Alternator sizing question

[flyzone]

I have a Cherokee 180 and it has a 60 amp alternator. Apparently you can never get the load higher than 30-35 amps no matter how much you turn on. So why is a 60 amp alternator installed in the plane if you can never use it? It is an excess reserve issue? I've noted that smaller Cherokees have a 40AH alternator but I don't know what their maximum load is. However it doesn't seem that a 180 would require 20 more amps over a 140 that it can't use.

Having just replaced my alternator this is just one of those technical mysteries that I've always wanted to ask someone who knows.

 

[Norman]

Simply put, you don't want the alternator working near full capacity because that will shorten it's life considerably. 50% more than the system normally draws creates a decent reserve and the alternator doesn't have to work so hard.

 

[flyzone]

Simply put, you don't want the alternator working near full capacity because that will shorten it's life considerably. 50% more than the system normally draws creates a decent reserve and the alternator doesn't have to work so hard.

OK, Thanks, Norman - like an engine I guess. My alternator (Chrysler) went for 16 years so I guess it is sufficiently "reserved" since it seems to be rated (60AH) at nearly 2x my heaviest load (35AH), which I don't see that often. I read once from a mechanic that said he never saw one electrically fail - only mechanically.

 

[Joshuajayg]

OK, Thanks, Norman - like an engine I guess. My alternator (Chrysler) went for 16 years so I guess it is sufficiently "reserved" since it seems to be rated (60AH) at nearly 2x my heaviest load (35AH), which I don't see that often. I read once from a mechanic that said he never saw one electrically fail - only mechanically.

What Norman said and also, it gives a little growth for electrical load if needed.

And electrical generating equipment gets measured in amps and volts, not amp hours. Amp/hours (Ah) is for capacity such as in batteries. It is a rated load for an amount of time.

Just some info you might like.

 

[Dan Thomas]

Back in the '50s and early '60s, generators were the norm. They had typical capacities of 20, 30 or 35 amps. They were big and heavy. Those old airplanes had vacuum-tube radios that drew a whole lot of amps just to light the filaments in the tubes. Add in incandescent flashing beacons that pulled 10 amps, or rotating beacons that pulled the same plus some more for the motor that drove the rotation, and the overall electrical load got pretty big. At night the generator couldn't keep up while taxiing. When alternators came out it made sense to install a 60-amp alternator, and besides, these alternators were just automotive alternators adapted to airplanes, and 60-amp units were common in cars and pickups.

Now airplanes have solid-state radios that use far less current than the old boat anchor radios, and LED lighting is taking over as well. Flap and gear motors haven't changed yet.

Alternator output is dependent on a couple of factors, one of which is RPM, as is a generator's. They cannot produce full power at low RPM, so night taxiing could be a problem. A bigger alternator will produce more at idle than a smaller one would, so even though we might never need 60 amps, we use that alternator because it will make things easier at idle or taxi speeds when loads are high.

 

[nrpetersen]

Besides 60A gives more smoke and fireworks when something shorts or runs away.

Given newer electronics etc I think these high capacity alternators are potentially a real hazard.

 

[GRG55]

Besides 60A gives more smoke and fireworks when something shorts or runs away.

Given newer electronics etc I think these high capacity alternators are potentially a real hazard.

If this is a serious concern for you, suggest you downsize the battery in your airplane significantly...and see how well that works.

 

[Clark1961]

Ummmm, gotta re-charge the battery after starting the engine so the comment about the load being half the alternator rating is just a bit on the deceptive side.

 

[unsafervguy]

In reality the only draw back to having a large capacity altanator is weight,size and cost.p

It only produces the amount of current that is needed.

Bob

 

[Norman]

Ummmm, gotta re-charge the battery after starting the engine so the comment about the load being half the alternator rating is just a bit on the deceptive side.

True, but: You don't want it to have to work at full capacity all the time. Doing so will develop heat and fry the windings.

 

[Dan Thomas]

True, but: You don't want it to have to work at full capacity all the time. Doing so will develop heat and fry the windings.

No, it won't. Manufacturers don't give bogus ratings like that. The windings seldom burn out even if they're worked hard. I have only seen one alternator with a burned winding, and that was the rotor (field) winding that got plenty hot when someone left the master on. The field gets full regulator current when the alternator isn't turning, because the system voltage is low enough that the regulator tries to fix it by strengthening the field. And when the alternator isn't turning, the fan isn't either. That winding still worked and I found it when changing the brushes.

