There are two intervals for ac motor speed adjusting which is constant torque interval and constant power interval.
0Hz to motor rated frequency is constant torque interval, in this interval, the motor torque keeps not change and the motor power increases according to the motor speed. That’s also the reason why when the inverter decreases the output frequency to decrease the motor speed, it can save energy.
When the running frequency is higher than the motor rated frequency, the motor runs in constant power interval. Now the motor power keeps not change but the motor torque will decease according to the speed increasing.
Now the key is coming.
How to increasing the motor power and speed together?
We have tested it well that when a 3PH 230V 50HZ motor runs at 3PH 400V 87Hz, the motor power and speed will increase by 73%.
Actually you can also run a 3PH 380V 50Hz motor at 63Hz to increase the power by 26% and the torque not decrease.
The motor remains correctly fluxed all the way through to 87Hz.
This is because a 400V Inverter controls Voltage and Frequency through 230V 50Hz on its way to 400V 87Hz.
What we should be noted is size the inverter or VFD to suit the motor full load current ,not motor power.
Any questions you can contact me,thanks!