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Variable frequency drive (VFD) motors

Variable Frequency/Variable Speed Drives are considered non-linear loads, which are used to control the speed of induction motors, induce distortion in generator output voltage. Larger alternators are required to prevent overheating due to the harmonic currents induced by the VFD and to lower system voltage distortion by lowering alternator reactance.

For example, VFD loads on a generator must be less than approximately 50 percent of generator capacity to limit total harmonic distortion to less than 15 percent.

Minimum generator set load/capacity:

Running a generator set under light load can lead to engine damage, reducing reliability. Cummins Power Generation does not recommend running generator sets at less than 30 percent of rated load. Load banks should supplement the regular loads when loading falls below the recommended value.

Maximum allowable step voltage dip (starting and running):

As you reduce the maximum allowable step voltage dip during initial startup, when loads cycle under automatic controls or when high peak loads operate, you need to increase the size of the generator set specified. Choosing lower allowable voltage dip requires a larger generator set.

Maximum allowable step frequency dip:

As you reduce the maximum allowable frequency dip, you increase the size of the generator set needed.

Altitude and temperature:

Based on the site location, the size of the generator set must increase for a given level of performance as altitude and ambient temperature rise.

Duty cycle:

Generator set size is also influenced by whether the application is for standby power, prime Power or utility paralleling. Standby power systems generally have no overload capability. Prime power systems generally have a minimum of 10 percent overload. Generator sets that are intended to operate extended hours at steady constant load should not be operated in excess of the continuous rating.


The preference for gas, diesel, or LP will affect generator set choices. Often, generator sets running on gas or LP must be oversized due to de-rating. Emergency systems typically must be supplied by fuel stored locally.


Select either single- or three-phase. The three-phase selection permits single-phase loads but the assumption is that the single-phase loads will be balanced across the three phases.


Either 50 Hz or 60 Hz.


Voltage choices are usually a function of selected frequency.

Check out our full range of General Purpose VFDs, V/Hz VFDs, High Performance VFDs, and Sensorless Vector VFDs to see our drive options.

Sizing based on total loads of all devices must also be taken into account. For further assistance is sizing your VFD, contact us today and our tech support team will be happy to assist you!

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