Power conditioner from a 5KVA power transformer?

AllenB

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Paid Member
2008-10-18 11:31 am
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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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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AllenB

Moderator
Paid Member
2008-10-18 11:31 am
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.
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?
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?
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
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?
 
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....
 
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.
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
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.
 
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.
 
suppose at 1 volt input you get .9 volts out at 1 amp, 10% regulation, .1 watts power loss.

at .5 volts in you get .4 volts out at 1 amp, 20% regulation. 50% power output, same .1 watt power loss, same temperature rise.

at .5 volts in you get .45 volts out at .5 amps, 10% regulation, 25% power output.

this isn't complicated Andrew, that's why i said 50% power output if limited by temperature rise, 25% if limited by regulation.

reduced core losses should be insignificant at 5KVA but sometimes they run those transformers at 30C no load temp rise and 115C full load temp rise. incredible really.


Given constant flux density, a transformers power density follows the 4/3rds power of the machine dimensions.
so does leakage inductance.
this is why a gigawatt sized transformer has an efficiency of 99.9% but an impedance of 16%.

You would be better off building transformers purposely leaky so to get better power factor and increased conduction angle on those rectifiers. yeah, the regulation sucks but you could even program that into it with a resonant voltage regulator, which takes advantage of the saturated magnetic shunts to store energy in the 3rd harmonic of the resonant tank, yeah it wastes energy but so do linear regulators.
another option would be saturateable reactors.. but I'm not sure you could make either of those lower noise than a linear regulator.
 
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TonyTecson

diyAudio Moderator Emeritus
2003-05-29 2:57 am
Maybunga, Pasig City
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.

now that it is confirmed, split the primaries and use them as isolation traffo with 1:1 ratio..
the traffo is very big, bigger than you will ever need...

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..
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
The 415 + 10% tap (456) is within 5% of 480. Will this be close enough?

When I measure the magnetising current on this winding, will this increase fourfold when I split the winding?

How important is it that each half has precisely the same number of turns?
 
The 415 + 10% tap (456) is within 5% of 480. Will this be close enough?

When I measure the magnetising current on this winding, will this increase fourfold when I split the winding?

How important is it that each half has precisely the same number of turns?

5% increase could be a 10% increase in magnetizing current and a 20% increase in core losses. depends.

i suggest adding turns instead.

yeah if you cut the coil in half and input voltage in half, magnetizing current will double, because magnetizing va's and watts will stay the same

if the coils are running in parallel, they must be the same turns. a transformer this sized won't have a problem letting you know if it isn't.. just, use an amp meter to check circulating current.
 
i have an analog watt meter. you can find them on ebay for 20$ sometimes. i suggest you buy one. rewind the current sense coil with 50+ turns of oh, 16 awg instead of 2 turns of 4 awg wire and they are very sensitive.. just don't get any dirt in the bearings..

i have an old tube transformer from a 1965 era John Fluke differential volt meter, should i use that?

or, should i give you a chart of a microwave oven transformer running at a stock 1.95T flux density :D

hmm.. the transformer from a 1970's era kepco linear should be a good benchmark.. unfortunately, only the MOT will have a stock 50C temp rise no load..

has OP even plugged his in yet?

sweet.. i still have it.
http://johansense.com/bulk/motlossbig.png

the step, in the chart, is because my kill-a-watt meter (now deceased) must have had a bad callibration for low watts.. but once you get to 80 volts it should be accurate..
that chart is this MOT transformer: http://johansense.com/bulk/tappedmot_2.JPG (i tapped the secondary in 15 locations)
then did it again here: http://johansense.com/bulk/tappedmot_3.JPG man that was a lot of work.
life was pretty boring those years though..

here you go http://www.ebay.com/itm/Landis-Gyr-...ndard-Residential-240-volt-60-hz/111364336922
 
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suppose at 1 volt input you get .9 volts out at 1 amp, 10% regulation, .1 watts power loss.

at .5 volts in you get .4 volts out at 1 amp, 20% regulation. 50% power output, same .1 watt power loss, same temperature rise.

at .5 volts in you get .45 volts out at .5 amps, 10% regulation, 25% power output.

this isn't complicated Andrew, that's why i said 50% power output if limited by temperature rise, 25% if limited by regulation..........................
I don't follow your argument.

If the transformer has a 2:1 turns ratio then 400Vac input will give an open circuit output of 200Vac.
If that transformer has a 10% regulation then the output voltage will drop to
100/(100+10)*200 = 181.8Vac when the maximum rated output current is drawn by a resistive load.
Now explain what you are referring to?
 
I don't follow your argument.

If the transformer has a 2:1 turns ratio then 400Vac input will give an open circuit output of 200Vac.
If that transformer has a 10% regulation then the output voltage will drop to
100/(100+10)*200 = 181.8Vac when the maximum rated output current is drawn by a resistive load.
Now explain what you are referring to?

Will using my existing 240V power transformers at ~140V increase copper losses negating the benefits from reduced saturation?

I answered these in great detail.

In your example, cutting the input in half, 200 volts, will deliver, not 181.8 divided by two (90.5 volts), but rather, around 81 volts under the same output current.

The transformer run at half rated input voltage will deliver half the rated output current at the same rated impedance (which for small transformers is 90% copper losses)

in your example, you have to supply 40 volts ac to the input just to get the rated current to flow through a shorted secondary..
 
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