PSI - Issue 31

Sanjin Braut et al. / Procedia Structural Integrity 31 (2021) 45–50 Sanjin Braut et al. / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 1. (a) Equivalent stresses and temperatures in the magnet pocket upper fillet; (b) Equivalent stress of rotor structure in time instant 1550 s

3. Stress-based fatigue analysis with the mean stress effect The fatigue life of PMSM rotor is estimated according to stress-based approach and variable amplitude loading. As equivalent stresses, shown in Fig. 1, have pronounced mean stresses its effect is additionally evaluated. At the beginning, variable equivalent stress amplitudes of equivalent stresses for critical rotor position were identified with rainflow counting technique implemented in Python, Pastorcic et al. (2019). Stress amplitudes were then corrected with three mean stress correction models, namely, modified Goodman, Morrow (Vučković e al. 2018) and Smith Watson -Topper (SWT), according to Lee et al. (2012):

S

(Modified Goodman)

(2)

S

S

=

tU −

ac

a

S S

tU m

f S S ′ ′ − S

(Morrow)

(3)

S

S

=

ac

a

f

m

(

)

S S S S = + (Smith-Watson -Topper)

(4)

ac

a

m a

The total damage of the rotor is finally estimated according to linear Palmgren-Miner rule and modified S–N curve is a way to include reliability and rotor surface condition. According to Lee et al. (2012), Lee et al. (2005) and Haibach (2003), baseline S–N curve is modified in such a way that fatigue strength coefficient S f ´ is reduced by reliability factor C R . The fatigue limit S e is further reduced by the product of reliability factor C R and roughness correction factor C σ R . In this analysis, a reliability of 90% was assumed, which corresponds to a factor C R of 0.897 and roughness correction factor under normal stresses C σ R is calculated to the value of 0.943 which corresponds to the average surface roughness of R Z = 5 μ m (Abrasive water jet cutting of electrical steel lamination), Bayraktar et al. (2016). Finally, a new modified parameters of Basquin equation (1) were obtained, S fm ´ = 603.92 MPa, b m = – 0.09984, Mlikota et al. (2018), Baragetti et al. (2019), Babić et al. (2020). 4. Results and discussion The transient thermo-mechanical 3D analysis of reduced rotor model found magnet pocket upper filet as the critical stress region. Maximum equivalent stresses (von Mises) in the critical region were calculated for 3 laps i.e. 1550 s, Fig. 1. Rainflow counting technique found 159 cycles, which represents one block, with the corresponding stress ranges and the mean stresses. Stress amplitudes were then corrected with three different mean stress correction models, namely, modified Goodman, Morrow and SWT and compared to modified S–N curve, considering the contribution of