Can one run a transformer backwards, as in use the secondaries as the primaries?
I made a miscalculation and I need 240VAC for my mains and the wall voltage here is 220VAC....I found a transformer with the following specs.
190/200/208/220/240:220VAC, 500Watt.
If I run it backwards it will be a slight step-up. I know big jumps will bring up issues like insulation breakdown and such , but this is a slight increase. Are there other issues with operating transformers like this?
______________________________________Rick..............
I made a miscalculation and I need 240VAC for my mains and the wall voltage here is 220VAC....I found a transformer with the following specs.
190/200/208/220/240:220VAC, 500Watt.
If I run it backwards it will be a slight step-up. I know big jumps will bring up issues like insulation breakdown and such , but this is a slight increase. Are there other issues with operating transformers like this?
______________________________________Rick..............
Re: Re: Run a transformer backwards?
Not quite. A 1:1 transformer will have (should have) its primary wound from heavier gauge wire to allow for the magnetising current as well as the wanted current. Applying power to the secondary means you pass more current through a thinner wire and are in danger of overheating.
Tony said:By convention, primaries are connected to the power source and secondaries to load..... so as long as voltages are right, it should be no problem.
Not quite. A 1:1 transformer will have (should have) its primary wound from heavier gauge wire to allow for the magnetising current as well as the wanted current. Applying power to the secondary means you pass more current through a thinner wire and are in danger of overheating.
It will probably be fine with a slight derating. At 50 Hz, there may not be much margin though... check the current when unloaded - less than 20% of the full load current will be OK. If you get more than this, it's on the edge of saturation.
The rated voltage is under load, so the 220V winding is probably wound for about 230V to allow for resistive loss at full output. So you may not get as much step-up as you think...
The rated voltage is under load, so the 220V winding is probably wound for about 230V to allow for resistive loss at full output. So you may not get as much step-up as you think...
Apparently there is an important point being missed here.
Transformers are wound with internal losses in mind. Let us consider a simple isolation transformer for easyer demonstration of what goes on:
because transformers are wound so that for a given load there is a given percentage of 'regulation', in other words, a given percentage of loss is tolerated, an isolation transforemr, that nominally has a 1:1 winding ratio, actually does not - instead, for +-5% regulation, it's more like 1:1.05.
So, with no load, the output voltage will actually be 5% higher, to account for losses when it is fully loaded, in which case the voltage on the secondary will be 5% lower. It should also be noted that for a properly executed transformer, under nominal conditions, the magnetization current is a very low percentage of the total maximum primary current under load, so this is actually rarely accounted for when choosing wire gauges. It can be completely disregarded in this case.
So, if the transformer is a 240 to 220V model, it is likely that to compensate for losses, instead of a 12:11 ratio, it will be more like 12:11.55.
A 'real' 220 to 240v transformer would, by the same token, have a 11:12.6 ratio.
A 'backwards' connected 240 to 220V transformer will actually have a 11.55:12 ratio, or, to compare with the 'real' 240 to 220V one, instead of 11:12.6 it will be an equivalent of 11:11.43 - in other words, the turn ratio correction originally intended to cover the internal losses, ends up being subtracted instead of added. As a result, the transformer will perform more like a 220:220 isolation unit - in fact, by slightly less than 1% less, as a real isolation transformer of the ame VA rating would have a 11:11.5 ratio, which is higher!
Also, because the current rating of the winding stays the same, yet the voltage reduces, in theory you get a smaller VA rating. in practise, because wire sizes are selected from a limited discrete set of sizes, and you are supposed to round upwards when designing the transformer, the difference may be small or even nonexistent, it depends on the quality of the transformer and the actual voltages in question.
To conclude - you might get an unexpected surprise by reversing it.
Transformers are wound with internal losses in mind. Let us consider a simple isolation transformer for easyer demonstration of what goes on:
because transformers are wound so that for a given load there is a given percentage of 'regulation', in other words, a given percentage of loss is tolerated, an isolation transforemr, that nominally has a 1:1 winding ratio, actually does not - instead, for +-5% regulation, it's more like 1:1.05.
So, with no load, the output voltage will actually be 5% higher, to account for losses when it is fully loaded, in which case the voltage on the secondary will be 5% lower. It should also be noted that for a properly executed transformer, under nominal conditions, the magnetization current is a very low percentage of the total maximum primary current under load, so this is actually rarely accounted for when choosing wire gauges. It can be completely disregarded in this case.
So, if the transformer is a 240 to 220V model, it is likely that to compensate for losses, instead of a 12:11 ratio, it will be more like 12:11.55.
