When using droop control to balance the torque across multiple motors driving a coupled mechanical load such as a chain conveyor, what is the actual variable that is being altered by each drive? From what I understand, at rated conditions the output of each drive is altered by the selected droop value (i.e. 5%) and scaled linearly to 0% at zero RPM. What I am unsure of is exactly what variable is used to instigate the % droop, is it frequency independent of voltage, or is it the combined V/F signal?

The application in question is a chain conveyor, with 30+ individual induction motors each supplied by a small scalar control converter. The type of VSD specified will depend on what speed control method is appropriate. Most likely Droop control or Speed Trimming. I am attempting to build Simulink model to showcase the use of droop to regulate the motor speeds under variable loading of the conveyor but I am unsure exactly how the output of the drive should be altered when the load torque limit is reached.

Can provide more details if necessary. Regards


1 Answer 1



simulate this circuit – Schematic created using CircuitLab

Basically it reduces the speed with regard to the torque. All drives have the same speed setpoint value and they reduce speed equaly if they are all loaded the same. When one of the drive is more loaded, it reduces the speed further and lets the other to accept the extra load. Kind of simple equalization, without cross feedback.


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This is Siemens VFD Droop. Precontrol means an additional torque due to mass of inertia. Ideally would be using selector at position 2 (load torque only) and appropiate Droop factor.

  • \$\begingroup\$ Thanks for the response. From my understanding of V/F control, the reduction in speed is typically performed by reducing the frequency and magnitude of the drives output voltage so that it maintains the same V/F ratio as at the motors rated conditions, and this keeps the torque constant at reduced speed. Is this what is happening during droop control, or is the voltage magnitude staying constant with a reduced frequency, so that the motor is trying to maintain the same speed, but with a reduced torque, allowing other drives to 'pickup' the required torque? \$\endgroup\$
    – Jacob M
    May 25, 2021 at 2:14
  • \$\begingroup\$ @JacobM The voltage and frequency is changed according to basic rules of motor control, in case of IM it's V/f or vector control in the same manner as you would change manually the setpoint speed \$\endgroup\$ May 26, 2021 at 5:46

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