A Fault-Tolerant Permanent Magnet Synchronous Motor Drive with Integrated Voltage Source Inverter Open-Circuit Faults Diagnosis Jorge O. Estima and A. J. Marques Cardoso UNIVERSITY OF COIMBRA – INSTITUTO DE TELECOMUNICAÇÕES Department of Electrical and Computer Engineering, Pólo II – Pinhal de Marrocos, P – 3030-290, Coimbra, Portugal Tel.: +351 239 796 232 Email:
[email protected],
[email protected]
Acknowledgements The authors gratefully acknowledge the financial support of the Portuguese Foundation for Science and Technology (FCT) under Project No. SFRH/BD/40286/2007 and Project No. PTDC/EEAELC/105282/2008. The authors also acknowledge the PMSM gently offered by the Drives and Motion Division from Yaskawa Europe.
Keywords Fault Tolerance, Fault Handling Strategy, Variable Speed Drive, Permanent Magnet Motor, Diagnostics.
Abstract This paper presents a variable speed ac drive based on a permanent magnet synchronous motor, supplied by a three-phase fault-tolerant power converter. In order to achieve this, beyond the main routines, the control system integrates a reliable and simple algorithm for real-time diagnostics of inverter open-circuit faults. This algorithm performs an important role since it is able to detect an inverter malfunction and gives the information about its faulty phase. Then, the control system acts in order to first isolate the fault and then to proceed to a hardware and software reconfiguration. By doing this, a fully automated fault-tolerant variable speed drive can be achieved. Simulation and experimental results are presented showing the effectiveness of the proposed system under several operating conditions.
Introduction Thanks to their advantages such as high efficiency and power density, permanent magnet synchronous motors (PMSMs) are becoming more and more popular and at the present, they are replacing induction motors in some application fields. On the other side, it is known that variable speed ac drives are very sensitive to different kinds of faults. Therefore, when these faults occur, typically the drive operation must be stopped in order to proceed to a nonprogrammed maintenance schedule. This can be extremely undesired, especially for critical applications, where unplanned stoppages can result in very large costs or in even more catastrophic consequences. For these reasons, the development of fault-tolerant strategies applied to variable speed ac drive systems has been addressed by many researchers over the last years. In [1]-[3] a fault-tolerant converter for variable speed ac drives is presented, capable to handles with faults in a power switch. After the fault occurrence, the motor phase is disconnected from its corresponding inverter faulty leg and connected through a TRIAC to the midpoint of the capacitor bank in the dc link. Comparing with the normal six-switch three-phase inverter, with this topology, the inverter applies to the machine onehalf of the voltage. As a consequence, the drive post-fault operation will be limited to one-half of the rated speed.
In [4], a similar fault-tolerant converter topology is reported, where the machine phase is disconnected from the faulty inverter leg and connected through a TRIAC to an extra inverter leg, used as hardware redundancy. By doing this, the post-fault operation can achieve the same performance level of the normal situation. A fault-tolerant converter considering the connection of the motor neutral point to the dc link capacitor bank midpoint is addressed in [5]. This topology also allows the machine to develop the rated torque at one-half of the rated speed. Despite that this configuration requires just one extra power switch (TRIAC), the machine neutral point must be available. In addition, with the aim to reach the same magnetomotive force obtained under normal operating p conditions, for the post-fault operation, the motor phase currents will increase by a factor of 3 . Hence, the inverter semiconductors must be overdimensioned, increasing the overall converter cost. Another possible fault-tolerant converter design consists in the connection through a TRIAC of the machine neutral point to an extra inverter leg [6]. Although it is possible to achieve the machine rated speed p and torque under post-fault operating conditions, the motor phase currents increase by a factor of 3 , leading to the overdimensioning of the inverter power switches. Furthermore, this topology provides a path for the current third harmonic and consequently may cause torque pulsation. It is clear that, typically for all these fault-tolerant converters, a hardware reconfiguration is required. However, some changes at the software level must be also considered