Power P = 1x A
By Jim Sparks
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 subsequent weight savings, plus AC generators can operate without brushes, which reduces maintenance.
The majority of aircraft will have some use for alternating current power. However, the way this power is produced can vary drastically. Most common are: mechanical means where the North and South poles of a regulated magnetic field interact with fixed windings, and a unit where solid state electronic components are used to produce a very close tolerance output.
Aircraft that are equipped with a Direct Current (DC) power may utilize inverters to develop AC for specific applications. In other cases, engine-mounted alternators are used as a supply source. In several new generation aircraft with primary DC power, individual components that require AC have the ability to convert the DC to alternating current. This is true for many motorized devices, such as fuel boost pumps and blower fans.
Early inverters used a motor supplied from the aircraft's main electrical system. This motor would then drive an AC generator at a predetermined constant speed to produce a specific frequency. Output voltage was regulated by varying the DC power to a main excitor winding included in the AC generator. This type of unit had several draw backs. First of all, if a DC-powered motor was used to drive the AC generator, brushes were required and therefore obliged periodic maintenance. These units were also susceptible to variations in motor speed which resulted in frequency drift.
Frequency of an AC signal is often viewed as a sign wave and is referred to as a cycle. This begins when a magnetic field begins to interact with a coil of wire. As the magnet begins to move relative to a stationary coil, voltage begins to build and will continue to increase until the magnet has moved 90 degrees. From this positive peak voltage will begin to decline until the magnet is 180 degrees from the starting point. At this point there is a zero potential in the coil and continued movement of the magnet will result in another increase in voltage, but in this case, it is moving to a negative peak that occurs at the magnets 270 degree point from the start position. Then the voltage will begin to decline until the magnet is back to its point of origin. The positive 90 degree potential is the same as the potential occurring at the 270 degree point, however the charges are opposite. Many AC generators and Inverters will include three of these sine wave generators spaced at 120 degree intervals. The result is three independent AC outputs. By utilizing different circuit wiring techniques these three phases can be utilized to produce a voltage almost twice as high as each phase independently. The most common voltage levels in aircraft are 115 VAC and 26 VAC. By combining phases, 208 VAC can be achieved while a 115VAC can be used simultaneously.
Most AC generators are brushless self cooled units with an integral Permanent Magnet Generator (PMG) and current transformers. The quill drive shaft is connected to a source of mechanical energy, such as an engine gear box. Changes in RPM of this mechanical drive will result in a frequency change. In applications where the generator is supplying power for heating elements, this variable frequency is not a problem, but in most other situations constant frequency is essential. Fixed RPM can be achieved by using a Constant Speed Drive (CSD) which employ hydraulics or a system of mechanical clutches to maintain a constant velocity output. Most CSDs will require periodic inspections and servicing. In many cases, these units are electronically monitored to ensure proper operation.
The drive shaft going into the generator will have a permanent magnet, excitation rotor, and main rotor all attached. Within the generator housing are the PMG stator, excitation coil, and the main stator.
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.
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.
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.