Capacitive compensation of thyristor-controlled slip-energy-recovery system

The steady-state performance of a slip-energy-recovery scheme incorporating an induction motor and a static frequency changer is described. A considerable improvement in the low-speed efficiency may be obtained at the expense of a poor power factor. Power-factor correction by the use of primary-side capacitance is found to increase slightly the 5th-harmonic component of the supply current and to double the torque/supply-current ratio. Compensation by the use of fixed secondary capacitance is found to give increased torque at low speeds, increased torque/supply-current ratio, higher efficiency at some speeds, better speed regulation, much improved step response to changes of control signal and good power-factor correction at low speeds. The closed-loop steady-state performance is that of a good stable variable-speed drive, with speed regulation of the order of within 1¿3% over the entire speed range.