Overspeeding Motor with Variable Frequency Drive

Over speeding the motor with a variable frequency drive will depend a lot on the existing load on the motor. If it is a fan load and you are currently at 100% of NP rating, you have no head room left to play with. If you have a roll spinning lightly loaded you will have much more head room. In some applications we run 240vac motors on a 480vac VFD and over speed from 60 Hz rated to 90 Hz- and develop 1.5 x the HP at rated torque. If you have a 480vac motor on a 480vac VFD then you need details about the motor and the load.

The overspeeding of the motor is limited by the mechanical characteristics of the motor and the load. From electronics point of view, it is limited by maximum frequency of the VFD. For constant torque operation it is limited by the input voltage to the VFD. In the field weakening mode, it is limited by maximum inverter frequency. All above factors need to be considered for motor operation above rated speed.

Please verify with manufacturer of motor which is mechanical limit of max speed. Normal motor can run at +20% of max speed (IEC) without problems but generally be prudent of course. Special AC motors, example for spindle, can run up to 30000rpm. The master limit is generally on bearing system. Also for VFD depends of manufacturers, and marked of its application. Possibility curves of constant torque and constant power are described by VFD manufacturers, and best is to ask to them for type intended to use. All general purpose VFD can work at +100% of nominal frequency in constant power mode, but special types also at 2000hz.

We have successfully worked with AC induction motors at 200,000 RPM. Very small motor (1.6" diameter Stator). You need to have a motor designed for high speed, you need a VFD with output frequency able to get you to the speed you are trying to achieve. Make sure your driven equipment can handle the speed you will drive it to. Make sure your motor and VFD can handle the load at the speed required. Most very high speed motors have a very high base speed and low inductance - so a load reactor may be required.

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