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What happens if you have a blown rotating diode


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  #1  
Old 07-18-2015, 06:27:16 PM
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Default What happens if you have a blown rotating diode

I am about to dig into my DKC
http://www.smokstak.com/forum/showthread.php?t=147568
to diagnose the low (56V) output, this happens even at no load. It occurred to me that it might be a dead rotating diode (or even two). I know when you blow a diode in an automotive alternator it reduces the output but does not completely eliminate charging. Would this show up as a reduction in power output rather than in voltage. I am guessing that the VR would drive the output to meet the voltage setpoint regardless of load. I am also aware that if one diode is blown it takes that phase out so it would be a 1/3 reduction. Has anyone experienced this before with similar symptoms. I am more curious than anything else , I still expect to find a faulty voltage reg. Cheers Dan
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Old 07-18-2015, 09:40:34 PM
ArodaPowerCo ArodaPowerCo is offline
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Default Re: What happens if you have a blown rotating diode

I have experienced blown diodes twice, but neither were Onans. One had only a few volts of output, another had zero output. Both had been overloaded with high surge loads and the breakers were also ruined.
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Old 07-18-2015, 10:04:43 PM
pegasuspinto pegasuspinto is offline
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Default Re: What happens if you have a blown rotating diode

The voltage tends to dip badly under load. Easy to check them once you get there, tho. check the windings, since you have to disconnect one leg of the diode to check them, might as well get the windings done as well.
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Old 07-18-2015, 10:58:39 PM
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Default Re: What happens if you have a blown rotating diode

It does not sound like a high order possibility in that case. I will probably attack the voltage reg first since I don't like to mess with the rotating wiring if it is not suspect, too much chance of difficulty getting things back together without breaking something or screwing it up. Cheers Dan
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Old 07-19-2015, 10:55:09 AM
gnucklehead gnucklehead is offline
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Default Re: What happens if you have a blown rotating diode

Diodes can open or short which might present different symptoms, but I've never seen more than just a few volts resulting from one open diode. If you have already checked all the stator windings and connections, it's worth checking the diodes and rotor windings if they are easy to get at. Have you tried re-configuring it for something different? I haven't had the opportunity to deal with a -18 head, but I have read where earlier DK units had some stator issues (yours is a later one)

Do you have access to an oscilloscope? Curious to know what the output actually looks like.
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Old 07-19-2015, 03:11:35 PM
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Default Re: What happens if you have a blown rotating diode

Depends. If you have a shorted diode, you'll not have any output. 1 open and it might do fine except near full load and that will depend on the type of load. A resistive load, probably not notice it, a reactive load and you will.

2 bad diodes and it probably not work at all, you will probably have residual voltage and the regulator will be working its donkey off trying to get the voltage to where it needs to be and will eventually kill the regulator or trip the exciter breaker.

Measure excitation voltage, shou dbe around 12VDC at no load and the expected output. If a lot higher, like 20VDC (YD and such), look for an open diode.

An open diode has no effect on one phase making more voltage than the other. Its all about how strong the rotor gets magnetized and in turn the output voltage level.

If I were you, measure excitation voltage and go from there. With 56 VAC output (about half of nominal) I'd suspect that you have a regulator issue. Measure at the regulator F1 and F2 (or X and XX). Going to guess and predict it will be 0 or close to it. If so, disconnect the regulator (F1 and F2) and apply 12VDC from the battery is OK withe the engine running at 1800 rpm. Voltage should hop right up to pretty close to nominal. If that happens, be sure that the regulator sense and power wiring is good, can leave the battery powering the excitation while you are doing that. If the regulator is bad, then call Flight Systems and get a new one. Have your old one rebuilt by them and keep a spare on the shelf.
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Old 07-19-2015, 08:36:59 PM
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Default Re: What happens if you have a blown rotating diode

Conclusion field exciter coil open circuited. I may have misread the decimal my volt meter , I was expecting 56V cause the guy said it put out 50ish volts but it was actually 5.6v. When I checked the field resistance it was open circuited. I pulled the coil out and went over it with a 10X magnifier and found 2 spots where green corrosion was present. I was able to pick the wires out of the bundle and solder in a repair then heatshrink over the solder joint. Back to 13 ohms , put it back in and fired it up , got 230V leg to leg 116V to neutral, so I am happy. Incidentally I noticed that the exciter coil seems to interchange with a J series coil that I had lying around so that is useful information. I like interchangability. Cheers Dan
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