High speed machines

Abstract

The speed of a.c. machines increases with increase in the input frequency. High frequency of the armature current reduces the dimensions of electrical machines, as the electromagnetic torque is proportional to the electromagnetic power and number of pole pairs and inveresely proportioal to the frequency. High speed gearless electrical machines find many applications as spindle motors, pump motors, large chiller motors, gas compressor motors, microtur-bine generators and aircraft generators. Elimination of gear trains improves the efficiency of the system, reduces the dimensions and noise, and simplifies the construction and maintenance. Cage induction, wound synchronous and surface type PM synchronous machines with retaining sleeve are the most economical candidates for high speed applications. At present, the maximum power of high speed synchronous generators does not exceed 500 kW. Several airborne power missions are now evolving that will require lightweight multi megawatt electrical power systems, e.g., directed energy weapon (DEW) and airborne radar [197]. New high power airborne and mobile military systems will require 1 to 6 MW of electrical power generated at speeds 15 krpm. As potential candidates HTS rotor synchronous generators or all cryogenic generators (synchronous or homopolar) have been considered. 4.1 Requirements Fig. 4.1 shows the construction of a high speed electric machine with magnetic bearings. There are two radial magnetic bearings and one axial magnetic bearing. Basic design requirements for high speed machines include, but are not limited to: • compact design and high power density; • minimum number of components; • ability of the PM rotor to withstand high temperature; • minimum cost-to-output power ratio and cost-to-efficiency ratio;