On-line Efficiency Estimation Of Induction Motor Based On Air-gap Torque Method

Motor and its system consume most of the industrial electricity,so it has huge energy saving potential and plays a great role on energy saving and emission reduction.In order to save energy and improve the motor system of the enterprise scientifically and reasonably,the operation state of motor system must be on-lined monitored,by which we could put forward energy saving measures based on different situation to achieve the purpose of saving energy and reducing consumption.In this paper,the efficiency online estimation of the induction motor based on the air gap torque method is studied.First,several conventional efficiency testing methods for motor are introduced,then characteristics of each method in online efficiency estimation are comparatively analyzed.Conclusion shows that none of such methods could satisfy the requirements of both accuracy and non-invasion.After that,the principle of air gap torque method is introduced in detail.The analysis shows that it is the most suitable method for energy efficiency estimation under field conditions.For the air gap torque method,it is necessary to solve three problems when cope with the motor efficiency,they are: stator resistance identification,motor speed identification and determination of stray load and mechanical loss.In this paper,the method of recursive least squares(RLS)is used to realize the identification of stator resistance.Through deducing the state equations in least square form for induced motor based on two phase stationary coordinate system,we could simulate and analyze it in MATLAB by programming.Finally,the feasibility of the scheme is verified.In order to identify the speed,rotor slot harmonic method is adopted.The motor terminal current is collected and analyzed by Fast Fourier with Bimodal Interpolation and Blackman Window Function,from which the harmonic frequency of rotor frequency could be extracted to determine the rotor speed.Through comparative analysis of speed measured value and slot harmonic identification value of 750 W fan at different voltage level,the accuracy of slot harmonic method could be verified.On the problem of determining the load stray loss and mechanical loss,several test methods formulated by current international and national standard are introduced.Analysis shows that such methods are relatively complex and difficult,not suitable to perform on site operation of the rotor system.In this paper,sum of load stray loss and mechanical loss is estimated as 3% of the input power,then the online efficiency estimation model of motorcould be established considering input power,stator resistance and identification of rotor speed.Finally,the motor drag comprehensive experiment is perform in laboratory,which measures the speed,torque and efficiency.Comparative analysis of simulation and experiment results shows that motor efficiency online estimation by air gap torque method is valid.