3-Phase half bridge rectifier strange issue when neutral is removed - Page 1
Forum section: electronics / projects

author: ustaad
author_rank: Contributor
timestamp: September 01, 2024, 10:23:06 am
content: 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 belowThe 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?

author: wasedadoc
author_rank: Super Contributor
timestamp: September 01, 2024, 10:30:52 am
content: 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.

author: ustaad
author_rank: Contributor
timestamp: September 01, 2024, 11:06:37 am
content: Quote from: wasedadoc on September 01, 2024, 10:30:52 amNeutral 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 removedCan you explain me more how it works then?

author: wasedadoc
author_rank: Super Contributor
timestamp: September 01, 2024, 11:18:05 am
content: 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.

author: ustaad
author_rank: Contributor
timestamp: September 01, 2024, 11:27:02 am
content: Quote from: wasedadoc on September 01, 2024, 11:18:05 amThat 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

author: wasedadoc
author_rank: Super Contributor
timestamp: September 01, 2024, 11:40:09 am
content: 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.

author: BennoG
author_rank: Frequent Contributor
timestamp: September 01, 2024, 11:42:06 am
content: 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.htmlBenno

author: Andy Watson
author_rank: Super Contributor
timestamp: September 01, 2024, 11:53:32 am
content: Quote from: ustaad on September 01, 2024, 11:27:02 amWasedadoc's answers are accurate.Quote from: ustaad on September 01, 2024, 11:27:02 amYes,pluswhatever current flows through the load at the line voltage.

author: ustaad
author_rank: Contributor
timestamp: September 01, 2024, 12:31:46 pm
content: Quote from: Andy Watson on September 01, 2024, 11:53:32 amWhich load? The SMPS? Isn't it be effectively disconnected when there is no neutral?okay, I am getting it now. Thanks though

author: ustaad
author_rank: Contributor
timestamp: September 01, 2024, 12:40:34 pm
content: Quote from: BennoG on September 01, 2024, 11:42:06 amI 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

author: dustooff
author_rank: Contributor
timestamp: September 01, 2024, 01:39:06 pm
content: 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.

author: ustaad
author_rank: Contributor
timestamp: September 01, 2024, 01:49:41 pm
content: Quote from: dustooff on September 01, 2024, 01:39:06 pmI am also thinking the same