Inverters

Keeping up with the demand for in-flight technology


The usage of electrical energy in the aircraft of today would no doubt boggle the minds of Orville and Wilbur. Instead of manipulating mixture controls and pulling through propellers, a simple press of a switch brings current generation aviation powerplants to life. Whereas a magnetic compass and a railroad map could get most aviators successfully from point A to point B, now a constellation of earth orbiting satellites provide worldwide position information rendering most other forms of navigation all but obsolete. In aircraft of yesteryear electrical power systems were, for the most part, considered convenience systems rather than necessities.

Passenger expectations have also changed over the years. It used to be air travel was a good way to escape the reality of day-to-day life, offering impeccable cabin service and an atmosphere conducive to catching up on correspondence, reading, or just looking out the window. In the fast-paced environment of today, many folks find the need to “be in touch,” and this has significantly altered the flying experience.

AC and DC
Over the years manufacturing design philosophy has made the determination regarding the means of powering specific equipment. Size and weight constraints impose significant obstacles to an electrical system architect. Alternating current (AC) electrical power provides the advantage of being able to be transformed — that is, stepping up or down the voltage to compensate for an end user’s power consumption. The unit of measure for AC is the volt amp (VA) and by stepping up the voltage, lesser amperage is required and enables the circuit to function with smaller diameter wires and possibly smaller controlling devices. This inductive property is not, however, without drawbacks.

Care needs to be exercised during installation so that the fields produced during operation will not impact surrounding wiring or components. The FAA has published several documents on a subject titled Electrical Wiring Interfacing Systems (EWIS). One example is Advisory Circular AC 25.1701-1.

An inverter is an electrical or electro-mechanical device that converts direct current (DC) to AC and is so named due to the fact that early mechanical AC to DC converters were made to work in reverse, and thus were “inverted” to convert DC to AC.

An inverter’s resulting AC output can be at any voltage and frequency where the levels are set with the use of a high-power electronic oscillator, appropriate transformers, switching, control, and monitor circuits. Common aviation applications utilize 400 cycles and voltages at 5, 26, and 115v AC. Inverters supplying the needs of a passenger compartment often possess the electrical characteristics of the country of origin. In the United States this is 115v AC at 60 cycles while many European countries use 220v AC at 50 cycles.

Static inverters, as used in aircraft, have a limited number of moving parts and may include a blower fan which is often actuated by a thermal switch. Static inverters are used for a wide range of applications in aircraft and in many cases the presence may not be easily noticed — such as the fuel pump running on a 28v DC input but containing an AC motor. Proximity switches are another good example of a device with a concealed inverter.

The appropriate output rating of the inverter is directly dependent on the load to be powered. It is not uncommon to find reliability problems in systems where the limits of the inverter are regularly exceeded — that is, an inverter which is not powerful enough to operate system specific loads.

Inverter selection
The first step in inverter selection is to calculate the total (watts or amps) of all appliances you plan to power. Virtually all AC-powered equipment will bear a label (usually placed near where the power wire enters the unit) indicating how many volt amps (VA) or watts of electricity the equipment uses.

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