Fish Method based state estimation for controlling inverter driven machines

Torque and speed of inverter fed squirrel cage induction motors are estimated in this paper, based on just measured voltages and currents. Neither shaft sensors nor machine parameters are needed for the presented estimation method. When current- and stator terminal flux-vectors are projected onto a rotating reference frame according to the Fish Method, a flux-fish and a current-fish curve of almost identical shapes result. Differences between these curves incorporate machine parameter and loading influences. A consistent periodic current-fish movement caused by `slipping-by´ rotor-bars is used to determine the shaft speed and relative shaft position relative to the airgap flux