Efficiency - Danfoss VLT Series Design Manual

®
VLT HVAC Drive Design Guide

3.3. Efficiency

Efficiency of VLT HVAC (η
The load on the frequency converter has little effect on its efficiency. In general, the efficiency is
the same at the rated motor frequency f
torque 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/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
efficiency will also be slightly reduced if the mains voltage is 480 V, or if the motor cable is longer
than 30 m.
Efficiency of the motor (η
The efficiency of a motor connected to the frequency converter depends on magnetising level. In
general, the efficiency is just as good as with mains 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 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, in
motors from 11 kW and up, the advantages are significant.
In general, the switching frequency does not affect the efficiency of small motors. Motors from
11 kW and up have their efficiency improved (1-2%). This is because the sine shape of the motor
current is almost perfect at high switching frequency.
Efficiency of the system (η
To calculate the system efficiency, the efficiency of VLT HVAC (η
of the motor (η
η
SYSTEM
Calculate the efficiency of the system at different loads based on the graph above.
)
VLT
MOTOR
SYSTEM
):
MOTOR
) = η x η
VLT MOTOR
®
MG.11.B2.02 - VLT is a registered Danfoss trademark
, even if the motor supplies 100% of the rated shaft
M,N
)
)
VLT
3. VLT HVAC Selection
) is multiplied by the efficiency
3
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