Sensorless Control of a Fault-tolerant Multi-level PMSM Drive

This paper presents a new technique to track the saliency position in a permanent magnet synchronous motor (PMSM) post a single phase open-circuit fault. The PMSM is driven by a fault-tolerant multi-level inverter that is utilized to implement a fault-tolerant control strategy to minimize system performance degradation post the fault.The fault-tolerant multi-level inverter is consisting of a number of insulated-gate bipolar transistors (IGBTs). The dynamic current reponses of the PMSM motor due to the switching actions of these IGBTs are used extract the saliency position. This process is not introducing any modification to the operation of the fault-tolerant multil-level inverter as it uses only the fundamental pulse width modulation (PWM) waveform. Moreover,it considers the modifications introduced to the PMSM motor and the multi-level inverter post the fault.Simulation results are provided to verify the effectiveness of the proposed strategy of saliency tracking of a PMSM motor driven by a fault-tolerant four-leg multi-level inverter over a wide range of speeds in the case of a single-phase open circuit fault.


Introduction
Sensorless control of motor drives has been widely researched for systems employing standard two-level inverters [1][2][3][4][5][6]. These techniques introduce a significant additional current distortion which causes audible noise, torque pulsations and increases the system losses. In the other hand, a Multi-level inverter can achieve a higher voltage and power capability with conventional switching devices compared to two-level inverter and is now used for high power drives [7][8][9]. The particular structure of some of these converters offers significant potential for improving sensorless control of motors, as they employ H-bridge circuits with a relatively low DC link voltage. [10][11][12] are introducing different techniques to achieve sensorless control of multilevel inverter drives in healthy mode i.e no open circuit fault. Under faulty conditions, a number of fault-tolerant strategies to control two-level motor drives [13][14][15][16][17][18] and multi-level motor drives [19][20][21][22] have been used to enhance system operation under open circuit phase faults in sensor mode. [23][24] introduced a four-leg two-level (PMSM) drive to track the saturation saliency in the case of single-phase open circuit faults. This paper is introducing a new method to track the saturation saliency in a surface mounted permanent magnet motor in case of an open circuit fault. This motor is driven by a four-leg multi-level inverter. The objective is to maintain continuous system operation with a satisfactory performance to meet the safety procedure for the whole system and increase the reliability of the system.

Research Method 2.1. Fault Tolerant Multi-level Four-leg Converter Drive Topology
The proposed fault-tolerant multi-level drive topology is shown in Figure 1. A new Leg (fourth leg) is leg is added to the conventional multi-level inverter. The redundant fourth leg is permanently connected the motor neutral point. Under healthy operating conditions, the two IGBTs in this fourth leg will be inactivated making it redundant (not used). This will make the fault-tolerant multi-level inverter running as conventional three-leg mutlti-level as shown in Figure 2. When a single phase open-circuit fault is introduced to one phase of the PMSM motor, the switches on the faulty phase are disabled and the IGBTs the fourth leg are immediately activated in order to control the voltage at the neutral point of the motor.

Healthy Operation of the Multi-level Inverter
The seven-level cascaded H-bridge inverter cangenerate the voltage 3V, 2V, V, 0, -V, -2V and -3V from eah leg as shown in Figure 2. According to the switching state of each cell in the multilevel converter, it is possible to generate 2 7 switching states (vector in space). These vectors can be transformed into αβ coordinates (60 0 coordinate system) according to the following (1), (2) as shown in Figure 3: (-2,-1) (-1,-1) (6,-1) (1) where Vα and Vβ are the coordinates of the vector V in the 60 0 coordinate system, V and θ are the amplitude and phase angle of the reference vector respectively. The whole modulation technique that is used in this paper when the motor is running under healthy operating condition isillustrated in [8].

Open Phase Fault Operation
To maintain the system performance after a single phase open-circuit fault is introduced to one phase of the motor, the the rotating magnetomotive force should be maintained post the fault [18][19][20][21][22][23][24].To do so, many modification should be introduced to the operation of the PMSM drive system.Firstly, the phase where the open circuit fault is introduced to should be isolated by disabling the IGBTs in that leg. This is acieved by settingthe reference voltageof that phasezero. Secondly the IGBTs in the Fourth leg should be activated immediately to make the zero sequence current to circulate through it. And Finally, the two remaing motor currents post the fault shoud be increased by a √3 as well as phase shifted 30 degrees away from the faulted phase compared to the currents generated under normal operation, as given in (3).
These steps are illustrated in the vecture control structure shown in Figure 4. It is assumed that an open-circuit fault is introduced to phase'a' of the PMSM motor so that the current in phase 'a' drops to zero and the referenc voltage of phase 'a' (Va ref) is set to zero as well.Whereas the motor neutral current, which is the sum of the two remaining output currents (Ib and Ic), can circulate through the fourth phase of the multi-level inverter.
The simulation of the vector control structure shown in Figure 4 was carried out using SABER. Figure 5 shows the simulation results of a four-leg multi-level inverter PMSM drive system under healthy and faulted conditions. The motor was driving a 30 Nm load torque at 300rpm speed and running under healthy operating condition. Then a successive speed steps are applied to the system till the speed reaches 1500 rpm. It is clear that the controller could regulate the motor speed to follow the reference speed properly. The controlled currents id and iq are stable at the reference levels while the motor currents are balanced 3-phase sinusoidal. At t=1 s , an open circuit fault is introduced to phase 'a'. At this time the switch of the fourth leg is activated, which means that the neutral is connected to the supply. Then different reference speed commands are applied to the system. The simulation results show that ripple in the torque is almost the same as under normal operating conditions. The currents id and iq are now controlled properly.