A 'real' 220 to 240v transformer would, by the same token, have a 11:12.6 ratio.
A 'backwards' connected 240 to 220V transformer will actually have a 11.55:12 ratio, or, to compare with the 'real' 240 to 220V one, instead of 11:12.6 it will be an equivalent of 11:11.43 - in other words, the turn ratio correction originally intended to cover the internal losses, ends up being subtracted instead of added. As a result, the transformer will perform more like a 220:220 isolation unit - in fact, by slightly less than 1% less, as a real isolation transformer of the ame VA rating would have a 11:11.5 ratio, which is higher!
Also, because the current rating of the winding stays the same, yet the voltage reduces, in theory you get a smaller VA rating. in practise, because wire sizes are selected from a limited discrete set of sizes, and you are supposed to round upwards when designing the transformer, the difference may be small or even nonexistent, it depends on the quality of the transformer and the actual voltages in question.
To conclude - you might get an unexpected surprise by reversing it.
just how much is the magnetising current? i wound a power transformer with 230volts input/120volt center tapped on a core 2.25 in by 5.5 inch stack, primary wire was ga#12 whilst secondary was ga#10, measured magnetizing current was 0.27A .
anyway, the basic question was, can it be done, and the answer is yes...
Different primary..AHah!
Great info!
I knew all you guys out there would be overflowing with the brains on TX quirks. ???
It all makes sense now...The "Winding for losses" winding ratios & such.
Last night I too had the proverbial lightbulb go on....saying "Use another primary winding" Duh!
So attempting to hit my target voltage of 330 VDC, I try this:
190/200/208/220/240:220VAC
Using the 208 primary..220/208=1.0576923*220=232.6923 VAC
232.6923*1.41= 328.09614 VDC minus 1.4 diode drop
326.69614 VDC for B+....
Using this VDC (@ 500 Watts) to power two 2A3s' and one 5691 even if I derate...will leave a large buffer.
What is the concern of saturation using this alternate winding....yes I have alot to learn about transformers & with your help..I'll know more.
Perhaps the phrase "Transistors have specifications, Tubes have guidlines" applies here!
_____________________________________Rick......
Great info!
I knew all you guys out there would be overflowing with the brains on TX quirks. ???
It all makes sense now...The "Winding for losses" winding ratios & such.
Last night I too had the proverbial lightbulb go on....saying "Use another primary winding" Duh!
So attempting to hit my target voltage of 330 VDC, I try this:
190/200/208/220/240:220VAC
Using the 208 primary..220/208=1.0576923*220=232.6923 VAC
232.6923*1.41= 328.09614 VDC minus 1.4 diode drop
326.69614 VDC for B+....
Using this VDC (@ 500 Watts) to power two 2A3s' and one 5691 even if I derate...will leave a large buffer.
What is the concern of saturation using this alternate winding....yes I have alot to learn about transformers & with your help..I'll know more.
Perhaps the phrase "Transistors have specifications, Tubes have guidlines" applies here!
_____________________________________Rick......
Attachments
At LF, a real transformer looks like a perfect transformer in parallel with an inductor. By Ohm's law, that inductance draws current I = V/XL (XL = the reactance of the inductor, = 2*pi*f*L). If L was infinite, the inductor wouldn't draw any current, but it isn't, and that's part of the magnetising current. Connect 240V instead of 220V to a 220V tap and you will pass more magnetising current. That might not seem too bad, but often transformers are run very close to saturation, and if you hit saturation, the inductance falls to zero and the magnetising current rises dramatically. Thus, magnetising current and saturation can be a pretty catastrophic thing. Trouble is, you don't know what choices your transformer designer made.
Transformerless..
OK....Directly rectifying wall voltage is EXTREMELY dangerous and is considered verboten on this forum.
The high voltages present in tube gear is dangerous enough, and removing a simple safety item like an isolation transformer(Nearly isolation) is penney wise but pound foolish.
Deleting this transformer provides an easy path for electrons to pass thru your body.ZZZZZZZZZZZZZZ..end of story, goodbye, forever & ever.
_________________________________Rick.............
Some of the guys here can fill you in on the inherent dangers of direct rectification.
OK....Directly rectifying wall voltage is EXTREMELY dangerous and is considered verboten on this forum.
The high voltages present in tube gear is dangerous enough, and removing a simple safety item like an isolation transformer(Nearly isolation) is penney wise but pound foolish.
Deleting this transformer provides an easy path for electrons to pass thru your body.ZZZZZZZZZZZZZZ..end of story, goodbye, forever & ever.
_________________________________Rick.............
Some of the guys here can fill you in on the inherent dangers of direct rectification.
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