SENSOR AND INVERTER FAULT TOLERANT CONTROL IN INDUCTION MOTORS

SENSOR AND INVERTER FAULT TOLERANT CONTROL IN INDUCTION MOTORS Roberto Arnanz ∗ Antonio Mendoza ∗∗ Luis J. Miguel ∗∗∗ Jos´ e R. Per´ an ∗∗∗ ∗

Fundaci´ on CARTIF, Valladolid, Spain ∗∗ Gamesa Solar, Madrid, Spain ∗∗∗ University of Valladolid, Spain

Abstract: Induction motor has increased its importance in industry over the last two decades thanks to its robustness and control performance improvement. Direct Torque Control (DTC) has been the base for multiple developments or modifications in control algorithms and models obtaining great performances against torque variations at any motor speed (including zero speed). In this paper DTC scheme has been chosen as the starting point to design a fault tolerant control for induction motors. Also, a study of the possible faults and solutions in the inverter–motor set is presented. The control algorithms used for open–switch and open-phase faults are developed and their results are presented. The supervisor and fault diagnosis system has not been included in this work though they have c 2006 IFAC been taken into account during the research process. Copyright ° Keywords: Fault Tolerance, Direct Torque Control, Induction motors, Sensor failures, Inverter failures

1. INTRODUCTION

the motor (Blaschke, 1972; Leonhard, 1996), have improved the performance of the control so much that AC motors are nowadays the better option for most applications. Robustness, low weight– power factor and low maintenance needs of AC motors, joined to the cost reduction in power electronics over recent decades have made it impossible for the DC motors to compete with them in standard applications. In the late 80s, (Takahashi and Noguchi, 1984) developed Direct Torque Control (DTC), that allows the direct control of flux and torque through an optimal switching selection. Only the stator resistance value is needed for a high speed–high performance control even with load torque variations. From a theoretical point of view, it is enough to know a few motor parameters to implement VFOC and only one for DTC. In practice, the motor parameters change over time due, for example, to temperature change. Besides

Electrical motors are one of the most crucial production components, and many are of vital importance for factories to be operational. Thus, the correct working of the motors could avoid unnecessary stoppages and imply important savings in indirect costs. This is why motor operations need to be very reliable. Also, an appropriate performance of the motor control will produce important power savings if the motor is controlled with appropriate current or power factor values. For many years, only traditional control techniques were applied to AC motors, which made these very expensive and imprecise for use in industry applications. Since the 70s, the evolution of power electronics and microprocessors made it possible to implement vector flux oriented controls (VFOC) that, using an internal model of

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2. MODIFIED DTC FOR ENERGY SAVING

this, the flux model used in VFOC and the torque model used in DTC are not valid for all the operation points of the motor, especially at low speed. So, different methods and models have been developed to improve the control using parameter identification, observers, adaptive models or artificial intelligence (AI) (Habetler et al., 1998; Holtz and Quan, 2002; Lascu and Trzynadlowski, 2004; Maes and Melkebeek, 2000; Kazmierkowski et al., 2002).

DTC control is based on a switching table that selects the optimal voltage vector that has to be generated by the inverter to obtain the required increase/decrease of torque and flux variables. In classical DTC, flux and torque are the controlled variables, and two hysteresis controllers, whose inputs are the torque and flux errors respectively, are used to generate the increase/decrease commands for the optimal switching table. As only eight voltage vectors can be selected in the inverter (six active and two zero vectors), the control will produce high torque ripple, because it is not possible to control the amplitude or the angle of the voltage vector needed to reach the references exactly. Only the sign variations of the variables can be controlled with these eight vectors (Vas, 1998). (A voltage vector has a direct relationship with the impressed real voltages trough Park’s transformation, that converts the three phases in two components of a vector).

To improve AC motor application reliability, multiple predictive maintenance techniques can be applied to detect incipient faults. These include, first of all, off–line electrical analysis such as hipot, partial discharges, isolation test or surge comparison testing. Based on signal analysis, on–line methods, such as termography, vibration, stator current signature or Park’s current vector analysis have also been introduced in the industry. Fault detection developments based on parity equations, observer based residuals, parameter estimation or AI have also been used for fault diagnosis of the motor (Arnanz et al., 2000; Jiang et al., 2001; Filippetti et al., 2000). The set inverter–motor has also been the object of several proposals to detect faults in the actuator (inverter) and how this affects the plant (motor), as is presented by (Bellini et al., 2000; Mendoza et al., 2003).

