Aircraft Electrical Power

Aircraft Electrical Power

• A.C Power generation 
• D.C Power generation
• Emergency Power generation
• Voltage Regulation
• Power distribution 
• Inverters 
• Transformers

A.C Power generation

Alternating current (ac) generators supply the electrical energy for operating aircraft avionics equipment. A generator is a machine that converts mechanical energy into electrical energy by electromagnetic induction.

Alternating current (AC) electrical systems are found on most multi-engine, high performance turbine powered aircraft and transport category aircraft. AC is the same type of electricity used in industry and to power our homes.

The AC power is typically a three-phase wye generator at 115VAC using 400Hz. Use of 400Hz power has been a standard for decades as the power can be produced with smaller and lighter generators than 50/60Hz systems. Although the use of higher frequencies is not ideal for long distance power transmission (more sensitive to voltage drop), the benefit of the lighter system is ideal for aerospace applications.

           The electrical generating capacity of the generator will vary depending on the application, but can be more than 200kVA per generator.

Some common characteristics of A.C generators -

1. The stator (stationary armature winding) provides the AC output.
2. The AC generator field (rotor) is a rotating magnetic field with fixed polarity.
3. Regulating the rpm of the rotating magnetic field controls the voltage frequency.
4. Controlling the strength of the magnetic field controls the voltage frequency.

D.C power generation

Direct current (DC) is a flow of electrical charge carriers that always takes place in the same direction. The current need not always have the same magnitude, but if it is to be defined as dc, the direction of the charge carrier flow must never reverse.

DC generators transform mechanical energy into electrical energy. As the name implies, DC generators produce direct current and are typically found on light aircraft.

In many cases, DC generators have been replaced with DC alternators.Both devices produce electrical energy to power the aircraft’s electrical loads and charge the aircraft’s battery.

Direct current (DC) is used on systems that must be compatible with battery power, such as on light aircraft and automobiles.

Sources of direct current includes power supplies, electro-chemical cells and photo voltaic cells and panels.

DC generators require a control circuit in order to ensure the generator maintains the correct voltage and current for the current electrical conditions of the aircraft. Typically, aircraft generators maintain a nominal output voltage of approximately 14 volts or 28 volts.

There are many benefits of AC power when selected over DC power for aircraft electrical systems -

• AC can be transmitted over long distances more readily and more economically than DC, since AC voltages can be increased or decreased by means of transformers. Because more and more units are being operated electrically in airplanes, the power requirements are such that a number of advantages can be realized by using AC (especially with large transport category aircraft). 
• Space and weight can be saved since AC devices, especially motors, are smaller and simpler than DC devices. 
• In most AC motors, no brushes are required, and they require less maintenance than DC motors. 
• Circuit breakers operate satisfactorily under loads at high altitudes in an AC system, whereas arcing is so excessive on DC systems that circuit breakers must be replaced frequently. 
• Finally, most airplanes using a 24-volt DC system have special equipment that requires a certain amount of 400 cycle AC current. For these aircraft, a unit called an inverter is used to change DC to AC.

Emergency Power System

An emergency power system is an independent source of electrical power that supports important electrical systems on loss of normal power supply.

A standby power system may include a standby generator, batteries, and other apparatus. 

Emergency power systems are installed to protect from the consequences of loss of primary electrical power supply.

Voltage Regulation

A voltage regulator controls the magnetic field strength. Current generating the magnetic field is known as excitation current. The auxiliary DC generator (called the exciter) or a rotating three-phase rectified AC exciter generator supplies this current. The exciter is on the same shaft as the AC generator to make it an integral part of the generator.

Modern power systems operate at some standard voltages. The equipment working on these system are therefore given input voltages at these standard values, within certain agreed tolerance limits.

The voltage regulation can be defined in two ways :-

• Regulation Down :- This is defined as "the change in terminal voltage when a load current at any power factor is applied, expressed as a fraction of the no load terminal voltage."  
  
• This is the definition normally used in the case of the transformers.
• Regulation Up :- The regulation is expressed as the ratio of the change in the terminal voltage when a load at a given power factor is thrown off and the on load voltage.
  
• This definition is more commonly used in the case of alternators and power systems as the user end voltage is guaranteed by the power supply provider.

