A ROTOR SPEED DETECTOR FOR INDUCTION MACHINES: THE SIGNAL PROCESSING PART

A new method to detect rotor speed electrically from rotor slots harmonics of a three-phase squirrel cage induction motor fed with a 20 kRz pulse width modulated (PWM) inverter is presented. The rotor speed is detected in the range 20% to 100% of nominal speed directly by the rotor slots harmonics and in the range from 2% to 30% of nominal speed an additional 1 kHz three phase signal is fed from the inverter into the machine to produce an additional rotor-slot-modulation, which is used to detect rotor speed in this range. Principal signal detection schemes are shown. A simulation model based on current distribution in the motor is explained by signal processing methods. Practical EMC problems are discussed additionally.