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 Post subject: 25Hz Transformer on 60Hz
PostPosted: Jul Wed 21, 2021 10:24 pm 
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I am sure my RCA tombstone has a 25Hz transformer due to the IE stack size is about 4.5". Should be ok on 60Hz and what are differences operating a 25Hz transformer on 60Hz?

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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Wed 21, 2021 10:29 pm 
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It will run much cooler.

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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Wed 21, 2021 10:34 pm 
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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Thu 22, 2021 12:36 am 
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Does size matter?

I have a couple of rather large HV transformers designed for 25 Hz. that I want use. Somewhere around 3000 volts. I can run those at full power on 60 Hz without issues, right?


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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Thu 22, 2021 1:06 am 
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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Thu 22, 2021 1:16 am 
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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Thu 22, 2021 6:52 am 
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What do you call a capacitor that had just been soldered in, and not cleaned? A flux capacitor! Har Har Har *Slaps knee*. Bad joke, I know...

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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Thu 22, 2021 9:48 am 
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The maximum flux density Tm, in Teslas (or Webers/square meter) experienced by the transformer's iron core is:

Tm = Volts rms / (4.44 x frequency x primary turns x cross sectional area of the core in square meters).

Generally, most iron cored power transformers are designed so the Tm is one Tesla or less. If it is a lot higher, the primary magnetization current increases and the core is pushed up to the non linear part of its B-H magnetization curve and the off load primary current becomes high.

There are generally three types of losses with the transformer that make it heat up, I^2R(or copper losses) these depend on the winding currents and winding resistances, magnetic hysteresis losses which are proportional to the Tm and the frequency. And eddy current losses, these are reduced by laminating the core to help interrupt circulating currents.

Therefore since the core magnetization current (primary off load current) is inversely proportional to the frequency, and you have a transformer designed for say 25Hz, and for example you doubled its drive frequency to say 50Hz, the peak flux density Tm drops in half and the primary magnetization current drops too. The Hysteresis losses stays about the same because although the Tm is halved, the frequency is doubled, cancelling out any savings there.

So this means, with the transformer off load (or on load) the I^2R or copper losses in the primary due to the magnetization component of the primary current drop. But the thing is, in a normally designed transformer, the magnetization current is a low value compared to the on load current. And if all is well with the power transformer, if you run it on no load, it should barely warm up. Most of the losses, that cause a transformer to warm up are the I^2R (copper losses) when the transformer is fully loaded. So you should find that increasing the operating frequency will not make your transformer run a lot cooler, unless it was being pushed with very high magnetization currents on 25Hz in the first place.

From the relations though, if a transformer was designed for 50Hz and driven by a 25Hz supply, this sort of thing would result in drastic increases in the magnetization current, pushing the core too hard and resulting in large primary currents, even off load, and transformer heating from the primary winding.

Many vintage 60Hz transformers were sailing close to the wind when designed and the core peak flux densities run high. Even dropping these from 60 to 50Hz operation, they heat significantly, but since the Tm is also proportional to the applied voltage, one thing that helps it, is to drop the line voltage from 115V to around 100V, to offset the effect of the lower operating frequency.


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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Thu 22, 2021 1:43 pm 
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Very nice explanation, thank you! :)


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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Thu 22, 2021 10:16 pm 
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PhilF wrote:
Very nice explanation, thank you! :)

You are welcome.

I could have mentioned with the eddy current losses, they are both exactly proportional to the square of the max flux density and the square of the frequency. So doubling the frequency would increase them by a factor of 4, but because the max flux density, halves, the losses drops by a factor of 4, so both effects tend to cancel out, making the overall eddy current losses, stay about the same if the frequency gets doubled.

In the case of the hysteresis losses, the loss is not directly proportional to the peak flux density Tm, but rather Tm to the power of 1.6. So the effect of the peak flux density halving and the frequency doubling, don't exactly cancel as (1/2)^1.6 = 0.33 factor reducing the hysteresis loss , and frequency x 2 increasing the loss = 0.33 x2 = 0.66. So doubling the frequency, all else equal, results in the Hysteresis losses dropping to 66% of what they were before.

But as I mentioned, these are not the main transformer losses that cause heating, normally it is the I^2R or copper losses when the transformer is under load. If a transformer heats up significantly off load, it is a very bad sign.


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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Fri 23, 2021 1:13 am 
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I would imagine that it would run cooler because the very substantial size of it has more area to dissipate heat.


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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Fri 23, 2021 1:50 am 
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Scott wrote:
I would imagine that it would run cooler because the very substantial size of it has more area to dissipate heat.


Yes, the entire transformer body for a 25Hz transformer will be larger than a 60Hz one, all else equal, then as a thermal radiator it will have a lower Deg.C/watt temperature increase.


