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Join Date: Mar 2011
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ABT Rectifier

01/28/2012 11:58 AM

Dear Engineers,

May be the question I am asking is looks silly,but i am confused about it so please give clarification.

In 3 phase rectifier we are using 6 thyristor and if we give 415v ac it will come around 560v dc at output when we measure with our voltmeter.

Now if one of this 6 thyristor was not getting firing signal or it was bad (not conducting) then output voltage when we measure with voltmeter will show 560 v dc only? Is this right, or is there any changes in o/p voltage? As our voltmeter measures peak value. How we can find out there is problem in 1 thyristor?

Thank you

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#1

Re: ABT Rectifier

01/29/2012 12:18 AM

Simple way will be thus:

Filter the harmonics out of a "DC Volt-Out " measuring circuit--branching from Rectified output-- which is loaded with a constant heavy resistive load RL .

You are suspecting that 1 of the 6 -not working:

Take the reading=V11.RL is the load.

Now disconnect any one thyristor. Load as before .Take the reading=V12.RL is the load

Likewise 4 more times --take readings V13>>V16-all tested. RL is the load each time .

Now see which being disconnected made no difference. That is the FAULTY one .

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#2

Re: ABT Rectifier

01/29/2012 8:29 AM

In that case only 2 Φs are conducting in full wave. It will give as much output but ripple may increase on full load.

If you have a CRO try to measure ripple on both conditions [disconnect filter]

See Rectifier

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#3

Re: ABT Rectifier

01/30/2012 6:55 PM

You won't see much voltage output difference at all when one diode opens, you will see whatever the rectifier is powering get VERY hot, then you need a scope to find which is bad/not-firing. Now if you have a blown fuse DON'T replace it until you check if one of the diodes/SCR's are still shorted, then you won't have to use a scope to find the open/not-firing one.

in the real world the voltage drop isn't useful for troubleshooting 6 diode circuits

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