In the control scheme proposed in this paper (figure 1), an SVM technique is used to generate ¯ ¯∗ ¯s ¯ . The way to obtain the any voltage vector ¯U required vector is through a lineal combination of two of the six active vectors u ¯k plus a zero vector during a control period. The hysteresis controllers are replaced by controllers that are used to generate the amplitude and angle of the voltage vector from the torque and flux errors respectively. In the flux loop, an inverse model of the stator is used to calculate the angle between flux and voltage that allows the new flux reference to be reached using the reference voltage module. A compensation term is also included to overcome model errors. In the torque loop, when a torque error is present, the voltage module will be modified, so the acceleration of the flux angle will produce the torque variation. A speed loop with a PI controller is added for motor speed control, and its output is the torque reference of the modified DTC.

The next step in motor research has been to achieve fault tolerance. Most works study different inverter topologies to overcome with switch/leg short–circuit or open-circuit (Welchko et al., 2004; Ribeiro et al., 2004). These solutions, based on physical redundancy, considerably increase costs because they need additional power electronic devices. Inverter topology becomes more complex, but the post–fault operation is identical to the no–fault case. Other solutions requiring additional switches are proposed by (Kastha and Majumdar, 2000; Welchko and Lipo, 2001) but are not used as redundancy, and the motor works as a single or two phase motor. From the diagnosis field (Patton and Lopez-Toribio, 2000) present a fault tolerant scheme for induction motor based on a multiple model control structure. Each operation point or fault has been previously modeled and a fuzzy logic system defines the combination of the models that have to be used in the control.

The flux reference is used to vary the operation point of the motor. As shown in (Casadei et al., 2003), for a fixed load torque a flux variation will produce an increase or decrease of current amplitude, and is usually used to improve the classical field weakening strategy over base speed, so maximum torque can be obtained at any speed. In the same way, flux reference can be used to change other motor variables, such as power factor or active and reactive power, for fixed speed reference and load torque. In figure 2 reactive power variation is shown for a fixed load torque and different speed and flux values. In the proposed control, the flux reference is selected from a previously acquired table where the minimum reactive point for each speed–torque pair is stored. Though this is valid for stationary situation, there can be

In this paper a modification of a DTC is presented where the optimum switching table is substituted by a Space Vector Modulation (SVM) algorithm and the flux reference is used to optimize the motor operation. Possible faults of the inverter– motor set are enumerated to define which can be recovered with new algorithms and which must be detected and isolated by protection elements. Finally, some solutions are proposed for the rest of the faults and a modified switching algorithm is used to control the motor with only two phases.

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Fig. 1. Modified DTC scheme strated how some of the faults cannot be candidates for fault tolerant control and their catastrophic results have to be avoided using protection devices such as fuses. Below, a list of inverter and motor faults is explained with possible remedial actions:

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(1) Input supply single line to ground fault: an increase of dc voltage ripple appears and a pulsating torque due to that ripple is generated. Changing control parameters can reduce the effects of this fault and a control performance degradation must be considered. (2) Rectifier diode short-circuit fault: has the same effects as the input supply single line to ground fault. (3) Earth fault on dc bus: an excessive current stress affects the line fuses and this causes one or more input fuses to blow. If the fuse in the faulty phase blows first, the rectifier will operate in single–phase mode, but otherwise, the power to the rectifier will be interrupted. (4) DC link capacitor short–circuit fault: the system must be shutdown and protection is needed due to the high currents. (5) Transistor base drive open fault: dc current is impressed to the machine and a dc offset appears in the three phases. A negative torque appears due to the offset components and the switches increase their stress. A solution is proposed below in this paper. (6) Transistor short–circuit fault: is a commonly occurring fault due to current or voltage stress. The other switch of the leg cannot be switched on to prevent a shoot-through fault. A dangerously large dc component is introduced and the faulty phase current grows. It is necessary to open the faulty phase. (7) Line to line short circuit at machine terminal: the system must be shutdown immediately. (8) Single lines to ground fault at machine terminal: the system must be shutdown immediately. If the fault can be detected predictively, an open–phase algorithm is presented in the next section to overcome this fault.

