We have seen that the dynamical stability of an electromechanical system is determined by the damping and synchronising torque coefficients and the inertia constants. In a mechanical system damping and spring constants can be easily visualised—damping, for instance arises due to friction. In an electrical system, mechanical friction constitutes a small part of the total damping, the main damping torque being of electrical origin. In trying to understand the electrically generated damping, we can tell intuitively that this is caused by power dissipation due to copper loss. In a machine, during steady-state operation, copper loss takes place continually and largely accounts for the power difference between input and output. If, however, the rotor begins to oscillate about its steady-state angular velocity, oscillating currents induced as a result, generate additional copper loss. For instance, if an oscillating current ∆I sin (ωot + α) in a winding is superimposed upon a 50 Hz steady-state current 1 sin ωt, where ω0 = hω, then the additional average copper loss iswhere the integration period is the common repetition time for the two oscillatory currents. This additional copper loss appears to be the only dissipation (neglecting mechanical damping) which may suppress the rotor oscillation and bring it back to its normal uniform angular velocity. This argument, however, cannot explain why the rotor oscillations may sometimes build up, indicating the presence of negative damping—apparently produced by copper loss which is always positive. We shall see presently that the physical nature of damping is more complicated than that.
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© D. P. Sen Gupta and J. W. Lynn 1980