in order to take into account the fault isolation and to adapt the main control routines to the new inverter topology, optimizing in this way the global drive performance. Another important issue for the development of a fault-tolerant drive system is the fault diagnosis. In order to allow the control system to isolate the faulty phase and to proceed with the hardware and software reconfiguration, the inverter leg affected by the power switch failure must be firstly identified. For these reasons, the development of on-line methods that can detect and localize power switches open-circuit faults in inverter-fed ac drives, has become an important research topic. Some of these methods are based on patterns recognition and artificial intelligence [7]-[8], which are very complex and not suitable for real-time analysis. Other developed methods which take into account the comparison of actual and reference voltages, are effective and relatively fast [9]. However they require extra voltage sensors which is not desirable since it increases the system complexity and costs. Among the methods needing less implementation effort, are the ones based on the analysis of the motor phase currents average values [10]. These algorithms are quite simple and suitable for integration into the drive main controller without great effort. However, besides being load dependent, they also have the tendency to issue false alarms resulting from misinterpreting large transient variations of the currents average values. These problems were mitigated by the normalization of the diagnostic variables, as proposed in [11]. By the calculation of several diagnostic variables, the robustness against false alarms was considerably improved, having simultaneously the capability to detect multiple power switches open-circuit failures. Despite of these advantages, these enhancements bring some drawbacks such as a higher complexity and larger detection times. A novel approach for real-time open-circuit fault diagnostics in inverter-fed PWM motor drives was firstly proposed in [12] and later improved in [13]. This algorithm also presents a very high immunity against the issue of false alarms and it does not depend on the motor load level neither on its respective mechanical speed. In addition, it is quite simple and can be easily integrated into the main control system without great effort. More recently, a method to detect and localize multiple open-circuit faults in motor drive inverters was proposed in [14] by the analysis of the current Park’s Vector phase and the currents polarity. A new
method for single power switch open-circuit faults diagnosis based on the reference current errors was also presented in [15], showing a fast detection time equivalent to 5% of the motor phase currents fundamental period. Although there are several works addressing inverter fault diagnosis and fault-tolerant reconfigurations, the majority of these studies discuss these subjects separately. Therefore, there is a lack of research regarding the integration of fault diagnostic methods and fault-tolerant hardware/software reconfigurations into a single fault-tolerant motor drive system. For this reason, in this work a fault-tolerant variable speed PMSM drive is presented, integrating an algorithm for real-time diagnostics of inverter IGBT open-circuit faults. A rotor field oriented control strategy employing PWM hysteresis current controllers was applied to the inverter in order to control the PMSM mechanical speed. Simulation and experimental results are presented showing the fault detection and localization by the on-line diagnostic method as well as the fault isolation and hardware/software reconfiguration for the fault-tolerant converter.
Fault Detection and Localization Algorithm The diagnostic method used for the open-circuit faults occurrence in the inverter power switches is based on the algorithm proposed in [13]. This technique utilizes variables already used by the main control, avoiding the use of extra sensors and the subsequent increase of the system complexity and costs. Furthermore, since the diagnostic variables are normalized, it does not depend on the machine operating conditions. Additionally, beyond its high immunity against the issue of false alarms and its relatively fast detection time, it is not computationally demanding, making it suitable for integration into the main controller. Considering this, a simplified version of this algorithm is used, as shown in Fig. 1. in
¯ ¯ ¯is ¯
× inN ÷
»
u (t ) jinN j
Park’s Vector Modulus
Average Values
hjinN ji
+ −
en
Fault Detection and Localization
Faulty Phase
Fig. 1 – Block diagram of the fault diagnostic algorithm.
