The most common NEMA Design Classes of AC induction motors have speed-torque characteristics that can give rise to unstable self-excitation of torsional vibration in rotational systems during start-up. A torsional vibration computational model for start-up transients has been developed as a design tool for induction motor applications. Torsional instability can occur at speeds in the positive sloping segment of the motor’s speed-torque curve and is particularly acute when the mass moment of inertia of the load device is more than two times the mass moment of inertia of the motor rotor. The computational model is compared with an exact solution method and with a laboratory test of a motor-driven inertial load. Applications of the computational model to electric submersible pump (ESP) design cases are discussed.

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