Operation of Large two Pole Sleeve Bearing Induction Motors with well Damped Resonant Speeds

Rotor designs for two pole induction motors, especially those of larger size up to 5000 HP, exist in either "truly rigid" (operation below resonant speed) or "flexible" (operation above the first resonant speed) designs. API 541 5^th edition (4.4.6.1.2) allows for a third mode of operation given specific criteria are met for a well damped resonant speed which can be of great benefit for adjustable speed drive applications where a speed lockout range is not acceptable. Test data and analytical models are presented and compared to differentiate between a "flexible" rotor design and a rotor design that operates with a well damped resonant speed.