¯ ¯ The three motor phase currents in, where n = a; b; c , are divided by the Park’s Vector modulus ¯is ¯ which is defined as: ¯ ¯ q ¯is ¯ = i2 + i2q d
(1)
The Park’s Vector components id and iq are obtained from the three motor phase currents using the transformation: id =
r
2 1 1 ia ¡ p ib ¡ p ic 3 6 6
1 1 iq = p ib ¡ p ic 2 2
(2)
(3)
As a result of this operation, the normalized motor phase currents inN are obtained. The three diagnostic variables en are obtained from the errors of the normalized currents average absolute values hjinN ji:
(4)
en = » ¡ hjinN ji
where » is a constant value approximately equal to 0.5198. Comparing with the original proposed scheme, in this simplified version, the average values of the normalized motor phase currents, responsible for the identification of the faulty switch in the same inverter leg, are not calculated. This is justified by the fact that for this purpose, it is just required the information about the faulty inverter phase, which is directly given by the three diagnostic variables en. As a result, this makes the diagnostic method even more simpler and less computationally demanding.
Fault-Tolerant Converter Topology The considered fault-tolerant inverter is based on a typical six switch (IGBT) three-phase voltage source inverter topology, feeding a PMSM. Three additional TRIACs (TRa, TRb and TRc) are used with the objective of connecting each motor phase to the midpoint of the capacitor bank in the dc link, as shown in Fig. 2.
TRa
T3
T1
T5 ia ib
TRb
PMSM ic
TRc
T2
T4
T6
Fig. 2: Topology of the fault-tolerant power converter used.
After the diagnosis and isolation of the faulty inverter leg, the hardware reconfiguration is achieved by using the corresponding TRIAC that is triggered on.
Fault-Tolerant Control Algorithm With the aim to develop a fully integrated fault-tolerant drive, the control system must be able to accomplish four important steps: fault diagnosis, faulty leg isolation, hardware reconfiguration and post-fault software control (Fig. 3). Fault-Tolerant Control System Actual Speed Motor Currents Reference Speed
Main Control PMSM Vector Control
Fault Diagnostic Algorithm
Hardware & Software Reconfiguration
Fault Isolation
T1 . . . . . . . . T6
TRIAC Control
TRa . . TRc
Fig. 3 – Fault-tolerant control system block diagram.
Regarding the first stage, through the algorithm previously described, the control system is capable to detect an inverter power switch open-circuit fault. If this happens, information about the converter faulty phase will be also obtained, which is then used for the hardware and software reconfiguration. After the fault detection and the affected phase is identified, the control system will remove the gate command signals to the power switches of the inverter faulty leg in order to isolate it. Although it is not considered in this work, for the case of short-circuit faults, a more complex procedure must be followed in order to isolate the faulty leg from the corresponding motor phase [16].
Considering the fault-tolerant converter topology shown in Fig. 2, after the isolation of the inverter faulty leg, a hardware reconfiguration is performed by triggering the corresponding TRIAC in order to connect the isolated motor phase to the midpoint of the capacitor bank in the dc link. Finally, some software modifications must be implemented with the aim to adjust the control system to the post-fault operation. Considering that the inverter is supplied by a three-phase diode bridge rectifier, the dc link voltage will have practically the same value under normal and post-fault operating conditions. Therefore, and taking into account that comparing with the normal situation the voltage space vector applied to the machine under post-fault operating conditions decreases by 50%, the control system must limit the motor mechanical speed to one-half of its rated value. This action is mandatory in order to avoid the electromagnetic torque oscillation and to enable rated load torque operation. At last, and depending on the used PWM strategy, it can be also improved by adapting it to a fourswitch three-phase inverter topology. This can be particularly important when a space vector modulation (SV-PWM ) technique is used. In this work, since hysteresis current controllers are used, the motor phase currents are controlled directly by the hysteresis comparators, becoming unnecessary any additional changes at this level. Beyond this great advantage, this PWM technique does not need to take into account a possible voltage asymmetry at the dc link capacitors, which may be considered if a SV-PWM technique is used.
