Synchronous Motor Speed Comparison (1200 vs. 300 rpm)

[Laplacian]

Problem:

We have a 13.2kV, 14000HP, 6 pole (1200rpm, TEWAC synchronous motor driving a two-stage reduction gearbox. The load is a large twin screw extruder turning at 300rpm. It is suspected that torque oscillations are reducing the reliability of several bearings in the complex gearbox as the motor accelerates to synchronous speed. The noise generated and vibrations felt through the ground seem the most extreme just before the motor reaches synchronism. Of course the gearbox manufacturer says their equipment can handle this.

Question:

In a future project, we are looking to eliminate the complexity of the gearbox by using a 24 pole (300rpm) synchronous motor. I am already communicating to some manufacturers regarding the cost difference between the two speeds. I would like to gather input from others with experience on large synchronous motors.
Is torque oscillation phenomena less destructive in a lower speed machine?
Assuming the machine will be operating with a leading PF, are there efficiency differences between the 300 and 1200 rpm machines?
Besides the slower motor being physically larger, are there any differences in maintenance requirements between the two?
Is there a significant difference in locked rotor current during start?

Any additional information would be appreciated.

(I know the 300 rpm machine would be gargantuan; a sister plant has a 34 pole, 11000HP machine in a different application)

 

[jbartos]

Comment: Efficiency will be somewhat lower. Also, the no-load current will be higher. Vibration will be marginally higher. However, it all should be a matter of good trade off between the 24-pole machine without the gearbox and 6-pole machine with the gearbox. The motor manufacturer may be asked to provide the 24 pole motor characteristics and parameters for your evaluations.