Speed droop enables load sharing between multiple inverters when they are controlled in speed reference. Load sharing can be needed when more than one inverter is controlling the same motor or multiple single driven motors are driving the same power train, see the articles Multi-inverter drives and Load-sharing between inverters for more information. Note that also in an application where two separate power trains are used to move a vehicle, for example a machine with two individually driven axles, drooping is needed if these two power trains are coupled via ground.
Speed droop decreases speed reference when applied torque increases and vice versa. An inverter consuming more power than others at certain speed is forced to drive slower due to drooping. While this inverter is braking the other/others need to increase torque to achieve desired speed. Eventually inverters find the balance and load share is equal.
Speed droop is set in parameter speed_control.speedcon_ref_drooping_rate. Value is given in decimals, 1 equaling 100% of the nominal speed. E.g. value 0.04 means that at nominal motoring torque speed reference is drooped by 4%. Drooping rate of 0.04 has been experienced to be a good initial value. Same drooping rate need to be configured to all inverters. It needs to be taken into consideration that user given speed reference needs to be higher than desired speed correspondingly to the rate of drooping: E.g. when nominal torque is applied and drooping rate is 0.04 (4%), speed reference of nominal speed results in speed of 0.96*nominal speed. Therefore, user given reference needs to be ~4.2% (1/0.96 = 1.0417) greater than nominal speed in order to achieve the nominal speed.
In situations where load changes rapidly a low pass filter can be tuned to make the drooping action more fluent. Time constant of this filter can be set in parameter speed_control.filt_time_speedcon_drooping_term.
In figure 1 an example of drooping effect to a speed reference is presented. Figure shows how drooping changes the used speed reference by the speed controller. User given speed reference in this example is 2000rpm, nominal torque 400Nm/inverter and drooping rate is 0.05.
Figure 1. User given speed reference of 2000rpm in relation to torque applied by an inverter when speed drooping activated.