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Alternator field wire - is the current draw continuous?

L

lndwarrior

Well Known Member
I am trying to summarize my continuous vs. intermittent electrical loads. As near as I can figure the field wire circuit draws between 2 and 4 amps.

I would like to know if the alternator field wire current is a continuous load - or a intermittent load that drops off once the engine/alternator are running?

TIA
Gary
 
Consider the field current at its max as a continuous current (because it is). Most 12V alternators have about a 4A field current at max alternator output. Some of the 70A alternators might take 5A.
 
and i've learned that if the field wire pops, you are likely hitting the "over voltage circuit" on the B&C regulators

my culprit is the cessna style master switch, which over time ~500hrs or so, they get internally corroded and cause false alarms
 
The field has a relatively linear power draw based upon current demanded from the alternator. The regulator will keep feeding more current on the field (forcing the alt to produce more current) until the voltage reaches it's set point or alternator reaches it's maximum capacity. i.e. the field current increases with the alternator output. Max draw will vary on both the alternator and regulator design. My 50A alternator would trip the 5A breaker at loads over 40 amps. Unsure if it was truly drawing 5 amps or if my breaker was tripping early. I replaced with a 7.5A breaker.

As I learned, you need to size for continuous use at full draw. If you ever run your battery down, you will run at max capacity for a period of time long enough to trip the breaker. Not something you want to learn "in the air" as I did.

Larry
 
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You may also have a self exciting alternator. Many of the automotive alternators, particularly chevy, have this feature. Once up to an rpm, off it goes on its own and killing any field wire you may have to get it going, does nothing after it is. You can kill the field and it will keep on charging.
 
Kahuna said:
You may also have a self exciting alternator. Many of the automotive alternators, particularly chevy, have this feature. Once up to an rpm, off it goes on its own and killing any field wire you may have to get it going, does nothing after it is. You can kill the field and it will keep on charging.
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Kahuna,

Very few internally regulated auto alternators have a field connection. There is a field circuit, but you usually have to cut them open to get access to the field circuit. some leave an opening in the case for a technician to get a test lead on them. Some atlernators, separate the B lead, or output, from the Excitation feed (most contemporary alternators are single input). The excitation feed is typically just input power/voltage sense for the regulator and is not a field lead. It essentially creates on on/off situation for the voltage regulator. Those with separate excitation inputs will typically not produce output without voltage on this lead. In the case of single input, the excitation is bridged internally and they cannot be prevented from producing power.

Larry
 
Available Alternators ??

Have an RV6A with a Duralast 14184 alternator that needs to be replaced. This alternator was used in late 70s and early 80s Hondas and a replacement is not "off the shelf" taking several days to arrive. Anyone have any alternatives for an alternator that may have a more recent application?
 
Had same issue with the 14129.

Snagged one online, will rebuild the other for education.

Looking at switching to an IR model and adding OVP to it.
 
TomDPilot said:
Have an RV6A with a Duralast 14184 alternator that needs to be replaced. This alternator was used in late 70s and early 80s Hondas and a replacement is not "off the shelf" taking several days to arrive. Anyone have any alternatives for an alternator that may have a more recent application?
Click to expand...

I use a 14158; same era as yours. Can find them on amazon with one day delivery or local auto parts store sometimes. 50 amps externally regulated and CC rotation to match our engines.

Larry
 
moosepileit said:
Looking at switching to an IR model and adding OVP to it.
Click to expand...

It's basically pointless to add OV protection to an automotive-style IR alternator, as shutting off power to the field wire (which is what the OV protection does) will not stop the OV condition.

With an IR alternator, a failure mode exists where it "feeds itself" and rather than using external voltage from the field wire. This failure mode does not exist in externally regulated alternators, hence the reason they are used in aircraft.

A caveat is the PlanePower IR alternators which have eliminated this failure mode. There may be others but I'm not aware of them.

Yes, automotive alternators are cheaper but you do have this risk. :eek:
 
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