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...

When the drive shaft is rotated, the Permanent Magnet begins to move within the PMG stator, and the result is low voltage variable frequency AC signal. In most AC generators incorporating PMG excitors the cockpit control switch will rarely interrupt this excitation power, so electric shock can occur if caution is not used in the troubleshooting process.


A Generator Control Unit (GCU) is on the receiving side of the excitation signal. GCUs are electronic devices used to regulate and protect the AC Generator and corresponding circuitry. By monitoring each output phase of the alternator, the GCU can provide a specific regulation voltage to the main excitor winding. This DC voltage is variable and is changed anytime the main Generator output changes. By increasing the electric load on the alternator, the output voltage would tend to drop. By sensing this momentary drop, the GCU can immediately increase the excitation level causing the generator voltage to remain relatively constant.

Protections such as overvoltage, undervoltage, and overload are also provided by the control unit. Typically these protections are available for each phase of the generators output. A fault in any one will result in the generator being uncoupled from the aircraft electrical system and possibly deactivation of the excitor winding.

Devices producing AC power are rated in Volt Amps (VA) and Cycles. Wattage is a term that means volts multiplied by amps. However, in an AC circuit, voltage and amperage are rarely in phase, so without further mathematical correction the calculation will not be exact. AC generators are also rated in cycles at a specific RPM. Converter regulators are devices that can use a wild frequency AC input and convert it through internal electrical means to a constant frequency AC output.

Another widely used electronic device for producing a constant frequency AC signal is the inverter. A rotary inverter uses an AC Generator and is driven at a constant speed by a motor. The static inverter is usually smaller than the rotary and uses a DC input to electronically produce AC. Each airframe manufacturer, or sometimes completion centers, will make the determination of inverter rating depending on the electrical loads. Like AC generators, inverters are rated in VA and cycles.

Extreme care needs to be exercised when working around or troubleshooting inverter systems, as it is sometimes difficult to tell when they are alive! A Direct Current input is supplied to the inverter through some form of circuit protection. In many cases, airframe manufacturers will control the inverter by installing a power contactor. Some systems may require in excess of a 50 amp DC supply.

With many of today's devices power is always supplied to the unit and a remote switch (usually flight deck mounted) will complete a triggering circuit to turn on the inverter. Within the inverter is a circuit called an oscillator that is made up of a capacitor and a coil. In electric circuits, capacitors and coils are used to affect the phase relationship between voltage and amperage. A coil or inductive device will cause voltage to lead current while a capacitor will cause voltage to lag current. This relationship is important and can be remembered by "ELI the ICE man." The letter "L" is used as an identifier of an inductive circuit, and the letter "E" means voltage, while "I" is current. ELI means in an inductice "L" circuit, voltage "E" leads current "I." "ICE" applies the same principle with the "C" meaning capacitive circuit. With a capacitive circuit current "I" leads voltage "E." The electrical ratings of the specific components will determine the exact amount of lead or lag. The oscillator will determine the frequency output of the inverter. In some cases, an adjustment is provided to compensate for drift. Adjustments should only be made in accordance with manufacturers recommendations. Output from the oscillator is typically a very weak signal, so the next step is to give it strength. This is accomplished by several stages of amplifier and sets the main AC output voltage. This may also be field adjustable. Once an adequate signal is produced, it is then supplied to the input or "Primary" stage of the transformer. The secondary stage can be segmented to provide several different voltage levels. Most common are 115 and 26 Volts. However, specific levels can be provided to meet any specific power requirement. Airframe manufacturers may elect to install circuit protection on the different AC outputs, so a short to ground on one leg will not disable the entire inverter.

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