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Old 12th Dec 2021, 10:12 pm   #41
ChristianFletcher
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Default Re: Early Unusual Transformer Design

Yes I can do that after work tomorrow.

Thanks Regards Chris
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Old 13th Dec 2021, 11:06 am   #42
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Default Re: Early Unusual Transformer Design

We did iron and copper loss tests in the C&G electrical courses and a temperature test. We checked the workshop temperature (T1) and the transformer winding resistance at the beginning of the test (R1). After a hour on load, measured the resistance once again (R2). then used the formula R2 divided by R1, then minus 1 and dived that by .004.(Temp Co. Eff of copper).We had a thermo couple in the windings and it is surprising how accurate the simple temperature test is. Ted
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Old 13th Dec 2021, 12:58 pm   #43
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Default Re: Early Unusual Transformer Design

Yes. Although it's the average temperature of the whole winding rather than a buried hot-spot, the thermocouple is unlikely to be as accurate unless you can actually bury it in the winding to get good thermal coupling!
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Old 13th Dec 2021, 8:27 pm   #44
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Default Re: Early Unusual Transformer Design

Quote:
Originally Posted by kalee20 View Post
Quote:
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I have run the transformer for about an hour and it just gets warm to the touch. Just by feel I don’t think the temperature change is very much different for on or of load. Again I guess this indicates that for this transformer it the core losses that dominate.
It sounds very much like it!

While you have the measuring equipment out, can you share with us the winding resistances; the open-circuit voltage (tell us the primary voltage!); and the full-load voltage (at the same primary voltage).

The no-load and full-load primary currents would be good to know, too.

I can do some number-crunching then!
P1 Primary
P2 low voltage tapping 200-220V
P3 high voltage tapping 230-250V

For no particular reason I did the test using the P2 Tapping at 210V


Primary Resistance P1 - P2 292.5 Ohm
Primary Resistance P1 - P3. 337.0 Ohm
Secondary S1 - S2 372 Ohm

primary Voltage 210V Secondary Voltage 171V unloaded

primary Voltage 210V secondary Voltage 159V @ 20mA load

Impedance Voltage 14.9V
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Old 14th Dec 2021, 9:37 pm   #45
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Default Re: Early Unusual Transformer Design

I was hoping to do a frequency sweep but unfortunately my power source only varies from 45 to 60 hz. The loaded output was a few volts lower at 45hz and a few volts higher at 60hz. The battery eliminator has 50-100 hz stamped on the casing. Iím guessing back in the day mains frequency wasnít yet standard across the country or perhaps even across the street.
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Old 14th Dec 2021, 10:15 pm   #46
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Default Re: Early Unusual Transformer Design

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Originally Posted by kalee20 View Post
If you connect the transformer to a variable supply (with a voltmeter across the supply to be sure you're hitting it with full voltage) via a wattmeter, leaving the secondary completely open-circuit, the power drawn is virtuall all core (iron) losses. (The primary current is relatively low, and secondary currents are zero, so winding losses are negligible).
Not sure about this as the primary resistance will cause very significant losses.
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Old 15th Dec 2021, 7:11 am   #47
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Default Re: Early Unusual Transformer Design

There is an error, sure, but it's (hopefully) not huge in a well-designed transformer.

Even if the off-load primary current is as much as a third of the on-load current, the primary losses are still only 1/9 of what they are on-load. And as for minimum winding losses the wire gauges will be chosen so that primary losses and secondary losses are equal (this gives the lowest figure for the sum) we can infer that the off-load primary losses are 1/18 of the on-load winding losses. That's an error of under 6%.
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Old 15th Dec 2021, 9:04 am   #48
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Default Re: Early Unusual Transformer Design

As others have said, these were quite simple devices and were designed at a time when high efficiency devices were not required as they are today and were built to be affordable.
It is what it is, is unlikely to burst into flames if correctly fused, so other than as an historical perspective little point in characterising it in great detail; it is now unlikely to be used at other than 50Hz

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Old 15th Dec 2021, 11:00 am   #49
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Default Re: Early Unusual Transformer Design

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Originally Posted by ChristianFletcher View Post

Primary Resistance P1 - P2 292.5 Ohm
...
Secondary S1 - S2 372 Ohm

primary Voltage 210V Secondary Voltage 171V unloaded

primary Voltage 210V secondary Voltage 159V @ 20mA load

Impedance Voltage 14.9V
OK. So, playing with figures:

Assuming the off-load primary current is small (so primary resistance loss is negligible) we have:

Turns ratio = 171V / 210V = 57/70.