Tracking the Saturation Saliency of PMSM under Healthy Condition
The stator leakage inductances of the induction motor are modulated by anisotropy either from the rotor slotting or from the saturation of the main flux. The modulation can be expressed by the following (4) to (6).
where 0 is the average inductance and ∆ is the variation of leakage inductance due to the rotor anisotropy ( =2for saturation anisotropy) This modulation of the stator leakage inductances will be reflected in the transient response of the motor line current to the test vector imposed by the multi-level inverter.So by using the fundamental PWM wave form and by measuring the transient current response to the active vectors it is possible to detect the inductance variation and track the rotor position for three-leg multilevel inverter. After obtaning the scalar quantities pa, pb and pc then the position of the saliency can be constructed as shown in (7).
The algorithm to track the saliency in 3-leg multi-level inverter that is proposed in [9] is used to track the saliency for four-leg multilevel inverter and the results is shown in Figure 6. Under the healthy operation, the switches in the fourth leg will not be activated and so the algorithm proposed in [9] could track the saliency from t=0.1 s till t=1 s at different speeds. In case of an open phase fault (t=1s to 2.2s), the algorithm is no longer able to track the saliency as shown in Figure 6. This because the measurement of the current response (di/dt) associated with the faulty phase will become zeros as ia=0. Which means that these position scalars will be zero as seen in Figure 6.
the following equations are obtained using Figure 8 finally when V0 is applied as shown in Fig 8 (c), the following equations hold true: Assuming that the voltage drop across the stator resistances are small and can be neglected and the back emf can be cancelled if the time separation between the vectors is small. Subtracting (12) from (10) and (13) from (9) respectively yields: Finally: can't be obtained from measuring the current response in phase a as it is zero in the case of an open circuit phase fault. But it can be deduced from and according to the following (18): By doing the same procedures for other sectors, Tables 1 to 6 can be constructed to track the saliency in case of phase 'a' open circuit. Figure 9 shows the results of the simulation after modifying the tracking saliency algorithm as introduced in Tables 1 to 6. The figure shows that the new algorithm can track the saliency in the case of an open-circuit fault is introduced to phase 'a' fault with the same quality as in healthy conditions.       Figure 9. The algorithm to track the saliency in of multilevel inverter using the fundamental PWM algorithm given in [12] for helathy mode and the algorithm presented above for unhealthy condition

Results and Analysis 3.1. The Sensorless Vector Control Structure
The sensorless speed control for PMSM motor driven by a fault-tolerant multi-level inverter has been implemented in simulation in the SABER modeling environment. The saturation saliency postion signal Pαβ are used as the input to a mechanical observer [25] to obtain a filtered signal for the estimated speed ω^ and a cleaned position θ^. Note that the simulation includes a minimum pulse width of 10 u s when di/dt measurements are made, similar to the experimental results of [6]. This estimated speed ω^ and position θ^ are used to obtain a fully sensorless speed control as shown in Figure 10.  Figure 11 shows the results of a fully sensorless speed control of a PMSM motor driven b afault-tolerant muli-levelinverter at loaded condition using the algorithm proposed in [12] for the healthy case and the method proposed above in the case of phase 'a' open circuit fault.The results of this test illustrates the capability of the PMSM motor drive system to work at very low and zero speeds in helathy operating condition and when a single phase open-circuit fault is introduced to the operation of the motor. The motor was working in sensorless healthy mode at speed 30 rpm, and then at time t=7.5 s a speed step change from 30 rpm to 0 rpm (until t=9 s) is applied to the system. Figure 11 shows that the motor responded to the speed step with a good transient and steady state response. At time t=9 s, an open circuit fault in phase a was introduced to the system, and this can be noticed from In #0 and the current waveforms ( Ia=0, Ib, and Ic are multiplied by√3and and shifted by 60 degrees). Figure 11 shows that performance of the whole system under faulty conditions is in the same level of performance in healthy mode. Moreover, the motor responded to a speed step from 0 rpm to 30 rpm at t=10.5 s with acceptable transient and steady state performance.

Conclusion
This paper has outlined a new scheme for tracking the saliency of a motor fed by a fourleg multi-level inverter in the case of a single phase open circuit fault through measuring the dynamic current response of the motor line currents due to the IGBT switching actions. The proposed method includes software modification to the method proposed in [12] to track the saliency of the motor under healthy conditions to make it applicable in the cases of open circuit phase condition. The new strategy can be used to track the saturation saliency in PM motors (2 fe) and the rotor slotting saliency in IMs (14*fr) similar to the method used in a healthy motor drive and the only difference between the PM and IM will be the tracked harmonic number. The results have shown the effectiveness of the new method in increasing the safety measures in critical systems that need continuous operation. The drawbacks of this method are increasing the total harmonic distortion of the motor's current, especially at a very low speed, due to the minimum pulse width in addition to the need for 3 di/dt sensors.