Battery/Ground Power/Electrical question

[vontresc]

So this morning I get a call from one of our club pilots that the battery was dead. Turns out, he was the last one to fly it, and didn't turn off the master three days ago. So lets assume a fully discharged battery.

He got the FBO to bring over the start cart, fired it up, but now he was seeing the Alt Fail Battery only message. He said that the main tie breaker was not tripped, both alt and batt masters were on, and the essential bus switch was off.

This brings me to my question. Does plugging in the external power plug do anything funky like isolate one of the buses? I didn't think so, but the fact that the alternator wasn't producing power had me wondering what was going on.

While looking at the diagram I was wondering what the purpose of that solenoid in the external power wiring is.

Anyway the battery is going on the charger at the shop, and hopefully isn't too fried.

[Steve]

While looking at the diagram I was wondering what the purpose of that solenoid in the external power wiring is.

Peter:

That solenoid is to isolate the external power jack from the Battery Bus unless external power is connected (that middle pin is energized by the external power source as well as the main positive pin). This way, you don't have a live, unprotected positive pin present in the jack to accidentally short out the battery bus.

Steve

[DaveS1900]

The Alternator requires battery voltage to excite the windings. Commonly about 7 volts or less from the battery and the Alternator does not work. Follow the manufacturers guidelines but this usually involves pulling the battery and charging overnight.

Dave S.

[CFIDave]

To reinforce what Dave S said, alternators need "current to make current" in the form of excitation current from a battery. Without that, an alternator can't generate any additional current.

Many older aircraft have generators with permanent magnets, and thus can create current by simply spinning a magnet (that always creates a magnetic field) inside of coils. No battery needed.

In contrast, alternators need a small amount of current (i.e., the excitation current) to create a magnetic field instead of using a permanent magnet -- and this current has to come from a charged battery. An advantage of an alternator over a generator is that a voltage regulator can vary the *amount* of excitation current input to the alternator to maintain a more-constant output voltage. Thus an alternator can more efficiently create a constant output voltage across a wider range of RPMs.

So you have to at least partially charge the battery to make your alternator work.

[curts63]

Does your flying club charge by the Hobbs meter? IF so, boy is he in for a surprise. In most cases, the hobbs meter will continue to run when the battery gets low. Mine has almost an extra 50 hours of time from the dying battery.

[vontresc]

LOL no we go by the tach hour. We will bill him for the battery charge though.

[Rich]

To reinforce what Dave S said, alternators need "current to make current" in the form of excitation current from a battery. Without that, an alternator can't generate any additional current.

Many older aircraft have generators with permanent magnets, and thus can create current by simply spinning a magnet (that always creates a magnetic field) inside of coils. No battery needed.

In contrast, alternators need a small amount of current (i.e., the excitation current) to create a magnetic field instead of using a permanent magnet -- and this current has to come from a charged battery. An advantage of an alternator over a generator is that a voltage regulator can vary the *amount* of excitation current input to the alternator to maintain a more-constant output voltage. Thus an alternator can more efficiently create a constant output voltage across a wider range of RPMs.

So you have to at least partially charge the battery to make your alternator work.

Not universally correct. The majority of generators also used field current, but many had weak permanent magnets to sort of "boot up" from a dead battery. But like most alternators, output voltage is controlled by managing the current in the field windings. Even a generator that might not have been supplied with permanent field magnets would often acquire magnetism as a side effect of field current constantly being applied to the windings about a ferrous core structure. The major distinction between alternators and generators is that generators deliver DC current from the rotor through the brushes/commutator. Alternators develop AC (typically 3-phase) current from the stator instead of the rotor which is then rectified to DC via diode bridge. This rectifier may be built into the alternator housing or may be externally located. The brushes of an alternator (if there are any) only carry field current, which is far less than the output current so the brushes are much smaller and only use slip rings, which are less prone to wear.

Alternators can be built with permanent magnets in the rotor with no external field current provided. To control voltage such alternators have regulators that simply shunt current on the output side. This kind of alternator is very common in modern motorcycles. It is less efficient than a field-controlled alternator, because the alternator always puts out the maximum current for any given RPM, so its power is mostly wasted and is therefore a bit of a parasitic drain on engine power.