FC 300 Design GuideFC 300 Selection" EfficiencyEfficiency of FC 300 Series (η VLT)The load on the adjustable frequency drive has little effect on its efficiency. In general, theefficiency is the same at the rated motor frequency fM,N , even if the motor supplies 100% ofthe rated shaft torque or only 75%, i.e., in case of part loads.This also means that the efficiency of the adjustable frequency drive does not changeeven if other U/f characteristics are chosen.However, the U/f characteristics influence the efficiency of the motor.The efficiency declines a little when the switching frequency is set to a value of above 5 kHz. The efficiencywill also be slightly reduced if the line voltage is 500 V, or if the motor cable is longer than 98.43 ft. [30 m].Efficiency of the motor (ηMOTOR )The efficiency of a motor connected to the adjustable frequency drive depends on magnetizing level. In general,the efficiency is just as good as with line operation. The efficiency of the motor depends on the type of motor.In the range of 75-100% of the rated torque, the efficiency of the motor is practically constant, both whenit is controlled by the adjustable frequency drive and when it runs directly on line power.In small motors, the influence from the U/f characteristic on efficiency is marginal. However, inmotors from 14.75 HP [11 kW] and up, the advantages are significant.In general, the switching frequency does not affect the efficiency of small motors. Motors from14.75 HP [11 kW] and up have their efficiency improved (1-2%). This is because the sine shapeof the motor current is almost perfect at high switching frequency.Efficiency of the system (ηSYSTEM )To calculate the system efficiency, the FC 300 Series efficiency (ηVLT) is multipliedby the efficiency of the motor (ηMOTOR ):ηSYSTEM ) = η VLT x ηMOTORCalculate the efficiency of the system at different loads based on the graph above.66 MG.33.B6.22 - VLT is a registered Danfoss trademark
