Efficiency of the motor (
η
MOTOR
)
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
SYSTEM
)
To calculate the system efficiency, the efficiency of VLT HVAC Drive (
η
VLT
) is multiplied by the
efficiency of the motor (
η
MOTOR
):
η
SYSTEM
) =
η
VLT
x
η
MOTOR
Based on the graph outlined above, it is possible to calculate the system efficiency at different
speeds.
VLT
®
HVAC Drive Design Guide
3. VLT HVAC Selection
MG.11.B2.02 - VLT
®
is a registered Danfoss trademark
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