Variable speed AC-generators in wind energy convertors

The authors describe two variable-speed AC-drive systems: a synchronous generator with frequency converter for large speed ranges and a rotor cascade induction generator for small variations of speed. Results of transient state calculations using computer models and verifying laboratory tests are presented. The synchronous generator system together with a digital control system is being tested on a 40 kW windmill. The converters introduce harmonics into the currents in the generators and thereby into the electrical torque and the shafts of the generators. A temperature rise is caused by the harmonic currents. By current control of the inverters the electrical torque is controlled, and by this control the maximum torque in the drive train of the windmill is limited. Optimal operation can also be achieved in low wind conditions