The concept of Minimum - Base - Maximum speed of a motor in VFD operation

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In general, in an enquiry sheet for motors which will be running with VVVFD, the speed range may read as below

0-500-1200

0-1000-1500

100-750-1000

The meaning of this speed range is that the within the first two speed the motor should provide constant torque, and at the last or the highest speed the motor is expected to provide constant power.



For a 50Hz motor, Constant Torque Zone will be limited up to 50Hz. For a self cooled motor it may be from 30Hz to 50Hz and for a forced cooled motor it may be from 10Hz to 50Hz. 

Constant power zone is that zone which is beyond motor rated frequency. in this zone Voltage becomes constant and the frequency is increasing. hence V/f ratio will not be maintained and thus the torque will fall. However, upto certain speed the power will be constant.

In constant power zone the rated torque of the motor will be 

Tnom ∝ 1/ Nop

Tnom = nominal or rated torque

Nop = operating speed.

Hope this is okay to understand. 

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