3.4 Peak voltage on motor
When a transistor in the inverter bridge switches, the voltage across the motor increases by a du/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 stabilizes 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.
In motors without phase insulation paper or other insulation reinforcement suitable for operation with voltage supply (such as a frequency converter),
fit a sine-wave filter on the output of the frequency converter.
To obtain approximate values for cable lengths and voltages not mentioned below, use the following rules of thumb:
1.
Rise time increases/decreases proportionally with cable length.
2.
U
PEAK
= DC link voltage x 1.9
(DC link voltage = Mains voltage x 1.35).
3.
dU
/
dt
=
0.8 ×
UPEAK
Risetime
Data are measured according to IEC 60034-17.
Cable lengths are in metres.
FC 202, P7K5T2
Cable
length [m]
Mains
voltage [V]
Rise time
[
μ
sec]
Vpeak
[kV]
dU/dt
[kV/
μ
sec]
5
230
0.13
0.510
3.090
50
230
0.23
2.034
100
230
0.54
0.580
0.865
150
230
0.66
0.560
0.674
FC 202, P11KT2
Cable
length [m]
Mains
voltage [V]
Rise time [
μ
sec]
Vpeak
[kV]
dU/dt [kV/
μ
sec]
36
240
0.264
0.624
1.890
136
240
0.536
0.596
0.889
150
240
0.568
0.568
0.800
FC 202, P15KT2
Cable
length [m]
Mains
voltage [V]
Rise time
[
μ
sec]
Vpeak
[kV]
dU/dt
[kV/
μ
sec]
30
240
0.556
0.650
0.935
100
240
0.592
0.594
0.802
150
240
0.708
0.587
0.663
VLT
®
AQUA Drive Design Guide
3 VLT AQUA Selection
MG.20.N5.02 - VLT
®
is a registered Danfoss trademark
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