A Comparative Study of Three Starting Strategies for an Aero Flywheel Motor Using the Modified DTC Method

An aero flywheel motor (AFM) is an integration of a starter, generator, and inertial flywheel on the traditional aero piston-engine (APE). When performing as a starter, the AFM is responsible for dragging the APE to a high speed (600-800 rpm) in a short time (0.2-0.4 s). In order to obtain better performance, two modifications are applied to the direct torque control (DTC) method. First, the switching table is modified by using only the positive voltage vectors. Second, a new low-pass filter (LPF) flux estimator is proposed to eliminate the initial flux error using the pulse width modulated (PWM) flux orientation method and the resistance estimation error by using real-time varying (RTV) equivalent resistance. Three DTC-based starting strategies are developed for the AFM, namely, the constant acceleration strategy (CAS), the constant torque strategy (CTS), and the modified constant current strategy (MCCS). The modified and traditional DTC methods are compared to the three starting strategies. The results show that the proposed switching table significantly reduces the switching frequency, and the proposed LPF flux estimator improves the convergence of the DTC method; in addition, the robustness against the resistance error is improved. The performances of the three starting strategies based on the modified DTC method are also compared. The results show that the MCCS significantly reduces the peak value of the battery current, thus reducing the capacity, weight, and size of the battery.