AC Generators

• Most aircraft use AC electrical power, making the AC generator a primary component.
• AC generators, also called generators, vary in size based on their power output requirements.
• Large hydroelectric plants utilize enormous generators capable of producing thousands of kilowatts at high voltage levels.
• A typical automobile uses a very small generator, weighing only a few pounds, which produces 100-200 watts at approximately 12 volts.
• AC generators, the rotating part is the rotor, and the stationary part is the stator.

AC Generation

• AC generator voltage and current build from zero to maximum in one polarity, return to zero, and then build to maximum in the opposite polarity before returning to zero, graphically appearing as a sine wave.
• There are three basic AC generator types, permanent magnet, rotating armature, and rotating field generators.

Permanent Magnet Generators

• A permanent magnet generator uses an engine-driven rotor with a permanent magnet where magnet rotation causes its magnetic field to cut the stationary output windings.
• Alternating voltage is produced, making it suitable for integration into brushless AC generators.

Rotating Armature Generators

• A rotating armature generator is similar in construction to a DC generator where the rotor rotates in a fixed field.
• EMF is extracted using slip rings.
• Rotor windings are arranged axially along the rotor periphery in slots, and the armature is laminated to reduce eddy current losses.
• The stator has DC excitation windings on pole pieces to produce alternating north and south poles.
• Coils around like poles are wired in series.
• One voltage cycle occurs each time the conductor passes 360° past a pair of poles (N-S).
• Multiple pole pairs will induce multiple cycles of AC.
• Frequency (f) is determined by the number of pole pairs (p) and the rotor speed (N) where f = (N*p)/60.
• N represents generator speed in revolutions per minute (RPM)
• p signifies number of pole pairs
• hertz (Hz) is the frequency in cycles per second
• Formula for a two-pole Machine = N = 60f / p, and if the output is 400 Hz, the rpm is 24,000.

AC Generator Rotor Types

• Two rotor types are used in rotating-field generators, cylindrical and salient-pole rotors.
• Cylindrical rotors are typically used with high-speed turbines where windings form two to four distinct poles.
• Windings are embedded to handle centrifugal forces at high speeds.
• Salient-pole rotors are used in low-speed generators where the rotor comprises separately wound pole pieces bolted to the rotor frame.
• A salient-pole rotor has a greater diameter which results in higher centrifugal force at the same RPM.
• Salient-pole is only used in low-speed designs.

Rotating-Field AC Generators

• Rotating-field AC generators, also called alternators, are common in automobiles and small piston engine aircraft.
• The AC output is often converted to DC via a rectifier for aircraft electrical distribution.
• The design features a stationary armature winding and a rotating-field winding where the generated voltage can be connected directly to the load.
• Rotating armature designs require slip rings and brushes which can lead to insulation issues, arc-overs, as well as short circuits at high voltages. High-voltage generators avoid this by using rotating-field designs with low-voltage DC applied to the rotating field.
• The stator consists of a laminated iron core housing the armature windings and secured to the stator frame.

Generator Rating

• AC generators are rated by their voltage output and maximum current supply capability.
• Maximum current output is limited by the heating loss sustained in the armature.
• Generators are rated in volt-amperes for small units, and kilovolt-amperes for larger units.
• Generator operating characteristics, such as speed, voltage, and current limits, are fixed at the factory and indicated on a nameplate.

Single-Phase, Two-Phase, and Three-Phase Generators

• AC generator outputs are categorized as single-phase, two-phase, or three-phase, based on the winding configuration and outputs.

General - Single Phase Generators

• A 'single phase generator' provides output from a single winding connected to a pair of slip rings.
• If there were two windings at different angles connected to four slip rings then this would give two outputs and would be known as a two-phase generator.
• If there were three windings connected to six slip rings then it would be a three-phase generator.

Generators - Three Phase

• In three-phase generators, coils are positioned 120° apart, creating a three-phase output where it behaves like three generators in one, each 120° out of phase.

Applications of Three Phase Generators

• Three-phase generator outputs are extensively utilized in aircraft and national grid systems.
• The generator is limited as a main generating source due to insulation/cooling limitations when sourcing off the rotor and AC output interference problems.
• Considerable centrifugal forces are present on rotor windings.

Three-Phase Generator Connections

• Three-phase generator connections are achieved through star (Y) or delta configurations
• A star (Y) connection configures interconnected phase leads, resembling the letter Y where single-phase voltage is supplied from the neutral point to each phase.
• A delta connection links phases end-to-end where line voltages equal phase voltages, but line current equals 1.73 times the phase current.
• Aircraft use three-phase generators, which are more efficient than single-phase or two-phase. The benefits of star connections include the availability of two voltages with the neutral line (200V and 115V) and are generally connected in a star configuration.

Output frequency

• Output frequency is reliant on rotor rotation speed and number of poles, where increasing speed increases frequency.
• The frequency (f) is proportional to the number of poles (P) and the speed of rotation (N) and is expressed as f = NP/120.

Voltage regulation

• Voltage regulation refers to the change in voltage from full-load to no-load, expressed as a percentage of full-load voltage.
• The amount of voltage induced depends on the number of conductors, generator RPM, and strength of magnetic field.
• Voltage control happens by controlling the strength of the rotating magnetic field through current adjustments in the field coil.

Brushless Generators

• These generators eliminate commutator brushes.
• Brushless AC generators offer advantages in terms of reduced wear and sparking.

General Brushless Principle

• Brushless alternators uses separate fields, permanent magnetic, exciter, and main output field.
• Permanent magnets generate magnetic flux to initiate output.
• Magnetism induces voltage into an armature, sending current to a generator control unit (GCU) to rectify AC for the exciter field winding.
• The exciter field generates voltage in the exciter output winding.
• Exciter output is then rectified via six diodes to empower the output field winding.
• The three-phase output stators has a voltage of 115 V and, if used, a Y configuration, the output is 200 V.

Stationary Magnet Brushless Type

• Its design has a stationary control section, rotor, and star-wound main stationary output windings controlled by a voltage regulator.
• Stationary permanent magnets are placed by the exciter's poles where rotation produces a current in the exciter windings, which then pass current to a rotating rectifier circuit
• Silicon diodes on the rotor convert the output into DC where rotation produces EMFs into the main 3-phase stationary output which externally give AC phase voltages of 115 V with line-to-line at 200V.

Functions of Generators

• Exciter field windings are governed output voltage
• A feature allows the exciter has two field windings, with F1 providing over-voltage protection and F2 managing operating temperature.
• Another feature is under steady operations, EMF is only induced during quick field current fluctuations.

PMG Brushless Type

• Consists of a rotating permanent magnet generator (PMG), the main exciter, and the main AC generator.
• The rotating permanent magnet generator (PMG), acts as the pilot exciter with stationery start wound coils, and has the exciter with a fixed stationary field winding,
• The PMG generates AC voltage in its three-phase windings, which is then supplied to the GCU.
• The rectified PMG voltage is utilized by the GCU for controlling the generator's exciter field, thus the GCU power supply.