Generator fuse blowing

[Psmith]

I have a 35A generator on my "51 Model A. All works fine until I turn on my landing light (leading edge type), then quite often the Gen 15A fuse opens. Before the fuse goes, which is not immediate, the amp meter shows a pos charge of ~20Amps. Observing the gage a few minutes later shows a discharge indicating the fuse has opened. I believe this fuse is protecting the field circuit. Any Ideas as to the cause and any idea what the max amp draw to the field should be?


Pete N1320D 170A

P.S. Regarding the Aeromatic prop question, Eric Preston is the Clipper man I was refering to. He mentioned that there is about a 2500 foot range where the prop cycles normally. Beyond that you have a fixed pitch cruise prop or what ever you have set as the high pitch setting.

[N1277D]

In my 170A the Gen fuse protects the entire bus from the output of the generator, and can see the full generator output of 35 amps. If the generator/regulator has a fault to ground it protects the bus from too many amps.

I will check but I believe mine has a 50 amp fuse in that position.

[GAHorn]

There were several generators which could be installed in the 170. The original was a 12 Amp, but there were also 20, 25, and 35 Amp gens. The generator is controlled by the regulator, and it is important to have the correct regulator for things to work properly.
If you have a 12 Amp generator but a 20 Amp regulator, your regulator will call for 20 Amps from your gen until the gen overheats or the fuse blows. After enough times your gen will be cooked.
In a pinch, it's OK to install a smaller regulator to control a larger generator without harm, but the regulator will never ask for more than what it's rated at. I.E., A 20 Amp regulator will call for only 20 amps, which is all a 35 Amp generator will deliver because that was all it was asked for. No harm done, but the owner is not getting his generator's full output.
You should check to make certain you have a correct pairing of regulator/generator. But if you are certain you have a 35 Amp gen (pn: 1101898 is the usual one, but there are others that also fit) you should have a 1118704 or corresponding/superceded regulator (such as a VR300-14-35), and they should all be protected with a 40 amp fuse and the armature/bat wiring should be no smaller than 10 ga with 8 ga preferable.
Charging circuits are allowed a 5 amp overcapacity fuse/cb since surges can happen and since any prolonged or serious overcharge will still operate the circuit protection.

[Psmith]

Thanks for all your good info. I will check for a component mismatch. Since I have a 35A generator, it appears I probably have a faulty component rather than a mismatch. If the generator field is asking for more amps than the regulator is designed for, it would appear the generator is the problem. Also since I = E/R, possibly the field winding resistance is on the decrease. Does anyone know what it should Ohm out cold?


Pete N1320D '51 170A

[N1277D]

I am suspecting that all you need to do is replace the 15Amp fuse with a 40Amp one and your problems will go away.

[GAHorn]

Thanks for all your good info. I will check for a component mismatch. Since I have a 35A generator, it appears I probably have a faulty component rather than a mismatch. If the generator field is asking for more amps than the regulator is designed for, it would appear the generator is the problem. Also since I = E/R, possibly the field winding resistance is on the decrease. Does anyone know what it should Ohm out cold?


Pete N1320D '51 170A

Pete, I think you're making your life way too complicated. (And the troubleshooting technique you just mentioned is not correct, so forget about that.) First, look at the pn on your regulator. If it's correct (a 35 am regulator) then replace the fuse with a 35-40 amp fuse and run it up on the ground, turn on the lights and check it out.
If you still have a problem, then let us know and we can start proper troubleshooting.

[Psmith]

I verified I do have the correct regulator for my 35A generator, thanks for the part numbers. I also discovered that the generator to regulator and regulator to fuse holder wiring was way to small and yea it showed signs or over heating. It was 20 gage and should have been 8 to 10 gage. Good thing I only had a 15 A fuse in the holder. Why does the original panel stenciling call for a 15A fuse? I rewired and will test fly it tonight. Thanks for all your help, I will let you know.


Pete N1320D 170A

[GAHorn]

If your airplane originally left the factory with a 12 Amp generator, then a 15 Amp fuse would have been correct. It appears as if someone upgraded your gen/reg to 35 Amp but didn't do the job completely.
(This should serve as a warning. Make certain you also have a dampened crankshaft, indicated by a "D" in the engine serial number, or by the absence of an extra 1/4" drilled hole between prop bolt holes in the prop flange.)

