## 6 Discussion and Conclusions  
This paper presented the analysis of the dynamic performance of a squirrel-cage induction machine integrated to a flywheel storage with a developed model in MATLAB/Simulink. The structure of the control system as well as the comprehensive loss model consisted of the electrical, mechanical and power electronics losses were also discussed and embedded in the model. A detailed simulation model of a 100 kW IM drive was presented and analysed in different states of charging, discharging and standby modes. The presented results show the analysis of an induction machine driven flywheel storage system during different modes of operation including constant and variable torque conditions. The dynamic performance of the system was analysed for the two general cases of constant power and constant torque operations and the results of the analysis were presented. The SCIM-FESS was operated within a speed range of 10-20 krpm at a shaft torque of 90 N-m storing 5 kWh of energy in 2.17 min. The presented system with the proposed control system and parameters of the machine, can store more energy at higher speed (more than 20 krpm) at the expense of higher windage losses. When the torque on the flywheel is zero during standby mode, an energy loss of 215 Wh occurs in a period of approximately 18 min where the flywheel speed reduces to 19,480 rpm due to the windage effect and frictional component of the bearing torque. The standby losses of the system can be reduced by maintaining a low vacuum and using a better technology for the bearing system such as magnetic bearing but this will increase the cost of the system. Aerodynamic drag of the rotor can be reduced by housing the rotor in low a vacuum chamber, and can reduce the losses considerably as shown in Figure 13. Copper and iron losses in the stator of IM are negligible during standby mode as no currents flow in the stator of the IM (Figure 15) and, therefore, no magnetic field exists due to use of electromagnets in the stator windings. This is a significant advantage of IM over PMSM as it reduces total losses of the system providing better efficiency. Stable operation of the system implies that the use of the proposed control system in IM drive provides better performance and quick reversal electromagnetic torque during the transition from generating to motoring mode and vice versa. Operation scenarios of FESS presented in Sections 5.2 and 5.3 are the benchmarks for flywheel applications in transport and in an electric grid under different load conditions. The presented results show a realistic system with drag losses working stably in different conditions which justifies the proper working of control systems proposed in the paper. Also, the SCIM-FESS can be explored and analysed for use with a renewable energy source and a diesel generator set which can be used for energy management and ride through applications. This can improve the efficiency of the intermittent renewable energy source and save fuel costs of the associated diesel generator set. The results of the analysis show that the SCIM-FESS has satisfactory characteristics in energy regulation and dynamic response during load torque variations. SCIM drive system makes use of the vector control technique to control power electronics converters in order to follow the reference value commands of torque, speed and voltage. The machine side field-oriented control system providesquick reversal of magnetic field at the time when torque is reversed and the IM switches from generator to motor mode in less than 5 ms (Figure 9b). This can be a benefit in applications requiring fast response and involving critical loads such as in data centers. The vector controller of grid converter maintains a constant dc voltage during different load scenarios, which helps to improve the stability at the point of connection when the flywheel is integrated with renewable sources in an electric grid. Further research on this model will focus on making use of a SCIM-FESS in electric grid incorporating wind energy system.