10/05 AWB8230-1541GB
Motor and circuit type
23
When an individual motor is switched into the frequency inverter’s
output, it behaves as if is connected directly to the electrical mains.
When you select a frequency inverter, take into account the
highest possible inrush current and use a motor reactor or a
sinusoidal filter.
Motor cables
To ensure EMC, use only screened motor cables. The length of the
motor cables and related components has an influence on control
mode and operating behaviour. In parallel operation (multiple
motors connected to the frequency inverter output), the resulting
cable lengths l
res
must be calculated:
l
res
= Sl
M
x WnM
Sl
M
: Sum of all motor cable lengths
n
M
: Number of motor circuits
To optimize drive behaviour, keep the motor cables as short as
possible.
Motor chokes, du/dt filters, sinusoidal filters
Motor reactors compensate for capacitive currents with long
motor cables and with grouped drives (multiple connection of
parallel drives to a single inverter).
The use of motor reactors is recommended for (observe the
manufacturers’ instructions):
• grouped drives
• three-phase asynchronous motors with a maximum frequencies
above 200 Hz,
• for the operation of reluctance motors or permanently excited
synchronous motors with maximum frequencies above 120 Hz.
du/dt filters are used for limiting the rate of voltage rise at the
motor terminals to values below 500 V/ms. They should be applied
for motors with unknown or insufficient withstand voltage for the
insulation.
When sinusoidal filters are used, the motors are supplied with
near-sinusoidal voltage and current.
h
With long motor cables, the leakage currents caused by
parasitic cable capacities can cause the “earth fault”
message. In this case, motor filters must be used.
h
If the cable from frequency inverter to motor is longer
than about 10 m, the existing thermal relays (bimetallic
relays) may malfunction due to high frequency harmonics.
Install a motor reactor at the frequency inverter’s output
in this case.
Caution!
During the engineering phase, keep in mind that the
voltage drop across motor filters and du/dt filters can be
up to 4 % of the frequency inverter’s output voltage.
Caution!
During the engineering phase, keep in mind that the
sinusoidal filter must be matched to the output voltage
and to the frequency inverter’s clock frequency.
The voltage drop across the sinusoidal filter can be up to
15 % of the frequency inverter’s output voltage.
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