The brushes are what suffer when the alternator is worked harder. They carry more field current to get the desired output. Even then they don't burn out; they just wear faster.

Most common alternator internal problems: Worn-out brushes from too many hours. Brushes and slip rings gummed up by too much grease put in the rear bearing. Corroded bearings. All of this is due to poor or no maintenance.

 

[flyzone]

I never remember my mechanic performing or needing to perform maintenance on my 16 yr old alternator accept infrequently adjusting the belt which he says most mechanics make too tight. I dont think you can even lube them if you wanted to. Based on what I've seen and read these things are about as maint free as hardware with moving parts get.

 

[weirdjim]

Simply put, you don't want the alternator working near full capacity because that will shorten it's life considerably. 50% more than the system normally draws creates a decent reserve and the alternator doesn't have to work so hard.

Not to start a holy war, but I'm honestly ignorant. WHy would an alternator life be determined by capacity? If a 40 amp alternator and a 60 amp alternator are both putting out 30 amps at full load, the brush life ought to be identical. What's going on that I don't see?

Jim

 

[Dan Thomas]

I never remember my mechanic performing or needing to perform maintenance on my 16 yr old alternator accept infrequently adjusting the belt which he says most mechanics make too tight. I dont think you can even lube them if you wanted to. Based on what I've seen and read these things are about as maint free as hardware with moving parts get.

As a mechanic, I regularly encounter alternators that have been ignored. Bearings corroded, brushes nearly shot. If the brushes get short enough to pop right out of their holder, the springs behind them chatter and arc on the slip rings and totally wreck them. Now you're faced with a new alternator, not just a set of brushes. If bearings fail, it similarly trashes the alternator, and if that alternator is gear-driven, it can trash the drive, too. Bearings and brushes are readily available.

Magnetos are usually ignored, too, and the savings of not doing 500-hour checks is completely gone when the mags finally fail and they're found beyond economical repair.

As far as belt tension, Cessna (at least) has a specification on that. It's a torque reading at which the belt should slip, with two different torques for new and used belts. A torque wrench on the pulley nut does the trick.

 

[denverpilot]

Ummmm, gotta re-charge the battery after starting the engine so the comment about the load being half the alternator rating is just a bit on the deceptive side.

This... It's about engine start current draw recovery time. It's not about static in-flight load.

After a departure into IMC you want to be closer to "full" than not...

 

[Norman]

Not to start a holy war, but I'm honestly ignorant. WHy would an alternator life be determined by capacity? If a 40 amp alternator and a 60 amp alternator are both putting out 30 amps at full load, the brush life ought to be identical. What's going on that I don't see?

Jim

I was not thinking in terms of the brushes but the stator windings. When they are producing current they are also producing heat. Too much of the latter may do them in. The Honda 1200 Gold Wing ran the alternator full bore whenever the engine was running and bled the excess to ground. Replacing the stator was pretty common.

 

[Graueradler]

The failure mode on the Chrysler alternator on my Cherokee 140 was breaking off of the leads on top of the diodes, seemingly from vibration.

 

[Dan Thomas]

The 60-amp alternator, at full load and 14 volts, is generating 840 watts. Assuming a really conservative 90% efficiency, that means that around 84 watts will be lost to heat. That ain't much heat, and the alternator's cooling will easily dissipate it. And some of that heat doesn't come from the windings; it's generated in the core, by induction heating caused by the changing magnetic flux. Similar to a microwave oven's effect.

Alternators that burn out likely aren't getting sufficient air. And aircraft alternators are seldom at full capacity. Regulation is done by controlling field strength, not by dumping excess current. Dumping is commonly done by cheap machinery such as lawn mowers and the like. They have a simple solid-state regulator, mounted to the cooling shroud, that varies its resistance to the output current as RPM and load changes to keep the system at about 13 or 14 volts. Those alternators use permanent magnets in the rotor instead of a field winding.

 

[flyingbrit]

Assuming a really conservative 90% efficiency.

The typical efficiency of a 14V automotive style alternator is more like 50%:






A significant loss is in the rectifier diodes. At the high operating currents they can each drop 1.5V or so plus you have two in series (full wave three phase bridge) so you have e.g. 3V X 30A or 90W dissipation.