Power Transformers 50Hz vs 60Hz

[tubeswell]

Up until now I've used NZ-made PTs for my builds and have had a number of people commenting on how big the iron looks.

Last month I got a Weber 6A14 kit with their EU model PT and the PT is quite a bit smaller than the ones I get made locally.

Then I read on the Weber forum about guys in Oz and UK (where they also have 230V @50Hz mains) reporting how the same model PT as the one I just bought in my kit don't seem to last very long before they short. (Okay so I've gone back to my local winder for another Pt for this project).

But I thought I would share out what I found and hopefully spark some useful debate that might lead to (goodness knows what?) improvements is US export transformers. There seems to be a good reason why my local PT winder makes such big PTs.

50 Hz vs. 60 Hz Power Transformers

Transformers

[Diablo]

From what I read, the transformer size should be roughly inversely proportional to the frequency. So, export power transformers should be around 20% larger in order to survive at 50 Hz. Obviously, the output transformers don't care what frequency drives the power transformer - so they can be universal.

Utility frequency - Wikipedia, the free encyclopedia

[Steve Conner]

I've written about this before, in the context of the old Crown SXA amp I restored. The transformers were 60Hz only, and they ran hot and buzzed loudly here, which is as you say, 230V, 50Hz.

I ended up replacing them for Hammond 290KXs, which are marked "50/60Hz" and have about 20% more iron, in spite of being wound for bridge rectifiers, which should make them more efficient.

scopeblog Transformerectomy!

[Diablo]

While rooting around at work today, I came across a new old Stancor transformer in the box. It was made by Emerson / Stancor at the St. Louis plant in the old days. The spec sheet also rates it at 50/60 Hz - so that must have been a common practice in the old days to make transformers suitable for either frequency, but today the bean counters would say that's too large for 60 Hz.

[Old Tele man]

The spec sheet also rates it at 50/60 Hz - so that must have been a common practice in the old days to make transformers suitable for either frequency, but today the bean counters would say that's too large for 60 Hz.

...back then things were designed for robustness, today they're right-sized, ie: marginally adequate.

[zzzapfizzz]

Up until now I've used NZ-made PTs for my builds and have had a number of people commenting on how big the iron looks.

Last month I got a Weber 6A14 kit with their EU model PT and the PT is quite a bit smaller than the ones I get made locally.

Then I read on the Weber forum about guys in Oz and UK (where they also have 230V @50Hz mains) reporting how the same model PT as the one I just bought in my kit don't seem to last very long before they short. (Okay so I've gone back to my local winder for another Pt for this project).

But I thought I would share out what I found and hopefully spark some useful debate that might lead to (goodness knows what?) improvements is US export transformers. There seems to be a good reason why my local PT winder makes such big PTs.

50 Hz vs. 60 Hz Power Transformers

Transformers

My impression was that this issue has been known for years? Do Weber VST still ship this faulty PT to 240V customers? Seriously, I like to know as I have two of them here ready to go into builds intended for a working musician.

[Diablo]

OK, I just looked at Weber's transformer page with the specs.
The 6A14 kit comes with the 25130 PT. However, you can choose USA or EU model. The EU model does not say it is suitable for 50/60 Hz. They also sell another 25130 PT, and this one is the International model! It costs $4 more than the EU model, and the spec sheet says it's suitable for 50/60 Hz. I also calculated that it uses 27% more steel than the EU PT - based on the lamination stack outside dimensions.

[R.G.]

The facts is that power transformers must make a big enough primary inductance to limit the magnetizing current below some safe level. They must also provide a sufficient number of turns and core cross sectional area to limit the magnetic field intensity to below the saturation value. These two conditions are in fact two different ways of saying the same thing.

The limitation on the amount of power a transformer can provide is the internal temperature rise versus the temperatures the insulation system can withstand. In general, higher temperature insulation classes are more expensive, so the design optimization process tends to run the transformer right up against the edge of temperature rise for the given insulation class.

