3.4. Acoustic noise
The acoustic noise from the frequency converter comes from three sources:
1.
DC intermediate circuit coils.
2.
Integral fan.
3.
RFI filter choke.
The typical values measured at a distance of 1 m from the unit:
Encapsulation
At reduced fan speed
(50%)
Full fan speed
A2
51
60
A3
51
60
A5
-
-
B1
61
67
B2
58
70
C1
52
62
C2
55
65
3.5. Peak voltage on motor
When a transistor in the inverter bridge switches, the voltage across the motor increases by a dV/
dt ratio depending on:
-
the motor cable (type, cross-section, length screened or unscreened)
-
inductance
The natural induction causes an overshoot U
PEAK
in the motor voltage before it stabilises itself at
a level depending on the voltage in the intermediate circuit. The rise time and the peak voltage
U
PEAK
affect the service life of the motor. If the peak voltage is too high, especially motors without
phase coil insulation are affected. If the motor cable is short (a few metres), the rise time and
peak voltage are lower.
If the motor cable is long (100 m), the rise time and peak voltage increases.
If very small motors are used without phase coil insulation, connect an LC filter to the frequency
converter.
3. VLT HVAC Selection
VLT
®
HVAC Drive Design Guide
62
MG.11.B2.02 - VLT
®
is a registered Danfoss trademark
3