Power P = 1x A Part Two By Jim Sparks April 1998 A alternating Current (AC) is the power of choice for the manufacturers of large airline equipment. One primary reason is the ability to use smaller diameter wires resulting in a...

Most frequently outputs are wired to provide a "High" and a "Low," which is another way of saying an output and a return. Other inverters, if checked, will have 58 volts available on each of the two wires. In this case, each output is phased so that when an appliance is connected, it will benefit from the rated 115 Volts. It is important to be aware of the inverters capabilities before connecting or troubleshooting.


Some inverters will require an external lead/lag capacitor to compensate for systems with an abundance of reactive components.

Some aircraft will require two or more inverters operating together. In this situation if the output of the two operating units is not in phase, significant abnormalities can occur. "Out of Phase" operation can result in greatly altered frequencies and voltage and may eventually damage one or both inverters. Systems that are using the power will experience unusual operating characteristics. Aircraft compass systems as well as flight guidance have tight tolerance for frequency and voltage.

Most inverter manufacturers provide a means of synchronizing operating devices. This is accomplished by installing a wire between the oscillator circuits of the "On Line " inverters. Should one fail while connected to this synchronization system, the possibility exists that the remaining inverter would be dragged down along with the defective unit. Most airframe manufacturers that use an inverter sync system will make provisions that in the event of failure, the synchronization is interrupted, and the remaining inverter then operates on its own.

Monitoring of Inverter output is an important issue and can be accomplished internally. Circuits are provided to detect any abnormality in either voltage or frequency. With newer technology, it is common to find inverters operating within a one percent tolerance, but should frequency drift or voltage drop ,an automatic shutdown may occur.

Typical inverter.

Letter designations only apply to this diagram. Do not apply to a specific aircraft.

A = 26VAC output
B = 115 VAC output
C = Common or ground
D = 28VDC input
E = Remote control switch output
F = Inverter ground
H = Remote control switch input
I = Sychronization wire

Airframe manufacturers may provide methods for the flight crew to monitor inverter performance. These include voltage and frequency meters, along with fault warning lights that would sense a loss of voltage or a deviation from the specified frequency.

As these solid state AC power sources function, one byproduct is heat. In some cases, internal blower fans are installed and may be controlled by an internal temperature sensitive resistor (thermistor); others choose to have the fan operate any time the inverter is on. Fan motors may be either AC- or DC-powered. Earlier devices typically incorporated the DC motors which, of course, in time would require brushes. Most blower fans employed in later models are AC-powered. Solid state construction, which is not concerned with changes in atmospheric pressure, will allow inverters to be mounted within or outside the pressure vessel of the aircraft.

Passenger convenience systems are often supplied with AC power. In all but a few cases these will use 60 cycles, like house current. It is very important to keep systems of different frequencies segregated.

Should a problem arise in an AC power system, airframe manufacturers CAUTIONS should always be observed. In addition, all power sensitive components should be isolated from the problem system prior to testing or troubleshooting. An example of this would be a gyro. In many cases, they are wired direct to an AC bus with no means of isolation other than a circuit breaker. It may also be advantageous to utilize self preservation techniques when working around AC. This includes using only insulated tools and working with one hand behind your back and both eyes WIDE open.

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