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Fig. 2. Reactive power versus flux and speed variations for 20% of load torque some problems to overcome with abrupt changes of load torque or speed reference. In the second case, as speed reference is a user command, it is possible to select first a new flux reference that allows maximum torque and then change the speed. This introduces a delay in the motor response, but avoids unstability or overcurrents in stator windings. A load torque variation cannot usually be predicted (only when the load is also controled), so a compromise between reactive power minimization and torque perturbance robustness has to be achieved. 3. FAULT SOURCES IN AC MOTOR CONTROL SYSTEM The first step for fault tolerant control is to analyse every component looking for all possible faults, and select which of these faults must be candidates to design specific fault tolerant actions to overcome them. For this, it is important to know how each fault is propagated in and affects the system, so as to determine which of them are not recoverable, which have the goal of a graceful degradation behaviour and which can be completely recoverable by means of a control system reconfiguration. Several inverter faults are enumerated and analyzed in (Kastha and Bose, 1994). It is demon-

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Machine faults are reduced thanks to the use of a power inverter. Winding insulation failures, caused by excessive voltage or current stress, are practically eliminated because the line voltage surges are absorbed at the converter input, and overcurrent is avoided by the inverter control. Broken rotor bars, mainly due to direct on–line start, are also practically eliminated by soft starting with the inverter. Other possible faults that can appear in the motor are electric circuit asymmetries and rotor eccentricity that will cause vibration and noise, overheating that causes parameter change and bearings faults and misalignment that cause additional vibration and can be the origin of other mechanical faults in the motor.

4.1 Current sensor fault During normal operation three current sensors are used, and when one of them fails it is possible to continue working with the same control algorithm simply by substituting the faulty sensor signal using the relationship: iR + iS + iT = 0. Most authors use only two current sensors in normal operation, but in this case it has been considered as a fault mode because, in the experiments carried out, it has been demonstrated that with only two current sensors it is necessary to add a filter to the measured signals to mantain the performance at high speeds and obtain good responses at low speed. When two current sensors fail, a graceful degradation in control can be considered. As no models can be used to calculate motor variables with only one current, a voltage–frequency control is chosen for this fault mode. This control does not guarantee the same performance against torque variations as a DTC control, but allows good control without current sensors.

Another important source of faults are the sensors, which most diagnosis research works focus on. An erroneous value in current sensors will produce a bad estimation of flux and torque values and, consequently, the degradation of control performance or system unstability. The same effects will have a failure in the voltage sensor, used for DC link voltage monitorization. The encoder is used for measuring speed, which is the controlled variable, and its erroneous values will cause an error in the output of the system or unstabilities. One of the main sources of perturbances in encoder measurement is vibration of the encoder, usually caused by an incorrect coupling between encoder and motor. A common possible fault for all the sensors will be a power supply fault. In this case the output value is zero, and this can be considered as an abrupt fault that can be caused by power source fuse blow or control devices main supply malfunction. It is also possible to have short or open circuits in the wires that connect sensors and the acquisition system.

4.2 Open switch fault When an open switch fault occurs in the power converter of figure 1, and the control strategy for the inverter is not changed, the motor in some cases is able to continue working, but the flux and torque estimations are not correct, so the continuous operation cannot be guaranteed. The origin of these problems is that three of the six active voltage vectors of the fault–free inverter cannot be generated. Figure 3 represents, as an example, the available voltage vectors in the case of T–phase down switch always open. In this case u ¯2 becomes a zero voltage vector, and u ¯1 and u ¯3 are transformed into u ¯F T 1 and u ¯F T 2 respectively. If the voltage reference vector is in sector FT I, it has to be constructed as a lineal combination of adjacent active voltage vectors, but only vectors with u ¯F T 1 and u ¯F T 2 directions are being generated. In some cases this will cause an opposite action to the desired one. As can be ¯ref is needed seen in figure 3, if a voltage vector U ¯ s , the for a torque increase in a motor with a flux Ψ ¯f ault , real voltage vector in the inverter will be U whose effect is a torque decrease. This example can be generalized to any open switch in the inverter, and in any case, a complete recoverability of the control is not possible. As it is only possible to generate all the reference vectors with physical redundancy, a graceful degradation must be the goal of the designed control. The maximum torque that can be obtained from the motor in this case is lower than the rated torque. An increase in current and vibration is also a result of this fault.

