How do you synchronise two VFDs?

The motors are mechanically coupled to use two VFDs, with one unit configured as the master in speed or frequency control. The slave unit will be torque controlled and will receive its torque set point from the master. To accomplish this, the torque setpoint from the master can be sent to the slave via analogue I/O, but for greater precision, peer-to-peer serial communication is recommended.


If the motors are not mechanically coupled, the application must decide which is more important: load sharing or speed matching. If speed matching is desired, operate each VFD in closed loop vector mode. If load sharing is desired, a comparator can be used to bias the speed of one VFD to equalise the loads.


Determine the voltage that is present at each terminal in relation to the ground. The majority of VFDs generate 10 V and ground across the it, with the center connection feeding back the voltage; therefore, 5 V is equivalent to 50%, 7 V is equivalent to 70%, and so on. Find out what each VFD is feeding out and what needs to be fed back into it for any given speed, and then act accordingly. There is a possibility that there will be some inconsistencies, such as 4 V equaling 38% of full speed on one and 43% on the other.


Unplug the connection to the slider contact, then insert a resistor with a value of 10K into the series circuit with the lead. If the results of the test on the master VFD are good, users can proceed to link the “voltage in” (slider) of the second VFD to the “voltage in” (slider) of the master VFD by connecting a 10 K resistor between the two. With that optimism, it should operate effectively.

If the two variable frequency drives (VFDs) use different voltages or their voltages are referred to something other than earth (for example, 20V and 35V with 1/2 speed = 27.5V), then a solution will be complex.

  1. Verify that both (VFDs)- Variable Frequency Drives are powered by the same power supply and that it is stable and reliable.

  2. Connect both VFDs to the same communication bus and protocol, such as CAN or Modbus.

  3. Set the VFDs to the same parameters, such as frequency and voltage.

  4. Match the speeds of both VFDs.

  5. Turn on the synchronisation feature on both VFDs.

  6. Keep an eye on the synchronisation and ensure that both VFDs are operating at the same speed.

  7. If necessary, adjust the speed of one of the VFDs to match the speed of the other.