Rectify a double phase 50hz tension

circuitlearner73

Joined Jan 7, 2016
2
Dear forum members,
I bought a single phase power bridge rectifier to rectify the tension (socket on the wall) that I thought it were provided by a single a phase and the neutral conductor.
When I visualised the output of the bridge by an oscilloscope, with great surprise I found out that the waveform was rather different from the usual full-wave rectified curve. Only after some tests I realiesed that the electric power is not provided by a phase conductor at 230 V and a neutral conductor, but with two phases at 115 V with a shift of 120 degrees between them in order to obtain 230 V. The anomalous output is due to the fact that because of the two phases, there are certain intervals of time when the two couples of the bridge are both in conduction.
I am a bit puzzled as to how I may obtain a fully rectified waveform (as it is usually reported in the textbooks) with this bridge. I do not know whether I should get a triple phase bridge or maybe use a transformer with centre tap.

Could you please give me some hints as I am stuck.

Dodgydave

Joined Jun 22, 2012
10,945
What country are you from?

tcmtech

Joined Nov 4, 2013
2,867
Only after some tests I realiesed that the electric power is not provided by a phase conductor at 230 V and a neutral conductor, but with two phases at 115 V with a shift of 120 degrees between them in order to obtain 230 V.
Unlikely being the numbers you give do not add up for a 120 degree phase shift. To get 230 from a pair of 115 volt circuits they would have to be 180 degrees out of phase not 120. Basically a 230 volt single phase circuit with the center tap as the common line.

If they were 120 degrees out of phase you would have 115:115/200 or 133:133/230

I think you did your measurements wrong.

circuitlearner73

Joined Jan 7, 2016
2
What country are you from?
Italy. Sorry, I guess I have to fill in My Account page.

ebeowulf17

Joined Aug 12, 2014
3,307
I could be wrong, but without access to third phase, I don't think a triple phase bridge will improve your situation. I believe a transformer between your power source and the rectifier will solve your problem.

I should say that my knowledge of these things is strictly theoretical, not hands on, so I'd love to hear from more experienced members on this.

Incidentally, can you share an image of what you see on the oscilloscope from your rectifier right now? I think I'm visualizing it correctly, but I'd like to see what you're seeing.

Joined Jul 18, 2013
27,607
It sounds as though you have a 2ph system? L1, L2 and Neutral if truly 120°?
Therefore you would have a 4 conductor outlet which included the ground??
In any case, if you want single phase rectification either go across the 120v or across the 240v conductors, but I think you have something very odd there.
I would double check and confirm.
Italy shows 220v 50hz 1ph either 2 pin or 3 pin (earth) sockets.
Max.

Last edited:

ebeowulf17

Joined Aug 12, 2014
3,307
It sounds as though you have a 2ph system? L1, L2 and Neutral if truly 120°?
Therefore you would have a 4 conductor outlet which included the ground??
In any case, if you want single phase rectification either go across the 120v or across the 240v conductors, but I think you have something very odd there.
I would double check and confirm.
Italy shows 220v 50hz 1ph either 2 pin or 3 pin (earth) sockets.
Max.
From what I've heard, most of Europe is either single phase ~230-240 (3 wires: hot, neutral, ground) or three phase (4 wires: 3 hots, ground, no neutral) and it's not uncommon to find single phase devices powered by two of three legs of a three phase circuit. My only knowledge of this is with regards to commercial espresso machines, but in that context I've heard similar stories pretty consistently. I don't think there's anything like our split-phase power in Italy.

I'm guessing from the description that he's working with two of the three legs from a 3-phase power system. I played around with some simulations and it looks like you could get odd waveforms out of a rectifier in this scenario, but also like a transformer would solve the problem:

EDIT:
I'm attaching the LTspice files in case my sims don't make sense as they are.

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Picbuster

Joined Dec 2, 2013
1,045
the starpoint of ac site is connected to the minus of the bridge this will sort the negative part hence diode parallel to V2 /V2
remove that connection and measure over the 1K please report result.

ebeowulf17

Joined Aug 12, 2014
3,307
the starpoint of ac site is connected to the minus of the bridge this will sort the negative part hence diode parallel to V2 /V2
remove that connection and measure over the 1K please report result.
I'm not sure I understood your comment correctly, but here are two modifications to the simulation that isolate the AC ground reference from the DC negative. Let me know if either of these represents what you had in mind.

This first one appears at first glance to provide the desired output from the rectifier, but if you look at the AC voltage traces, they definitely do not match any normal three phase power source anymore. I think leaving the three separate AC sources floating has effectively broken the simulation and made it not represent real-world three phase power anymore.

This second sim leaves the rectified side floating relative to ground, which eliminates the clipping that was happening at the bottom of the rectified waveform earlier, but is definitely not providing the output one would expect when running single phase power through a rectifier. I believe this simulation does represent a plausible real-world scenario, and an undesirable rectified voltage waveform.

After running these two modified versions of the sim, I still think a simple transformer between the mains power and the rectifier is the best way to get the rectified waveform one would expect from a normal single phase power source.

Picbuster

Joined Dec 2, 2013
1,045
I agree with transformer
What you see is normal if you add the 3th phase the time high will be even a bit longer.
I do not see a problem with this shape use a integrator (capacitor ) to smooth out the dips.
We normally use 3 diodes ( no bridge) to rectify 3 phase signals.
voltages available in Europe ( 3 x 120V 50Hz in very old parts of some villages) ( 3 x 240V 50Hz) or ( for extreme heavy equipment like 100Kw++ motors 3 x 480V 50Hz) and then the main supply 10KV ,25KV 100KV