Peak voltage on motor
When a transistor in the inverter is opened, the voltage
across the motor increases by a dU/dt ratio that de-
pends on:
-
the motor cable (type, cross-section, length
screened or unscreened)
-
inductance
The natural induction causes an overshot U
PEAK
in the
motor voltage before it stabilises itself at a level which
depends 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, motors
without phase coil insulation are the ones that will pri-
marily be 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 will increase.
If very small motors are used without phase coil insu-
lation, it is recommended to fit a sine-wave filter after
the frequency converter.
Typical values for the rise time and peak voltage
U
PEAK
are measured on the motor terminals between
two phases.
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
=
0.5 ×
U
PEAK
Risetime
Data are measured according to IEC 60034-17.
VLT 5001-5011 / 380-500 V
Cable
length
Mains
voltage
Rise time
Peak
voltage
dU/dt
50 metres 500 V
0.5 µsec.
1230 V
1968 V/µsec.
150 metres 500 V
1 µsec.
1270 V
1270 V/µsec.
50 metres 380 V
0.6 µsec.
1000 V
1333 V/µsec.
150 metres 380 V
1.33 µsec.
1000 V
602 V/µsec.
VLT 5016-5102 / 380-500 V
Cable
length
Mains
voltage
Rise time
Peak
voltage
dU/dt
32 metres 380 V
0.27 µsec.
950 V
2794 V/µsec.
70 metres 380 V
0.60 µsec.
950 V
1267 V/µsec.
132 metres 380 V
1.11 µsec.
950 V
685 V/µsec.
VLT 5122-5302 / 380-500 V
Cable
length
Mains
voltage
Rise time
Peak
voltage
dU/dt
70 metres 400 V
0.34 µsec.
1040 V
2447 V/µsec.
VLT 5352-5552 / 380-500 V
Cable
length
Mains
voltage
Rise time
Peak
voltage
dU/dt
29 metres 500 V
0.71 µsec.
1165 V
1389 V/µsec.
29 metres 400 V
0.61 µsec.
942 V
1233 V/µsec.
VLT 5001-5011 / 525-600 V
Cable
length
Mains
voltage
Rise time
Peak
voltage
dU/dt
35 metres 600 V
0.36 µsec.
1360 V
3022 V/µsec.
VLT 5016-5062 / 525-600 V
Cable
length
Mains
voltage
Rise time
Peak
voltage
dU/dt
35 metres 575 V
0.38 µsec.
1430 V
3011 V/µsec.
VLT 5042-5352 / 525-690 V
Cable
length
Mains
voltage
Rise time
Peak
voltage
dU/dt
25 metres 690 V
0.59 µsec.
1425
1983 V/µsec.
25 metres 575 V
0.66 µsec.
1159
1428 V/µsec.
25 metres
690 V
1)
1.72 µsec.
1329
640 V/µsec.
1) With Danfoss dU/dt filter.
VLT 5402-5602 / 525-690 V
Cable
length
Mains
voltage
Rise time
Peak
voltage
dU/dt
25 metres 690 V
0.57 µsec.
1540
2.230 V/µsec.
25 metres 575 V
0.25 µsec. 2.510 V/µsec.
25 metres
690 V
1)
1.13 µsec.
1629
1.150 V/µsec.
1) With Danfoss dU/dt filter.
VLT
®
5000 Design Guide
112 MG.52.B2.02 - VLT
®
is a registered Danfoss trademark
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