I know this is the quot;field weakining regionquot; and that the toque will decrease non-linearly with speed, but is there anything else that needs to be considered? (over-heating)
In Excess of the base speed ie the rated speed, the slip of the ac motor( s=(ns-n)/n) becomes negative . at negative slip the power gets fed back ie the MOTOR NOW ACTS AS A GENERATOR. .
There are the mechanical aspects to consider. The bearings may fail if RPM exceeds their design speed. Depending on the type of motor, the armature windings may deflect outward erupt, rub, or cause vibration. An induction motor could also experience deflection and vibration. At sufficent speeds, any motor will fly apart. Overheating and smoking/fire are possible for reasons you already seem to understand.
My initial reaction is that you can't do this with either a synchronous or an induction type ac motor, unless you have a higher frequency supply available than the motor was designed for; for example by running a European motor in the USA. However if you do that you are continuously over-speeding the rotor so there is a considerably increased danger (speed^2) that the rotor or cooling fans may fail and that could be quite traumatic if it's a big motor. This danger could be compounded by your new speed being closer to a critical. You should have no problem with heating if you run it at the same voltage and power and the cooling will be improved if the fan can take the speed. You'd probably get away with it at the bearings provided the vibrations stay within limits but there might be some overheating there. Nevertheless with the rotor speed alone you may be confronting a major safety issue.
As you know, if you increase the frequency without proportionally increasing voltage, the torque capability is reduced. If the load is within the capability of the motor, the motor current will not exceed the rated motor current, copper losses will not increase, and the operating temperature will remain within the normal range. As the speed increases, the cooling fan and/or rotor fins will provide more cooling and the slight increase in iron losses will not overheat the motor. The increased windage torque from the cooling fan and/or rotor fins will further reduce the torque available to drive the load. At some frequency, the available torque will be reduced to the point that the motor will not be able to supply its own windage and friction torque. There is also the possibility of excessive vibration, excessive centrifugal stresses and exceeding the safe speed of the bearings. Depending on motor size and type, those problems can be prevented by balancing the rotor more precisely, using bearings that are rated for higher speed and reducing the size of the fan and the rotor fins. With the proper power supply voltage, a motor can be operated at constant V/Hz and thus constant torque above the base speed. However, the voltage rating of the winding insulation then becomes an issue. If a dual voltage motor is connected for its low voltage rating but operated up to its higher voltage rating, the winding insulation voltage should not be a problem. Note that operating a motor above base speed at constant torque, is operating the motor above its rated power. That is really not a problem because the power rating of a motor is really just a torque and current limitation.
