rectifier block, and surge supressor 1

[emiluz]

how do I test a rectifier block of a genset? How about the surge supressor?
How do I know if they are deffective or not?

 

[Skogsgurra]

Rectifier is easy. Just feed the right voltage into it and check that you get the right DC current out of it. It may need a good and variable (perhaps even a three-phase) source. There are other ways of doing it - using a diode tester - but I usually want to do it full scale. Do not do it if the block isn't screwed down to its heat sink.

Transient suppression is trickier. If you try to supply a voltage to check breakdown, you will probably overheat it (rated for very short pulses). But, you can connect it to an inductor and apply current to the inductor. Then remove current and catch the transient on a scope.

Other way is to charge a capacitor to double protection voltage and connect via resistor to MOV. Again, catch with scope to see limiting voltage.

I would replace the MOVs if in any doubt. It costs a lot less than having marginal testing leading to problems later.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

Then remove current and catch the transient on a scope.

What happens if the suppressor is damaged or defective? Decent size inductor + a couple of amps = bye-bye 'scope?


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[Skogsgurra]

Scotty!

You know better than that. Always use a 100:1 probe when measuring high voltages. You may (very unlikely) damage the probe. But the scope will survive.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[catserveng]

Most generator manufacturers have field verification tests for the rotating rectifier using a DMM and in some cases a megger.

Try this link to a very good manual for field troubleshooting generators

http://www.cumminsgeneratortechnologies.com/

http://www/en/common/pdfs/other/fault_finding_manual_8.pdf

 

[edison123]

catserveng

One more lps.

*Nothing exists except atoms and empty space; everything else is just an opinion*

 

[edison123]

One error in the article given by catserveng

Page 8, Polarization Index is given as (1 min value / 10 min value). It should be (10 min value / 1 min value)

*Nothing exists except atoms and empty space; everything else is just an opinion*

 

[Skogsgurra]

Yes, and the tip about using diode test to check power diodes isn't what I would do either.

The resistance measurement method on p.6 is also something that shouldn't be done. Doing this, you include DMM series resistance plus test lead resistance and contact resistance in your result. Not recommended when you measure .5 - 5 ohms. Error will be considerable.

The varistor testing with an ohm-meter in Megohm range is not recommended: "A good Surge Suppressor should have a very high resistance,(more than 100 Megohms in either direction)." See p.11.

This is pure non-sense. Varistors shall be tested at rated current - usually 1 mA - and with short pulses. If that is not possible, see if the varistor limits inductive kick-back as described earlier. It does not have to be a large set-up. Use a bench PSU and a coil of any kind (transformer primary works well), connect varistor parallel to coil, apply and break DC, measure resulting peak with scope. Simple, easy, safe.

It is possible that the Stamford book is good for some, but I see it as a source of confusion. You have to know what you are doing and Stamford does not encourage that. It just gives tips that are mostly difficult to follow or even downright wrong.

Feeling quite grumpy now, I cannot resist asking if negative polarity diodes have left-hand threads? P.7.
Also, the resistance numbers for diodes are worthless if you do not specify at what voltage and current they are taken. That is why DMMs have diode test ranges, where diode fwd drop is measured. That measurement makes a lot more sense. But I still wouldn't use that on power diodes.

I could go on forever pointing out things that confuse the unexperienced technician, but I will stop with this observation: Polarity is different on DMMs and analogue meters. So the pictures on p.7 will confuse those who use DMMs.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

Diodes on rotating rectifiers for large utility machines - multi-MW - are often large hockey puck type capsules rated at several hundred to about one thousand amps. Using a normal DMM to test these is meaningless because the test current is insufficient to bring the junction properly into conduction. DMMs can and do give results which indicate a good diode when in fact it is damaged. Reverse leakage on these big junctions is high when tested near rated voltage too so don't expect the 'infinite' resistance result from a reverse blocking test like usually happens when testing a little signal diode. When possible I test big capsule devices using a load capable of sinking at least a few tens of amps: a bank of eight tungsten floodlamps fed from the 110V turbine battery is typical. Reverse voltage withstand is usually carried out using a 1000V or 2500V Megger depending on the rating of the diode; anything over 1M[Ω] is usually good for service although better than 10M[Ω] is typical if the capsule is clean. Normally after a period in service they are covered in black crud which gives false results by creating a leakage path over the surface of the capsule.

