By natural, I mean a motor without a speed regulator will droop. When a dc motor’s load is increased, the armature current will increase, increasing electrical resistance losses (IR losses) and CEMF will drop, reducing the motor RPM. When an AC induction motor load increases, the motor will follow its slip vs. torque curve and the slip will increase, reducing the motor RPM.
Why would one pay for a precise speed regulator, and then implement a speed droop function which will reduce the accuracy of the speed regulator? There are many good reasons and applications for droop.
The first is the case of more than one motor on a single drive. We don’t see this much any more on new equipment, but it was very common on paper machine drives over the past 50 years. If one of the motors starts taking too much load, speed droop for that motor will reduce its speed (and thus load). Each of the motors on the drive will share load equally.
Another case is when two or more driven rollers are in contact. This could be a nipped roll or casting rollers or calender rollers. One of the drives will have a precise speed regulator. The others could have droop resulting in satisfactory load sharing.
Yet another case is a driven roll without tension measurement (load cell or dancer). A stiff material will not run well in open loop speed control, but droop may provide satisfactory (and inexpensive) tension control.
Speed Droop is available on most drives. It is an easy first step to solve a load sharing problem between motors. Droop is expressed as % speed droop when the torque is at 100%. The droop must be set by experiment. 5% speed droop at 100% torque is a good starting point.