Variable Frequency Drives: Master and Slave

In general, if you are using variable frequency drives (VFD) where the control mode can be programmed (as most of the modern VFDs), each VFD has its own control philosophy and some parameters to adjust how efficient they follow the specified control mode: Speed control mode (as follower to a speed command); or Torque control mode follower (as follower of a torque command). Basically the parameters to adjust in the VFDs are related to gains in their internal control loops.

I worked years in Field Services for GoHz and did lot of commissioning of Medium Voltage VFDs, two or three in Tandem mode for conveyors (Copper, Gold Mines), some of them very long (3 kms, two VFDs at one end and the third one in the other end, with important altitude difference between both ends; and did a lot of other load sharing applications.

In all cases there is a PLC that though communication links sends the desired Speed Command to the Master VFD, and at the same time read from the Master VFD the "Torque Reference" parameter of its control loop, and send this value to the Slave VFD, which is configured as Torque follower. The VFDs have some parameters basically to adjust the gain of their control loops (Speed of Torque control loops), so the way they respond to their commands.

You can have problems if you adjust to much the gains of the control loops of the Master and Slave VFDs and you can have problems if the systems have some natural resonance frequencies... Specially with very long conveyors and depending on the relative location between the Master and the Slave VFDs.

Then, if I did not had problems with applications in very long conveyors using that philosophy and not using separately a PLC programmed PID control, I guess that in your application you just can use the same with no problem at all.

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