How fast can I run a 4 pole AC motor ?

In our new project I tried to persuade our mechanics for using an AC motor instead of a normal DC that we normally use for the pinch roll. They know the power it would be 55KW.
it is cheaper for sure. regarding the speed I told them to choose a 4pole standard squirrel cage motor (for example 1LA7-1LA5 Siemens motor) I told them that I can give the nominal torque app. from zero speed to 1440 with vector control and I can have constant power in the speeds beyond 1440. but how far they can go ? in DC motors we had 2 speeds one as the max. torque limit and the other for the nominal power. but for AC motors How fast can they go?

If this motor has a cast aluminum rotor, at 55kw I would go as high as 100Hz without any concern. Certainly 80Hz would be a common thing. If you want to be double sure, check with Siemens on the rotor balance at those speeds. The bearings are of no concern. If you researched it, you would likely find the same bearings in a two pole motor of a different KW rating.

When using overspeed like this, be sure the available motor torque does not fall below the required load torque. If that happens at any speed, you will either not accelerate any further or the motor will have to go to overload to keep going.

One other issue, the short term overload available from the motor comes down as the inverse of the overspeed SQUARED. So, if there is 200% OL available at motor base speed, there is no OL left at double speed.

A 4 pole motor can also run 3600 rpm without any problem.
however i would use a normal 2p motor and use it half speed this way you are safe at all times. Half speed is better then overspeed. and yes with the FC now you can do what you like to do.