Aircraft Electrical Power Technology
From the article Why We Use 400HZ Power Supply In Aircraft by Areeb Ahmed: Is there any particular reason, why the standard aircraft power supply is 400Hz? As aircraft fly higher, faster and grow larger, the services that the power supply has to satisfy also grow more complex. In civil aircraft this means more power to the galley units, environmental control and passenger entertainment systems, while increased power demands for actuators, lighting systems, avionics and heating. The primary function of an aircraft electrical system is to generate, regulate and distribute electrical power throughout the aircraft. The aircraft electrical power system is used to operate (a) aircraft flight instruments, (b) passenger services. Essential power is that the aircraft needs to be able to continue safe operation. Passenger services power is the power that is used for cabin lighting, operation of entertainment systems and preparation of food. Aircraft electrical components operate on many different voltages both AC and DC. However, most of the aircraft systems use 115 volts (V) AC at 400hertz (Hz) or 28 volts DC. 26 volts AC is also used in some aircraft for lighting purposes. DC power is generally provided by “self-exciting” generators containing electromagnetics, where the power is generated by a commutator which regulates the output voltage of 28 volts DC. AC power, normally at a phase voltage of 115 V, is generated by an alternator, generally in a three-phase system and at a frequency of 400 Hz. However, higher frequencies are also more sensitive to voltage drop problems. There are two types of drops: resistive and reactive. Resistive losses are a function of current flowing through a conductor with respect to the length and size of the conductor. This is the most important factor in controlling resistive power loss and applies regardless of frequency. A higher frequency also increases loss of power over long distances, so using it on long transmission lines would be a bad idea. Running at 400Hz allows the use of smaller transformers. Less energy has to be stored in the transformer core per cycle, so the core can be smaller. A smaller core means a lighter transformer, and reducing weight is a good thing in an aircraft. Some equipment require a 3 phase supply i.e. motors. Some use only one if you had single phase generators you would need 3 for 3 phase and then you would require a very complicated timing circuit to make sure the phases were the correct angle apart. You would also need to duplicate this system or triplicate it to get an airframe certified the weight would be large all the power required and attach them to a constant speed drive that rotates the gen at a speed that gives you 400Hz depending on how many poles your choice of gen has because Frequency = RPM x pairs of Poles/60. We know how to gear the constant speed drive to get 400 Hz while varying engine RPM. It is the standard frequency in aircraft so all equip is made to operate at 400 Hz and when ask Why? The answer is we can use 3phase generators to save weight and simplify power production because 400Hz means a complete cycle occurs every 1/400 th of a second therefore a pole passes a coil of the generator and generates max voltage every 1/1200second (remember 3 phase so 3 coils. The less the coils again the greater the weight saving but three is the minimum) we therefore get the 3 phases 120 degrees apart because the same pole passes the next coil another 1/1200 of a sec later and then the third and all our electrical need's right down to Coffee is done on each 360 degree cycle of the generator or 400 times a second. Increasing the operating frequency can reduce the size of these magnetic. This desire for smaller size forces the power supply designers to increase the switching frequency. Some Important relationships regarding aircraft frequency are mention below: If an ordinary wire is used to carry high frequency currents, the wire acts as an antenna, and the high frequency currents radiate off the wire as radio waves, causing power losses. Does 3-phase effect? 3 phase means basically three AC circuits. On an oscilloscope you would see three sinus curves shifted by 120 degrees. Much more efficient than single phase and perfect for transmission to pointer gauges.
Power Electronics Handbook by M. H. Rashid