VFD Sizing: The main onsiderations

By Paul Wright, P.Eng.

nema nameplate
NEMA Part 30 Nameplate

In VFD Sizing, the main considerations are:

1.       The VFD continuous current shall be higher than the motors’ current at maximum load.  It is not necessary to size the VFD to the motor FLA rating if the motor application is oversized. A motor that operates under worst case conditions at 80% FLA current flowing, only requires a VFD that can continuously supply this 80% current value.

2.      For Variable Torque Applications, the intermittent current shall be 115% greater than the motor current rating for the worst case starting scenario.  If the motor rated current is 65 amperes, then the intermittent current rating should be greater than 74.8 amperes (65A x 115%).

For Constant Torque Application, the intermittent current shall be 150 to 200% of the motor rated current.  The equipment OEM can provide the actual worst case starting torque that they would normally expect for the application.  By ensuring that the intermittent current capability of the VFD is higher than this value, ensure the ac drive will produce sufficient torque loads to start the hp motor under abnormal situations.

3.      The single phase input voltage tolerance rating for the VFD shall be higher the highest line voltage and lower than the lowest line voltage that last longer than two seconds duration in the facility.  VFDs have a 100% voltage rating of 200, 230, 460 or 575 Volts. Each site has different high and low line levels that the VFD must be able to safely operate at without being damaged.  If the measured low line voltage is 550 volts and the high line voltage is 625 volts, then we need a 575 Volt rated VFD with a voltage tolerance of 575 volts plus 10.9% (625V), minus 4.6% (550V) or greater. Actual common tolerance values for VFDs from the rated voltage level of the VFD are+/- 5%, 10%, 12%, 15% and 20%. Some 575V commercial rated VFDs are 575V +5%, -10% (518V to 603V), whereas and industrial rated VFD may be rated 575V +/- 15% (489V to 660V).  If the tolerance rating of the VFD does not meet the high line actual site voltage condition, then the VFD is subjected to potential catastrophic failure. 


Other VFD equipment to be upgraded or changed:

There are many factors that need to be considered when conducting VFD Sizing in an existing application:

  • Is the existing motor suitable for the speed range and is the insulation designed to withstand the dv/dt stress caused by the fast switching of the voltage pulses to the motor?  If not use a NEMA MG1 Part 31 designed motor.
  • All VFD manufacturers’ recommend that the motor feeder cable be replaced with a shielded VFD symmetrical cable.

Motor specifications:

In North America, NEMA MG1standards are used for the specification of motors. In 1992, a new section (Part 31) was added to specifically address the motor design requirements to work on a VFD application without installing motor feeder filtering.  The original Part 30 design motor name plate specification allowed for VFD operation if the proper sized motor filters were installed in the VFD package.


Figure 1: NEMA Part 30 & 31 Nameplates


 Since each application required a custom engineered solution for the proper selection of the motor filters, most users and VFD distributors were not qualified in the proper selection process to guarantee a successful filter selection. By installing the NEMA MG1 Part 31 designed motor, there is no requirement to install a motor filter.  The deletion of the requirement for a motor filter usually offsets the slight cost increase for the Part 31 motor.  The Part 31 motor has an enhanced nameplate which provides the motor performance data when the motor is operated at the lowest operating speed, the base 60Hz speed and the highest speed above 60 Hz. 

System component used with VFDs:

There are many factors that are required when the decision for installing a VFD is made: The VFD solution should be supplied with all the necessary equipment to provide for a long life and reliable VFD solution:

  • Circuit Breaker,
  • Transient Surge Protection,
  • Line Filtering, Harmonics and Power Factor
  • Standard or High Performance VFD chassis as needed for the application. 
  • VFD cable
  • NEMA MG1 Part 31 motor should always be used
  • Ambient Temperature
  • Derating the VFD


Figure 2: Circuit breaker, MOVs, Reactor, Harmonic Filter, and VFD Cable


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