electrical systems

• Basic AC generator

  ·         Wire loop rotated inside magnetic field

  ·         Contact made with slip rings and brushes

  ·         Produces sinusoidal voltage
• Basic DC generator

  ·         Slip rings and brushes replaced with commutator arrangements

  ·         Ensures EMF reverses polarity every 180 degrees

  ·         All +Positive or -Negative voltage

  ·         Commutator can be further divided for more stable voltage
• Left Hand Rule

  ·         Thumb out: motion of conductor

  ·         Index straight: direction of magnetic field N to S

  ·         Middle out: EMF current – to +
• Induced Voltage Rating is dependent on

  1.       Strength of magnetic field (lines of force / unit area)

  2.       Speed of the conductor crossing lines of flux
• Elimination of DC Ripple

  ·         Increase the number of coils in armature and/or field coils

  ·         DC generation will always have slight commutator ripple

  -          Can cause hum in radio

  -          Solved with ripple filter in DC line in, inductance coil or “choke” coil in series, or a capacitor in parallel
• Residual Magnetism

  ·         Most generators use electromagnets as field coils, powered by the generator

  ·         All materials that have been magnetized retain some residual magnetism

  ·         This can be used to start a generator without external field coil excitation

  ·         As generator output Increases, field strength Increases, and output continues to Increase

  ·         If field windings lose their magnetism, it can be restored by “Flashing the Field”
• Shunt Wound DC Generation

  ·         Field winding and armature connected in parallel

  ·         Requires resistance / regulation in field circuit to prevent over-voltage

  ·         If run without load, Field Voltage and Output Voltage would both increase until the generator burns out

  ·         In theory it could run without regulation IF speed and load were constant
• Series Wound DC Generation

  ·         Field winding and armature connected in series

  ·         If load resistance Increases (electrical load decreases), Field and Output Voltage Decrease

  ·         If load resistance Decreases (electrical load increases), Field and Output Voltage Increase

  ·         Series wound generators cannot maintain constant voltage and are not suitable for aircraft
• Compound Wound DC Generator

  ·         Contains a field winding in series and in parallel with respect to armature

  ·         As load increases (resistance decreases), series field Increases, parallel field Decreases

  ·         As load decreases (resistance increases), series field Decreases, parallel field Increases

  This relationship keeps output voltage constant as load Increases or Decreases
• Shunt-wound generator with a voltage regulator is the most common on aircraft

  • can adjust current to the field as RPM and load change
• Armature Reaction causes the neutral plane to shift due to cross magnetization of the armature.
• Due to armature reaction, brushes of the generator must be set in the neutral plane to avoid short-circuiting “live” coils, causing loss of power.
• Armature Reaction can be solved with brush setting in small generators under constant load.
• Armature Reaction can also be solved with Interpoles, which are extra field poles within the generator.
• Each interpole has the same polarity as the next field pole in rotation.
• Interpoles “pull” the neutral plane into the correct position.
• Compensating winding can be used to increase the effectiveness of interpoles.
• Vibrator type Regulator

  ·         Uses a voltage coil in parallel with generator output

  ·         Coil controls contacts, when open the field current must pass through a higher resistance

  ·         Contacts open as generator Voltage reaches peak, which results in voltage drop, causing contacts to close again. Occurs multiple times / second

  ·         Only suitable for low field current, contact points will burn out at 78A
• Carbon-Pile type Regulator

  ·         Uses a stack of carbon disks as resistance

  ·         Compression of the stack lowers resistance

  ·         Electromagnet connected to generator output controls compression
• Equalizing Circuit

  ·         Controls field voltage for 2 generators connected in parallel

  ·         Lowers field Voltage of generator taking too much load, increases Voltage of the other

  ·         Can only adjust for small differences, = < 0.5 volts
• Reverse Current Cutout Relay

  ·         Voltage coil and current coil wrapped on same core controlling contact between generator and load

  ·         If generator voltage drops below battery voltage, contacts open to prevent battery from discharging through the generator
• Current Limiter

  ·         Reduces generator voltage when maximum safe load is reached

  ·         Uses a current coil connected in series to generator output to control resistance

  ·         Usually activates at 10% above generator rating
• Two-Unit Control panel

  ·         Voltage regulator & reverse current cutout relay mounted on same panel

  ·         Regulator has extra “accelerator winding” to allow contact points to open/close faster, resulting in smoother voltage regulation
• Three-Unit Control panel

  ·         Contains voltage regulator, reverse current cut-out, and current limiter

  ·         Very successful regulation in 12V or 28V systems

  ·         Used almost exclusively before transistor regulators
• Generally AC generators:

  ·         400 Hz / 3 Phase

  ·         Most alternators have rotating field, stationary armature, this allows for direct connections to output circuit

  ·         3-phase, 3 separate windings on stator, 120 degrees of separation

  ·         Stators wound in Y or Delta configuration
• 3-Phase, full-wave rectifier

  Used to rectify AC current into usable DC 12v or 28v

  • Made of 6 diode circuit, 2 connected to each output phase and then to + or – output lines
• AC – Rotating field, less current through brushes for excitation

  DC – Rotating armature, high current through brushes for power output
• DC alternators only require voltage regulators and current limiters (circuit breaker acts as limiter)
• AC generation in emergency

  1.       RAT – Ram Air Turbine

  2.       APU – Auxiliary Power Unit
• CSD – Constant Speed Drive

  ·         Needed on AC generator to maintain constant AC frequency

  ·         Constant output RPM with variable input RPM

  ·         Not used on small DC powered aircraft with alternators
• Generator Frequency

  -          2 poles = I cycle per rotation

  -          #poles / 2 * RPM = Cycles per minute

  -          Cycles / min / 60 = Hz (Cycles/sec)
• Advantages of Brushless High-Output AC Generators

  1.       No brush/run wear, low maintenance cost

  2.       Higher stability and consistency of output

  3.       Better performance at high altitude, no brush arcing
• PMG – Permanent Magnet Generator

  Contains

  1.       Permanent magnet generator (-> GCU)

  2.       Exciter generator

  3.       Main generator
• IDG – Integrated Drive Generator

  -          Contains both CSD and generator in one unit

  -          Reduces weight and size VS two separate units
• GCU – Generator Control Unit

  Provides

  1.       Voltage regulation

  2.       Current limiter

  3.       Protection from out of limit Frequency and Voltage
• Inverter

  ·         Converts DC to AC

  ·         Generally only used in emergency situations

  ·         Rotary Inverter is a simple DC motor driving an AC generator
• Static Inverter

  ·         Uses an oscillation circuit to produce 400Hz

  ·         Current passes through transformer and filter to produce desired waveshape and Voltage

  ·         Generally 115V single phase AC

  ·         Can be very small, used for electroluminescent instrument panels
• VSCF – Variable-Speed Constant-Frequency Power System

  ·         Eliminates need for CSD

  ·         Variable input RPM generator

  ·         Uses solid-state circuitry to convert variable frequency into usable 400Hz
• Starter Generator

  ·         Combines starter and generator in one housing

  ·         Typically found on small turboprop and turbine powered aircraft

  ·         Most have 2 field windings, Series wound for Starting, Shunt wound for Generating
• Starter Generator Advantage

  • Only one drive gear mechanism for start and generator, no engage/disengage of start gear
  • Reduced size and weight VS 2 separate units