Transient Analysis of Flux Weakening Operation for Synchronous Motor Considering NI HTS Field Coil Behaviors

In the case of a synchronous motor with armature input voltage limitation, flux weakening operation is performed to reduce the magnetic flux generated from the rotor for increasing the rotating speed greater than rated speed. However, in the case of a motor with a no-insulation (NI) HTS field winding, a discharging delay occurs due to radial leakage current when the operating current is lowered, and a motor control delay is generated in following the rotor magnetic flux and rotating speed reference command during flux weakening control. In this paper, in order to analyze the behaviors of the NI HTS field coil during field weakening operation, a dq-equivalent circuit-based transient analysis is performed. The following key aspects are discussed: (1) rotating speed response characteristics; (2) contact resistivity dependency on flux weakening control and (3) active control method of field winding current for mitigating motor speed variation delay.