4. FAULT TOLERANT MODES The fault tolerant control will use a multiple model strategy. Different control structures, or modifications of the DTC control, are designed to maintain the motor control when faults appear. The supervisor will switch the different algorithms using the information generated by the diagnosis system. Both diagnosis system and supervisor need a detailed and careful design to guarantee the robustness of fault diagnosis, stability in transitions between models and a short detection time to avoid malfunction before the fault tolerant algorithm is switched on. The design of such systems is beyond the scope of this paper and will be studied in future works. The developed algorithms have been tested in a workbench with a 5.5 Kw motor, a powder dynamometer as load and dSPACE hardware used for the inverter control.

With five active vectors, any voltage vector in sectors IV and V can be generated exactly, but

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substitutig the other four sectors, three new ones are defined: FT III, FT VI and FT I. Voltage vectors in sectors FT III and FT VI can be generated by modifying the equations of the SVM active vectors duty cycle, as the angle between active vectors is not π6 and the adjacent vectors have a different amplitude. This will mean that some vectors of high amplitude cannot be generated and will have to be substituted by a vector of the same direction but lower amplitude. The continuous line in figure 3 defines the maximum amplitude of voltage vectors for this type of fault. Finally, for vectors in sector FT I, is tried that the generated vector acts in the same direction as the reference vector. This is made by generating the projection of the reference vector on vectors u ¯F T 1 or u ¯F T 2 . As the new voltages and currents in the motor are not balanced, the flux estimation presents an offset and the module cannot be calculated as before the fault took place. To avoid errors in the estimations, a new control scheme is used based on voltage–frequency control. The flux reference is used to regulate the relation between voltage and source frequency, and it is variable with torque and speed references. As in the modified DTC the flux reference is used to minimize reactive power or current consumption. Figure 4 shows a speed ramp from 800 rpm to 1400 rpm and return with a 40% of nominal torque load. It can be seen that with the proposed algorithm it is possible to maintain the control of the motor even when an open switch fault is present. The speed ramp slope has been chosen to avoid overshoot and is part of the graceful degradation accepted for the fault.

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Fig. 4. Speed ramp with open switch in phase T (40% load torque). (a) Speed and (b) current phasor for t ∈ (0.9, 1.5)s control of the open switch case is used, and the voltage value is calculated as the mean value of voltage vectors amplitude. The frequency used is the amplitude modulation frequency of voltage. One of the problems of this method is that the transition between u ¯F T 1 and u ¯F T 2 implies reducing the voltage amplitude to zero voltage before increasing it again. To avoid motor demagnetizing a low–limit is imposed on the voltage amplitude, which results in a “jump” between both vectors and a vibration increase if this limit is not well chosen.

5. CONCLUSIONS A modified direct torque control has been developed where the flux reference is used to minimize the reactive power. Using previously stored data, the flux reference can also be used to minimize the current if the windings of the motor must be protected, or to minimize other variables of the motor if necessary.

4.3 Open phase fault If an inter–turn or a phase–ground short–circuit can be detected and isolated it is possible to keep the motor working by opening the two switches of the affected phase. In this case only two active vectors can be generated and these are u ¯F T 1 and u ¯F T 2 . The rotating flux vector of induction motor normal operation will be transformed into a variable amplitude vector with the direction of active voltage vectors. So, the same voltage–frequency

An analysis of possible faults in the inverter– motor set has been presented indicating the remedial actions required. Some of them involve protective elements, for example fuses, and others can be overcome with new algorithms or modifications of the developed control.

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A multiple–model structure has been used for fault tolerant control, including current sensor faults, encoder fault, open switch fault and open phase fault. A voltage–frequency based control is used when the faults do not allow good estimations of the variables involved in the control to be obtained. A complete recoverability of the system is not possible for faults in the inverter without physical redundancy but it is possible to keep the motor working. Real examples have been shown for speed control.

