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.
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.
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.
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