Larry Turner

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.

Filter Design for Estimating Parameters of Induction Motors With Time-Varying Loads

Periodically time-varying loads such as reciprocating compressors produce torque and speed oscillations in grid-connected induction motors. In each compression cycle, the torque and speed oscillations manifest themselves through periodic pulsations in the induction motors stator currents. In resistance-based rotor temperature tracking and overheating monitoring, low- or bandpass filters are frequently used to estimate induction motor electrical parameters from stator currents that contain periodic pulsations. Design of such filters relies on comprehensive knowledge of the load-related periodic pulsations in the stator currents. In this paper, a narrow-band angle modulation concept is first formulated in the framework of complex vector analysis to explain the periodic pulsations in the stator current. Carsons rule is then introduced to establish an empirical relationship between the stator currents modulating frequency and the appropriate bandwidth for low- or bandpass filters. The proposed filter design rules are experimentally validated, and explanations are provided using the phase angle between the motors complex stator current and voltage vectors.

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.

Application of Linear-Phase Filters in Induction Motor Speed Detection

In rotor-slot-harmonic-based induction motor speed detection schemes, a motors rotational speed is resolved from the phase of a rotor slot harmonic signal. In such schemes, frequency-selective digital filters are often applied to attenuate interferences in sampled motor currents before rotor slot harmonic signals are extracted. This paper demonstrates that the use of linear-phase filters, which is a family of digital filters with linear phase responses, helps preserve the rotor slot harmonic signals phase during the filtering operation. The paper also shows that for induction motors with periodically time-varying loads, the rotor slot harmonic signals frequency is modulated by periodic speed oscillations, and hence contains multiple sidebands in the frequency spectrum. Experimental results further show that linear-phase filters allows for a more accurate detection of the induction motor speed than filters with nonlinear phase responses.

Supply Frequency Tracking in Resistance-Based Induction Motor's Rotor Temperature Estimation

In resistance-based rotor temperature estimation schemes, a grid-connected induction motors rotor temperature is derived from a resistance quantity based on an equivalent circuit model. This temperature is susceptible to fluctuations in the motors supply frequency. In this paper, the relationship between supply frequency fluctuations and accuracy in resistance-based rotor temperature estimates is analyzed, and a complex adaptive phase discriminator is proposed to track the supply frequency fluctuations for grid-connected induction motors. A complex voltage vector is constructed from polyphase voltage measurements and passed through a frequency-selective digital filter to attenuate harmonics and interference. The complex adaptive phase discriminator then operates on the filtered voltage signal, iteratively adapting a complex reference signals phase to align it with the filtered voltage signal. The power grids supply frequency is directly estimated from the adapted phase on a sample-by-sample basis. Experimental results demonstrate that the proposed complex adaptive phase discriminator helps produce rotor temperature estimates with increased accuracy.

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.