PMSM Dynamic Model Typical PMSM mathematical models found in the literature do not take iron losses into account. For this reason, in order to obtain a more accurate modeling, especially for the iron losses, a dedicated parameter has been considered aimed at accounting for the iron losses in the stator core, specifically the eddy current losses. These are modeled by a resistor Rc which is inserted in parallel with the magnetizing branch, so that the power losses depend on the air-gap flux linkage. Therefore, assuming that the saturation is neglected, the electromotive force is sinusoidal and a cageless rotor, the stator dq equations in the rotor reference frame are: dimd ¡ !Lq imq dt
(5)
dimq + !Ld imd + !ÃP M dt
(6)
vd = Rs id + Ld
vq = Rs iq + Lq
where vd and vq are the dq axes voltage components, Rs the stator winding resistance, id and iq the dq axes supply currents, Ld and Lq the dq axes inductances, imd and imq the dq axes magnetizing currents, ! the fundamental frequency and ÃP M the flux linkage due to the rotor magnets. The dq axes iron losses currents icd and icq can be calculated by: icd
icq
1 = Rc
1 = Rc
µ
µ ¶ dimd Ld ¡ wLq imq dt
dimq Lq + wLd imd + !ÃP M dt
(7) ¶
(8)
Finally, the PMSM electromagnetic torque Te can be obtained by: 3 Te = p [ÃP M imq + (Ld ¡ Lq ) imd imq ] 2
(9)
being p the machine pole pairs number. Although not considered in this work, hysteresis losses can also be taken into account. These losses are proportional to the machine phase currents frequency.
Therefore, in order to include them into the machine model, the iron losses resistance Rc is usually treated as a function of !.
Simulation Results The modeling and simulation of the PMSM drive system was carried out using the Matlab/Simulink environment, in association with the Power System Blockset software toolbox. Considering the experimentally used machine, the modeling takes into account the parameters of a 2.2 kW 1750 rpm PMSM, given in the Table I of the Appendix. A rotor field oriented control strategy with hysteresis current controllers was applied to the inverter in order to control the PMSM mechanical speed. For all the considered operating conditions, a load torque equivalent to 33% of the PMSM rated torque was considered. Fig. 4 presents the simulation results regarding the time-domain waveforms of the motor phase currents, mechanical speed and electromagnetic torque for an operating speed of 950 and 875 revolutions per minute, respectively. 10
10 ib
ic
5 0 −5 −10 0.12
0.14
0.16
0.18
0.2 0.22 Time (s)
0.24
0.26
0.28
−5
0.14
0.16
0.18
0.2 0.22 Time (s)
0.24
0.26
0.28
0.3
Hardware + software reconfiguration
Fault Post−fault Operation
Normal Operation
0.14
0.16
0.18
0.2 0.22 Time (s)
0.24
0.26
0.28
Hardware reconfiguration
800 700 600 0.12
0.3
15 10 5 0
0.14
0.16
0.18
0.2 0.22 Time (s)
0.24
0.26
0.28
0.3
Fault detection and isolation
900
Electromagnetic Torque (Nm)
Electromagnetic Torque (Nm)
0
Mechanical Speed (rpm)
Mechanical Speed (rpm)
Fault detection and isolation
800
−5 0.12
ic
1000
900
600 0.12
ib
5
−10 0.12
0.3
1000
700
ia
Motor Phase Currents (A)
Motor Phase Currents (A)
ia
Fault Post−fault Operation
Normal Operation
0.14
0.16
0.18
0.2 0.22 Time (s)
0.24
0.26
0.28
0.3
0.14
0.16
0.18
0.2 0.22 Time (s)
0.24
0.26
0.28
0.3
15 10 5 0 −5 0.12
(a) (b) Fig. 4: Simulation results regarding the time-domain waveforms of the motor phase currents, mechanical speed and electromagnetic torque for an operating speed of (a) 950 rpm and (b) 875 rpm.
