Inverters

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.

The next step is an evaluation of the type equipment to be powered, as AC loads most frequently fall into two categories: electronic loads, such as switch mode power supplies (SMPS), as found in computers and various audio visual equipment, and motors which may drive cooling fans or portable vacuum cleaners.

Various types of electrical loads require different levels of initial start up or surge power. This is often referred to as peak surge for electronic loads or locked rotor current for motor loads. The peak surge or locked rotor current is usually significantly higher than the continuous load (which is the power needed to operate the device after start) and must be considered when sizing the inverter — along with associated wiring, controls, and circuit protection.

Some SMPS-type devices can possess extremely reactive tendencies (circuit with impedance variations) while in normal operation. A poor power factor, along with a high peak current, will often result in overload either causing inverter shutdown or possible failure. Maintaining a ratio of 3:1 between the power output rating of the inverter in VA, and the rating of the routine load in watts, will usually provide a comfortable surge protection margin. For example, powering a 300-watt load will be well suited when using an inverter having a minimum power output rating of 900 VA. If the load is known and constant the ratio for power factor determination can be dropped to 2:1. There is a tradeoff for utilizing a lightweight, compact inverter if powering a possible wide array of electrical appliances. The available power should be considered prior to any circuit modification involving load increase.

Remote switching
Many installations provide remote switching to determine when AC power will be provided. In some cases the inverter is switched on as soon as aircraft power is selected ON.

Understanding the aircraft’s specific control circuits is very important for self-preservation. After all, AC can be a significant hazard, especially when its unexpected presence is detected by an unprotected body part.

Remote switching circuits often include involvement of sensors detecting if adequate aircraft power is available to sustain the loads carried by the inverter. In addition, cabin pressurization sensors may be part of the AC system switching logic. In the event cabin altitude exceeds 10,000 feet it may be prudent to eliminate high voltage to cabin entertainment systems.

Situations do arise where it becomes necessary to operate multiple inverters simultaneously. In some cases they power separate loads. In other cases they are connected to provide three-phase power to various consumers or are connected in parallel to boost the available volt amps. Most solid-state devices contain an external paralleling circuit to ensure the combined outputs are operating in phase. Even when multiple inverters are powering independent buses, having phase synchronization is a desirable condition due to the inductive properties of alternating current. Frequency deviations are notorious for causing very unusual problems.

Output monitoring
Most static inverters include output monitoring devices that observe regulated voltage and frequency and have the ability to deactivate the unit should a parameter exceedance occur. An internal overheat in most cases will also result in shutdown.

Ground fault interrupters (GFI) are now commonplace in aircraft and compare the current flow in the power wire to the return. In the event amperage is flowing through a non-intended circuit component (such as an aircraft technician), the GFI will detect a mis-compare between the two conductors and isolate the circuit from the power source.

In many cases these protective circuits incorporate self-test features which when actuated will simulate a ground fault. They do also include a reset feature that has to be selected once the ground fault has been rectified.

I can honestly say that technology today has enabled passengers to stay in touch, be connected, and stay productive while traveling, and much of it is attributed to our ability to make household power available at 40,000 feet.

Boy, do I miss the good old days!

Jim Sparks has been in aviation for 30 years and is a licensed A&P. His career began in general aviation as a mechanic, electrician, and avionics technician. In addition to extensive hands-on, he created and delivered educational programs for several training organizations and served as a technical representative for a manufacturer of business jets. Currently when not writing for AMT, he is the manager of aviation maintenance for a private company with a fleet including light single engine aircraft, helicopters, and several types of business jets.

Loading