Compensation of Loading Effects in Saliency based Position Estimation of Induction Machines using PWM-Induced Transient Excitation

This study deals with two main problems of sensorless control at zero stator frequency. By using a transient voltage excitation to exploit inherent machine saliencies, the time derivative of current must be estimated. The initial transient oscillation of the current slope prohibits to estimate the derivative from small signal durations. Therefore, the first part of this study presents a new robust and fast estimation algorithm which is capable to estimate the current derivation from short signal durations also containing the initial transient oscillation part. This proposed algorithm uses a fast Fourier transformation followed by a linear transformation. Moreover, a comparison of the proposed algorithm with the commonly used derivation estimation technique linear regression was conducted. The comparison was made for different signal durations and sampling rates. Whereby the proposed algorithm shows quite better results than the linear regression. The second part of this paper presents a new compensation technique for the application of sensorless control in combination with induction machines. A frequently used sensorless control technique in this specific combination is tracking of rotor slots, consequently the current modulation due to the rotor slots must be dominant. However, all other modulation effects appear as disturbances and have to compensated. Especially for high torque ranges the modulation caused by saturation becomes significant. The new compensation technique described in this study is capable to separate all disturbing modulations from the modulation caused by rotor slotting without any spectral analyses. Secondly the identified disturbing modulations were subtracted. As a result, the modulation due to rotor slotting becomes dominant.