## 5 Results and Analysis
### SCIM-FESS Losses  
The mechanical losses of the FESS are calculated based on the derived equations and the methods described in Section 3. The stray losses (eddy current and core losses) as well as the bearing and windage losses of the flywheel system as a function of the rotational speed is shown in Figure 13. Since the bearing loss is a function of the speed and the load on the bearing, and the windage loss is dependent on the speed and pressure of the flywheel rotor containment, these losses are lower in comparison to the stray losses (nearly 1.8 kW at 20,000) and are not related to the power rating of the motor/generator. For the maximum rated speed of 20,000 rpm, the bearing friction loss is approximated as 110 W and the windage loss at the atmosphere pressure level is about 90 W, giving a total of 200 W for combined windage and bearing losses. However, the system losses need to be kept at a minimum and particularly the standby losses, which is directly affected by the aerodynamic and bearing losses. Therefore, if the pressure level is kept below 10 Pa and the speed is maintained at 20,000 rpm, the windage losses will be 60% reduced as compared to the case of atmospheric pressure (Figure 13c). Figure 11: (**a**) % SoC of the flywheel at different torque; (**b**) Energy stored in the flywheel at different times. Figure 12: DC-link voltage during different states of operation and variable torque values. Stator and rotor specific core losses of SCIM-FESS are shown in Figure 14. The core losses occur in largest fraction in the stator of the IM because at low slip, the IM operates at nearly synchronous speed and the relative speed of magnetic fields over the rotor surface are slow, therefore, rotor core losses are very small compared to stator core losses [79]. As shown in Figure 14a, the specific core losses increase exponentially with increase in speed and magnetic flux density. The stator specific core losses are not significant for values of magnetic flux density up to 1 Tesla and 3 krpm speed. However, the specific core losses in stator of IM increase significantly above 3 krpms and even at 0.648 Tesla, the power losses due to hysteresis and eddy currents in stator core of IM operating at 20 krpm is 16.48 W/kg. Rotor specific core losses of IM are shown in Figure 14b and it can be seen that rotor specific core losses increases exponentially with increase in maximum magnetic flux density. However, even at a high speed of 20 krpm, the rotor specific core losses are not significant. The copper loss in the IM is directly proportional to square of the current and is a function of current and resistance. Copper losses in the stator and rotor of the IM are measured by simulating the copper loss models (Equations (16) and (17)) in MATLAB/Simulink software. The SCIM-FESS dynamic performance was analysed at different torque magnitudes in order to demonstrate copper losses at different values of three phase currents. The system was operated with initial positive torque of 50 N\(\cdot\)m  
Figure 13: Different mechanical losses of SCIM-FESS: (**a**) Bearing losses due to load and speed torque; (**b**) Stray losses due to magnetic flux leakages; (**c**) Windage losses at low and high pressures;(**d**) Total mechanical losses. for 50 s in charging mode and then the torque was increased to its maximum rated value of 90 N-m for 30 s. When the flywheel was at standby mode (\(t=80\) s), there were no currents flowing through the rotor and stator windings of the IM as shown in Figure 15a,b. At \(t=100\) s, the IM operated as a generator and flywheel started discharging for 20 s until switching back to standby mode at \(t=120\) s.  
Figure 14: Specific core losses of SCIM-FESS: (**a**) Stator core losses; (**b**) Rotor core losses. Figure 15: (**a**) Three phase rotor currents; (**b**) Three phase stator currents; (**c**) Torque commands; (**d**) Copper losses in rotor and stator of IM. The rotor and stator copper losses vary with currents as torque is varied on the shaft of the IM (Figure 15c).