alternator conversion

[lbell1945]

I am going to convert to an alternator. I looked on skytornics web & looked at the wire diagram & noticed that there are 2 circuit breakers & one switch/circuit braker. is all this needed & where would you mount them? can anyone that has been there help me?
Larry bell
N2359J

[GAHorn]

Alternators require current protection. That requires a circuit breaker or fuse...hence the requirement for a minimum 5 amp CB to protect the regulator. You can install that anywhere you wish such as on a small bracket on the firewall near the regulator, but most pilots prefer to install it within reach somewhere on the panel. A good location is on a short sub-panel (a piece of angle below the insrument panel row of fuses in most 170's is a favorite location. IF you do, don't ever reset it more than once in flight.) Also notice that although they recommend you use the appropriate size CB according/appropriate to the wire size used....they also happen to stipulate that all wire be 18 ga. except where noted....which effectively means you MUST use 18 ga wire in that location and MUST use a 10A CB in that location.
Another CB controls the alternator ON/OFF function via the field, and they arbitrarily/safely decided to use a CB/SWitch.... a very good idea in my opinion. (It only costs about $20 and adds a nice safety feature against fires.) I"d locate that near my main master switch if it were me, but it could also be located beside the other regulator CB on the angle bracket. Beware not to locate the angle bracket where it might interfere with flap operations. I suggest you install such a bracket below the glove box or below the radio panel.
Finally, the main CB (50A) replaces the existing GEN fuse or CB in your instrument panel, and attaches the system to the main elect. buss.

[Bruce Fenstermacher]

....they also happen to stipulate that all wire be 18 ga. except where noted....which effectively means you MUST use 18 ga wire in that location and MUST use a 10A CB in that location.

George we all should know that CBs protect the wiring and not necessarily the appliance. As I recall 18 ga. wire can handle 10 amps and so that would be the maximum CB for the wire. But why couldn't you protect the wire with a smaller CB like the 5 amp suggested. The wire would certainly be protected and you may actually be providing some protection for the voltage regulator in this case.

[GAHorn]

....they also happen to stipulate that all wire be 18 ga. except where noted....which effectively means you MUST use 18 ga wire in that location and MUST use a 10A CB in that location.

George we all should know that CBs protect the wiring and not necessarily the appliance. As I recall 18 ga. wire can handle 10 amps and so that would be the maximum CB for the wire. But why couldn't you protect the wire with a smaller CB like the 5 amp suggested. The wire would certainly be protected and you may actually be providing some protection for the voltage regulator in this case.

That's an excellent question, Bruce.
And the long answer is: Because that is the manner in which the STC wiring diagram is notated. (Notice the diagram/schematic at http://www.skytronicsinc.com/wiring_diagram.htm has FAA approval stamps on it. Your observation regarding the CB value VS wire ga. is correct, and your observation about the purpose of the CB (from FAR and electrical engineering viewpoint) is to protect the wire and not the appliance is also correct,.... and in fact your logic is actually appliable elsewhere in this STC (just not in that particular location) ... But...the fact that the FAA approved the data as drawn/depicted makes any other choice of CB amperage and wire ga. or alterations of the data illegal. (That diagram/schematic lists which wire gauges/CB amperages are applicable according to which wire gauge you choose to utilize in that particular service....implying that the regulator only needs 5 amps of service...which would necessitate only 22 ga. wire...and that indeed they even offer you a chart illustrating that CB-size VS wire ga. relationship...5A CB for 22 ga. wire... ...and according to Note 4 it seems to be that you are allowed to choose to put in other gauge wire. However, there is also a note contained elsewhere in the diagram/schematic that all wire is to be 18 ga. except output. I interpret that to actually restrict one to installing only 18 ga. wire in any location other than output (where a 50A CB and either 8 ga. (if run alone) or 6 ga. wire (if bundled with others) is required)...and their FAA approved schematic (again Note 4) specifies a 10A CB with 18 ga. wire...therefore no allowance is authorized for anything else but 10A CB and 18 ga. wire. in the regulator power-supply circuit.)
Notice also that the notation in the schematic "Note 1" specifies ("recomedes", actually) a particular Potter-Brumfield 5 amp CB, and "Note 2" recommends a 50 amp CB ....those are in fact not recommendations but are specifications and required ampereage sizes. The "recommedation" only pertains to mfr/model CB... not amperage. (However, the CB described in "Note 4" is not specified as to size except as pertains to the wire size selected for the service. And that is what all this additional discussion is about....the fact that on one hand we seem to be allowed to choose which wire ga. to use ....but on the other hand and in fact we are actually restricted to using 18 ga. wire except output.)
It's important to understand that the field circuit is controlled by a 5A CB/Switch for a particular reason despite the fact that 18 ga. wire is specified in that service . The reason for that is, that the mfr. requires the field circuit of the regulator and alternator be limited to 5A maximum current in order to stay in calibration. (I.E.-Not because of wire protection in the normal sense. It has both minimum and maximum current requirements, therefore to cover the minimum current required...line losses must be minimized and so they use larger than necessary 18 ga. wire, and to cover maximum current allowed, a 5A breaker is employed.)
Summary: You are correct as pertains to the field circuit...a smaller than standard breaker can be used on larger wire (but it does not protect the wire...it limits it) ... in the case of this STC'd installation the FAA approved notes require that all wire be 18 ga. (except output), which always requires 10A CB unless otherwise specified. (The CB addressed in Note 1 serves a regulatory function... specified as a maximum 5A current between regulator and field, service for which the wire ga. is deliberately oversized to avoid induction/line losses. This CB is also used in an on/off function.)