Given that, one has two choices when designing a power transformer to a particular insulation temperature rise/class. (1) Use enough core stack to work OK at 50Hz and make one model. If it works OK with low enough temp rise at 50Hz, it will have extra margin at 60Hz and one model fits all, except that you pay about 20% more for the iron and 10% more for the copper on every 60Hz transformer than you need to. (2) you can design one 50Hz device about 20/10% bigger/more expensive for the 50Hz market and another for the 60Hz market and get the lowest cost for each customized build. These are the competent engineering approaches. There are choices (3) through (N) which get cheap on some part of this. To the degree that the transformer runs hotter, its life will be shorter.

If a transformer runs OK at 50Hz, it will run OK at 60Hz at the same volts per turn, and will have a bigger power capability by the amount that the lower magnetizing current decreases heating, but will still not be as good as a 60Hz transformer specifically designed for that because the longer length per turn will cause additional resistive losses in the windings unless the core stack is also made bigger still to get fewer turns and get the copper losses lower.

A competent transformer designer can take this all into account depending on what he's told to design. A beginner will miss some things.

In general, for a given insulation class, the lower the lowest frequency on the primary, the bigger the transformer.

[Enzo]

I am no transformer engineer, but it would seem to me that if it takes an extra 20% materials to make a good 50Hz transformer, if a transformer maker is planning to sell 100,000 of them in the USA and another 100,000 of them for export, why would they want to add the uneeded extra 20% to the ones intended for US use? Make two different transformers.

[tubeswell]

They also sell another 25130 PT, and this one is the International model! It costs $4 more than the EU model, and the spec sheet says it's suitable for 50/60 Hz. I also calculated that it uses 27% more steel than the EU PT - based on the lamination stack outside dimensions.

Dang - Why didn't they include that with the full amp kit they sent me? I've just forked out another $140 for a replacement PT. Oh well

[tubeswell]

My impression was that this issue has been known for years? Do Weber VST still ship this faulty PT to 240V customers? Seriously, I like to know as I have two of them here ready to go into builds intended for a working musician.

Well I would like to know too. If you find out anything before I do, please don't hesitate to let me know. Meantime I'm playing it safe/cool by leaving this one on the shelf (at least until I find out some more)

(BTW the posts I referred to complaining about their EU PTs shorting out were made in the last couple of weeks, so obviously it is still happening I guess)

[unparalleled]

Do you know Basil Patchett the transformer winder? (Auckland)

[GainFreak]

Hi,

I have a question concerning rewiring a tube preamp made in Europe for 230V/50Hz operation to be used in US. Iron was initially intended for 50Hz mains so this is not an issue here.
The power transformer has two 115V primaries which are now in series. Except for connecting them in parallel (there is pace for that left on the PCB) what other changes should be made if at all?
AFAIK US devices have a switch connected to a "death cap" which this preamp doesn't have. The AC plug has 3 solder lugs the middle one is connected to the star ground and chassis. The primaries are not symmetrical though - they measure 274 Ohms and 314 Ohms.

[R.G.]

I have a question concerning rewiring a tube preamp made in Europe for 230V/50Hz operation to be used in US. Iron was initially intended for 50Hz mains so this is not an issue here.

Always a good first step.

The power transformer has two 115V primaries which are now in series. Except for connecting them in parallel (there is pace for that left on the PCB) what other changes should be made if at all?

If the transformer was designed specifically for doing this, only paralleling the two windings is needed. No other changes.

AFAIK US devices have a switch connected to a "death cap" which this preamp doesn't have.

That practice is substantially archaic, a relic from the day when AC power outlets mostly had only two pins, no safety ground. The death cap and death switch are things which caring techs disable every time they convert an amp to a safer three wire cord.

The AC plug has 3 solder lugs the middle one is connected to the star ground and chassis.

This is as it should be.

The primaries are not symmetrical though - they measure 274 Ohms and 314 Ohms.

That does not mean that they are magnetically asymmetrical, only that one is wound with more copper wire length than the other, as it would be if one was wound first, then the second on top of the first. The turns get longer as the wire layers build up. If they have exactly the same number of turns, they may be paralleled safely even if the resistances are different. If they have different numbers of turns, they may NOT be paralleled safely even if the resistances are equal.

[tubeswell]

Do you know Basil Patchett the transformer winder? (Auckland)

Padgett. Yes. He has wound several transformers for me over the last couple of years. I totally commend him to anyone looking for a Transformer in NZ