Page 11
VP-X Installation and Operating Manual
Rev. D (August 5, 2020)
3.2 Introduction
An aircraft electrical system can be divided into three parts:
1. “Backbone” components: aircraft battery, alternator, voltage
regulator, the
Vertical Power Primary Power System
(PPS) and
associated wiring. This is called the primary power distribution
system. In place of the PPS, you can also use traditional contactors.
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. The Vertical
Power
Primary Power System
(PPS) connects (or disconnects)
the high-current wires between the battery and the main power
distribution bus. In place of the PPS, you can also use a traditional
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 or
ground plane (aircraft ground) or running a ground wire from the
device to a central location such as a firewall grounding point.
3.3 Alternator Operation
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.
When planning your electrical system, assume the alternator provides 80% of
its rated output (in amperes), and therefore your total continuous load (don’t
worry about trim or flaps or other transient loads) should not exceed 80% of
rated alternator capacity.
3.4 Wire Sizes and Circuit Protection
A table below shows wires sizes versus loads for a typical homebuilt-size
airplane.
The wire size can be larger than necessary but should not be
smaller.
Circuit breakers (and fuses) protect the wiring, not the device. If the breaker
is too large, then the wire may overheat and fail. If too small, then the device
may fault (breaker trips) because it draws too much current.
Most kit aircraft companies and avionics companies provide
recommendations for sizing wires and breakers. You can use these
recommendations, or you can borrow or purchase an ammeter to measure
the current draw of each electrical device and then determine the sizes
yourself.
