Acoustic Noise - Danfoss VLT AutomationDrive FC 300 Design Manual

4. FC 300 Selection
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 500 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
4
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 FC 300 Series (η
ciency of the motor (η
η
SYSTEM

4.4.1. Acoustic Noise

The acoustic noise from the frequency converter comes from three sources:
1.
2.
3.
The typical values measured at a distance of 1 m from the unit:
FC 301/ FC 302
PK25-P7K5: @ 400 V
PK25-P7K5
Reduced fan speed
Full fan speed
When a transistor in the inverter bridge switches, the voltage across the motor in-
creases by a du/dt ratio depending on:
-
-
The natural induction causes an overshoot U
a level depending on the voltage in the intermediate circuit. The rise time and the peak voltage
U
PEAK
phase coil insulation are affected. If the motor cable is short (a few metres), the rise time and
peak voltage are lower.
74
MOTOR
SYSTEM
):
MOTOR
= η x η
VLT MOTOR
DC intermediate circuit coils.
Integral fan.
RFI filter choke.
the motor cable (type, cross-section, length screened or unscreened)
inductance
affect the service life of the motor. If the peak voltage is too high, especially motors without
MG.33.B9.02 - VLT
)
)
IP20/IP21/NEMA TYPE 1
IP55/NEMA TYPE 12
in the motor voltage before it stabilises itself at
PEAK
® is a registered Danfoss trademark
FC 300 Design Guide
) is multiplied by the effi-
VLT
51 dB(A)
60 dB(A)