Secondary load = 20mA so primary on-load current = 20mA x 57/70 = 16.29mA.

Primary voltage drop = 16.29mA x 292.5 ohms = 4.76V giving 'effective' primary voltage 210V - 4.76V = 205.24V.

And primary power loss = 0.01629≤ x 292.5 W = 0.078W.

Secondary voltage then, allowing for primary drop = 205.24V x 57/70 = 167.1V.

Secondary volt drop = 20mA x 372 ohms = 7.44V.

So actual load voltage is 167.1 V - 7.44V = 159.66V which is pretty close to your measured figure.

Secondary power loss = 0.02≤ x 372 W = 0.1488W.

Total winding losses thus come to 0.078 + 0.1488 W = 0.227W. This is somewhat lower than your measured 0.3W, though same order of magnitude.

One immediate comment is that with secondary loss about double the primary, a more efficient transformer could have been made with slightly smaller gauge primary wire and slightly larger gauge secondary (to fit in the same space). The losses would have been nearer equal then. However without getting into the designer's mind (or knowing the constraints he/she was under), there could have been a good reason for it being as it is.
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Old 15th Dec 2021, 11:43 am   #50
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Default Re: Early Unusual Transformer Design

Thanks for running the numbers that’s very interesting and close to what was measured. Based on the information we have is it possible to calculate the perspective short circuit current. I did measure the impedance voltage but could not complete the calculation as from memory the VA rating is required
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Old 15th Dec 2021, 12:38 pm   #51
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Based on the information we have is it possible to calculate the perspective short circuit current.
Yes it is.

As everything is linear, we can extrapolate (the only errors are rounding errors).

We have 171V off-load; 159.66V on-load. That's a drop of 11.34V for a load current of 20mA.

So the effective output resistance is 11.34 / 0.02 ohms = 567 ohms.

Then the short-circuit current will of course be 171 / 567 A = 302mA.

The primary current will be 302mA x turns ratio (= 57/70) giving 246mA.

(If you try to measure this, the transformer will be dissipating 210V x 246mA = 51.6W so it's not a good idea! But you could try at 1/3 primary voltage at which the currents will also be a third, and the power will be about 6W - it will get warm and you'll see the currents drop due to the windings heating and increasing resistance due to the temperature coefficient of copper).
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Old 15th Dec 2021, 8:31 pm   #52
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Default Re: Early Unusual Transformer Design

Thanks for all the help and information. I don’t think I will be trying a short circuit test as I’m planning on rebuilding the battery eliminator so don’t want to risk damaging the old transformer.

Best regards Chris
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Old 16th Dec 2021, 9:30 am   #53
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Default Re: Early Unusual Transformer Design

Transformer wallwarts were banned because they consume power when not loaded. To keep cost and size down mains transformers are designed with the fewest number of primary turns they can get away with and that means lower inductance, higher standing current and higher no load resistive losses.
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Old 16th Dec 2021, 9:50 am   #54
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Default Re: Early Unusual Transformer Design

It certainly seems the case for Chris's transformer - he measured 4W off-load losses. Which is quite high!

It's quite possible to design transformers for arbitrarily low off-load losses, the down side is that size increases (as PGL implies).
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Old 16th Dec 2021, 11:20 am   #55
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I think you will find that 4W off-load power consumption is not that high and larger transformers will have proportionally higher losses. The more turns the greater the resistance so for regulation you would need thicker wire but the extra turns will also create a greater flux density so you need more core material. It's fascinating that the material physics add up to allow transformers to work.
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Old 16th Dec 2021, 12:51 pm   #56
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I think you will find that 4W off-load power consumption is not that high
It is for this one!

Chris's load is 159V 20mA which is 3.18W, less than the core loss!

And it's high considering the absolute maximum power this transformer can possibly deliver is 12.9W (the O/C voltage being 171V; the S/C current 302mA; and apply the Maximum Power theorem).
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Old 19th Dec 2021, 1:17 pm   #57
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Default Re: Early Unusual Transformer Design

Thanks Again for the help and information. I really learned a lot this time which for me is just as important as the actual restoration work. Had good reactions to my video and I have included a link for anyone that wants to take a closer look at the actual transformer. Any comments good are bad are welcome. We come to learn.

Thanks regards Chris

https://youtu.be/LJScHhKJggc
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