Mains transformer for 50Hz and 400Hz use?

[Buk]

the smaller the core needs to be. I just don't see 50x more core loss.

Perhaps the lesson here is: that if you try to use a core designed for 50Hz, at 400Hz, it is bigger than required, so has more core material in which to induce eddy currents; so greater losses.

 

[crutschow]

I find it interesting that 400hz transformer are smaller than 50/60hz transformers.

You understand that's just because it requires significantly more iron at 50/60 Hz to keep a transformer core from saturating due to the magnetizing current as compared to a 400Hz, since the magnetizing current is inversely proportional to frequency at a given voltage?

 

[rjenkinsgb]

As ChrisP58 said in the first reply to this thread, high power audio transformers exist; both line matching and just amp power output.

They cover a vastly wider range than 50-400Hz and do not have particularly large cores or ultra-thin laminations, compared to 50/60 Hz transformers of similar power, even though they can be rated to 20KHz or more.

Example: 200W, roughly 4x3x3"

Home

70V 200W High Power Speaker Line Matching TransformerThis high quality line-matching transformer is designed to convert the output of any standard 4 or 8 ohm amplifier for use with a 70.7 volt public address sound distribution system.Specifications: • Power handling: 200 watt RMS • Frequency...

www.parts-express.com

It does appear that some of the theory being quoted does not fit with real world products?

 

[Buk]

It does appear that some of the theory being quoted does not fit with real world products?

What a wonderfully polite way of telling me I am wrong

The "theory" being quoted are numbers that come out of FEMM written by David Meeker, which has been the go to 2D magnetostatic simulation software for the research community since 1998.

It's numbers are proveably reliable. My interpretation of them may not be.

 

[rjenkinsgb]

It's numbers are proveably reliable. My interpretation of them may not be.

I have no clue at all where the discrepancies are, if any.

It's just an inescapable fact that high power audio transformers exist, with a frequency range sometimes near 1000:1 and acceptable losses, using materials available 60 years or more ago.

I'd expect a transformer with just 8:1 frequency range to surely have significantly lower loss still, if appropriately designed to span the required range??


ps. I believe 400Hz became a standard as being the optimum generator/drive frequency for aircraft synchros and gyros etc?
I used to have quite a collection of old aircraft synchros & resolvers etc., all of which were designed for 400Hz. Presumably that gives a good balance of rotational speed for motors and physical size requirements for such as synchros for instrumentation.

I pretty much only have photos left now.. eg.

https://flic.kr/p/8gdwiz

 

[Flyback]

These are all great replies thanks, do you think it is a common thing to design a transformer for a PSU for 50Hz and 400Hz use?

It seems to be unwise, as why would anyone use a transformer deisnged for 50Hz on an aircraft, where the main reason for 400Hz is to get it lighter.

It sounds believable that a transformer deisgned for 50Hz use, would have 8x the core loss if then used at 400Hz

 

[Diver300]

It sounds believable that a transformer deisgned for 50Hz use, would have 8x the core loss if then used at 400Hz

That would be true if the number of Volts-seconds were kept the same, so that the peak magnetic field were the same.

However, that would need 8 times the voltage at 400 Hz compared to what was used at 50 Hz.

If the voltage at 400 Hz and 50 Hz were similar, the number of Volt-seconds would be far less at 400 Hz, so the magnetic field would be far less as well at 400 Hz.

Transformers designed down to a price / weight will run the core near saturation, and associated with that is larger magnetic hysteresis. The core loss in a transformer will be quite a lot less than half if the supply voltage is halved, due to the reduction in hysteresis loss. At constant frequency, other losses may be proportional to the voltage, but hysteresis loss is generally a major part of the no-load losses.

I would not expect that the iron losses at 400 Hz would be 8 times that at 50 Hz if the voltage is the same. The iron losses are likely to be small compared to the full-load copper loss, so a small reduction in current from the rated value at 400 Hz will keep the power lower than the design power at 50 Hz and at the rated current.

Few online resources seem to mention how iron loss changes with supply voltage, but I found one with some graphs here:- https://www.researchgate.net/public...s_Modeling_and_Experimental_Research/download

 

[Nigel Goodwin]

These are all great replies thanks, do you think it is a common thing to design a transformer for a PSU for 50Hz and 400Hz use?

No, it's utterly pointless - you 'could' design a steel core transformer to do so, as the vast number of audio transformers show it's easily do-able. But as the point of a 400Hz PSU is to make the transformer smaller and lighter, why would you?.

As everything now is switch-mode, using MUCH higher frequencies makes supplies far smaller, lighter, and efficient in any case.

BTW, an audio transformer isn't really any different to a 50Hz mains transformer - and in these days of EXPENSIVE valve output transformers it's quite common to use a suitably chosen 'off the shelf' mains transformer in place of one. From that I would assume a 50Hz transformer would happily work at 400Hz?.

 

[Buk]

t's just an inescapable fact that high power audio transformers exist, with a frequency range sometimes near 1000:1 and acceptable losses, using materials available 60 years or more ago.

I suspect that audio transformers 'get away with it', because unlike an AC supply voltage, the signal they carry is not a pure sine wave; but I do not know enough on the subject to justify that suspicion.

But I will find out.

 

[ronsimpson]

One thing about switchers, you convert the 50/60/400hz to DC and then chop the DC to make 100khz AC and now do the isolation.

 

[Nigel Goodwin]

I suspect that audion transformer 'get away with it', because unlike an AC supply voltage, the signal they carry is not a pure sine wave; but I do not know enough on the subject to justify that suspicion.

But I will find out.

Fairly obviously sinewaves pass through audio transformers with no issues, and is how they actually measure their distortion.

