One standard EC-C1200 inverter can provide a maximum of 350 A current. If the application requires more current, multiple inverters have to be used. The best way to implement multi-inverter drives for motor control is to use a motor with multiple separate winding systems. This means that the winding systems, and thus the inverters' AC outputs, are not galvanically connected to each other. This article explains some terminology and the basics of multi-inverter drives.
Parallel setups for motor control
In a parallel installation (also called a master-master setup) multiple inverters are used in parallel and treated as individual devices. All commands are sent to all inverters, and the inverters do not communicate with each other directly.
In order to balance the load on the inverters and prevent unwanted power flow, a separate load sharing function needs to be used in specific control modes:
| Control mode | Load-sharing needed | Available load-sharing functions |
| Speed control | Yes | Speed reference droop, Parallel load balancing |
| Torque control | No | N/A |
| DC-link voltage control | Yes | DC-link voltage reference droop |
| Power control | No | N/A |
| Position control | Yes | Contact Danfoss Editron for more information. |
| Scalar frequency | N/A (single-inverter use only) | N/A |
See the article about load-sharing functions for more information.
A master-master setup also offers redundancy, as the operation of one inverter does not depend on the other inverters. If one inverter stops, the other inverter(s) can keep running the motor and the system can continue operating, albeit with reduced power.
Parallel setups for AFE, uG, and DC/DC converters
The aforementioned principles apply also for parallel setups with other application types. It is strongly recommended to have galvanic isolation on either the AC or DC side of the parallel devices to prevent circulating currents and control issues. Droop function must be used to balance the load of parallel devices in all DC or AC voltage control modes. See the article about load-sharing functions for more information.
Master-Follower [deprecated, firmware 7 only]
A master-follower setup acts almost like a single inverter from the user's or system controller's point of view. The run command and the reference values are sent only to the master inverter. The master inverter calculates the required torque values for all inverters and transmits the torque reference and run command to the followers via CAN bus. The followers run their own control software and are always in torque control mode.
Because the followers are in torque control mode, the master inverter determines the output power and torque based on the given reference. Thus no load-sharing functionality is needed.
A major disadvantage of the master-follower setup is that it does not offer redundancy, as the master inverter is always needed for operation. The slave inverters also need to receive the fault clear command directly from the system controller or via PowerUSER, so some communication between the controller and the followers is needed in most cases.
See the master-follower configuration article about details on the parameter setup and configuration.