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Old 08-20-2020, 11:30 AM   #1
pintglass
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Default Back to Back Transformers

Im currently building an isolation transformer from 2 240v-12v transformers, wired with the 2 12V secondaries joined to give 240V out, which I am getting.

The Issue I have is that I'm getting 100Hz out from 50Hz in.
Can anyone tell me why that may be?
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Old 08-20-2020, 12:10 PM   #2
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Default Re: Back to Back Transformers

Either you're measuring it wrong or the source transformer has a rectifier output... ?
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Old 08-20-2020, 02:21 PM   #3
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Default Re: Back to Back Transformers

Both the transformers are from old 12V battery chargers both of which had rectifiers in them aswell.
If the transfomers had rectifacation would they still need a seperate rectifier.
I'm just measuring with my multimeter and going off what its reading which is 100Hz
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Old 08-20-2020, 02:48 PM   #4
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Default Re: Back to Back Transformers

closest i could find without using diodes is this .but your set up isnt like it ... maybe try moving the transformers further away from each other ...
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Old 08-20-2020, 04:00 PM   #5
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Default Re: Back to Back Transformers

Show us how you have them all connected up.

You need to connect the 12V winding directly together, no diode, nothing but just the 12VAC winding of each transformer.
240VAC into first transformer 240VAC winding, 12VAC winding of first transformer to 12VAC winding of second transformer, 240VAC winding of second transformer for 240VAC Isolated output.
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Old 08-20-2020, 04:40 PM   #6
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Default Re: Back to Back Transformers

Thats how I've got them setup.
I've attached an image of how there wired. The transformer in the bottom of the image is the 240V in, with a fuse before it, then the two 12V's are connected together then the output is the transfomer at the top.This goes into a power meter display then into a DPST switch.
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Old 08-20-2020, 05:01 PM   #7
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Default Re: Back to Back Transformers

So direct connection without any diodes anywhere, you are seeing 240VAC 100Hz at the output?
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Old 08-20-2020, 05:17 PM   #8
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Default Re: Back to Back Transformers

you may be getting transfer between the transformers because they are oriented in line with each other.
magnetic flux through the air or even the metal baseplate.
do you have a scope?
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Old 08-20-2020, 06:14 PM   #9
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Default Re: Back to Back Transformers

The coupling is a nice theory but doesn't really hold water, you need a phase shift of some sort to get doubling and coupling is in phase or at least very close to be in phase. Still doesn't explain...

What about the frequency of the 12V between the two? What about after the measurement jig before the first transformer?

I have to say that there's some measurement issue or some other noise source like the measurement jig... Scope would indeed be nice.
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Old 08-20-2020, 06:47 PM   #10
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Default Re: Back to Back Transformers

PZEM-021 energy/power meter uses a capacitive-dropper for power and likely causing some ringing on the AC waveform output of the second transformer when it has no load on it. Are you sure you've got the right winding, I see some steel lamination cut off and extra windings on it and 247V label?
Connect a light bulb load on the output and I'd guess the frequency reading would mysteriously half.
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Old 08-21-2020, 12:03 AM   #11
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Default Re: Back to Back Transformers

Yes with direct connection and no diodes my multimeter is showing 100Hz

Yes I have a scope

I only get 50Hz on the 12V windings

Even if I disconnect the energy/power meter I still get 100Hz on the output Transformer.

The output transfomer does have more than one tap but I still get 100Hz on both of them.
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Old 08-21-2020, 03:52 AM   #12
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Default Re: Back to Back Transformers

You do have the right phasing or you will not get the right voltage or current

On the primary and secondary winding

I done this before and you have to get it right for it to work correctly
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Old 08-21-2020, 04:03 AM   #13
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Default Re: Back to Back Transformers

What do you mean by having the right phasing ?
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Old 08-21-2020, 04:31 AM   #14
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Default Re: Back to Back Transformers

Have you noticed that on some transformers have a dot on them on one corner one side then on the side of the transformer there is another dot on the opposite corner

This is the phasing of the transformer when you hook up two in series you hook it one way when you hook it up in parallel you hook up another way

This also depends on the transformer you are using

Most transformers are not marked by the way

If you google how to determine the phasing you should be able to figure it out
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Old 08-21-2020, 07:07 AM   #15
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Default Re: Back to Back Transformers

Again this does not make sense. Phasing at most will invert or not invert the sinusoid of the AC current, but the downstream equipment should not care what phase it's running at. You can try swapping the connection of the 12V/low voltage wires to put things in phase.

This must be some noise problem that your meter is picking up though I'd say the 100Hz is a second harmonic that the meter is picking up. The only thing that sticks out at the moment is that your secondary transformer is using two windings hooked up in parallel or this should be the case. A mismatch may cause some weird behavior. What if you use the two transformers the other way around -- use the low impedance of line power to override the parallel winding mismatches?
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Old 08-21-2020, 09:04 AM   #16
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Default Re: Back to Back Transformers

Do you mean that the output transformer's 240V side has dual windings in parallel, as I am only connected to one of them.
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Old 08-21-2020, 10:10 AM   #17
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Default Re: Back to Back Transformers

yeah you have a mess there, can't exactly tell how you're connected. Might be some noise being introduced there too, dunno...
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Old 08-21-2020, 04:27 PM   #18
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Default Re: Back to Back Transformers

Quote:
Originally Posted by eccerr0r View Post
Again this does not make sense. Phasing at most will invert or not invert the sinusoid of the AC current, but the downstream equipment should not care what phase it's running at. You can try swapping the connection of the 12V/low voltage wires to put things in phase.

This must be some noise problem that your meter is picking up though I'd say the 100Hz is a second harmonic that the meter is picking up. The only thing that sticks out at the moment is that your secondary transformer is using two windings hooked up in parallel or this should be the case. A mismatch may cause some weird behavior. What if you use the two transformers the other way around -- use the low impedance of line power to override the parallel winding mismatches?

No what I am talking about is if you have it out of phase then you will not get any voltage or current or you are shorting both transformers out
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Old 08-21-2020, 05:07 PM   #19
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Default Re: Back to Back Transformers

The windings are all in parallel and the signal (AC line) is symmetric/sinusoidal, so you don't have to worry about it.

If one was planning to make a voltage doubler out of these transformers, they'd need to be in series, and this would make a difference.

However 0 or 180 phase difference, the peaks and valleys would all appear at the same time, cancelling each other out. Need a phase shift to double the frequency somehow...
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Old 08-21-2020, 05:48 PM   #20
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Default Re: Back to Back Transformers

This schematic diagram might help. If OP can verify.

Phasing is important for the dual-primary tranny but more so if they were autotransformer connected which is what sam is talking about.
Adding a small load will get rid of the frequency doubling OP is seeing.

I want an earth ground connection and don't see it.
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