

Since the output is rated at 220Vac, it could be reversed, if your supply voltage is not too high.
It would be worth checking the unloaded primary current in the normal direction and when reversed.
I suspect reversing will get too near saturation.

What about using the 415Vac primary on the lowest tapping (maybe 400Vac) and feed in your 245Vac.
the rating will be reduced quite a bit to ~ 3000VA.
still a pretty hefty transformer.
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Last edited by AndrewT; 24th August 2014 at 03:10 PM.
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Old 24th August 2014, 11:57 PM   #3
AllenB is offline AllenB  Australia
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Presumably, the benefits of using an isolation transformer are the reduction of harmonics in the supply, and in the transmission of HF through the interwinding capacitance which is in this case ~7nF.
Quote:
Originally Posted by AndrewT View Post
It would be worth checking the unloaded primary current in the normal direction and when reversed.
As in mains voltage across each winding (separately) and sense the current in each case? Would it be wise to ramp it through a variac?
Quote:
What about using the 415Vac primary on the lowest tapping (maybe 400Vac) and feed in your 245Vac.
The lowest tapping is 415V -10%. Just measured mains..240V at 10am. ie: 140Vac sec when loaded, which this won't be. Though if it were 1:1 I'd be running a couple of hundred watts more from it.

Will using my existing 240V power transformers at ~140V increase copper losses negating the benefits from reduced saturation?
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Old 25th August 2014, 02:24 AM   #4
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depends on the loading of the transformer, but yes running a transformer at half regular line voltage will cut your power down to 25% if you are limited by output regulation.
50% power output if you are limited by copper losses.
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Old 25th August 2014, 02:33 AM   #5
AllenB is offline AllenB  Australia
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Furthermore, if I can't establish a motive for preferring to step either up or down, and decided to put down one extra layer of secondary for no reason other than the compatibility with existing equipment and available power transformers, how would this affect the ratings of this transformer?
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Old 25th August 2014, 04:29 AM   #6
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depends how you define rating.

the core is going to get warm yeah, but at 5 KW you're looking at probably 60C temperature rise.

So if you pull turns off both sides to reduce the voltage, keeping the same current will reduce the heat load proportionately.

Because the surface area won't change significantly, you can increase the current to get equivalent heat output. also note that the outer turns are proportionately more resistance per volt..
So if you take out half the 240vac coil it could be run at 1.41*22.7 is 32 amps and you get 120vac at 32 amps so that's 1/1.41 rating or 71%

for the 415 volt coil cutting it down to 240 is ratio of 1.72 so that's not as bad as cutting the secondary in half.

Transformers this big are easy to rewind you know....
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Old 25th August 2014, 04:47 AM   #7
AJT is offline AJT  Philippines
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the question i have in mind is, are the primary and secondaries isolated?

if so, you can arrange the primary winding split in two, and parallel connected.

or use the secondary for primary connection, and the split primary as secondaries..
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Old 25th August 2014, 09:27 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by johansen View Post
depends on the loading of the transformer, but yes running a transformer at half regular line voltage will cut your power down to 25% if you are limited by output regulation.
50% power output if you are limited by copper losses.
I don't follow you here.

I can see that the current rating of the primary and the secondary will remain the same.
The VA rating based on this current limit will reduce in proportion to the reduction in the primary voltage.
start with 415Vac and we have 5000VA.
feed in 245Vac and we will have 5000*245/415 ~ 3000VA
I think we agree for VA rating.

But you have said that for 50% primary voltage, the regulation limited VA rating would be 25%.
Could you explain?
I thought the regulation (at the lower voltage) at the rated current would be slightly better.
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Old 25th August 2014, 01:11 PM   #9
AllenB is offline AllenB  Australia
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The secondary measures 60mH with a DMM based inductance meter, if this can be trusted, which would suggest a magnetising current of 12A. This could be a problem. Primary measured 160mH (on one of the taps, can't remember which).

Each winding is shared between the two half cores and could probably be split and run as parallel sections.
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Old 25th August 2014, 02:06 PM   #10
AndrewT is offline AndrewT  Scotland
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The split windings allow for very good coupling between the primary and secondary.
This reduces interference.


This is often omitted in the Rcores we see recommended. These cheap Rcores put the primary on one limb and the secondary on the other limb. Reduces capacitance but ruins coupling.

Could you add some primary turns to increase the 415 to 480Vac?
That when split will give you a paralleled 240Vac primary.
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