Regarding the results shown in Fig. 4a, when the drive is operating under normal operating conditions, the PMSM mechanical speed is equal to 950 rpm, the motor phase currents present a sinusoidal waveform (apart from the high frequency noise due to the inverter supply) and the electromagnetic torque is constant. At the instant t=0.173 s, a single power switch open circuit fault in IGBT T1 is introduced. As a result, the positive alternation of phase a cannot be generated and the motor starts to be supplied by nonsinusoidal currents. These unbalanced supply conditions lead to the development of a pulsating electromagnetic torque which also makes the speed to decrease and oscillate. This inverter fault is detected and identified by the integrated fault diagnostic algorithm at the instant t=0.1974 s. Immediately, the fault-tolerant control system proceeds to the fault isolation by removing the IGBT gate command signals to the faulty inverter leg. Therefore, after this moment the PMSM is
supplied by just the two remaining inverter healthy phases. In this case, and since the phase a current is zero, the other two phases will have a similar waveform, shifted by 180º. These supplying conditions lead to the generation of an even more pulsating electrometric torque, leading to a greater decrease of the PMSM mechanical speed. Finally, 20 ms after, the control system proceeds to hardware and software reconfiguration by turning on the TRIAC TRa and limiting the operating speed to one-half of the machine rated value, that is 875 rpm. By doing this, it can be seen in Fig. 4a that the phase a current starts to flow again and the PMSM is supplied by a balanced sinusoidal current system. As a result, the PMSM electromagnetic torque becomes again constant and the mechanical speed increases until it reaches the new set-point of 875 rpm. This new reference speed for post-fault operating conditions is always imposed by the faulttolerant control system whenever the initial value is greater than half of the motor rated speed. The results presented in Fig. 4b are very similar to the ones shown in Fig. 4a and, therefore, the analysis previously done is also valid. The biggest difference is that under normal operating conditions, the reference speed is set to 875 rpm, which corresponds to exactly one-half of the PMSM rated speed. Consequently, the same mechanical speed is maintained under post-fault operating conditions and no software reconfiguration is necessary. In fact, if the initial reference speed is lower or equal to half of the motor rated speed, no software changes are performed by the fault-tolerant control system. It must be noticed that, for all the steps performed by the fault-tolerant control after the fault occurrence, a delay of 20 ms was imposed in order to show the drive behavior. However, in practice, for some steps (fault isolation and hardware/software reconfiguration), a delay of some microseconds may be sufficient, and thus, all the reconfiguration process starting from the fault occurrence, can be accomplished in less than one current period. Considering this, the time taken by the fault tolerant control to accomplish all the process will be strongly influenced by the performance of the fault diagnostic method.
Experimental Results The experimental setup basically comprises a PMSM coupled to a four-quadrant servomotor test system, a three-phase diode bridge rectifier, a Semikron SKiiP three-phase inverter, a dSPACE DS1103 digital controller and two precision digital power analysers (Fig. 5). The rated parameters of the used PMSM for the experimental tests are reported in the Appendix.
(a) (b) Fig. 5: Experimental setup: (a) detail of the PMSM coupled to the servo machine and (b) general view of the power and control stages.
A rotor field oriented control strategy employing hysteresis current controllers and the developed fault-tolerant control system were also implemented for the DS1103 digital controller board, using a sampling time of 25 µs. The rotor position is obtained by an incremental encoder with 1024 pulses per
revolution, being the PMSM mechanical speed obtained by filtering the derivative of the rotor position. The motor phase currents are measured using two LEM LA-55P current sensors. Inverter power switch open-circuit faults are controlled by the user using the dSPACE ControlDesk software. These are accomplished by removing the gate command signals of the required IGBTs. Fig. 6 presents the experimental results regarding the time-domain waveforms of the PMSM phase currents, mechanical speed and electromagnetic torque for a reference speed of 875 revolutions per minute and a load level equivalent to 33% of the motor rated torque. The electromagnetic torque is estimated by measuring the dq current components and using equation 9. 10
Motor Phase Currents (A)
ia
ib
ic
5 0 −5 −10 0.25
0.27
0.29
0.31
0.33 0.35 Time (s)
0.37
0.39
0.41
0.43
Mechanical Speed (rpm)
1000 900
Fault detection and isolation Hardware reconfiguration
800 700
Electromagnetic Torque (Nm)
600 0.25
Fault Post−fault Operation
Normal Operation
0.27
0.29
0.31
0.33 0.35 Time (s)
0.37
0.39
0.41
0.43
0.27
0.29
0.31
0.33 0.35 Time (s)
0.37
0.39
0.41
0.43
15 10 5 0 −5 0.25
Fig. 6: Experimental results regarding the time-domain waveforms of the motor phase currents, mechanical speed and electromagnetic torque for an operating speed of 875 rpm.
