Thomas M. Wolbank

Stator-Current Spectrum Signature of Healthy Cage Rotor Induction Machines

Before applying current-signature-analysis-based monitoring methods, it is necessary to thoroughly analyze the existence of the various harmonics on healthy machines. As such an analysis is only done in very few papers, the objective of this paper is to make a clear and rigorous characterization and classification of the harmonics present in a healthy cage rotor induction motor spectrum as a starting point for diagnosis. Magnetomotive force space harmonics, slot permeance harmonics, and saturation of main magnetic flux path through the virtual air-gap permeance variation are taken into analytical consideration. General rules are introduced giving a connection between the number of stator slots, rotor bars, and pole pairs and the existence of rotor slot harmonics as well as saturation-related harmonics in the current spectrum. For certain combinations of stator and rotor slots, saturation-related harmonics are shown to be most prominent in motors with a pole pair number of two or more. A comparison of predicted and measured current harmonics is given for several motors with different numbers of pole pairs, stator slots, and rotor bars.

Using PWM-Induced Transient Excitation and Advanced Signal Processing for Zero-Speed Sensorless Control of AC Machines

The sensorless control of induction machines, particularly for operation at low speed, has received significant attention in recent years. To realize a field-oriented control of AC machines that is able to work at zero speed, the most commonly used methods are either sensor-based models or transient-signal-excitation methods. The major disadvantage of present signal-injection methods is that they are intrusive to pulsewidth modulation (PWM). An additional switching sequence has to be embedded in the control that will cause a torque and current ripple. In order to overcome these problems, a new flux-estimation algorithm that uses the phase current derivative to extract the flux-position information is presented. In contrast to previously introduced methods, this new approach operates without additional transient excitation of the machine and requires only fundamental-wave excitation using standard PWM or slightly modified PWM. Furthermore, only the current response in the two active states of PWM is used. This makes it possible to use sensorless control for the whole speed range including overmodulation and removes the distortion and parasitic influence of the zero switching states during the estimation of the flux. Experimental results are presented to validate the applicability of the presented approach.

A modified PWM scheme in order to obtain spatial information of AC machines without mechanical sensor

To realise the sensorless control of AC machines which are able to work even at zero speed, an additional high frequency excitation is necessary. Different schemes have been presented recently, which use an transient excitation caused by the switching of the voltage source inverter. As any additional excitation also has an impact on the current ripple and the noise emission of a machine, investigations have been made to integrate the fundamental wave and the transient excitation into one inverter control scheme. In this paper, a new modified PWM scheme is presented which enables both the fundamental wave excitation to generate the flux and the torque of the machine as well as the transient excitation necessary to obtain sensorless control signals. Simulations as well as measurement results are given to show the advantages and limits of the proposed scheme.

Monitoring of Rotor-Bar Defects in Inverter-Fed Induction Machines at Zero Load and Speed

Rotor-cage fault detection in inverter-fed induction machines is still difficult nowadays as the dynamics introduced by the control or load influence the fault-indicator signals commonly applied. In addition, detection is usually possible only when the machine is operated above a specific load level to generate a significant rotor-current magnitude. This paper proposes a new method of detecting rotor-bar defects at zero load and almost at standstill. The method uses the standard current sensors already present in modern industrial inverters and, hence, is noninvasive. It is thus well suited as a start-up test for drives. By applying an excitation with voltage pulses using the switching of the inverter and then measuring the resulting current slope, a new fault indicator is obtained. As a result, it is possible to clearly identify the fault-induced asymmetry in the machines transient reactances. Although the transient-flux linkage cannot penetrate the rotor because of the cage, the faulty bar locally influences the zigzag flux, leading to a significant change in the transient reactances. Measurement results show the applicability and sensitivity of the proposed method.

Current-controller with single DC link current measurement for inverter-fed AC machines based on an improved observer-structure

In modern industrial applications even low-cost drives have excellent dynamic behavior, which is achieved by field-oriented control combined with high-dynamic current regulation. Usually at least two phase-current sensors are necessary to realize such an operation. In this paper, a new current-control scheme is presented, which enables high-dynamic control of ac machine-line currents with improved accuracy by measuring only the current of the dc-link. The controller is based on three individual and adaptive phase-current observers, which guarantee very accurate phase-current estimates even at low-modulation indices. Thus, operation is possible even at low-speed without modifying the switching pattern of the controller and the necessary modification at zero-speed can be clearly reduced. Different realizations of the proposed scheme are given and a comparison is made to previously published schemes which are also based on a single dc-link current sensor. Measurements performed on an induction-machine drive at different points of operation show the applicability of the proposed structure. It is shown that the long-term accuracy is clearly increased and the maximum deviation of the observer estimate from the actual phase-current is reduced when using the proposed scheme.

