Bigger Size VFD for a smaller load

[sobeys81]

Hi,
A client intends to use an existing 350 HP VFD for a new 200 HP fan Load on 600 V. These are for variable torque loads(vent fan application).

I was wondering, if I should be concerned about any issues while using a higher size VFD for a smaller load.

Q#2, What is the criteria for sizing over current protection for VFD load. For example for a 200 HP load, what should be the size of MCP.
Thanks

 

[ozmosis]

sobeys81
For this size of VFD to the size of motor then there should not be a problem. Most VFD's have the facility to program motor data into the drive parameters. This will ensure the motor characteristics are therefore 'known' by the VFD. Internal protection and overload will typically be carried out by the VFD such as IsqT protection-the VFD show be able to calculate the load at certain speeds and know if it is being overloaded subject to the cooling method of the motor (i.e. forced cooling or the motor cooling is attached to the motor shaft)-again, it depends on the type of VFD if these functions are available.
Check if there is thermistor protection in the motor anyway and if so, you can always connect these into the VFD, if it has the capability.
Regarding the MCP then you should typically size to the motor load based on local regulations. Having a bigger VFD should not be the base for greater protection unless you change the motor size up.