[Psmith]

George?,
I flew last evening and all worked fine. Thanks for your help. I will check for the "D" this evening. Thanks again.


Pete

[lowNslow]

If your airplane originally left the factory with a 12 Amp generator, then a 15 Amp fuse would have been correct. It appears as if someone upgraded your gen/reg to 35 Amp but didn't do the job completely.
(This should serve as a warning. Make certain you also have a dampened crankshaft, indicated by a "D" in the engine serial number, or by the absence of an extra 1/4" drilled hole between prop bolt holes in the prop flange.)

George, what is the largest capacity generator allowed WITHOUT a dampened crank??

Karl

[GAHorn]

There are only three (3) generators approved on this airplane by the Type Certificate. The standard was the Delco-Remy 1101876 12Amp. The optional extra was the Delco-Remy 1101879 (also known by it's Cont. PN 40734) 25 Amp. (According to the Type cert., it must be installed with the Cont. PN 531325 drive gear.) The Delco-Remy 1101880 or 1101898 35Amp generators which the follow-on note states: "Eligible only on engines with dampened crankshafts."

(Notice that the 20 Amp generator, found on some 170's I've seen are not approved by the Type certificate. There must be some other basis of approval.)

I know of one 170 that had the 50 Amp generator installed, but it was done illegally with no approval. (And it was a foolish thing to do, since the regulator installed with it was a 25 Amp reg., which limits that 50 Amp generator to only 25 amps. I have no idea what kind of strain that must put on the crankshaft, and I have no idea what the firewall looked like, as it must surely contact it from time to time since that generator is longer than the 35 A even.)

On top of all that, the Type certificate lists only the 1101385 regulator. It does not mention any approval for any other regulator, despite the fact that the optional generators would all require a different regulator than that.

[lowNslow]

George, thanks for the info, I thought of checking the TC after posting my question. It is amazing how much info there is in such a small document, everyone should get a copy of one is they don't have it.

Regarding the 1101385 regulator, I noticed it had an asterisk (along with a lot of other stuff on this list) and it states above:

"The asterisk denotes that approval has been obtained by someone other than the aircraft manufacturer. An item marked with an asterisk may not have been manufactured under a FAA monitored or approved quality control system, and therefore conformity must be determined if the item is not identified by a Form ACA-186, PMA, or other evidence of FAA
production approval."

Does this mean when can use ANY FAA-PMA item where there is an asterisk - interesting?

[N1478D]

Just a note:

My plane has the 12 amp generator. Runs panel mount GPS with com, 2nd com, encoding transponder, electric turn & bank, landing light, intercom, and panel lights without any problem. I've replaced the battery this summer for the 2nd time in 5 years, which is probably better than average. Have flown cross country at night with plenty of electrical power.

[GAHorn]

Just a note:

My plane has the 12 amp generator. Runs panel mount GPS with com, 2nd com, encoding transponder, electric turn & bank, landing light, intercom, and panel lights without any problem. I've replaced the battery this summer for the 2nd time in 5 years, which is probably better than average. Have flown cross country at night with plenty of electrical power.

In the old days 12 amp generators were adequate for aircraft that typically only had some lighting and not much else. The 25 amp gen was designed for those with 2 or 3 old vacuum-tube radios which really drew a lot of power. The 35 amp gens answered the call for those with dual landing/taxi lamp, vacuum-tube radios (several) and anti-collision lights (which originally had drive-motors which also drew a lot of power.)
Now things have come full circle, and radios that are solid-state and surface-mount technology use so little power that large gens simply aren't necessary any longer.
And that's good, especially for Joe, because he can get by with a lighter 12 amp generator. Otherwise that green turtle would be even slower than it is.

[GAHorn]

The asterisk does not grant approval to substitute any other parts for those listed. It merely denotes that Cessna didn't obtain the approval for that item's listing in the Type Cert. In the case of the 35 amp gen, for example, it may have been obtained jointly by General Electric and Delco-Remy, and done as an addendum to the Type certificate so that an STC would not be required for the installation. (This helped sell more radios and larger gens.)