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Seminole Electrical System

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Active member
Apr 28, 2002
Can anyone tell me what exactly a "Shunt" does in the Seminole electrical system? I think it's a "reducer" of some sort, but would welcome any more specific information.


I don't have any Seminole info avialable here, but a "shunt" has a specific meaning.

Railroad signal systems, for example, refer to a shunt as the electrical pathway created between the two rails when a train occupies a section of track that contains a detection circuit. Instead of current being allowed to run to the end of the track section and energize a relay, the current runs through the wheels and axle of the train, starving the relay at the other end of the track of its energy. This is why crossing gates go down as a train aproaches.

By definition, a shunt is an alternate, and intentional path for electricity or magnetism in a circuit. You could probably apply this term, by extension, to a flow of liquid, too, as in shunt the flow of oil. Sometimes, a high current shunt can be used in a charging system to indicate charging current flow. A voltage drop is measured across the shunt, and a proportional value of circuit current is calculated. This isn't used much because it is an inefficient method, and was favored before the days of the "amp clamp" for current diagnosis.

If you can describe where you see this mentioned in the case of the Seminole, I can probably be of more help.
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Thanks for the reply. The shunt is located next to the alternator between the voltage regulator and the ammeter. The POH claims that "the positive output of each alternator is fed through individual shunts to the tie bus."

I guess it ensures that the proper current is being sent to the tie bus.

Let me know what you think!
Thanks, Buckman
I'd call that a "gen buss" in a jet, but it sounds as though the output could be sent to more than one location, i.e.: the tie buss and/or an individual L or R buss. If that's the case, they are using the word "shunt" to mean "divert or direct" the current flow where desired. Does that seem right to you? I'd need a circuit diagram to be certain of why the word "shunt" was used here.

Hope that helps.

Oh, one more definition.

In medicine, a shunt is used to drain intracranial pressure from the skull, usually after surgery or concussion. I's a tube that goes into the skull, and craniospinal fluid comes out, helping to reduce pressure.

Before I wanted to be a broadcaster, medicine was a passion for me. MD's: please forgive any spelling errors here!
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buckman said:

Thanks for the reply. The shunt is located next to the alternator between the voltage regulator and the ammeter. The POH claims that "the positive output of each alternator is fed through individual shunts to the tie bus."

The way it has been explained to me, a "shunt," in the way Piper likes to use the term, is used to display amperage on the loadmeters. It's a calibrated resistor through which current flows to enable the ammeter to display the correct load.
I believe that Booker has the correct answer. The shunt is a calibrated resistor. The ammeter does not measure the amperage directly, but actually measures the voltage across the shunt (as timebuilder noted) The instrument itself is graduated in total anperage. If there is a shunt attatched to each alternator, that would suggest that you may measure the amperage from each alternator individually. Does the Seminole have 2 ammeters, or perhaps one ammeter which may be switched from the left to the right alternator?

Correct- the shunt is simply a tap off of the primary output of the alternator that lets the ammeter read it. If all of the alternator's output ran through the ammeter, Piper would have had to install a huge ammeter to handle the amperage. With a shunt, a small panel-mounted ammeter can read the output of the alternator by sampling a small percentage of the output.
Shunt wound systems are used for both generators and motors. This can include alternators, and also starter motors. The shunt is a parallel field winding which carries the same current and voltage being fed to the generator/alternator/starter.

The shunt-wound generator isn't suitable for rapidly changing loads, if constant voltage is required. Any increase in load causes a decrease in output voltage. This decrease in voltage causes a decrease in field strength, and voltage is further reduced.

Shunt wound generators are suitable for light or medium use duty in aircraft. These units have mostly been replaced by DC alternators.

The shunt is controlled with a field rheostat which is in turn controlled by voltage sensitive devices to form a voltage regulator.

Typically aircraft use either a shunt-wound generator, or a compound wound generator. Both use shunt windings (a shunt circuit), but in the shunt wound generator, the shunt field windings are in parallel with the external circuit, and in the more common compound wound generator, the shunt circuit and the series circuit are connected in series. This arrangement may be made in several ways, which do different things with the generator.

Most commonly, the arrangement is "overcompounded," which means that the generator has a full load voltage that is higher than the no-load value. In other words, voltage increases the more the load is increased on the generator. This is done by connecting a variable shut across the series field, which is called a "diverter." Compound generators are used where voltage regulation is critical.

A shunt is used to control voltage, and the placement of that shut will determine how this is accomplished, and the characteristics of the generator.
Thanks, avbug. I hadn't considered the whole winding aspect of this situation.

How old is that Seminole? I haven't seen a shunt measurement of current in a while, maybe back in the sixties in a Heathkit auto analyzer I built.

A long, long, time ago, in a galaxy far, far, away.....
A shunt is basically like a resistor that is connected in parallel with a loadmeter, the shunt is used to measure the resistance of the load going through it, this resistance is propotional to the amount of current in the system....thats about all that i remember off the top of my head..

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