# A Survey on Mission-Profile-Based Stress Testing of Electric Motor Drives
## 4 Reliability and Lifespan Improvement Methods for EMD Components
### Methods for Mitigating Stress Factors  
In recent years, the problem of restarting free-running induction motor drives (IMDs) has gained more attention. A significant amount of stress is imposed on EMD components due to the occurrence of inrush currents. In [105], a detailed review of restarting methods is given. According to the authors, restarting of IMD can be implemented by frequency search methods, back electro-motive force (EMF), and DC current injection methods. A frequency search method based on stator current has been implemented in [106]. The proposed method uses a simple algorithm based on stator current measurement to estimate the motor rotor speed during the gradual reduction of the input frequency from the rated to zero. Compared to previous solutions, the method does not require any controllers and is not dependent on motor parameters, making it simple to implement and highly reliable. However, the method relies on the control circuit staying powered when the main power supply is lost and being able to detect when power is lost and restored. In real situations, this might not always be the case, which could limit how practical the method is in some applications. A more complex approach is implemented in [107], based on back-EMF. The authors introduce a current controller that mitigates current-induced stress during the restart of the IMD by applying compensation voltage. This method requires a controller, making it more complex than the previous one, but offers a faster response since it does not need the frequency sweep. The authors of [108] reduce the impact of inrush currents by implementing a full-order observer. The proposed strategy is based on the DC current injection method, where the speed is estimated from the AC components of the rotor flux. The experimental results show a significant reduction in the time required for initial speed estimation and the restoration of normal operation compared to previous strategies. EMD can also be protected through predictive algorithms that anticipate potentially damaging events and take preventive action. These strategies aim to improve system reliability and prevent permanent failures. In [109], a survivability-based protection algorithm is developed and implemented to enhance the reliability and fault tolerance of IMD systems. The IMD operation depends on the calculated survivability index, which indicates the ability of the system to withstand dynamic events, such as step changes in speed and torque, and drops in DC-link voltage. The method introduces a proactive protection strategy, improving system response to dynamic disturbances. Its reliance on accurate system modeling and real-time computation could limit its practical use, and the performance under electrical faults is yet to be evaluated. A similar solution for PMSM drives is presented in [110]. The authors [111] define a fault index to determine the number of broken bars in the rotor of an induction motor. The intensity of vibrations associated with faults grows proportionally to the square of the number of broken bars. The authors also discuss issues related to tangential stress, radial displacement, and the skewing effect, which require further investigation. The reliability and lifespan of EMD components can also be improved by reducing the system's energy losses while increasing efficiency. However, most existing methods rely on simplified loss models for the sake of simplicity, often neglecting certain types of losses, which reveals a notable scientific gap in accurately capturing the full loss profile. In [112], the speed profile of the elevator system is optimized to minimize losses in the IMD. The authors considered only the induction motor losses, which were optimized using [PERSON]'s minimum principle. A limitation of the proposed method is that it neglects power losses in the frequency converter, which can increase even when the motor is optimally controlled, leading to a potential mismatch in overall system efficiency.