   Types of Voltage regulator -

• Reference Voltage Regulator :- One type of voltage regulator that has no mechanical moving parts (except the exciter control relay) is the solid-state regulator.
• Sensing Voltage Regulator :- Another type of solid-state voltage regulator operates by sensing the voltage existing on the lines. It amplifies the changes in this signal, and varies the average current supplied to the field winding of the integral exciter. The voltage regulator consists of a sensing circuit with input rectifiers, a temperature compensated Zener diode, reference and error-detecting bridge, and a three-stage transistor amplifier. The output of the bridge circuit is a voltage inversely proportional to the difference between generator voltage and regulator set voltage. This output is referred to as the error signal.

Power Distribution System

• Carries power from source to various components in cockpit.
• It is most crucial and reliable system of aircraft
• Key points while designing  1. More than two sources should supply power to each load. 2.Critical electrical loads must be supplied from more than one busses

Various parts of power distribution system :-

•   Two starter generator units
•   The system is typically defined as a split-bus power distribution system since there is a left and right generator bus that splits (shares) the electrical loads by connecting to each sub-bus through a diode and current limiter.
•   Small amounts of AC are supplied by two inverters
•   The hot battery bus
•  Main Battery bus: supply power for engine to start.
•  Isolated bus: Connects left and right generator in parallel
•  Current limiter: Connects generator busses to isolation bus
•   dual-feed electrical busses

Inverters 

• 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.
• A modern inverter is a solid-state device that converts DC power into AC power.
• 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. 
• Supplies both - 26 Volt AC & 115 Volt AC
• Two types of inverter may be found on aircrafts - 1. Rotary inverters  2. Static inverter

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.
  
• Static inverters use solid state components, they are considerably smaller, more compact, and much lighter in weight than rotary inverters. 
• Depending on the output power rating required, static inverters that are no larger than a typical airspeed indicator can be used in aircraft systems. 
• Some of the features of static inverters are:-

1. High efficiency.
2. Low maintenance, long life.
3. No warmup period required.
4. Capable of starting under load.
5. Extremely quiet operation.
6. Fast response to load changes.

Static inverters are commonly used to provide power for such frequency sensitive instruments as the attitude gyro and directional gyro. They also provide power for autosyn and magnesyn indicators and transmitters, rate gyros, radar, and other airborne applications.

Rotary Inverters

• There are many sizes, types, and configurations of rotary inverters. Such inverters are essentially AC generators and DC motors in one housing. 
• The generator field, or armature, and the motor field, or armature, are mounted on a common shaft which will rotate within the housing. 
• Common type of rotary inverter are - Permanent magnet rotary inverter and Inductor type rotary inverter.

Permanent Magnet Rotary Inverter

• A permanent magnet inverter is composed of a DC motor and a permanent magnet AC generator assembly. 
• Each has a separate stator mounted within a common housing. 
• The motor armature is mounted on a rotor and connected to the DC supply through a commutator and brush assembly. 
• The motor field windings are mounted on the housing and connected directly to the DC supply. 
• A permanent magnet rotor is mounted at the opposite end of the same shaft as the motor armature, and the stator windings are mounted on the housing, allowing AC to be taken from the inverter without the use of brushes.  
• The generator rotor has six poles, magnetized to provide alternate north and south poles about its circumference.
• The frequency of this type unit is determined by the speed of the motor and the number of generator poles.

Internal wiring diagram of rotary inverter

Inductor-Type Rotary Inverter

• Inductor-type inverters use a rotor made of soft iron laminations with grooves cut laterally across the surface to provide poles that correspond to the number of stator poles. 
• The field coils are wound on one set of stationary poles and the AC armature coils on the other set of stationary poles. 
• When DC is applied to the field coils, a magnetic field is produced. 
• The rotor turns within the field coils and, as the poles on the rotor align with the stationary poles, a low reluctance path for flux is established from the field pole through the rotor poles to the AC armature pole and through the housing back to the field pole. 
• In this circumstance, there will be a large amount of magnetic flux linking the AC coils.
• The frequency of this type of inverter is determined by the number of poles and the speed of the motor. 
• The voltage is controlled by the DC stator field current.

        aircraft AC power distribution system

Transformer

•   A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction.
•  Increase or decrease the alternating voltages in electric power applications.
•  Transformers works on Faraday law in conjunction with high magnetic permeability core properties.
•   Two types of transformers

       1. Step-up transformer

         2. Step- down transformer
Click here for detailed notes on Transformer


Aircraft Electrical Power (Part 2)