General
vacon • 5
Local contacts: https://www.danfoss.com/en/contact-us/contacts-list/
Grounding
Screening
1.3
General
AC drives do not supply a smoothly changing AC voltage to the motor, but a pulsed DC voltage. The
sum of the three phase voltages does not sum to zero, as in the case of mains supply, leading to the
presence of a common-mode voltage between the motor coils and ground.
The pulses fed to the motor have very steep flanks, and the rate of voltage change (dU/dt) may reach
several kV/µs. Unloaded IGBTs will switch at speeds of about 6 - 8 kV/µs, when a motor cable and
motor is connected, and the speed will drop to about 2 kV/µs. The voltages caused by these steep
pulses may be dangerous to the insulation of the motor.
The high switching speeds give rise to EMC effects, both as radiated disturbances as well as
disturbance currents in galvanically connected cables. The radiated interference is usually
grounded by the metallic enclosure of the drive as well as through the use of shielded cable, and
does not cause any problems in the environment. The galvanically coupled interference may have to
be filtered in order to achieve EMC compliance. The VACON
®
NX range of AC drives contain an
internal filter for bringing the drives into compliance with the standard (specifically IEC 61800-
3:2004). In some cases, additional filtering is required. Both input and output filters are available for
the VACON
®
NX range of drives. The drive may also have to be protected against various
overvoltages occurring in the supply. On the input side RFI filters and additional chokes can be
installed, on the output side dU/dt filters or sine filters.
WARNING
: The unit must be grounded before switching the power on.
Please refer to the data sheet for detailed information.
It is recommended to use shielded cables to reduce noise, electromagnetic
interference and to prevent malfunctions in the installation.
Preferably:
• Screened and/or twised cable between the converter output and unit input (U1, V1, W1).
• Shielded cables between the unit output (U2, V2, W2) and the motor.
• In case of unscreened cables it should be ensured that the installation minimizes the
possibility of cross-couplings with other cables which are carrying sensitive signals.
This can be achieved by mea-sures such as cable segregation and mounting in earthed
cable trays.
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