Importance of direct- and quadrature-axis damper impedances in starting performance of salient-pole motors

Asynchronous torque characteristics of salient-pole motors may exhibit cusps at various speeds, with the possibility of consequent detrimental effect on the starting performance. These cusps may occur at half the synchronous speed and near to the synchronous speed. The paper shows that, in addition to being dependent on the value of armature resistance, the magnitude of a torque dip near halfspeed is dependent on the difference between the damper-circuit impedances on the direct and quadrature axes of the salient-pole rotor. Tests conducted on a 7.5kVA laboratory machine demonstrate this relationship, and the experimental results are correlated with computed characteristics based on established analytical methods. It is concluded that a factual knowledge of damper-circuit impedances is essential to enable reliable predictions of starting performance to be made.