Load sharing in motor control use
When several parallel inverters are running in certain control modes, for example speed control or DC-voltage control, the inaccuracy of the inverters' control systems might cause a small estimation difference between parallel devices, which will lead to the inverters creating torque or power output against each other. This can in some cases lead to, for example, one inverter trying to accelerate the motor with full torque while another inverter tries to decelerate the motor with an opposite torque. To prevent this, a load-sharing function must be used.
See also the article Multi-inverter drives for more information regarding parallel inverter use.
Speed control mode
For speed control mode, Speed reference droop can be enabled from parameter group 2.7.3 Reference drooping. [Firmware v.7: speed_control] by setting the parameter Speed reference drooping rate [Fw7:speedcon_ref_drooping_rate] to any positive value. For usual motor control applications, a value between 0.01 and 0.1 is recommended. Increasing the droop value makes load sharing more even and steady between inverters, but it reduces the speed of the motor when running under load. See the article speed droop for more information.
Alternatively, the parallel unit balancing feature can be used instead of (or in addition to) the droop function by enabling the feature from the parameter group 2.12 Speed control parallel unit balancing. See the software manual or quick guide for more information on the parallel unit balancing feature.
DC-voltage control mode
Droop can also be used in DC-voltage control mode, and it works similarly to speed control droop. DC-voltage droop can be enabled from parameter group 2.7.3 Reference drooping [Fw7: udccon] by setting the parameter DC link voltage reference drooping rate [Fw7: droop_udccon] to a positive value, with the recommended values between 5 and 40.
Other control modes
When running a motor controller in torque or power control mode, load sharing functionality is not needed since the torque is directly controlled by the user, and thus no risk of controller unbalance is present. For parallel inverter use in position control, contact Danfoss Editron for more information.
Load sharing in AFE or microgrid use
Droop settings for microgrid control can be found in the parameter group 4.3.3 Reference drooping [Fw7: microgrid], where the parameter Frequency reference drooping rate [Fw7: frequency_drooping_rate] can be used to set the drooping for active power (output frequency is affected), and Voltage reference drooping rate [Fw7: voltage_drooping_rate] can be used to balance reactive power (output voltage is affected). For usual microgrid applications, a value between 0.01 and 0.1 is recommended for both droop settings.
For AFE control, droop is only needed when running in DC-link voltage control mode. The droop setting can be found in parameter group 3.3.3 Reference drooping [Fw7:udccon] and adjusted via the parameter DC link voltage reference drooping rate [Fw7: lc_droop_udccon], with recommended values between 5 and 40.
Load sharing in DC/DC converter use
Droop settings for the DC/DC converter can be found in the parameter group 5.4.3 Reference drooping [Fw7: dcdc_application and dcdc_control]. Droop settings are available for both LV voltage control mode as well as HV voltage control mode. Load sharing functionality is not needed in current control or power control modes for the DC/DC converter.