All this sounds like a lot of work, but it is better than putting dead diodes back in the rectifier and having to shut down a unit, dismantle it a second time to repair a fault that was missed, and explain to the management why it was missed! The above test works well with big capsule thyristors too: trigger the gate using a pulse of at least a few hundred mA.


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[edison123]

The stamford manual is for its range of generators (max 2 MVA ?). So they know their marbles or resistances ?

*Nothing exists except atoms and empty space; everything else is just an opinion*

 

[catserveng]

Hey guys, when your drowning a life ring is as good as a mega-yacht. Are there better ways to do what is decribed in the manual, you betcha! But let's be realistic here, in the size generators this guy has been talking about, some simple field verifications tests with common test equipment will give you a pretty good indication (gross as it is) about if the component is good or bad.

Go do a web search on "AC Generators" and see how much really recent info you find on 100 to 2000 kVA size generators. Lots of pretty good info out there on utility size stuff, but with the changes in how newer generators are manufactured, applied, and worked on, there sure isn't much out there to really help a guy just trying to fix his machine.

In 20+ years doing generators in the field, using a DMM and doing the "simple" tests have found the problems about 99.99% of the time. And yes, there has been times when they didn't find the problem, but the boss is happy with the trend, and at least it gives us a starting point.

If someone was going to publish an up to date field troubleshooting guide for "small" synchronous generators, with repeatable tests using readily available test equipment, I'd be one of the first in line to buy it. Without that, forums like this at least are helping get info into the hands of people who need it, and the info I have picked up here has been invaluable.

 

[edison123]

"In 20+ years doing generators in the field, using a DMM and doing the "simple" tests have found the problems about 99.99% of the time."

Yep, it worked for me too for over 2 decades.

*Nothing exists except atoms and empty space; everything else is just an opinion*

 

[ScottyUK]

Sorry guys, I wasn't trying to rain on the parade!

Most of my work is on 'big' machines where the imperfections of just about everything take on greater significance than on normal sized equipment. Tests that work just fine on small components go straight to hell on big ones. I appreciate the link to that Cummins document - it might give me a clue when one of smaller sets which we sometimes hire in for primary injection tests breaks down at the most inopportune moment. Does our supplier knowingly send us sick machines because they know we abuse them with massive single phase loads?

I thought it was worth pointing out some of the potential shortcomings of some of the tests if they are taken as gospel. I take little at face value these days - I have been lied to by too many normally trustworthy test instruments when I have been testing big semiconductors and accepted the reading at face value. The diode test example is one which has bitten me on the ass rather hard a couple of times: in both instances we had to dismantle virtually all of a rotating rectifier to find a failed diode that the simple test had passed.


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[catserveng]

Not to worry, for years diodes and surge suppresors were the least of our problems in the size machines I spend the most time on. "Value engineering" has changed that to a certain degree. And when you factor in labor costs and cost of downtimes, and the new thinking of only selling a complete rectifier assembly because the factory can't trust the field tech to use thermo compound and properly tighten the diode blocks to the heat simk, doing signficant checks usually get sidelined in favor of just throwing a new part on.

I don't work for a Cummins dist, but have found their field manual, even with it's faults, to be one of the best overall generator troubleshooting manuals I have found. Wish the other manufacturers would think the same way, and make this kind of info more "open".

And as for instruments, we are starting to dictate to our field staff brands and models top use, since even with regular calibration intervals, some of what is on the market today just ins't reliable.

I hope the fellow who started this thread is getting to the bottom of his problem.