ELECTROMECHANICAL MOTION DEVICES
Rotating Magnetic Field Based Analysis
3^rd Edition

A salient-pole three-phase synchronous machine is shown below. In addition to the symmetrical three-phase stator windings ($as$, $bs$, and $cs$) the rotor is equipped with a field winding ($fd$) and damper windings $(kq$ and $kd).$

There are three types of torque involved; the main torque due to the interaction of mmf$_s$ and the field mmf$_r$ due to $i_{fd}$, reluctance torque due to the salient nature of the rotor, and the damping torque which is an induction motor torque when $\omega_r$ is different from $\omega_e$. Although we are aware of the first two torques we are not aware of the induction motor torque. It was found that the synchronous machine without damper windings oscillated after a disturbance and this was minimized with the damper windings by the torque developed due to currents induced in these short-circuited windings when $\omega_r$ was not equal to $\omega_e$. This damping torque was such that it returned the rotor to $\omega_e$.

We ran into a synchronous machine in Chapter 6. The permanent-magnet ac machine, when operated as a brushless dc machine, operates from a variable frequency source. The frequency of the applied voltage is controlled so that it is equal to the rotor speed. The synchronous machine in this chapter and in this animation is connected to a power system. The power system is a constant frequency system with a fixed voltage magnitude and phase.

The synchronous machine supplies the bulk of the power in the United States and in order for the synchronous machine to supply power to the grid it must run in synchronism with the rest of synchronous machines connected to it. Maintaining synchronism is a huge problem for power system engineers. Ensuring system stability is a career in itself.

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