Patent ID: 8781761

Claim:
A detection method of a rotary speed of a rotating shaft of an induction motor by using a calculation processing unit, wherein the detection method is that when a power input P is supplied to the induction motor, the rotary speed of the rotating shaft of the induction motor is detected, the detection method comprising: a step I, comprising: setting a loss power of when the power input P is supplied to the induction motor to be P L , dividing the loss power P L into a loss power A that does not depend on the rotary speed of the rotating shaft of the induction motor and a loss power B that depends on the rotary speed; considering a difference between the power input P and the loss power A as a first order approximation value PM 1 of mechanical output of the induction motor; and from an equation PM 1 =αS 1 (α is a motor constant) of an output PM 1 given in the induction motor and a slip S 1 , obtaining a first order approximation value of the rotary speed of the rotating shaft N 1 =N S (1−S 1 ) (N S is a synchronous speed); a step II, based on the first order approximation value N 1 , obtaining the loss power B 1 ; and a step III, comprising: considering a second order approximation value PM 2 of the output of the induction motor as P−(A+B 1 ); and from an equation PM 2 =αS 2 (α is the motor constant) of the output PM 2 given in the induction motor and the slip S 2 , obtaining a second order approximation value of the rotary speed of the rotating shaft, N 2 =N S (1−S 2 ) (N S is the synchronous speed) wherein calculation of the step I, the step II and the step III is performed by the calculation processing unit.