Abstract
It has been shown in previous papers that the torque-speed curves of the two phase servomotor are widely varying with time at non-zero speed even in steady state, with a frequency of twice line frequency. So, the inclination of the curves can be positive at some periodic time intervals, even for a stable servomotor whose time-averaged inclination is negative.
Then, the system in which the rotor is fixed through some finite stiffness can be approximated to “2nd order time-varying damping system”. From the analysis on stability of the system, it is clarified that the parametric resonance with line frequency occurs even in the case of sinusoidal excitation for stable servomotor, when system frequency is in the vicinity of the line frequency.
Therefore, in force servomechanisms, the system frequency must be chosen higher than twice line frequency in order to prevent the parametric resonance.
