The “rules of thumb” for multiple drives are;
1. Each motor is required to have its own separate overload protection. If you use an Over Load Relay, all of them must be wired to shut down the drive if any one motor overloads. But be aware that most bimetallic OLRs are NOT rated for use at any frequencies other than 50 or 60 Hz. The alternative is to use a Motor Protective Switch that actually opens under load to disconnect one motor, which also allows you to run the rest of the motors if necessary. Opening the MPS on the output of a VFD is not something you want to do on a regular basis, but for this purpose it’s fine.
2. Size the drive amp rating for a minimum of 10% higher than the combined motor FLA ratings. More is better, less will risk the drive tripping.
3. Use a load reactor on the VFD. When you add motors, you add circuit length in terms of capacitive charging current and in terms of risk exposure to external conductor faults. So if you have 4 motors each 10m from the drive, it’s like having one motor that is 40m away. This is the reason some drives will trip trying to start multiple motors at once (it’s not the motor inrush). The output reactor slows the rise time of that current spike to where the drive doesn’t interpret it as a dI/dt event and in the case of a conductor fault, the slowing of that current spike to where the VFD protective features have time to react.
4. Don’t cycle the separate motors with contactors. The drive will control the starting current as it ALWAYS does. But once the drive has ramped up the first motor, then you energize the second one (and every one thereafter), those subsequent motors will NOT receive the benefit of being ramped from zero, so there WILL be a motor inrush for each one. If you are lucky, the VFD will go into current limit when each one starts, but this is putting significant dI/dt stress on the transistors and they will have significantly shortened life.
Also, although it SHOULD be intuitive, one thing I have run into many times is that people are shocked that all of the motors will run at the same frequency! This is just a warning to “manage expectations” with the users who request this.
