EMI/EMC Considerations with Respect to Distortion Requirements in Aircraft Electric Power Generating Systems 901801

Electromagnetic Interference (EMI) is often a subject of great controversy because it is not generally well understood by non EMI people. Because of this there exists an incompatibility between EMI specifications and Power Quality (PQ) specifications for electrical power generator systems. Typically the problem lies at the low frequency end (up to 500 kHz) of the EMI spectrum and is due to harmonics permitted by PQ specifications which exceed the EMI specifications for both radiated (air) and conducted (wires) EMI emission requirements. The disparity is quite large (orders of magnitude) and cannot continue to be ignored in hopes that it will go away. The problem is not peculiar to a specific type of power generator system. It is an aircraft or vehicle problem because any source or load can distort the PQ to a level which remains within distortion spectrum limits but forces the EMI well beyond its allowable limit. EMI is a science, not black magic, which must be addressed and properly resolved. This paper defines the conflict between PQ and EMI specifications which exist in the industry. It addresses Variable Speed Constant Frequency (VSCF) systems which seem to be prone to closer scrutiny than conventional systems. It illustrates the problem so that non EMI personnel can understand it and offers possible solutions to the dilemma.