400 HZ Aerospace Appllications

Fundamental physics - the voltage in an output coil on a generator is proportional to the rate of change of flux. For a given flux from a generator field at 400 Hz, compared to 50Hz, one gets 400/50 = 8 times the voltage - or 1/8 the turns are needed for the same voltage. Practically, if you apply a given direct current voltage to a coil of wire on a magnetic core, it can only store a limited energy before the ampere-turns reach a saturation limit at which magnetic flux stops rising, there is no back-emf and no transformer action. When the voltage is reversed before saturation, the core can be magnetized in the opposite direction and so on again in a continuous cycle. The smaller and lighter the core (for a given number of turns on the winding), the shorter the time it takes to reach saturation current. This truth applies just the same for a sinusoidal AC voltage. The practical effect is that for low frequencies, a relatively large and heavy core is needed to avoid saturation, whether it be in a transformer, inductor or a motor.

1. But there are disadvantages to higher frequencies.
2. The resistance of cables is increased at higher frequencies. Bigger cross section is needed for same current. Copper and aluminium are expensive.
3. Inductance, even of cables, becomes more significant, causing 8 times voltage drop at 400 vs 50Hz.
4. Transformer/motor magnetic core losses increase with square of frequency - 64 times at 400 vs 50.
5. Special low-loss magnetic materials have to be used - they are expensive compared to the steel used in 50Hz machines.
6. The distance power can be transferred without uneconomic loss is much reduced
7. The simplest motor (2 pole induction) at 50Hz, runs at the useful speed of 3000 rev/min , !500 rev/min needs 4 poles ( more complex) etc.
8. The same motor type at 400 Hz runs at 24000 rev/min - 3000 rev/min needs 16 poles.

On aircraft, the size and weight benefits overrule the debits and distances are small. The advantages of high frequency are very obvious in compact fluorescent lamps and mains power supplies, in which rectification of primary power to direct current, followed by high frequency (10 kHz and up) electronic switching, gives great reduction of size and weight of transformers and inductors compared to 50Hz.