Re: Pulsing Alternator Output
If you will examine the wiring diagram for your airplane, you
will likely find that the battery is connected to a bus bar through
the master switch. Everything that uses battery power is connected to this bus. The bus bar is a copper bar attached
directly to circuit breakers behind the panel. The alternator output is connected thru the ammeter and then through the alternator circuit breaker to this bus bar.
To excite the field winding of the alternator a voltage regulator
receives power from the bus through an over voltage relay,
the alternator field switch, and the field circuit breaker. (in series)
The purpose of the voltage regulator is to regulate the output
voltage of the alternator and not the battery voltage. After a little
pause for reflection one will see that the point of connection
to receive power to operate the field circuit should be obtained
right at the alternator output terminal and not at the battery bus.
An aircraft wired this way has had the voltage regulator loop normal high frequency switching characteristic (switches so rapidly the flicker is not seen) changed by the presence of the battery between the regulator and the alternator output terminal. The battery acts like a very low pass filter that lowers this swiching frequency to such a low frequency that the flicker on the bus voltage can be seen. The causes the jumping of the ammeter. It is not the ammeter's fault or the fault of any other of the components associated with the alternator.
It is way the system is wired and the long wire runs involved. If you will carefully observe over a period of time, the flicker rate is subject to the state of charge of the battery. When the battery charge is low from heavy usage and little recharge, the internal resistance of the battery is higher than when fully charged. This extra resistance reduces the effect of the battery's presence somewhat and the flicker frequency increases. It may increase sufficiently so that the ammeter no longer flickers. As the battery recharges and the internal resistance drops,the observable flicker gets more pronounced again.
So what can one do to improve on this situation? Three choices:
1. nothing. Ignore it.
2. obtain another ammeter that has high damping and that
will mask the flickering so that you don't see it. Such an
instrument may not exist. An electronic version may. There
was a Piper service bulletin a few years back calling for the
replacement of the ammeter. I don't know if this one called
for a unit with sufficient damping to mask this condition or not.
3. Cut away the bus bar where it connects to the alternator field
circuit breaker and connect the newly opened field circuit breaker terminal through a 16 gauge wire to the alternator output terminal.
One might question why the field circuit goes through a switch in the first place. No switch like this is involved in automobiles. The reason is that in setting the aircraft up for a forced landing, it is desirable to be able to disable the alternator output to
reduce the potential for electrical sparks should the alternator be damaged in the event. The master switch and the alternator switches are located side by side for this same reason.
The overvoltage relay is required to protect the electrical system from a run-away-alternator. This would occur if the voltage regulator failed and applied full battery voltage continuously to the field winding. The alternator output voltage would rise and
could destroy equipment using the bus.
This condition will probably never be the subject of an AD since in spite of the poor wiring arrangement, safety or airworthyness is not involved. So you likely can't look to Piper for any aftermarket support on this conditiion. Who knows?
Good luck.