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LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408
Posted: 06:57pm 19 Mar 2019
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I know nothing about metal core transformers but want to change that (for the better).
I was reading that transformers have a dot on each winding and that if connecting transformers in series, the dots must be aligned, just like you would align dc cells, or bad things can happen.
However it's not clear to me:
1. what the dot signifies 2. if a transformer does not come with a dot/marking, HOW to deduce where the dot/marking should be? 3. For example, if the secondary has 3 windings : 10V, 28V and 70V and I wanted to wire all of them up in series, how should I go about it?
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 09:20pm 19 Mar 2019
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Dot usually corresponds to the physical start of each winding. You often see a dot on the schematic also.
It just means that all the ends with dots swing either positive or negative together in the same phase. Its also very helpful when connecting different windings in series or parallel.
If there are no dots, the wire colours and the rating label often make things clear.
Without any information at all, its a simple matter to measure some relative ac voltages between windings connected in series.
For instance if you connected a 50v winding in series with a 10v winding, you could measure either 60v or 40v across the combination. Its then easy to see that the 10v winding either adds if in phase, or subtracts if connected out of phase with the 50v winding.Cheers, Tony.
LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408
Posted: 09:33pm 19 Mar 2019
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OK, if the only issue with getting the dots wrong is that the amplitude from the connection reduces due to getting the phase incorrect, then it's less of a concern.
However, I read online that getting the dots wrong could lead to the transformer and windings melting/catching fire and so on - when could that happen?
Now THAT would be very very concerning!
If that was some fearmongering I am happy to ignore that (I'll be trying to look the source up to ensure I'm not imagining things)
Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406
Posted: 10:12pm 19 Mar 2019
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Only if you start connecting windings in parallel do you need to be super careful. Cheers, Tony.
mackoffgrid Guru Joined: 13/03/2017 Location: AustraliaPosts: 460
Posted: 10:18pm 19 Mar 2019
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It can be very important. You must series the secondaries in Tony's StepInverter using the correct polarity (dots). As Tony said it is simple to check for polarity. Having said that I still managed in my addled brain, to get one wrong. Makes the output look rather Gothic - it also blew one of those chinese power meters. (they use a series mains capacitor in there power supply - not a good idea)
LadyN Guru Joined: 26/01/2019 Location: United StatesPosts: 408
Posted: 02:00am 20 Mar 2019
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Yes, you see this is what I am trying to understand: why did the series cap blow up? I understand if you were supposed to connect them 'anti-series' so that the voltages subtract than if you connected them in series, where the voltages add up Is that what you ended up doing? (the voltages added up exceeding the series cap limit while you expected the voltages to subtract instead)
What's the issue with them being in parallel the wrong way? The windings will be heating each other up as their amplitudes rise and fall out of sync with each other? Is that the only issue or are there other physics at play?
wiseguy Guru Joined: 21/06/2018 Location: AustraliaPosts: 1156
Posted: 02:47am 20 Mar 2019
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The Meter died (not necessarily the cap) because it was designed for sinewaves which "gently" ease energy into and out of a capacitor for powering up the meter module.
Square waves are not gentle they are the equivalent of hitting the capacitor with a hammer as high instantaneous currents flow as the voltage changes to a new step. It is normally series components zeners resistors diodes and sometimes capacitors can fail, when driven by square waves.
This type of power supply is usually termed as a charge pump power supply and they are good with sinewaves but lousy with squarewaves - phasing wrong must have caused Andrew to have a much bigger jump in voltage than the usual small step..Edited by wiseguy 2019-03-21If at first you dont succeed, I suggest you avoid sky diving.... Cheers Mike
mackoffgrid Guru Joined: 13/03/2017 Location: AustraliaPosts: 460
Posted: 02:51am 20 Mar 2019
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Paralleling transformers the wrong way is like paralleling batteries the wrong way.
The series capacitor didn't blow, but the output experienced large vertical swings, so, high frequency rise times and fall times, the impedance of the capacitor drops dramatically allowing high voltage across the device.
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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