INCREASE OF DYNAMIC STABILITY IN A DOUBLY-FED MACHINE BY A NEAR UNITY POWER-FACTOR, CURRENT MINIMUM CRITERION

A doubly-fed machine may be considered to produce induction-motor type speed dependent torques in addition to the load-angle dependent synchronising torques which form the main part of its output. As an educational concept to help understanding, the two torques may be considered to be superimposed. Stable operation about any selected synchronous speed can only occur if transient load changes do not momentarily cause the machine to swing beyond either peak on the induction-motor torque-speed characteristic. Stability is obviously increased by reducing the slope of the machine's induction torque-speed characteristic and increasing the speed bandwidth between the torque peaks. This has been traditionally carried out at the expense of energy dissipation by an increase of rotor resistance. This paper shows that this stability criterion can be achieved by minimising either the rotor or the stator current by near unity power-factor operation. The selection of an appropriate stator to rotor voltage ratio achieves this objective. This ratio is computed in terms of the machine parameters and the result demonstrated in a closed loop system.