Benoit Leprettre

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.

Bearing Fault Diagnosis in Induction Machine Based on Current Analysis Using High-Resolution Technique

In this paper, we propose to perform early fault diagnosis using high-resolution spectral analysis of the stator current to detect bearing faults in electrical induction machine. While most research works focus on mechanical vibration analysis, the originality of our work relies on the use of high-resolution methods to detect modulations in the stator current. We present the results obtained for real data to detect inner and outer raceway bearing defects made articially as well as bearing defects obtained through on-site ageing. The obtained results show that the proposed method yields better detection than classical spectum analysis.

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.

Comparison of protection requirements in IE2-, IE3-, and IE4-class motors

In this paper, an overview of the main motor features influencing the protection devices settings is presented, pointing out key differences between three-phase direct-on-line (DOL) motors of different efficiency classes. The results of experimental tests and simulations are presented and discussed in the scope of the motor protection against overload, short-circuit, current unbalance, and locked-rotor conditions, for 7.5-kW, four-pole, IE2- and IE3-class squirrel-cage induction motors (SCIMs) and for IE4-class line-start permanent-magnet motors (LSPMs), offering important data related to their electrical and thermal behaviors. Two commercial digital thermal-magnetic motor protection devices, whose thermal models were designed for IE1- and IE2-class motors, are evaluated in terms of operation effectiveness in the tested motors. This paper is important for industry due to the significant penetration of IE3-class induction motors, as well as to the recent introduction into the market of IE4-class induction motors and LSPMs.