Motor Feedback-Torque or Current

Yes, this is done by most soft start and VFD manufacturers that make units for pumps. The sense the back emf of the motor to tell where the motor is in it's operating curve. As to the details, probably talk to someone at a company like Rockwell Automation or GE. They can tell you a lot more.

Measuring the Torque, I heared no any measuring device to measure the torque but it can be only claculated form the shaft.

Direct measurement of torque is always possible - you do have transducers. But usually motor load is usually calculated by the electrical parameters.

Could you please offer an example?

Current alone cannot offer an accurate measure of torque, because current will vary with applied voltage. Current also varies at a non-linear rate as the motor becomes less loaded because the percentage of total current represented by magnetizing (flux) current becomes greater. Power Factor is sometimes used as a method of determining torque, but that to is flawed because it too in non-linear; as the motor becomes more loaded, the power factor difference becomes less measurable.

There are true "shaft torque" relays available that provide a output torque calculation by using the current profile AND the Power Factor profile in the calculations to provide a linear analog output or set point trip value. Probably the most well-known of these is from a company called Emotron. Siemens also offers one but it is only for single phase monitoring.

You can calclate the torque by measuring theRPM.Slip is directly prportional to the output so the torque caultedwill be proportional to the actual load.As u are aware when motor is loaded speed drops and Torque is more till it reaches Pull out torque beyond which it stalls.The torque vs speed curve is linear between POT to Syn RPM at which it is zero(in In duction motors).Starting (0RPM) To POT is non linear