over voltage protection

[Kevin Horton]

But, if someone isn't willing to spend the time and money to put in OV protection, what are the odds they will spend the time and money to do battery capacity testing and/or replace batteries regularly? Any takers on this one? I think we have to assume that those batteries will remain installed until they will no longer reliably crank the engine.

Once that battery is a few years old, I wonder how its ability to hold down the voltage in an OV event is affected.

[mannanj]

GMC et al:

Years ago I had a 72 Toyota Pick Up that had an alternator go bad. Replaced it with a rebuilt unit.

Miles from home of course, I smelled a strong odor of sulfuric acid. Pulled over, popped open the hood and the battery was literally boiling. Suspecting the rebuilt ND alternator, I disconnected the "B" lead and drove to the next town large enough to have a parts store and bought another rebuilt alternator.

Cured the problem. Have no idea what the voltage went to but there was no apparent damage to any of the electrical wiring or eqpt. It sure put out enough current to boil out a few ounces of electrolyte.

Of course it never got reported to the DOT

Mannan Thomason
RV-8 N161RL (No One Girl)

[airguy]

I had a similar episode that ended with the coil exploding and expelling its guts all over the engine compartment, which effectively killed the engine and solved the overcharging problem.

[AlexPeterson]

Originally Posted by Kevin Horton

SNIP

Once that battery is a few years old, I wonder how its ability to hold down the voltage in an OV event is affected.

It would be very interesting to do this testing. One would need one used battery and a big power supply. Perhaps the big power supply could be a couple of big 12 volters in series. Controlling and measuring the current would be the difficult part. Hmmmm, need to think about this.

[airguy]

Use a welder transformer and a BIG variac! Commercially-built rectifier bridges for several hundred amps are available relatively cheap.

[Geico266]

gmcjetpilot,

Mick Ice asked about overload protection in this (his) thread. I have stated my opinion and experience in this matter. Not having OV protection cost me $1,000, and cost my friend $2,500. I stand by my statements as facts, not rumors. Both planes involved were RV-9A's.

Van's now recommends and sells Plane Power alternators with OV protection. If you want to disreguard what the manufacturer of the RV kits recommends do what you want. May I suggest you call Vans and quiery their electronics engineer why they recommend OV protection.

It is interesting, in your informative thread about alternators you do make a footnote worth repeating;

**(Why are Van's ND alternators unreliable? Well for one its a matter of poor aftermarket parts and rebuilds. OEM ND units where of a different and higher quality apparently. Unfortunately they stopped making the models we use 10-15 years ago, so aftermarket is all we have now that the salvage yard supply is dry. Not all aftermarket parts are bad, and they certainly are not all of the same quality. Van has a generous return policy, and they now have a better quality supplier I believe. Also the "Plane Power" brand of ND alternators is high quality, and they do add an OV module that cuts power to the alternator in the unlikely event of an OV, if that makes you more comfortable. The price is set right and they are nice.)

Enough said.

You build and fly your plane the way you want, and I'll do the same.

[jcoloccia]

Originally Posted by AlexPeterson

My little 40 amp alternator running through a 50 amp CB will simply not drive the battery voltage up much no matter how hard it tries.

I would submit, to much flaming I'm sure, that if the system voltage has gone up to damaging levels, something else is amiss in the wiring design or battery integrity.

No flaming here, and you're right to some extent. The battery will absorb an awful lot in terms of short "spikes" and the like...it makes an awesome filter. A fully charged battey, though, is drawing very little current at the 14V (or whatever) we're holding the buss at. You can basically say that about any voltage you put on the buss. Once the battery's charged, and it'll charge at practically any voltage over 13.8, the current drops off sharply. It's quite difficult to increase the current just by craking the voltage, just like it's difficult to get any usable energy out of a practically discharged battery (although you may get a very brief, very large "short circuit" current for some very brief time).

So if you had a regulator/alternator failure that dumped the full bus voltage right onto the field, the battery would absorb some but I think you'd quickly have some excitment under the cowl. You're CB probably won't trip since your alternator probably can't put out enough sustained to trip it (just a guess) but your bus voltage could be significantly higher than 14V and your battery/avionics may be very sore when you land

I'm not saying any of this is likely but that is the rational behind OV protection.

[szicree]

I know precious little about electrons, but I have read a few books lately and I keep up on the list here as well. I'm confused. Somebody asks about how to correctly wire things without OV protection and George steps in to say that OV problems are mostly unsubstantiated myth anyway. Several folks chime in to give first hand accounts to the contrary. They are then given the third degree as if they are making this stuff up. Even a dope like me can get a few things out of this dialogue:

1. In theory, OV can occur.

2. According to eyewitness accounts, it has occurred in cars and planes.

3. An OV event can be expensive and possibly very dangerous.

[Geico266]

Originally Posted by szicree

I know precious little about electrons, but I have read a few books lately and I keep up on the list here as well. I'm confused. Somebody asks about how to correctly wire things without OV protection and George steps in to say that OV problems are mostly unsubstantiated myth anyway. Several folks chime in to give first hand accounts to the contrary. They are then given the third degree as if they are making this stuff up. Even a dope like me can get a few things out of this dialogue:

1. In theory, OV can occur.

2. According to eyewitness accounts, it has occurred in cars and planes.

3. An OV event can be expensive and possibly very dangerous.

4. OV protection is a CHEAP insurance policy.

[elippse]

If your external VR is tied to your buss on the buss-side of an alternator breaker, when you open the circuit the VR will detect the decreased battery voltage and send max current to the alternator field. This will cause the alternator voltage to go out of sight! Then when you close through the alternator breaker, you put an incredible transient on your buss, which, depending on the wiring inductance from the buss to the battery, could cause a substantial voltage transient in your avionics. With the VR tied to the alternator-side of the alternator breaker, the VR will try to maintain regulated voltage.
I designed and built a VR with series OV protection with hysteresis. When the voltage goes above 14.5 volts, the series transistor in the field supply turns off. When the voltage goes below 10V it turns back on. If there is a failure in the regulator portion of the circuit, there will probaly be an ON-OFF oscillation, but at least the time-constants won't let the alternator put out damaging transients! I would guess that the voltage cycling up and down like that should get the pilot's attention! If not, a visual indicator could be tied to the hysteresis network to show the problem.
I did some investigation of alternator noise a few years back. That whine is due to a fly-back condition that exists when the ON diode in the rectifier turns off when its winding voltage falls below that of the next rectifier in the bridge. That diode's current suddenly tries to go to zero, which causes a transient oscillation whose frequency is determined by circuit capacitance and inductance. It's got lots of harmonics which can be both radiated and conducted!