Adjustable frequency drives with rotating machine power supply

THE squirrel-cage induction motor so well known to manufacturers and users of machinery of all kinds is usually-considered to be a constant-speed drive because the power available for energizing the motors nearly always has a fixed frequency. This is the frequency of the available power supply from a utility company or local power plant. If an adjustable frequency power supply with the proper characteristics is provided, these motors will operate over a wide speed range with essentially constant torque capability. The rugged, simple construction of the squirrel-cage motor or the similar reluctance synchronous motor can thus be combined with the flexibility of an adjustable speed drive. Fig. 1 shows the basic elements of an adjustable frequency drive consisting of a synchronous generator, its adjustable speed drive, starting equipment, and the drive motors.