VFD AC Motor Issue

ABB type AC motor: QABP180L6A, 380/660V can operate between 5-50Hz. But according to ABB, during motor start-up must have the frequency set between 30-50Hz, and once the motor started, the frequency can be set as low to 5Hz until 50Hz.

I have two questions here:
1. Why must the motor start between 30Hz to 50Hz? My understanding is that below 30Hz it will be difficult for the motor to start.

2. What will happen if a running motor set at 10Hz for long period?

Thanks,