|
Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
|
Thread Tools |
12th Dec 2021, 11:12 pm | #41 |
Heptode
Join Date: Dec 2003
Location: South Yorkshire, UK.
Posts: 900
|
Re: Early Unusual Transformer Design
Yes I can do that after work tomorrow.
Thanks Regards Chris
__________________
Every Silver Lining Has Its Cloud https://youtube.com/channel/UCvBpiuUUnErJlNBm6DWb3Ww |
13th Dec 2021, 12:06 pm | #42 |
Heptode
Join Date: Nov 2009
Location: Scarborough, North Yorkshire, UK.
Posts: 507
|
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
|
13th Dec 2021, 1:58 pm | #43 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
|
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!
|
13th Dec 2021, 9:27 pm | #44 | ||
Heptode
Join Date: Dec 2003
Location: South Yorkshire, UK.
Posts: 900
|
Re: Early Unusual Transformer Design
Quote:
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
__________________
Every Silver Lining Has Its Cloud https://youtube.com/channel/UCvBpiuUUnErJlNBm6DWb3Ww |
||
14th Dec 2021, 10:37 pm | #45 |
Heptode
Join Date: Dec 2003
Location: South Yorkshire, UK.
Posts: 900
|
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. I’m guessing back in the day mains frequency wasn’t yet standard across the country or perhaps even across the street.
__________________
Every Silver Lining Has Its Cloud https://youtube.com/channel/UCvBpiuUUnErJlNBm6DWb3Ww |
14th Dec 2021, 11:15 pm | #46 | |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
|
Re: Early Unusual Transformer Design
Quote:
|
|
15th Dec 2021, 8:11 am | #47 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
|
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%. |
15th Dec 2021, 10:04 am | #48 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,171
|
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 Ed |
15th Dec 2021, 12:00 pm | #49 | |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
|
Re: Early Unusual Transformer Design
Quote:
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 = 0.01629² 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 = 0.02² 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. |
|
15th Dec 2021, 12:43 pm | #50 |
Heptode
Join Date: Dec 2003
Location: South Yorkshire, UK.
Posts: 900
|
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
__________________
Every Silver Lining Has Its Cloud https://youtube.com/channel/UCvBpiuUUnErJlNBm6DWb3Ww |
15th Dec 2021, 1:38 pm | #51 | |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
|
Re: Early Unusual Transformer Design
Quote:
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). |
|
15th Dec 2021, 9:31 pm | #52 |
Heptode
Join Date: Dec 2003
Location: South Yorkshire, UK.
Posts: 900
|
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
__________________
Every Silver Lining Has Its Cloud https://youtube.com/channel/UCvBpiuUUnErJlNBm6DWb3Ww |
16th Dec 2021, 10:30 am | #53 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
|
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.
|
16th Dec 2021, 10:50 am | #54 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
|
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). |
16th Dec 2021, 12:20 pm | #55 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
|
Re: Early Unusual Transformer Design
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.
|
16th Dec 2021, 1:51 pm | #56 | |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
|
Re: Early Unusual Transformer Design
Quote:
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). |
|
19th Dec 2021, 2:17 pm | #57 |
Heptode
Join Date: Dec 2003
Location: South Yorkshire, UK.
Posts: 900
|
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
__________________
Every Silver Lining Has Its Cloud https://youtube.com/channel/UCvBpiuUUnErJlNBm6DWb3Ww |