I have designed controls for automated machinery for many years and have always used actual full load current draw under normal operating conditions to calculate full load current. This is very easy to calculate when you are operating a motor using a motor starter. The issue I have is that we asked a vender to install VFD's in front of the motors for speed control. They installed VFD's that were oversized and used the rated input current to calculate the FLA.
I assumed that adding VFD's would only increase the FLA by a slight amount due to the power loss of the VFD. In actuality, because they used the rated input current of the larger VFD's it increased the FLA by 30%. Would it have been correct to use the motor current plus current required for power loss instead of rated input current or is the vendors method correct. Clearly the way the vendor calculated the FLA was the safe way of doing this but it caused us increase the transformer size, bus feed, wiring etc.
I assumed that adding VFD's would only increase the FLA by a slight amount due to the power loss of the VFD. In actuality, because they used the rated input current of the larger VFD's it increased the FLA by 30%. Would it have been correct to use the motor current plus current required for power loss instead of rated input current or is the vendors method correct. Clearly the way the vendor calculated the FLA was the safe way of doing this but it caused us increase the transformer size, bus feed, wiring etc.
