why three phase system is not preferred in aircrafts power circuits....

Thanks- I wasn't sure. However, there are some advantages to high speed motors as the weight/HP/volume ratio is better than for slower motors. Of course there are more reactance problems- both L and C. On land or on ships- there is less need to save weight. However, I do recall a compressor which was driven at these higher speeds -in a refinery. The details are now pretty hazy.

In some countries there is still electrical railway systems operating at 16.7 Hz electrical power. Many other countries run their electrical transis with 25 kV 50 Hz power.

"The countries Germany, Austria, and Switzerland use a traction power network for railways, distributing single-phase AC at 16.7 Hz. A frequency of 25 Hz was used for the German railway Mariazeller Bahn and some railway systems in New York and Pennsylvania (Amtrak) in the USA."

"Germany, Austria, Switzerland, Sweden and Norway use traction power networks for railways, distributing single-phase AC at 16. Hz. "

Then there are those special applications where DC power is used and ditributed. DC is commonly found in many low-voltage applications, especially where these are powered by batteries. Telephone exchange communication equipment, such as DSLAM, uses standard -48V DC power supply. Positive 12V, 24V, 48V, 110V and

220V voltage power supplies (battery backed up) are used in by control systems used by electrical utilities and some industrial automation applications. An electrified third rail carrying a DC is used to power both underground (subway) and overground trains (for example use 600V-1200V DC ).