Start-Up Torques in Synchronous Motor Drives

During run-up as an induction motor, the torque produced by a synchronous machine has oscillating components in addition to the natural frequencies of torsional oscillation of the complete drive system so that the shaft torque may differ substantially from the motor electromagnetic torque. The latter can be divided into four components: 1) a unidirectional or accelerating component, 2) an oscillating component due to magnetic saliency and nonuniform rotor windings, 3) oscillating components due to higher harmonics of air gap permeance and winding magnetomotive force, and 4) components caused by switching the supply. In addition, slot effects and magnetic saturation may also lead to oscillating torque components. The factors affecting the magnitude and frequency of these components are discussed and related to recordings of actual shaft torque.