ON SOME CHARACTERISTICS OF ELECTRICAL VARIABLE SPEED DRIVES WITH ELECTRONIC SWITCHING CONTROL.

After a short review of the development of this type of drive, it is shown how switching converters can be used to obtain conversion from ac to dc and dc to ac, as well as to control output amplitude of voltages. The fundamental relationships between mechanical output speed and electrical input frequency for electric drive machines are given, and related to the different types of machines. Power dissipation in electric machines is illustrated in terms of a general dissipation diagram including electric input at the rotor, stator and mechanical output at the drive shaft. A short review of some electrical variable speed drives controlled by switching power convertors is presented. Electrical variable speed drives with dc machines and armature voltage control by controlled rectifier, with ac machines and frequency control by inverter and with ac induction machines and variable slip control by stator or rotor converter are evaluated according to criteria of coverage of the speed torque plane, efficiency and losses, harmonic generation and power factor.