When working with your VFD, the motor, of course, is a vital piece of the puzzle in making sure that your system is running smoothly. Often times, we have clients and partners ask us about VFD sizing in terms of the motors.
In general, when it comes to the Danfoss VFDs, motors are sized to ensure that the VFD system has the ability to slightly exceed the max amp values, as show in the NEC Table 430-150. That NEC 430-150 table tells us a great deal of information, but the most important when it comes to motor sizing can be found in the excerpt below.
- 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, while many in the industry frequently turn to this table for budgeting, it’s important to remember that users should also always utilize the Motor FLA from the data plate to ensure that the VFD is sized appropriately. It’s so important that it bears repeating, always use the Motor FLA to verify that the VFD is the correct size. It’s incredibly important in ensuring that your VFD system can run smoothly, effectively, and safely.
As always, if you’re struggling to get the correct measurements or fit, or if you’re just not confident in your findings, reach out to our team for further assistance. Team AirReps is here for all of your HVAC system and HVAC design needs, and our experts are always ready to help!