Regarding the FAA stamps all over this schematic.... my previous comment regarding where to locate the CB's should also be viewed as recommendations only provided the STC instructions do not specify a location. Example: If an STC states that the Switch/CB must be placed in the cockpit and placarded...then so it must be. However if the STC instructions merely instruct that a CB must be installed...yet no location is specified...then it's ladies choice and my recommendation is merely an opinion you may choose to follow or not.....as usual.)

[Bruce Fenstermacher]

Wow George I'm sorry to put you through all that. I did look at the schematic briefly but didn't pick up on all that you did. Apparently the FAA didn't pick up on the inconsistencies either.

Unfortunately these kinds of inconsistencies seem fairly common with approved aircraft paperwork. The FAA machine has made it so difficult, time consuming and expensive to just get any kind of approval that once obtained the holder of such approvals are very very reluctant to seek an update or correction. I've run across this before and when questioned the owners usually say something like "of course you should do it like we intended which makes sense and not as the paper work technically says you should. No one will be the wiser"

One example of this is Univair ownes an STC which can only be applied to a Cub with metal wings spars on which the Reed Clip Wing modification has been performed. When I pointed out that you can't perform a Reed Clip Wing mod to a metal spar wing since it is only for wood spars the Univair owner said something to the effect that it was close enough and know one would know the difference, including the FAA.

As for my post I thought you were saying that since the wire HAD to be 18ga that the CB ALSO had to be a 10 amp breaker. I didn't realize that the paperwork spelled that out through a "back door" blanket statement.

[lbell1945]

the breaker on the 18ga wire must work as an overvoltage protector.
I used to have a comanche with an alcor conversion. it had an overvoltage relay that woud brake the field circuit and you were supposed to be able to power down the alt. main breaker & it would reset. I put on 3 overvoltage protectors. they would open the field circuit & never reset. after 3 in flight alt. failures I tossed it out and put on a volt guage and flew the airplane 10 years & 750 hrs. without another incident. never saw the voltage over 14.5 volts. if this is the purpose of this breaker it seems to me to be a lot better system. I never ask anyone else if they had overvoltage relay problems since I was running w/o one that was called for in the stc.

[GAHorn]

the breaker on the 18ga wire must work as an overvoltage protector.
I used to have a comanche with an alcor conversion. it had an overvoltage relay that woud brake the field circuit and you were supposed to be able to power down the alt. main breaker & it would reset. I put on 3 overvoltage protectors. they would open the field circuit & never reset. after 3 in flight alt. failures I tossed it out and put on a volt guage and flew the airplane 10 years & 750 hrs. without another incident. never saw the voltage over 14.5 volts. if this is the purpose of this breaker it seems to me to be a lot better system. I never ask anyone else if they had overvoltage relay problems since I was running w/o one that was called for in the stc.

...A L M O S T... not quite, lbell1945...but almost..... You almost got it exactly right and you understand the "intent"...but that's not exactly correct.
The Alcor alternator conversion (since marketed under a different name, InterAv) used a circuit breaker to control their overvoltage regulator (a seperate unit from their main regulator.) But you might have had difficulties with your installation for other reasons than you might have thought. That OV regulator also had a 5 amp CB to control the OV regulator, and the correct procedure is to power down the OV 5A CB,... NOT the main circuit breaker...and reset it. THAT would reset the OV protection.
Operation without that OV regulator was a dangerous practice as it could lead to a battery explosion/fire inflight. That's why the FAA (and any elect. engineer) requires OV protection to be in that circuit. You lead a charmed life that it never over-volted inflight.
Circuit breakers do NOT control voltage. The regulator does that. Circuit breakers only control AMPERAGE. (It's common to see CB's rated at a particular amp rating....over a very wide range of voltages. The ones likely protecting your home are probably rated anywhere from 105 volts to 240 volts...but their amperage rating is a constant. The same is true of switches.)*

* Caution: FYI-- Just because a switch may be rated at 10Amps 100-400 volts doesn't mean it will work in all circuits within that voltage range. You should always consider whether or not the application is Direct Current (DC) or Alternating Current (AC). The opposite is true than most folks believe.....DC current is harder on switches than AC current. This is because AC current rapidly alternates back and forth with a momentary "off" between alternate surges. This causes less stress on the switch due to the momentary "off" of AC current. Therefore...if ever you use a switch intended for AC ...in a DC circuit...you must always increase the amp capacity of the switch or it'll burn out in DC service. (Using a 5Amp DC switch in a 5A AC circuit....as long as the voltage ratings are similar....the swtich will be "loafing". The reverse however, using a 5A AC switch in a 5A DC circuit will cause the switch to fail.) It's important to use properly rated equipment for a reason.

[lowNslow]

I am going to convert to an alternator. I looked on skytornics web & looked at the wire diagram & noticed that there are 2 circuit breakers & one switch/circuit braker. is all this needed & where would you mount them? can anyone that has been there help me?
Larry bell
N2359J

Larry, I believe you need a dampened crankshaft to install a alternator on a O300. The UN-dampened crankshaft has a 1/4" hole drilled through the prop flange.