Concept of sensorless induction motor?

Question

How do you measure the speed of the induction motor without speed encoder. Explain me briefly.........

Answer 1

The speed of an induction motor can not be measured without using an encoder, tachometer generator, or some other sensor. Sensorless determination of motor speed is done by calculations based on known and measurable electrical parameters.

The synchronous speed of an induction motor is given by RPM = 120f/p where RPM is the speed in revolutions per minute, f is the frequency in Hz and p is the number of poles formed by the configuration of the stator windings. Therefore, the synchronous speed can be easily determined from the frequency of the applied stator voltage.

The motor’s operating speed is synchronous speed minus slip. For any given motor, slip is determined by the load torque and the torque vs slip characteristics of the motor. The motor characteristics can be calculated from an equivalent circuit for the motor. The equivalent circuit can be developed from motor test data. The load torque delivered by the motor can be calculated from measured motor current, motor voltage and the phase relationship between voltage and current plus the motor equivalent circuit information. Estimation of magnetic flux is a key element of these calculations.

The motor test data can be obtained by applying AC and DC voltages to the motor and measuring the resulting current vs time and the current vs voltage phase relationship. Variable frequency drives (VFDs) that include sensorless speed and torque determination have built into them, microprocessor controlled test routines that determine an equivalent circuit for the motor. Manual input of motor nameplate data is also required. The microprocessor also performs the speed and torque calculations and controls the motor voltage, current and frequency to produce the desired performance.