AC Electrical Power System

Frequency of AC equation

F = P/2 x N/60 = PN/120 F = frequency P = # of poles N = speed in RPM

Disadvantages of Rotating armature alts

- Coils are heavy and centrifugal forces are high - Efficient insulation of the rotating coil is difficult - Resistance arcoss the brushes to the slip rings is high - Rotating coils are difficult to cool - Poor power to weight ratio

Advantages of the brushless gen

- Extremely reliable - No brush wear problems - High power to weight ratio

Two of the more common CSD problems during operation

- Low oil pressure - High oil temperature CSD can be mechanically disconnected by a solenoid in flight in case of emergency, but can only be reattached on the ground when the engine is not running.

In order to load share or parallel the ac alternators, two operating parameters must be monitored:

- Real Load - Reactive Load

Classifications of AC alternators

- Single-phase - Two-phase - three-phase (sometimes six-phase or more.) Most use three phase

What is AC frequency determined by with an AC alternator?

1. The number of poles 2. The speed of rotation of the rotor More poles on a rotor = higher frequency generated for any given speed

What is the typical frequency of AC power on a transport category a/c?

400 Hz

What is typical frequency of commercial alternating current?

60 Hz

Real Load

Actual working load output - output avialable for supplying the various systems; measured in kilowatts (Kw) Directly related to mechanical power or torque

Frequency wild alternator

Alternators that have their frequency directly geared to changes in engine speed.

Integrated Drive Generator (IDG)

Differs from a DSC in that both the constant speed drive unit and the AC gen are both housed in the same unit.s

Differential fault protection

Gens have energize relays and sense malfunctions to provide protection by either: - warning the pilot - disconnecting the gen Most disconnect the gen in the condition of: - Over/under frequency, voltage or excitation - Short circuits between the bus bars or bus bars to ground, or open circuit faults

Two-phase alts

Have two or more single phase windings spaced so AC voltage induced in one is 90 degrees out of phase with the voltage induced in the other

Phase Voltage

If a voltmeter was connected from a phase to the neutral line.

Single-phase alt.

In this alt., the stator is made up of several windings connected in series to form a single circuit. To determine a single-phase AC alt's total output, the voltage induced into each winding must be added. With this, the total voltage produced by a stator with four windings is four times the single voltage in any one winding. However, this setup is impractical due to pulsation.

How is a frequency wild alternator controlled?

Inverters use the wild AC and change it to DC, which then powers a static inverter that can convert DC into constant frequency AC

AC electrical need in transport category aircraft

Large aircraft use larger pieces of electrical equipment and require alternating current (AC) as the electrical source that is a 115v/200v/400Hz, 3-phase power source. Therefore, large aircraft can take advantage of the appreciable weight savings and voltage speeding variety provided by using alternating current (AC) as their primary power source

Constant Speed Drive (CSD)

Most AC gens have these, which turn the gen at a set speed. CSD is turned by the engine and uses differential assembly and hydraulic pumps to turn the gen at a constant speed

How are most aircraft circuits wired for AC power from active generators?

Parallel Benefits: - If one gen fails, the others pick up the load without electrical service interruption - Automatically redistributes the load amoung the other active gens

Two types of AC alternator systems

Rotating armature Rotating field

Brushed alternator

The current used to excite the rotor field can come from the A/C's bus bar, or AC from the AC bus that is rectified to DC, which is directed through the brushes to the slip rings and on to the rotating field. Control is accomplished by a voltage regulator sampling output and adjusts the excitation current to maintain correct voltage

Line voltage

The difference between two line connections

Neutral Point

The fourth connection in a star connected alt. It is normally grounded and is the earth returning in the a/c

Frequency of AC

The number of cycles completed in one second. Expressed in hertz (Hz): 1HZ = one cycle/sec

What is the most common phase circuit in a/c and why?

Three-phase, because ac voltage induced in each winding is 120 degrees out of phase with the voltage in the other two. Can be connected via star/"Y" or delta type.

Split bus

Used my many twin engine transport a/c. - ac gens not operated in parallel - each gen supplies electrical power separately form the other gens to its respective bus Two gens are connected when - a/c on external power - a/c on APU power - One gen fails

Brushless Alternator

Uses an exciter generator mounted on the same shaft as the main generator. Exciter generator provides current for the main gen's rotating field. A rotating rectifier in the gen converts the AC produced in the exciter armature to DC, which is required for the main rotor field supply. Voltage regulation is accomplished by controlling the exciter field strength.

Reactive load

Wattless load - the vector sum of inductive and capacitive currents and voltages, expressed in KVAR. Reactive load sharing accomplished by controlling the exciter field current of each alternator connected in parallel so that their voltages are identical, which allows for the reactive load on each gen to be equal

rotating field type alternator

alternator has a stationary armature winding and a rotor, or rotating field winding that is connected directly to the load circuit; which eliminates problems with arcing at the brushes and brush wear Advantage: only low current is fed to the field windings through the field windings Most used alternator design Direct connection to the armature circuit using brushes (sliding contacts), making it possible to use large cross-section conductors that are adequately insulated for high voltage

Rotating Armature Type alternator

similar to a DC generator in that the armature rotates within a stationary magnetic field. As armature rotates, emf is induced into it and is taken out as AC through slip rings. Converted to DC via rectifier pack built into the unit so that unit supplies a source of DC to a/c. This type of alternator typically has a very small output and is used normally only in small aircraft electrical systems

What are AC alternators rated in?

volts-amps: a measure of the apparent power being produced by the alternator Or Kilovolt amperes (KVA) due to the great amount of power