Jiang, B., M. Staroswiecki, V. Cocquempot and C. Cristophe (2001). Application to sensor fault diagnosis for induction motor. In: Prep. of the 3rd IEEE Int. Symposium on Diagnostics for Electrical Machines, Power Electronics and Drives. Grado (Italia). pp. 265–269. Kastha, D. and A. K. Majumdar (2000). An improved starting strategy for voltage-source inverter fed three phase induction motor drives under inverter fault conditions. IEEE Trans. on Power Electronics 15(4), 726–732. Kastha, D. and B. K. Bose (1994). Investigation of fault modes of voltage–fed inverter system for induction motor drive. IEEE Trans. on Industry Applications 30(4), 1028–1038. Kazmierkowski, M., P. R. Krishnan and F. Blaabjerg (2002). Control in Power Electronics. Selected Problems. Academic Press. Lascu, C. and A. M. Trzynadlowski (2004). A sensorless hybrid DTC drive for high–volume low–cost applications. IEEE Trans. on Industrial Electronics 51(5), 1048–1055. Leonhard, W. (1996). Control of Electrical Drives. Springer Verlag. Berlin. Maes, J. and J. A. Melkebeek (2000). Speed– sensorless direct torque control of induction motors using an adaptive flux observer. IEEE Trans. on Ind. Applications 36(3), 778–784. Mendoza, A., L. J. de Miguel, R. Arnanz, M. A. Pacheco and J. R. Per´an (2003). On line diagnosis of induction motors for additive and short–circuit windings with EKF. In: Proc. SAFEPROCESS 2003. Washington. pp. 909– 914. Patton, R. J. and C. J. Lopez-Toribio (2000). Multiple–model fault–tolerant control of an induction motor in the presence of uncertainty. In: Prep. of SAFEPROCESS 2000. Vol. 2. pp. 1139–1144. Ribeiro, R. L. A., C. B. Jacobina, E. R. C. da Silva and A. M. N. Lima (2004). Fault–tolerant voltage-fed pwm inverter ac motor drive systems. IEEE Trans. on Industrial Electronics 51(2), 439–446. Takahashi, I. and T. Noguchi (1984). Quick torque response control of an induction motor using a new concept. IEEE J. Tech. Meeting on Rotating Machines pp. 61–70. Vas, P. (1998). Sensorles Vector and Direct Torque Control. Oxford Science Publ.. New York. Welchko, B. A. and A. Lipo (2001). A novel variable–frequency three–phase induction motor drive system using only three controlled switches. IEEE Trans. on Industry Applications 37(6), 1739–1745. Welchko, B. A., T. A. Lipo, T. M. Jahns and S. E. Schulz (2004). Fault tolerant three–phase AC motor drive topologies: A comparison of features, cost, and limitations. IEEE Trans. on Power Electronics 19(4), 1108–1116.

The fault tolerant system has to be completed with the careful design of a supervisor and diagnosis system that will be done in future works.

ACKNOWLEDGMENTS The research work developed in this paper was mainly supported by the funded project currently in progress: CICYT, reference DPI2004-06458

REFERENCES Arnanz, R., L. J. de Miguel, E. Moya and J. R. Per´an (2000). Model-based diagnosis of AC motors. In: Prep. of the SAFEPROCESS 2000. Vol. 2. pp. 1145–1150. Bellini, A., F. Filippetti, G. Franceschini and C. Tassoni (2000). Closed loop control impact on the diagnosis of induction motors faults. IEEE Trans. on Industry Applications 36(3), 1318–1324. Blaschke, F. (1972). The principle of field orientation as applied to the new transvector closed–loop control system for rotating field machines. Siemens Review 39(5), 217–220. Casadei, D., G. Serra, A. Tani, L. Zarri and F. Profumo (2003). Performance analysis of a speed-sensorless induction motor drive based on a constant–switching–frequency DTC scheme. IEEE Trans. on Industry Applications 39(2), 476–484. Filippetti, F., G. Franceschini, C. Tassoni and P. Vas (2000). Recent developments of induction motor drives fault diagnosis using AI techniques. IEEE Trans. on Industrial Electronics 47(5), 994–1004. Habetler, T. G., F. Profumo, G. Griva, M. Pastorelli and A. Bettini (1998). Stator resistance tuning in a stator flux field oriented drive using an instantaneous hybrid flux estimator. IEEE Trans. on Power Electronics 13(1), 125–133. Holtz, J. and J. Quan (2002). Sensorless vector control of induction motors at very low speed using a nonlinear inverter model and parameter identification. IEEE Trans. on Industry Applications 38(4), 1087–1095.

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