3-Phase half bridge rectifier strange issue when neutral is removed

[ustaad]

Hello guys,

I am facing a strange issue in a 230V 3-phase (~400V RMS Line to Line) bridge rectifier as shown in the image below



The rectifier's output goes to an isolated SMPS power supply.

Now when I remove the neutral the varistors go into conduction mode which is strange cause the varistors are 470V DC rated (7D471K) and two of them in series make it 940V dc (~665V RMS). Is this a known issue with 3-phase half bridge rectifiers that I cannot understand?

[wasedadoc]

Don't remove the neutral. It will not work properly if you do.

Bluntly, if you cannot understand that or why 2 varistors in series is not the situation you should not be working on AC mains, never mind 3 phase.

[ustaad]

Don't remove the neutral. It will not work properly if you do.

Bluntly, if you cannot understand that or why 2 varistors in series is not the situation you should not be working on AC mains, never mind 3 phase.

Neutral is not removed on purpose but it needs not be damaged if neutral is disconnected due to a fault. I do not want it to work after neutral is removed 

Can you explain me more how it works then?

[wasedadoc]

If neutral is removed, current from the most positive phase will attempt to return to each of the other two phases. The path to each is a single varistor (and parallel 300k). One of those phases will be at a significant negative voltage.

If you are the "designer", stop and get someone who is qualified competent.

[ustaad]

If neutral is removed, current from the most positive phase will attempt to return to each of the other two phases. The path to each is a single varistor (and parallel 300k). One of those phases will be at a significant negative voltage.

That is true whether neutral is connected or not. 300k resistor only makes it 1.33 mA at 400V phase-to-phase voltage.

If you don't know about this stuff, please stop and let someone competent answer this

[wasedadoc]

The 300k resistors are irrelevant. I have given enough hints for anyone who should be permitted to work with that circuit to calculate the maximum voltage across each varistor when neutral is absent.

Please explain why you think no-one else can answer until I stop.

[BennoG]

when only using 3 diodes you need the neutral line.
when using 6 (full bridge) diodes you can skip the neutral line.

https://www.electronics-tutorials.ws/power/three-phase-rectification.html

Benno

[Andy Watson]

If you don't know about this stuff, please stop and let someone competent answer this

Wasedadoc's answers are accurate.

... 300k resistor only makes it 1.33 mA at 400V ...

Yes, plus whatever current flows through the load at the line voltage.

[ustaad]

Yes, plus whatever current flows through the load at the line voltage.

Which load? The SMPS? Isn't it be effectively disconnected when there is no neutral?

okay, I am getting it now. Thanks though

[ustaad]

when only using 3 diodes you need the neutral line.
when using 6 (full bridge) diodes you can skip the neutral line.

https://www.electronics-tutorials.ws/power/three-phase-rectification.html

Benno

I know the difference between half bridge and full bridge rectifiers and your link does not provide any explanation about how to connect varistors in half bridge rectifier

[dustooff]

There is a difference between "3 phase bridge rectifier" and your circuit "3 phase Half wave rectifier".

Voltage clamping would be best on the DC side. (with suitable upstream circuit breaker)

Can a purpose built 3 phase SMPS be specified instead.

[ustaad]

Voltage clamping would be best on the DC side. (with suitable upstream circuit breaker)

I am also thinking the same