Investigation of Induction Motors Starting and Operation with Variable Frequency Drives

Induction machines are most widely used in various industrial applications. Variable frequency drives (VFDs) provide flexibility in speed control and improve starting and operating performance of induction motors. However, investigation of induction motor starting with VFDs is not considerably carried out. In some cases motor performance is different from that expected, which causes confusion to field engineers. In this paper, theoretical derivation of starting parameters of induction motors using the equivalent circuit is conducted for cross-line and VFD starting. Cross-line starting is considered as one special scenario of VFD starting with a starting frequency of 60 Hz. An industrial distribution system in the oil field is used as a case study for the cross-line starting. Factors that could significantly affect the starting performance are studied. A two-pole squirrel-cage induction motor is used in the case study for VFD starting. The relationship between the starting frequency, starting current and required starting voltage is derived mathematically. It is found that although both starting voltage and current are functions of starting frequency the required starting voltage shows large variation at the lower starting frequency between 2 Hz -10 Hz. The induction motor torque characteristics of speed control during normal operation with VFDs are also discussed.