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Overloaded VFD Due to Harmonic Current Generation
Copyright © 2001 Francis J. Martino


Under full load conditions, a typical induction motor driven by a variable frequency drive (VFD) will draw five to six percent more current than it would if operating on a pure sinusoidal waveform.

Thus, if a 50 HP, 460 volt, 1800 RPM motor is nameplated at 59 amps it will draw 59 amps at full load with a pure sine wave applied. When the motor is driven by a VFD it will then draw approximately 62 to 62.5 amps when fully loaded.

A typical 50 HP VFD will have an output current rating of 65 amps which will be sufficient.

However, a typical 50 HP, 900 RPM motor will have a nameplate rating of 66 amps. When powered by a VFD under full load conditions it will draw approximately 69 to 70 amps under full load. Thus, a 60 HP drive will be required.

With the same 900 RPM motor lightly loaded with a 62 amp load, the total amperage with harmonic current will be between 65.1 and 65.72 and, therefore, within the capability of a 50 HP drive.

If a 50 HP drive was inadvertently selected and the harmonic current is causing the drive to trip on overload, the additon of a load reactor at the output terminals of the drive will reduce the harmonic current output. If the motor is not fully loaded, the reduction in current might be sufficient to allow the drive to operate satisfactorily.

Keep in mind that a 3% reactor will cause a 3% reduction in output terminal voltage. Thus, a 460 volt output will be reduced to 446.2 volts, and a 5% reactor will reduce the output to 437 volts.

Nema specifications require a motor to operate at full load over a range of plus or minus 10% of nameplate voltage. Thus a 460 volt motor is rated to operate at 414 volts at a sinusoidal frequency of 60 HZ.

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