 

[Buk]

I suspect that audio transformers 'get away with it', because unlike an AC supply voltage, the signal they carry is not a pure sine wave; but I do not know enough on the subject to justify that suspicion.

But I will find out.

Seems my suspicions were correct -- or at least, not completely wrong.

I found this description of high voltage audio systems; I bolded the salient part:

Also known as high impedance or high voltage audio systems, a 100V line system works by taking any typical line level audio input signal and outputting it as a steady 100 Volts RMS to the speaker terminals.
The 100V line speakers feature a step-down transformer which serves two functions. It will convert the arriving 100V signal to a low impedance one that the speaker can use, and secondly it isolates the speaker coils low impedance load from the amplifier.

In essence, the audio transformers see a low current, essentially steady DC voltage with a ripple.

Where an AC transformer core sees the full current and magnetic field reverse 50/60(400) times a second, thus lots of reversing eddy currents, the audio transformer's core sees tiny fluctations.

 

[Nigel Goodwin]

Seems my suspicions were correct -- or at least, not completely wrong.

I found this description of high voltage audio systems; I bolded the salient part:

In essence, the audio transformers see a low current, essentially steady DC voltage with a ripple.

Where an AC transformer core sees the full current and magnetic field reverse 50/60(400) times a second, thus lots of reversing eddy currents, the audio transformer's core sees tiny fluctations.

I suspect you're confused by the highlighted part? a steady 100 Volts RMS

There's no 'steady' voltage there, the output voltage simply changes in direct proportion to the input voltage, depending on the turns ratio - it's just a bog standard transformer.

It's rather misleading been called '100V line', as in reality it's simply an easy method for the non-technical to connect multiple speakers to an amplifier with no concern over impedances. It also has the advantage of less loss in the cables, due to the higher voltage level - although many modern high power amplifiers will exceed that voltage level just feeding 8 ohms.

The output transformer in the amplifier converts the output signal to 100V RMS with the amp at full power, and a sinewave applied. With no signal applied, the output of the transformer is obviously zero.

You can then connect any number of speaker to the system, as long as their total power doesn't exceed 100W, so for example ten 10W speakers - the transformers in the speakers make it trivial to work out how many speakers, and of what type, you can connect to it. Another example would be one 50W speaker, two 20W speakers, and one 10W speaker - or even just one 10W speaker, impedances don't matter as it's all been done in the manufacturing design process. You can even get speakers with switched taps on the transformer, so you can set the power to individual speakers.

 

[Buk]

And I think you are wrong.

Correction. I know you are wrong. No one suggested ttransformer wasn't "bog standard"; its the signal driving it that is different to your expectations.

 

[Buk]

From Wikipedia **broken link removed**:

• Insertion loss: The transformers themselves commonly reduce total power applied to the loudspeakers, requiring the amplifier to be some 10% to 20% more powerful than the total power that is intended to be applied to the loudspeakers. Typical transformer insertion loss measurements are taken at 1,000 Hz to optimize the transformer's specifications.[4] Using this method, typical insertion losses are about 1 dB, a 20% power loss. Most of the power in voice-application audio systems is below 400 Hz, meaning that insertion loss at lower frequencies would be greater. The best transformers reduce mid-band frequencies by 0.5 dB (approximately 10% power loss) or less, resulting in a ten watt loudspeaker drawing 11.1 watts from the amplifier.[13]

 

[Ramussons]

Hi,
Is it likely that for a mains transformer based offline power supply....it's transformer would be perfectly ok for use with 50Hz or 400Hz? I mean, is it usual to see such designs whereby its ok for 50Hz or 400Hz?

Why not buy a mains operated heater instead? It will perform better and last longer.

 

[Lax Luthier]

I've had some Lambda power supplies (linear) that were rated to be powered from just under 60 Hz to just over 400 Hz, and seen old school test equipment with similar ratings. So such transformers do exist. One thing I would certainly expect is that they more expensive than a 60 Hz or 400 Hz only type.

 

[Nigel Goodwin]

I've had some Lambda power supplies (linear) that were rated to be powered from just under 60 Hz to just over 400 Hz, and seen old school test equipment with similar ratings. So such transformers do exist. One thing I would certainly expect is that they more expensive than a 60 Hz or 400 Hz only type.

As has been said throughout this thread, it's not an issue, as audio output transformers go from 20Hz to 20KHz with no problems - and it's now common practice to even use cheap mains transformers as output transformers in small valve amplifiers (easily available, and much cheaper).

I imagine the only issue is having a big enough transformer for the lower frequencies?, so a 50Hz transformer will be fine higher (to well above 400Hz), but a 400Hz transformer wouldn't be happy at 50Hz as it's too small.

I imagine using them for audio use, you simply get a more powerful transformer than you need, and that will extend your bass response to less than 50Hz.

 

[crutschow]

it's now common practice to even use cheap mains transformers as output transformers in small valve amplifiers (easily available, and much cheaper).

I would expect mains transformers to have significantly higher distortion and poorer frequency response than audio transformers, but I suppose those that want a tube amplifier, aren't particularly concerned about such things.

 

[Nigel Goodwin]

I would expect mains transformers to have significantly higher distortion and poorer frequency response than audio transformers, but I suppose those that want a tube amplifier, aren't particularly concerned about such things.

Exactly, quality from valve amplifiers is relatively low anyway - but what's the difference in the transformers?, it's basically copper windings on a similar core, and negative feedback will help reduce distortion etc. I suspect if you were looking for push-pull and ultra-linear, it would be a very different story though But as you're unlikely to find such a suitable mains transformer, it's not really an issue - but for single ended it should work fine.