When the statorand rotor poles are aligned, the current is turned off and the rotor keeps moving dueto stored kinetic energy. As the stator poles become unaligned, the stator windingagain is energized, producing an electromagnetic torque. A problem with singlephaseSRM operation arises only when the stator and rotor poles are in alignmentat standstill (i.e., at the time of starting) or the rotor is at a position where the torqueproduced may be lower than that of the load torque at starting. This problem isovercome by having a permanent magnet on the stator to pull the rotor away fromalignment, or to the appropriate position, to enable the generation of maximumelectromagnetic torque

The earlier chapters developed a procedure for designing an SRM, selecting a suitableconverter topology to drive it, and designing a current controller, torque-smoothingcontroller, and speed controller. The controllers integrate the machine, converter, andload to deliver a specified performance for the entire motor drive system. A dynamicsimulation of the drive system enables verification of the analytical designs and abilityof the motor drive system to match the load torque over its entire speed range both inits steady state and during transients. With such a verification, time and cost of productdevelopment are minimized by avoiding a trial-and-error approach to prototype constructionthat may lead to repetitive testing and redesign until specifications are met.

It is assumed that thepower devices in the converter are ideal in this illustration, hence their voltage dropsand switching times are neglected.