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Single VFD for Multiple Motors

A single variable frequency drive (VFD) may control 2 or 3 motors at a time, multiple motors control is usually done because of budget considerations. When in this example below, a cooling tower with 4 equally sized motors are controlled by one variable frequency drive.
Multiple motors on one VFD

Motors are to have the same capacity and operate at the same speed. In this example the 4 motors are 2 amps each.

The variable frequency drive parameters must be set to accommodate the total capacity. In the case the variable frequency drive parameter is set to 8 amps (2 amps x 4 motors). Each variable frequency drive parameters that are meant for one motor should be turned off. For example slip compensation (calculating the difference between the field speed and rotor speed) should be set to off in multi-motor controls.

A single motor has its own overload protection. In this example, if the cooling tower fans were operating with a very light load, say 1 amps each, and if one of the 4 motors became failure, the the amp draw on the failed motor would need to surpass, 5 amps (8 amps -– (3 x 1 amps)) before the variable frequency drive caused any problem.

Wiring multiple motors on a single VFD, certain conditions, considerations and safety must be met, here are few guidelines you'd better follow to avoid failures:
  • Variable frequency drive should be at least 20% more power of the two or three motors' power clubbed together.
  • Set variable frequency drive to V/F mode, no vector control at all.
  • Set correct current rating.
  • Don't perform Auto tuning.
  • At the variable frequency drive output put right OLR for individual motor.
  • NC of OLR should be wired to variable frequency drives enable terminal, so that any of the motor goes to fault variable frequency drive should be disabled.
  • NOs of OLR and variable frequency drives can be used for fault display.
But please keep in mind that you can only set the VFD to V/F mode control if you control multiple motors by one variable frequency drive, never set variable frequency drive to any vector control, even at open loop with vector control, otherwise the variable frequency drive will fail.

So the bottom line, go ahead with multiple motors. It is absolutely fine to run more than one motor on a single variable frequency drive till the power capacity is matched and the parameters are set correctly. This is the key of any variable frequency drive application.

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