A. Ferrah

The effect of rotor design on sensorless speed estimation using rotor slot harmonics identified by adaptive digital filtering using the maximum likelihood approach

Adaptive digital filtering has been demonstrated as an effective technique for extracting a real-time, sensorless, speed signal from rotor slot harmonics (RSHs) embedded in the line current waveform of induction motor drives. It is known that sensorless speed estimation techniques using RSHs may exhibit poor performance with certain motor designs. This paper examines the reasons for that poor performance which reflects differences in the magnitude of the slot harmonic signals consequent upon rotor design. Experimental results for a 30 kW motor with 6 different rotors are presented. The significant parameters are the number of rotor slots, skew and the accuracy of construction. It is clearly shown how inferior performance can arise. Conversely, improvements in reliability of speed estimation and transient response can be obtained by recognising those aspects which provide an enhanced signal, by minimising the background noise of the inverter/machine, or by enhancing the adaptive filter. The recursive maximum likelihood technique is presented as an improved algorithm for tuning the digital filter which aids transient response and reliability of speed estimation. Real time, experimental transient performance is demonstrated for the different rotors used in this paper and the performance failure of a particular slot combination demonstrated.

Sensorless speed detection of inverter fed induction motors using rotor slot harmonics and fast Fourier transform

A novel approach to sensorless speed detection for adjustable-speed AC drives is described. No a priori knowledge is required about the motor construction, electrical parameters, or load condition. In addition, no external tuning is needed for the system. The technique is based on instantaneous spectral estimation using the fast Fourier transform, whereby the speed-dependent slot ripple harmonic frequency is determined. For the assessment of this technique, an all-digital speed detector has been built around a general-purpose 386 microcomputer. The performance of this detector over a wide range of inverter frequencies and load conditions is discussed.<<ETX>>

An FFT-based novel approach to noninvasive speed measurement in induction motor drives

A novel sensorless speed detection method based on the fast Fourier transform (FFT) spectral analysis is described. The main concern is the extraction of the speed information contained in the rotor slot-ripple harmonics created in the airgap of the induction motor using digital signal-processing techniques. A nonintrusive all-digital speed detector suitable for steady-state operation has been designed around an Intel/80386-based microcomputer equipped with a 80387 coprocessor. The detector performance with and without load over a wide range of speeds is described. >

A speed identifier for induction motor drives using real-time adaptive digital filtering

A novel sensorless speed identifier for real-time application in induction motor drives under steady-state and transient conditions is proposed. It is based on the calculation of rotor slot harmonic (RSH) frequencies using an adaptive digital filter. It outperforms other analog or spectrum-based RSH speed identifiers in terms of accuracy and speed of response. The new identifier measures the speed with less than 0.1% error by processing the stator current on a sample-by-sample basis. It is also capable of tracking speed transients of high slew rates with high accuracy. The authors believe this to be the first effective tracking of RSHs during transients ever reported. The proposed algorithm is computationally very efficient and requires only a single processor for its real-time implementation. Simulated and experimental data were used to validate the algorithm.

High precision calorimetry for the measurement of the efficiency of induction motors

Calorimeters are used to measure directly the loss in electrical machines operating under various load conditions. This paper examines experimental errors resulting from calorimeter design and from operating procedures. The techniques of minimising these errors used by previous researchers are reviewed. An improved technique for obtaining high accuracy in the measurement of power loss is discussed based on two generations of a calorimeter. The performance of these designs is presented with the first providing an accuracy of 0.5% and the second better than 0.2% in measurement of total loss at the 30 kW level.

Efficient dynamic models for induction machines

A computationally efficient reluctance mesh model suitable for simulating the dynamic performance of practical induction motors is presented. The calculation time is minimized by direct computation of rate of change of flux, avoiding unnecessary evaluation of time and rotor position dependent inductances. The magnetic, electrical and dynamic models of the machine are coupled so that the simulation determines both steady state and transient behaviour including the effects of saturation and high order torque harmonics due to the rotor and stator teeth. The algorithm is developed from rigorous field theory. This is combined with and simplified by, practical experience, resulting in a procedure that can efficiently simulate 3D machines within an iterative design environment without requiring extensive computational resources. Full details of the implementation are presented along with simulations and experimental characterization of a practical induction motor which are found to be in ‘excellent’ agreement. Copyright

A transputer-based speed identifier for induction motor drives using real-time adaptive filtering

A novel sensorless speed identifier for induction motor drives is proposed. It is based on the real-time calculation of rotor slot harmonics (RSHs) using an adaptive digital filter. It outperforms any other analogue or spectrum-based speed identifier in terms of accuracy and speed of response. The new identifier measures the speed, by processing two stator currents, with an accuracy better than 0.1% on a sample-by-sample basis. It is also capable of tracking speed transients, of high slew rates, with similar accuracy. This is notable as the first effective tracking of RSHs during transients ever reported. The proposed algorithm is computationally very efficient, and requires only a single processor for its real-time implementation. Simulated and experimental data were used to validate the algorithm.

Applications of the FFT to the speed measurement of inverter fed induction motors

An alternative to the analog techniques and devices used in detecting the speed of induction motors in adjustable-speed drives is presented. The method involves digital processing of one stator phase current in order to retrieve the speed information contained in the slot-ripple harmonics, using FFT-based spectral analysis. A noninvasive digital speed detector, suitable for steady-state operation, was developed. The detector is designed around an Intel/80386-based microcomputer. Its performance with and without load over a wide range of speeds is reported.<<ETX>>

On the use of the maximum entropy method for the spectral estimation of PWM output signals

The fast Fourier transform (FTT) algorithm has always been an important tool for characterizing the frequency content of inverter output waveforms. As an alternative to the FFT, the parametric technique referred to as the maximum entropy method (MEM) is applied in the spectral analysis of the stator current of an inverter-fed induction motor under different operating conditions. With this method, windowing and interpolation techniques are not needed and a better frequency resolution is expected. Both the FFT and MEM methods are briefly reviewed and experimental results are provided.<<ETX>>

A new sensorless dynamic speed-estimator for induction motor drives using predictive adaptive filtering

A new sensorless speed estimation technique for induction motor drives is described. The new estimator is based on predictive adaptive spectral estimation of rotor slot harmonics. The predictive filter parameters are updated with time using the fast recursive least squares algorithm. The proposed speed estimator was implemented on a transputer-based i860 vector processor, end tested on experimental data from 3 different motors at different speeds and load conditions. The dynamic performance of the estimator was also assessed using simulated data. A comparison between the proposed estimator and conventional estimator, based on the fast Fourier transform, was also provided.