This paper describes the characteristics of the switched reluctance motor (SRM) used as a generator. In this mode of operation the SRM is unique in that it does not employ permanent magnets or a field winding on its rotor. Thus the SRM generator does not have the inherent problem of generating into a shorted winding like the permanent magnet machine (you cannot turn off the excitation), and the rotor structure is inherently simpler and more reliable than the wound field machine. Because the SRM generator does not use permanent magnets or a field winding, the nature of its excitation during generating is of particular interest. In fact, the SRM's lack of direct field excitation makes the SRM generator unstable when operating open loop in the square-wave mode and connected in the conventional manner. In this case, the generator's output voltage increases exponentially for loads less than a critical value and goes to zero for loads greater than this same critical value. This result has important implications for operation of the SRM generator during load faults. An alternative connection of the SRM generator is described that minimizes these effects.
Like all electrical machines, the switched reluctance generator is a dual of the machine as a motor. This duality not only exists in the electromagnetic performance of the machine, but it also carries over into its control. From an electromagnetic analysis of the duality of the SRM during motoring and generating, new control strategies are obtained for the machine operating as a generator that are derived from control strategies previously described in the SRM literature for motoring.
An experimental 33-kW SRM generating system is described and experimental results obtained with this system are presented. In particular, the experimental results are used to demonstrate the duality of the SRM as generator and motor. The experimental results also demonstrate the consequences of the switched reluctance generator having no permanent magnets or a field winding. The experimental results also provide the data needed to understand the open-loop instability of the SRM generator system.