Sensorless rotor temperature estimation of permanent magnet synchronous motor

The work proposes a method for estimation of the magnet temperature in permanent-magnet synchronous machines by exploiting the d-axis saturation effects in the steel stator core produced by the d-current and rotor flux excitation. The method implies an intermittent injection of a voltage pulse in the d-axis of the motor. The resulting d-current response is a function of both the initial value of the d-current itself and the magnetization level of the magnets. Thus, a temperature dependent variation in the magnetization level of the permanent magnets is reflected in a variation of the d-current slope upon the voltage pulse. Experimental validation of the method is demonstrated with surface permanent-magnet motor.

Compensation of Speed Dependence in Sensorless Rotor Temperature Estimation for Permanent-Magnet Synchronous Motor

This paper proposes an improved method for estimating the magnet temperature in permanent-magnet synchronous machines without using any temperature sensors. Originally, the method implies an intermittent injection of a voltage pulse in the positive d-axis of the motor while keeping the load current zero. Thus, the resulting d-current response depends on both the initial value of the d-current itself and the actual magnetization level of the permanent magnets. Since the magnetization of the magnets depends on the temperature, different d-current slopes are associated with given temperature levels of the magnets. At higher speeds, the current response is additionally affected by induced voltages of various sources which lead to erroneous estimation of the magnet temperature. By applying a voltage pulse in the positive and negative d-axis of the motor, symmetry of the induced voltages can be achieved in a manner that the difference of the current responses from the positive and negative pulses is no longer affected by the induced voltages.

Distance laboratory for teaching Industrial Electronics

The paper deals with basic philosophy and structure of remote controlled laboratory for experimentation in electrical engineering. PEMCWebLab project provides the user with a practical experience in industrial electronics education. It is designed based on leading ideas and has clear targets. In this paper laboratory for the course motion control is introduced. First of all description of the system and then two assignments namely calibration of resolver and current control loop design explained. Goal of the experiment, content of the experiment and verification of the achieved learning outcomes is illustrated on the mentioned two examples.

Inverter statistics for online detection of stator asymmetries in inverter-fed induction motors

A simulation model of an induction machine that enables the analysis of the influence of stator fault conditions on the machine control of inverter-fed drives is presented. Based on this model, the influence of the breakdown of the stator windings insulation on the behavior of the machine and especially on the current control scheme is shown. A new online method to detect such asymmetries caused, for example, by an interturn insulation failure in the stator windings is proposed and investigated. This new method utilizes the influence of stator asymmetries on the inverter current control scheme. By evaluating the statistical distribution of the different inverter switching states and switching times, asymmetries in the stator can be detected and isolated. The measurements required to implement this method are already available in modern inverter fed drives as they are used to realize the current control loop. Thus, no additional sensors are necessary. The practical realization of the fault detection algorithm is demonstrated in combination with a predictive single step current controller. Measurements performed on a drive test stand verify the applicability of the proposed online method to detect and isolate stator faults.

An improved observer-based current controller for inverter fed AC machines with single DC-link current measurement

In modern industrial applications even low cost drives have excellent dynamic behaviour which is achieved by field oriented control combined with a high dynamic current control. Usually at least two phase current sensors are necessary for such an operation. In this paper, a new current control scheme is presented, which enables dynamic control of AC machine line currents with improved accuracy by measuring only the current of the DC link. The controller is based on three individual and adaptive phase current observers, which guarantee very accurate phase current estimates even at low modulation indices. Thus operation is possible even at low speed without modifying the switching pattern of the controller and the necessary modification at zero speed can be clearly reduced. Different realisations of the proposed scheme are given and a comparison is made to previously published schemes which are also based on a single DC link current sensor. Measurement results performed on an induction machine drive at different points of operation show the applicability of the current controller and furthermore enables a grading of different structures.