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Rectifier Fried in Nippon-Denso Alternator

avrojockey

Well Known Member
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Needed to fly up to KGRB from KATW (10 min) to pick up some Camguard and warm things up for an oil change. Returning to KATW my wife and I decided to fly to lunch, so I stopped for gas at self-serve. After fueling, I cranked up to repo to hangar where my wife was waiting, and during the short taxi I noticed the distinct smell of burning rubber..not an acrid electrical burning smell. My braking was normal and there was adequate cooling during fueling, but I started looking for smoke for a brake fire or evidence I had a flat tire. I shut down at the hanger and popped out for a look, everything appeared normal and all systems were operating normally, so we continued to lunch because I thought the smell wasn't originating for my aircraft.

Mid-flight, I smelled the rubber burning smell again for a couple seconds and it went away. Everything indicated normal, and again looked normal after landing. I took an extra hard look around and in cowling and still nothing, because I was now convinced the smell was from my airplane. The totally confusing part was it smelled exactly like burning rubber...the smell you encounter when you're downwind of large aircraft touchdown. It did not smell electrical.

After lunch I returned back to KATW, but what I didn't notice is that my system voltage was battery voltage (~12.5) the entire flight (20 min). It wasn't until I stopped for gas again and restarted that I noticed cranking was different and saw there was no alternator output after start.

I changed my oil, and the next day attempted fixing the alternator. I initially thought it was bad regulator because mine is modified to receive field input only from the IGN wire. However, the problem ended up being a bad rectifier...it appears one of the diodes completely fried. I didn't save the part or take pictures, but it was burned up pretty good underneath, near the A stator output...assuming the diode in that spot? Luckily there's an alternator shop down the road and they had a rectifier in stock for $40. I changed it out and proper alternator output resumed.

Finally my question...what would cause this? I'm not a fan of things burning in flight and wondering if there's another root-level problem. The auto electric shop thought it was a weak battery or high resistance somewhere causing the rectifier to heat up. Though this doesn't seem to be the case...I was able to do two starts and fly almost 30 minutes with only 0.1v drop in voltage, and as indicated by the G3X graph, the charge recovery is looks quick and the loads all look normal prior to failure. The only thing that is different recently is the installation of a IBBS, but that and every other system is operating normally.

I've attached the Volts and Amps reading from the G3X...first picture shows the point in flight where I smelled burning rubber...you can see the voltage drop below normal battery voltage and amps go to -39 indicating a significant load on the battery originating from the alternator (my Hall effect sensor is installed on the B+ lead), but then recovery eventually to normal volts and amps. The second picture shows the beginning of the flight home where only battery voltage is indicated and the cursor pointing to the voltage drop during the start.

Thoughts?
 

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I asked the old guy at my favorite alternator repair shop what most likely takes out rectifiers in our alternators, his answer is heat. Alternators have cooling fans, some suck air in from the front behind the pulley, some are opposite and pull air from back through the alternator. He asked me where was the cold air blast tube, front or back & what was the orientation of the cooling fan, as different models of alternators may have either.
 
I asked the old guy at my favorite alternator repair shop what most likely takes out rectifiers in our alternators, his answer is heat. Alternators have cooling fans, some suck air in from the front behind the pulley, some are opposite and pull air from back through the alternator. He asked me where was the cold air blast tube, front or back & what was the orientation of the cooling fan, as different models of alternators may have either.

I have a blast tube near the front...maybe contributing. Its a standard Nippon-Denso Lester #14684 that normally is designed for CW rotation...not sure which way the winds blows when strapped to O320
 
I have a blast tube near the front...maybe contributing. Its a standard Nippon-Denso Lester #14684 that normally is designed for CW rotation...not sure which way the winds blows when strapped to O320

I took apart a PP alternator and not sure it (rotation) makes a huge difference. Previous experience with forward and backwards facing fan blades would say the air still flows, but they are crude in any case. A blast tube on the SRE to the bridge/regulator would at least provide the coolest air source. These are tested for ambient environments above 230F at full output, exhaust radiation notwithstanding. Time at temperature is a big factor, so after shutdown the bridge won't be adding any energy and may not be an issue in that condition. We would need data to confirm, but since a claw pole alternator is only 40-60% efficient that is a lot of heat generation that goes to zero when shut off.

I have done no testing however, so this is all relative.

Tim, how many hours were/are on your unit? There are two known bridge failures on VAF, for the PP EI60, one at 520 hrs and one over 900 hrs.
 
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Check the belt and alternator pulley for evidence of slipping and heat, possibly causing your burning rubber smell. This can happen intermittently if the alternator is loading and unloading quickly due to an internal fault (although usually it will be a regulator fault causing this..).
 
Tim, how many hours were/are on your unit? There are two known bridge failures on VAF, for the PP EI60, one at 520 hrs and one over 900 hrs.

Im not the builder but it appears the alternator could be original and the aircraft has 730 hrs on it.
 
Im not the builder but it appears the alternator could be original and the aircraft has 730 hrs on it.

Not a bad number (IMO) for life, but they will all fail at some point. A blast tube could certainly help, it should blow on the back end around the regulator fins might be a good spot.

Honestly, I wrestle with when to replace my plane power too. After looking at the insides, 500 hrs is selected as the take apart and check time. Then get a new one with warranty and clean, test and save the old one for long x-ctry trips where it might be needed.

Repair/replace and fly happy. :D
 
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