Danfoss VLT AQUA Drive Design Manual page 56

3. VLT AQUA Selection
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 line voltage is 480 V, or if the motor cable is longer
than 98.43 ft. [30 m].
Efficiency of the motor (η
3
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; however, it 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
To calculate the system efficiency, the efficiency of VLT AQUA (η
of the motor (η
η
SYSTEM
Calculate the efficiency of the system at different loads based on the graph above.
3.3. Acoustic noise
The acoustic noise from the adjustable frequency drive comes from three sources:
1.
2.
3.
Typical values are measured at a distance of 3.28 ft. [1 m] from the unit:
A2
A3
A5
B1
B2
C1
C2
56
MOTOR
SYSTEM
):
MOTOR
) = η x η
VLT MOTOR
DC intermediate circuit coils.
Integrated fan.
RFI filter choke.
Encapsulation
MG.20.N2.22 - VLT
)
)
VLT
At reduced fan speed (50%)
[dBA]
51
51
-
61
58
52
55
® is a registered Danfoss trademark.
VLT ® AQUA Drive Design Guide
) is multiplied by the efficiency
Full fan speed [dBA]
60
60
54
67
70
62
65