Three phase rectifier output voltage

[cupoftea]

Hi,
Do you agree that if a PSU is supplied by three phases, but no neutral wire, then its three phase 6 pulse rectifier is likely to have a ~330Vpk output, rather than a ~580Vpk output? (UK mains)

 

[FvM]

According to my knowledge, UK mains voltage is 230/400 V, same as in other European countries. Three phase bridge rectifier has 400*sqrt(2) peak output respectively.

 

[cupoftea]

Thanks, yes, but as in the attached, which connection is the most likely, given that there is no neutral to the PSU? (as you know, vout is different, depending on connection)
(LTspice and pdf schem attached)

 

[FvM]

Do you have 240/415 V nominal mains voltage in UK? I thought it has been unified to 230/400V?

Regarding your schemes, case 2 138/240 V can't be found in European mains. Case 4 with half-wave rectifiers involves illegal DC current consumption.
Case 1 and 3 are essential the same, just different load.

 

[BradtheRad]

When common ground is connected, diode current waveforms overlap briefly in additive fashion, resulting in the jump from 280V peak to 330V peak.

However the positive waveform peaks at a different time than the negative waveform, thus the greater amplitude is not evident and is not directly measurable. (Hence it shows as 570 VDC unfiltered.)

Notice by making a split bipolar supply (using the common ground as a centerline)...
and adding capacitive smoothing (omitted in my simulation below)...

then output voltage assumes the positive and negative peaks, producing overall 656 V.

 

[KlausST]

Hi,

if I´m not mistaken...
you have 3 phases from mains (no neutral) and you want symmetric DC free load to the phases, then there is only one possible wiring method.
--> use the 3 phase bridge recitifier and connect the three mains_phases to the three rectifier inputs.
(you can´t differ between start or delta wiring)

Each diode will be conductive for 60°
Each phase will be conductive for 2 x 60°

The rectifier output ripples
* from V_Star_RMS * 2.21
* to V_Star_RMS * 2.45
* with 6 times mains_frequency.

Klaus

--- Updated Jan 13, 2022 ---

Added: try your own excel simulation. It´s sarther simple.

 

[Easy peasy]

consider a single 4 diode bridge running off 2 of the available phases - the peak o/p will be Vph-ph peak

which is 400/415 * 1.414 = 565 / 586 volts peak

for 3 phases there is simply less ripple - the peak remains the same.

 

[cupoftea]

Thanks, i appreciate that...though if you were to run the sim of the top post then you would see what i mean.......the delta supplied 3 phase bridge has a much lower peak vout than the star supplied one........as such, which one do you think exists in a PSU which does not have a neutral supply wire?

 

[KlausST]

Hi,

Again: In my eyes you can't wire a 3 input device in star or delta.
You can do this only on a 6 input device (motor with star and delta capability).

Am I wrong?

Klaus

 

[FvM]

Thanks, i appreciate that...though if you were to run the sim of the top post then you would see what i mean.......the delta supplied 3 phase bridge has a much lower peak vout than the star supplied one........as such, which one do you think exists in a PSU which does not have a neutral supply wire?

You are asking about mains supply, isn't it? As stated in post #2, "case 2 138/240 V can't be found in European mains". A transformer is the only way to reduce the supply voltage.

 

[cupoftea]

Do you have 240/415 V nominal mains voltage in UK? I thought it has been unified to 230/400V?

Thanks i think its still 240VAC/415VAC

Thanks for your answers. Though what i am specifically asking is , given that there is no neutral wire coming into our PSU........what configuration is likely to be used...A or B in the attached? (LTspice sim and PDF schem attached)

The problem is that i am not allowed to measure the primary DC bus voltage with a DMM......so i have to make an estimated guess as to what is the primary DC bus voltage....as you can see, B gives a much higher Bus voltage than A.......but given i feel that the star connected one would be more likely to take a neutral to the PSU...even if only to put it across a capacitor to the phases. i feel that we are likely to have the higher primary dc bus voltage, as in B.....would you tend to agree?

I appreciate and agree with all your answers, but its just that i wish to assess the Pri DC bus voltage without actually being permitted to measure it....so have to ascertain it by other means.

The power supply in question has only the three phases wired to it...not the neutral.....i appreciate it doesnt use the neutral, but for the sake of keeping current loops small in area, i feel that EMC would be improved if the neutral were taken to the PSU if "B" were used.

 

[FvM]

what configuration is likely to be used...A or B in the attached?

What do you mean with "to be used"? The only meaningful question is which configuration represents your mains configuration. The answer is simple and has been already given in this thread.

You have three phase wires comming out of the wall outlet and connect it to the 3-phase bridge rectifier. No way to select between different configurations.

 

[KlausST]

Hi,

I fully agree with FvM.

Klaus

 

[Easy peasy]

If you have 230/400 coming in and no neutral - or even with a neutral - there are only 2 ( +1) configurations internally:

1) IF WITH NEUTRAL: 3 totally separate stages each with Brect P - N giving 380 - 420V after PFC, then 3 separate down converters with the output combined at the DC out - Meanwell does this , OR,

1a) NO NEUTRAL: each pair of phases is rectified and PFC'd ( 650VDC at least ), then DC/DC to common output for each pair of phases.

2) IF NO NEUTRAL: Any type of diode bridge off 3 phase will give 565V ( or higher ) bus, PF = 0.95 at beast

2a) AGAIN, IF WITH NEUTRAL: if you use a volt doubler, then the neutral goes to the centre point of the caps and the 3 phases go to the top and bottom each via diodes - this gives you 325 + 325 = 650 VDCpk ... PF = 0.95 at best

If the quoted PF is higher than 0.96 it is a fair bet that 1) is being used - if there is a neutral.

NB: with no neutral and 3 wires - you CANNOT get 330Vpk unless the supply is 190Vac Ph-Ph ( 109Vac Ph -N ).

 

[cupoftea]

NB: with no neutral and 3 wires - you CANNOT get 330Vpk unless the supply is 190Vac Ph-Ph ( 109Vac Ph -N ).

Thankyou, i see where i went wrong now...i put the wrong peak value in the ph-ph voltages by mistake.

You have three phase wires comming out of the wall outlet and connect it to the 3-phase bridge rectifier. No way to select between different configurations.

Ditto Thankyou

I fully agree with FvM.

Ditto Thankyou