Constant torqueWith constant torque loads, the torque loading is not a function of speed. Typical applications are: Traction drives, Compressors, Conveyors, Positive displacement pumps and Hoists. As the speed changes, the load torque remains constant and the horsepower changes linearly with speed. Constant torque loads cause motors to draw relatively high current at low speeds when compared to variable torque applications. This is why the same size drive may have a lower HP rating for Constant Torque applications.
Constant torque loads. These loads represent 90% of all general industrial machines (other than pumps and fans). Examples of these load types include general machinery, hoists, conveyors, printing presses, positive displacement pumps, some mixers and extruders, reciprocating compressors, as well as rotary compressors.
If you know the curve torque-speed in an electrical, you have to know that when the motor has a high velocity, then its torque is low and by the other way, if you have a high torque, the speed is low then.
The difference is, when you need a speed controller or torque controller. Normally, it depends of your speed consideration, if you have sized well your machine in speed or torque, (controlled and motor geared) you shouldn't have any problem, but whether you have not done that, or your client wants another considerations at the last moment, and you need to control your motor control under of 10-15 Hz, with a speed control, you feel a problem in the motor which his axis doesn't linearly. Then, you would have 2 possibilities, the most complicated but the most recommended (in my opinion) is to change the motor geared with the news specifications. The second solution is to get a VFD with Vector control (Torque Control).
It is the load that you have to be concerned about in the first instance. Is the load variable torque such as a centrifugal fan or pump or is a constant torque load such as a conveyor. If a variable torque load what is the power at the maximum speed you require. If a constant torque load what torque do you require and what turn down ratio are you looking for? When you know these things the selection of the motor and variable frequency drive is a very simple process.