Danfoss VLT Series Design Manual

Also see for VLT series: Service manual Installation guide Operating instructions Operating instructions Service manual

Contents

Danfoss VLT Series Design Manual Manual pdf 61 page image

3.3. EfficiencyEfficiency of VLT HVAC (η VLT)The load on the frequency converter has little effect on its efficiency. In general, the efficiency isthe same at the rated motor frequency f M,N, even if the motor supplies 100% of the rated shafttorque or only 75%, i.e. in case of part loads.This also means that the efficiency of the frequency converter does not change even if other U/fcharacteristics 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. Theefficiency will also be slightly reduced if the mains voltage is 480 V, or if the motor cable is longerthan 30 m.Efficiency of the motor (ηMOTOR )The efficiency of a motor connected to the frequency converter depends on magnetising level. Ingeneral, the efficiency is just as good as with mains operation. The efficiency of the motor dependson the type of motor.In the range of 75-100% of the rated torque, the efficiency of the motor is practically constant,both when it is controlled by the frequency converter and when it runs directly on mains.In small motors, the influence from the U/f characteristic on efficiency is marginal. However, inmotors from 11 kW and up, the advantages are significant.In general, the switching frequency does not affect the efficiency of small motors. Motors from11 kW and up have their efficiency improved (1-2%). This is because the sine shape of the motorcurrent is almost perfect at high switching frequency.Efficiency of the system (ηSYSTEM )To calculate the system efficiency, the efficiency of VLT HVAC (ηVLT ) is multiplied by the efficiencyof the motor (ηMOTOR):ηSYSTEM) = η VLT x ηMOTORCalculate the efficiency of the system at different loads based on the graph above.VLT® HVAC Drive Design Guide 3. VLT HVAC SelectionMG.11.B2.02 - VLT® is a registered Danfoss trademark 613