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VLT® FCD SeriesEfficiency of frequency converters (INV )The load on the frequency converter has little effecton its efficiency . In general, the efficiency is thesame at the rated motor frequency f M,N, regardlessof whether the motor supplies 100% rated shafttorque or only 75%, e.g. in case of part loads.This also means that the efficiency of the frequencyconverter does not change even if other U/f charac-teristics are chosen. However, the U/f characteristicsinfluence the efficiency of the motor.The efficiency declines a little when the switchingfrequency is set to a value above 4.5 kHz (parame-ter 411 Switching frequency). The rate of efficiencywill also be slightly reduced at a high mains voltage(480 V).Efficiency of the motor (MOTOR )The efficiency of a motor connected to the frequencyconverter depends on the sine shape of the current.In general, the efficiency is just as good as in mainsoperation. The efficiency of the motor depends onthe motor type.In the range of 75-100% of the rated torque, the effi-ciency of the motor is practically constant, bothwhen it is controlled by the frequency converter andwhen it runs directly on mains.In general, the switching frequency does not affectthe efficiency of small motors.Efficiency of the system (SYSTEM )To calculate the system efficiency, the efficiency ofthe frequency converters (INV ) should be multipliedby the efficiency of the motor (MOTOR ):SYSTEM = INV x MOTOR .Based on the graph above, it is possible to calculatethe system efficiency at different loads. Mains supply interference/harmonicsA frequency converter takes up a non-sinusoidal cur-rent from mains, which increases the input currentIRMS. A non-sinusoidal current can be transformedby means of a Fourier analysis and split up intosinusoidal currents with different frequencies, i.e. dif-ferent harmonic currents I n with 50 Hz as the basicfrequency:Harmonic currents I 1 I 5 I 7Frequency [Hz] 50 250 350The harmonic currents do not affect power con-sumption directly, but they increase the heat lossesin the installation (transformer, cables). Conse-quently, in plants with a rather high percentage ofrectifier load, it is important to maintain harmoniccurrents at a low level to avoid overload of the trans-former and high temperature in the cables.Some of the harmonic currents might disturb com-munication equipment connected to the sametransformer or cause resonance in connection withpower-factor correction batteries. Power factorThe power factor (Pf) is the relation between I 1 andIRMS.The power factor for 3-phase supply:P f =p3 U I1 cosp3 U IRM SThe power factor indicates the extent to which thefrequency converter imposes a load on the mainssupply. The lower the power factor, the higher theI RMS for the same kW performance. In addition, ahigh power factor indicates that the different har-monic currents are low.MG.04.A1.02 - VLT is a registered Danfoss trade mark 121All about FCD 300