3.2 Efficiency
Efficiency of VLT AQUA (η
VLT
)
The load on the adjustable frequency drive has little effect on its efficiency. In general, the efficiency is the same at the rated motor frequency f
M,N
, even
if the motor supplies 100% of the rated shaft torque or only 75%, i.e., in case of partial loads.
This also means that the efficiency of the adjustable frequency drive 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 greater than 5 kHz. The efficiency will also be slightly reduced if the AC line
voltage is 480 V, or if the motor cable is longer than 98.43 ft [30 m].
Efficiency of the motor (η)
MOTOR
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 when it 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, in motors from 15 hp [11 kW] and up, the advantages are
significant.
In general, the switching frequency does not affect the efficiency of small motors. The efficiency of motors from 15 hp [11 kW] and up improves by 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 AQUA (η
VLT
) is multiplied by the efficiency of the motor (η
MOTOR
):
η
SYSTEM
) = η
VLT
x η
MOTOR
Calculate the efficiency of the system at different loads based on the graph above.
VLT
®
AQUA Drive Design Guide 3 VLT AQUA Selection
MG.20.N5.22 - VLT
®
is a registered Danfoss trademark
3-19
3
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