Comparing with the simulation results shown Fig. 4, it can be verified that the experimental ones are also very similar. At the instant t=0.2965 s, an open-circuit fault in IGBT T1 is introduced, leading to the generation of a pulsating electromagnetic torque and the subsequent decrease of the PMSM speed. The implemented fault diagnostic algorithm detects this abnormal behavior at t=0.3184 s, and the information about the faulty phase is used to immediately isolated it by removing the gate signals for T1 and T2. As a consequence, there is no current on phase a and the PMSM is supplied by just the two remaining healthy phases. In addition, the electromagnetic torque oscillation increases, which contributes even more to the decreasing of the motor mechanical speed. The last step of the all process is accomplished at the instant t=0.3384 s, when the hardware reconfiguration is performed by the fault-tolerant control system. Therefore, a turn on signal is commanded to TRIAC TRa, connecting in this way the motor phase a to the dc link midpoint. As a result, the PMSM is supplied again by a balanced three-phase current system and the electromagnetic torque pulsation is considerably reduced to a level equivalent to the one shown under normal operating conditions. Finally, the mechanical speed will increase until it reaches the previous set-point of 875 rpm. It must be noticed that, as this reference value corresponds to exactly half of the PMSM rated speed, no action at a software level is performed in order to limit the motor speed under post-fault operating conditions.
In a similar way to what was done for the simulation results, a delay of 20 ms between each reconfiguration process was also introduced in order to show the drive behavior. As previously mentioned, this delay time can be significantly reduced and the all process can be completed in less than one current fundamental period.
Conclusions A fault-tolerant PMSM drive integrating a real-time fault diagnostic method for inverter IGBT opencircuit faults has been presented in this paper. The key component of the proposed drive system is the developed fault-tolerant control that incorporates the main control routines regarding the PMSM vector control and the diagnosis and reconfiguration process algorithm. The fault diagnostic algorithm performs an important role since it is responsible to correctly diagnose the voltage source inverter condition, by detecting and localizing the faulty phase. Considering this, the implemented algorithm is suitable for this purpose since it does not require the use of extra voltage or current sensors, avoiding the subsequent increasing of the drive system cost and complexity. Furthermore, due to its simplicity, it can be easily integrated into the controller since the required computational power is minimum. Finally, it has also other advantages such as operating conditions independence, simple tuning and a relatively fast diagnosis. Indeed, an inverter fault can be detected and localized in a time interval as fast as 1/9 of the motor currents fundamental period. The reconfiguration procedure comprises the inverter faulty phase isolation, by removing the corresponding IGBTs gate command signals, and hardware/software modifications. Regarding the hardware changes, after the fault isolation and taking into account the considered fault-tolerant inverter topology, the control system turns on the TRIAC associated to the faulty phase, connecting the corresponding PMSM phase to the dc link midpoint. The software modifications, for this specific case, just occur when defined reference speed is greater than half of the motor rated speed. If this condition is verified, the fault tolerant control automatically limits the reference speed to half of the motor rated value. The presented simulation and experimental results allow to successfully verify the effectiveness of the proposed fault-tolerant PMSM drive. The implemented control algorithm is able to detect an abnormal inverter operation, acting rapidly and automatically in order maintain the drive operation, although in a degraded/limited mode. The proposed fault-tolerant control system can also be extended to induction motor drives and to other fault-tolerant converter topologies.
Appendix Table I: Parameters of the used PMSM Power Speed Voltage Current Number of pole pairs Armature resistance Iron losses resistance Magnet flux linkage d-axis inductance q-axis inductance Moment of inertia
P N V I p Rs Rc ÃP M Ld Lq J
2.2 kW 1750 rpm 316 V 5.3 A 5 1.72 Ω 700 Ω 0.244 Wb 20.5 mH 20.5 mH 0.007 Kg.m2
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