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PPS Installation and Operating Manual
Rev. C.2 (January 19, 2018)
break. For some reason, while most experimental airplanes are built as
dependable but simple vehicles, their builders are enticed to attach every
electrical bell and whistle they can find in a catalog. By adding more
relays, busses, terminals, diodes, wires, and (let’s face it) toys, you are
actually adding more things that can fail and more things that make it
harder to troubleshoot.
Before you delve into the details of designing your electrical system,
please consider these three bits of advice. If you do so, the end result will
be an electrical system and avionics package that meets your real needs
when you get your project in the air.
3.2 Basic Concepts
An aircraft electrical system can be divided into three parts:
1.
“Backbone” components: aircraft battery, alternator, voltage
regulator, contactors and associated wiring. This is called the
primary power distribution system. Contactors are just high
capacity relays that are energized by low power signals but allow
large amounts of power to pass through.
2.
Busses, switches, smaller wiring, and circuit protection (fuses
and/or circuit breakers). This is called the secondary power
distribution system.
3.
Users of power and the wiring to and from those users. Users
may be lights, instruments, avionics, pumps, etc. The term device
or load is used in this manual to generically describe all the users.
More on electrical system basics:
•
The aircraft battery and alternator provide power to all
electrically-dependent systems. Normally, the battery powers
systems before and during starts and then the alternator takes
over charging the battery and providing power to the electrical
devices. A battery contactor, connects (or disconnects) the
high-current wires between the battery and the main power
distribution bus. The PPS functions as the battery contactor.
•
Power typically runs from the battery/alternator to electrical
busses behind the panel where power is split and sent to
individual devices through circuit protection devices (fuses and
circuit breakers) and switches. The VP-X assumes the role of
busses, circuit protection, and a host of single-function modules.
During construction, the VP-X greatly simplifies the task of
wiring your aircraft.
•
Wire sizes vary and the size of the wire to each device is
determined by the current load (amps) of that device as well as
the distance the current must travel. If a wire is too small for
the load or distance, it will heat up and possibly fail. If the wire
is too big, it will certainly carry the load but at the expense of
added weight.
•
To complete the electrical path, devices must have a ground. This
means connecting a ground wire to the metal aircraft structure
(aircraft ground) or running a ground wire from the device to a
central location such as a firewall grounding point.
3.3 Alternator Options
The alternator provides power to devices and also charges the aircraft
battery. The voltage regulator continuously monitors the bus voltage and
adjusts the output of the alternator. The regulator only works when it is
powered from a bus through a wire called the field wire. Some alternators
are internally regulated (the regulator is built in), and others have
external regulators (a separate box located outside the alternator).
Today’s experimental aircraft are powered by either 14 volt or 28 volts
systems. Often you may hear 12 volt or 24 volt systems. Why the
difference? The reason is because the batteries are rated at either 12 or
24 volts. When the engine is running and the alternator is turned on, the
alternator generates 14 volts or 28 volts, slightly higher than the battery
voltage so it will keep the battery charged.
If you have a primary alternator and a secondary (backup) alternator only
one alternator (field wire) should be powered on at a time. Therefore, we
refer to one alternator as the primary and the other as the secondary. If
both are on simultaneously, they do not equally “contribute” to powering
the loads. The one whose voltage regulator is set to the highest voltage
will draw all the current (sometimes called current hogging), possibly
overloading the alternator.
