Roy S. Colby

A Sensorless Adaptive Stator Winding Temperature Estimator for Mains-Fed Induction Machines with Continuous-operation Periodic Duty Cycles

An induction machines thermal behavior is determined by various machine components with dissimilar thermal characteristics. When a model with a single thermal time constant is used to characterize the machines thermal behavior under periodic duty cycles, the magnitude of the thermal time constant needs to be adjusted according to the duty cycles to reflect the machines dominant thermal dynamics during that specific interval. Based on the analysis of the internal heating effects of a small- to medium-sized mains-fed induction machine, a sensorless stator winding temperature estimator is proposed to compensate for the shift in the thermal time constant for motors with continuous- operation periodic duty cycles. First, the rotor temperature is estimated from the voltage and current measurements and is used as an indicator of the motors internal thermal operating condition. Then, a hybrid thermal model is employed to correlate the rotor temperature to the stator winding temperature. Finally, an observer is designed to take the estimated rotor temperature as a feedback signal into the hybrid thermal model. The correction provided by the feedback signal enables a reliable tracking of the stator winding temperature for motors with periodic duty cycles. The experimental results are given to validate the proposed method, and the overall scheme is shown to estimate the stator winding temperature efficiently without using any real temperature sensors.

A frequency demodulation approach to induction motor speed detection

Rotor slot harmonics are found in the stator current waveforms for most squirrel-cage induction motors. These harmonics are caused by the finite number of rotor slots in a motor, and their frequencies are inherently correlated with the motors rotational speed. A frequency demodulation approach is proposed in this paper to continuously and accurately track the rotational speed for induction motors that are operated at either dynamic or steady-state conditions from fixed-frequency power supplies. First, a complex current vector is synthesized from polyphase electrical current measurements. Second, a local oscillator and a mixer are cascaded with a digital filter to heterodyne a specific rotor slot harmonic and suppress adjacent interferences. A finite impulse response differentiator is then employed as a frequency demodulator to approximate the time derivative of the phase of this specific rotor slot harmonic and to resolve its instantaneous frequency. Finally, the induction motor speed is calculated from this resolved instantaneous rotor slot harmonic frequency. Experimental results demonstrate that the proposed scheme is capable of interleaving data acquisition with real-time computation, iteratively estimating motor speed on a sample-by-sample basis.

Narrowband angle modulations in induction motor complex current vectors

Periodically time-varying loads such as reciprocating compressors produce torque and speed variations in grid-connected induction motors that are used to drive them. In each compression cycle, the torque and speed variations manifest themselves through periodic pulsations in induction motors complex current vectors. Based on the concept of narrowband angle modulations, this paper analyzes the complex current vectors behavior in a typical motor-compressor system. Carsons rule is introduced to establish an empirical relationship between the complex current vectors bandwidth and the modulating frequency induced by the compressor. By reconstructing complex current vectors from raw experimental data through application of digital filters with different bandwidths, narrowband angle modulations and Carsons rule are experimentally validated.

A Sensorless Rotor Temperature Estimator for Induction Machines Based on Current Harmonic Spectral Estimation Scheme

This paper proposes a sensorless rotor temperature estimator for small to medium size mains-fed induction machines. With measurements obtained from only current and voltage sensors, the proposed estimator can capture the rotor temperature in an online fashion. First the rotor speed is extracted from the current harmonic spectrum based on the estimated rotor slot and eccentricity harmonic frequencies. Then the inductances are estimated from the induction machine equivalent circuit. The stator winding resistance at ambient temperature is the only motor parameter needed at this stage. Once the inductances are obtained, they are fed into the rotor resistance estimation algorithm to yield an estimate of the rotor resistance. Finally the rotor temperature is calculated from the linear relationship between temperature and rotor resistance. Experimental results from different motors are shown to validate the proposed scheme. The whole scheme is accurate and reliable and is therefore suitable to be implemented in a rotor thermal protection relay or a motor drive employing vector control scheme to compensate for the detuning effects

Application of linear-phase filters in induction motor speed detection

In rotor-slot-harmonic-based speed detection schemes, a grid-connected induction motors rotational speed isresolved from the phase of a rotor slot harmonic signal for condition monitoring purposes. In such schemes,frequency-selective digital filters are often applied to attenuate interferences in sampled motor currents before rotorslot harmonic signals are extracted. This paper demonstrates that the use of linear-phase filters, which is a family ofdigital filters with linear phase responses, helps preserve the rotor slot harmonic signals phase during the filteringoperation. This paper also shows that, for grid-connected induction motors with periodically time-varying loads, therotor slot harmonic signals frequency is modulated by periodic speed oscillations and hence contains multiple sidebandsin the frequency spectrum. Experimental results further show that linear-phase filters allow for a more accuratedetection of the induction motor speed than filters with nonlinear phase responses.