VFD sizing and NEC table 430-150
/Hello- a client recently asked about VFD sizing for motors- here is a quick explanation.
In general, Danfoss VFDs are sized to make sure that we slightly exceed the max amp values in NEC table 430-150, see an excerpt below.
That NEC 430-150 table tells us:
The MAXIMUM amps that are allowed for a given HP motor (see 7.6amps for a 5hp/460v motor). Most modern high-efficiency motors will be less than this max value by 10-30%.
Low RPM motors may need MORE amps then specified on this table— the table is for standard 1800rpm motors. If you remember your high school physics (heh!), HP = Torque x RPM. Since the RPM is lower, you need more torque to get the same HP. To get more torque you need more amps from the VFD. We see low RPM motors sometimes on direct-drive fans. Verify the motor FLA on low rpm motors before ordering.
Multi-motor applications may need to bump up one VFD size to accommodate the amps— To drive two 5hp/460v motors needs 7.6a + 7.6a = 15.2amps max. If you look at the 10hp/460v value in the table it is only 14amps max. In this example we may need a 15hp VFD to run those two 5hp motors.
In summary, we frequently use Table 430-150 for budgeting, but you should always use the motor FLA from the dataplate to be sure that the VFD is sized appropriately.
ALWAYS USE THE MOTOR FLA TO VERIFY THAT THE VFD IS THE CORRECT SIZE!
