# A Survey on Mission-Profile-Based Stress Testing of Electric Motor Drives
## 2 Influential Factors on the Aging and Reliability of EMD Components
### Electromagnetic Stress
#### 2.1.3 Current-Induced Stress  
Electric current has a significant impact on the aging and reliability of drive components. This impact is reflected in the heating of the drive components, leading to accelerated insulation aging, eddy currents, inrush currents, higher current harmonics, the occurrence of mutual inductance voltages, and power dissipation [28]. According to [29], the circulating currents in the machine windings increase resistive losses and create local temperature differences, which can accelerate the degradation process of the insulation.  
In most cases, current-induced stress is evaluated using Equation (2) [30]:  
\[E_{dis}=\int_{0}^{T}iu\,dt \tag{2}\]  
where \(E_{dis}\) is the energy dissipated, \(i\) is the instantaneous current, and \(u\) represents the instantaneous value of the voltage in each component. Based on current and voltage measurements over a certain time interval, the dissipated energy, which converts to heat and contributes to thermal degradation, is evaluated by integrating their product.  
In the case of short-term current surges above the nominal value, significant stresses occur due to [PERSON]'s forces, which are electromagnetic forces exerted on current-carrying conductors in a magnetic field, within the electric motor, which can result in damage to the machine windings [31]. In practice, to protect the motor from large current surges, overcurrent protection is configured such that overload curves are set to protect the motor according to its thermal limit curves. Modern microprocessor-based motor protection relays incorporate thermal models that approximate the heating effects on the stator and rotor components. However, in most cases, these thermal models rely on the motor's thermal damage limit curves, which manufacturers typically specify only at the nominal frequency of 50 or 60 Hz, representing one of their limitations [32]. In cases where thermal limits are available at different frequencies, an alternative approach could involve implementing adaptive characteristics to account for frequency-dependent thermal behavior, ensuring accurate overload protection and preventing unnecessary trips.  
Eddy currents in the magnetic core of an electric motor induce thermal stress that decreases efficiency and increases the probability of failure. The power dissipated due to the effect of eddy currents is proportional to the square of the frequency of the alternating magnetic field that induces voltage in the conductors and magnetic core [33]. However, the influence of PWM strategies and their average switching frequency on rotor eddy current loss is investigated in [34]. According to the authors, a high switching frequency of the voltage source inverter (VSI) can reduce high-frequency harmonic content in the voltage waveform, thus lowering rotor eddy current loss.  
High-order harmonic currents, transient currents, and pulsating currents create electromagnetic stress in voltage source inverters (VSIs), resulting in a rise in \(R_{DS(ON)}\) (the on-state resistance of metal-oxide-semiconductor field-effect transistors (MOSFETs), which leads to increased power losses and heat generation in the power device) and a reduction in overall efficiency [35].  
The occurrence of current spikes in the windings of electric machines driven by PWM inverters increases as the switching frequency of semiconductor devices rises. As stated in [36], these spikes are caused by PDs due to uneven voltage distribution across the windings.