Why is there no CSD between the electric generator mounted the APU and the aircraft electrical buses?
An airliner has electrical generators mounted to the engines. These generators generate AC current. Before this current is fed into the aircraft electrical buses it has to pass through a Constant Speed Drive (CSD) to stabilize the current. (APU = Auxiliary Power Unit)
Answer by RickH
The engines, by necessity, must operate through a wide range of rotor RPMs, and that RPM, absent a CSD or as they are called now, IDG (Integrated Drive Generator), then the frequency would be all over the place. In this case, AC motors would speed up and slow down with the frequency of the AC, electronics would go crazy, etc. A CSD keeps the frequency right at 400 hz which makes the system components happy.
An APU on the other hand, can be designed to run at a constant speed so that the generator can be designed to make 400 hz, or whatever, at that constant speed. The only trick is to make the fuel control of the APU keep it running at a constant speed.
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