Load Calculations with ASD's

[cabletray]

I've noticed that design consultants may ignore the ASD input current ratings when summing loads for ampacity calcs, voltage drop, etc. Instead, they only include the motor FLA values as though the motor was actually connected across-the-line and not ASD-driven where the ASD is wasting significant KW. Is this method correct, or should the ASD input current ratings be used instead of motor FLA when adding loads? Surprisingly, the NEC doesn't address this issue.

 

[DickDV]

I know of no normal operating scenario where the drive would be wasting "significant" amounts of KW.

In fact, worst case is about 2% of motor maximum load KW. That's hardly significant and is why motor KW is used.

 

[justinn4]

When a Variable Frequency Drive is controlling a motor, the performance is no longer as a traditional motor load.

The motor currents from starting to full load are now controlled by the driver and are different as compared to the current take from the power supply.

The Driver-motor is now the load. To be stricter the efficiency of the driver must be considered while calculating the line loads.

 

[aolalde]

If you have many VFD's consider the extra load due to the drivers efficiency.

 

[rbulsara]

Any loss of efficiency due to a VFD would be more than off set by the gain in energy savigs by the very use of the VFD at frequent use of less than rated speed.

If VFD were to add to losses than normal starters, they wouldn't be so popular after all.. not that it is the only reason a vfd is applied.

But it is true that the drive has some losses and can be separately accounted for, but using the nameplate current is not the way because that simply lists its maximum capabilty not the current it will draw all the time.