jraef
Member
You should ALWAYS have reactors or some other form of impedance protection ahead of drives, especially small drives. You need to slow down the rise time of ringing transients that can affect the DC bus, but are too fast to be captured by regular hand held meters. These transients can be caused by other large loads being switched on and off in your facility, even a neighboring facility or even grid switching transients from the utility themselves (which they will deny, deny, deny they exist, until you present them with evidence on paper).
The quick-and-dirty rule of thumb is that if your supply transformer is more than 10x the kVA of a VFD, you need to put an input impedance component ahead of the drive. It could be one reactor for one drive, or one reactor for 10 drives (properly sized), it's all the same. This is what DickDV had alluded to back in 2011 when this zombie thread was still alive, in his suggestion to the OP to add a drive isolation transformer. That may be a more extreme version of the same principal, because it too adds impedance. It also adds more protection against common mode noise coming in from other systems on the same bus, but that is rarely a problem with modern VFDs now. I only use drive isolation transformers when there is a Delta power source or a resistance grounded Wye that might be dangerous for the VFD. Otherwise, I just use reactors, they are cheaper and smaller. Either way though, you need to do something.
The quick-and-dirty rule of thumb is that if your supply transformer is more than 10x the kVA of a VFD, you need to put an input impedance component ahead of the drive. It could be one reactor for one drive, or one reactor for 10 drives (properly sized), it's all the same. This is what DickDV had alluded to back in 2011 when this zombie thread was still alive, in his suggestion to the OP to add a drive isolation transformer. That may be a more extreme version of the same principal, because it too adds impedance. It also adds more protection against common mode noise coming in from other systems on the same bus, but that is rarely a problem with modern VFDs now. I only use drive isolation transformers when there is a Delta power source or a resistance grounded Wye that might be dangerous for the VFD. Otherwise, I just use reactors, they are cheaper and smaller. Either way though, you need to do something.