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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Sat 24, 2021 12:26 am 
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According to theory, transformer voltage ratios are equal to their turns ratios, so there should be no difference in voltage when a 25-Hz transformer is operated on 50/60 Hz. This may vary a little depending on the designs of specific transformers, where frequency-dependent effects like impedance and interwinding capacitance could make a slight difference.

As for heating, it has to be remembered that in the end a transformer is designed to present a particular impedance to the AC line which depends on the load it was meant to carry and some other factors. Since impedance in inductors is proprtional to frequency, it follows that a transformer designed to operate at a high frequency may not have enough impedance to survive on lower frequency power. With insufficient impedance too much current will flow through the primary and it will burn up. Transformers designed for 25-Hz power were necessarily bigger and heavier than their 50/60 Hz counterparts because they were built to have more inductance, and hence sufficient impedance for the lowest frequency they would encounter. That's why they run nice and cool when used on 50/60 Hz.

Adding inductance to a transformer is not necessarily a good thing, for a number of reasons. Aside from higher cost, too much inductance can lead to voltage regulation problems and low power factor when the transformers are used on higher frequencies. But radio manufacturers from the mid-1920s on realized that radios equipped with 25-Hz power transformers would almost inevitably be used on 50/60 Hz power, so they designed the transformers to be satisfactory at both frequencies.

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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Sat 24, 2021 2:01 am 
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This is the 25Hz transformer. Looks like a .5KVA for 60Hz.


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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Sat 24, 2021 3:27 am 
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jimbenedict wrote:
This is the 25Hz transformer. Looks like a .5KVA for 60Hz.


It looks like the basic method they used here to convert what would be the design of a 60Hz transformer, into a 25Hz one, was to use the same laminations and brackets, but to increase the height of the lamination stack to increase the core's cross sectional area. At least this way the transformer would have the same chassis footprint / cut out hole.

Assuming they wanted the same core magnetization peak flux (after this conversion) and since the peak flux density is inversely proportional to the core's cross sectional area and inversely proportional to the frequency, with the frequency going down by a factor of 25/60, the cross sectional area would have to go up by 60/25 or about 2.4.

If you consider a core of unit cross sectional area of 1 x 1 square units, and you can only alter one dimension (in this case the stack height) then the stack has to be a factor
of 2.4 times taller for the 25 Hz transformer.

But, there is another factor where the 25Hz transformer might have to have the core stack a tad taller again. Since the winding window was unaltered, if this is what they did, if the same winding wire and winding architecture is used, the DC resistance of the windings will be higher in the 25Hz transformer, than the 60Hz one, because the average length of a winding turn is greater. To solve this requires going to a thicker wire, reducing the primary turns somewhat, so that would require an increase in the core cross section to a higher factor of maybe around 2.5 to 3 for the 25Hz transformer to keep the same peak flux.

So just off the cuff, I would expect that a 25Hz transformer, with the same footprint as a 60Hz one, to be roughly about 2.5 to 3 times taller.

But, if the laminations are larger, which increases the core cross section as well as the height increase helping and a bigger winding window, with a bigger footprint for the 25Hz transformer, it could be a lot more squat looking.


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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Sat 24, 2021 12:51 pm 
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I noticed on schematic the 60Hz 120 volt winding is rated at 6.1 ohm and the 25Hz winding at 11.6 ohm. The high voltage winding is rated at 415 ohm for 60Hz and the 25Hz winding rated at 760 ohm. The using the 25Hz transformer on 60Hz, the high voltage would be a little lower, but likely end up right on spec with the normal 110 volts AC years ago vs todays 120 volts. The transformer is rated 105 to 125 volts though.

High voltage on the Duncan PSU2 shows 285 volts DC with 60Hz transformer and 260 volts DC using the 25Hz transformer. Total high voltage current draw is shown on the schematic.

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 Post subject: Re: 25Hz Transformer on 60Hz
PostPosted: Jul Sun 25, 2021 1:02 am 
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jimbenedict wrote:
I noticed on schematic the 60Hz 120 volt winding is rated at 6.1 ohm and the 25Hz winding at 11.6 ohm. The high voltage winding is rated at 415 ohm for 60Hz and the 25Hz winding rated at 760 ohm. The using the 25Hz transformer on 60Hz, the high voltage would be a little lower, but likely end up right on spec with the normal 110 volts AC years ago vs todays 120 volts. The transformer is rated 105 to 125 volts though.

High voltage on the Duncan PSU2 shows 285 volts DC with 60Hz transformer and 260 volts DC using the 25Hz transformer. Total high voltage current draw is shown on the schematic.


That suggests they used nearly the same wize sizes in the 60Hz and 25Hz transformers and just accepted the higher winding resistances in the 25Hz transformer, and then as a result, the slightly lower output (on load) voltages in the 25Hz transformer case. I guess it meant a quick solution. If this was the case, it would be very interesting to measure the lamination stack height 25Hz vs 60Hz, in theory then, the stack of the 25Hz transformer will be (should be) close to 2.4 times taller than the 60Hz one.


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