4.1 RULES FOR EMC COMPLIANT INVERTER - MOTOR WIRING
Correct wiring of the INVERTER - MOTOR unit must be performed as follows:
1- The main earth of the building must be connected directly both to the inverter and the motor.
2- The power cables for inverter/contactors and contactors/motor connection must be as short as possible,
four-pole (three phases plus yellow/green earth wire) and the shielded type, or four non-shielded cables
banded to each other and inserted inside a duct or a metal pipe connected to earth. In other words, in the
same cable or pipe there must be an earth wire as near as possible to the power cables. In the case of
the shielded cable, the continuity of the earth braid must be guaranteed between the inverter/contactors
and contactors/motor tract.
It is recommended to connect the earth shield from both sides, with a 360° connection or with special
terminals, which can be supplied by SMS.
If the 360° earth shield connection is not possible inside the motor terminal board, the shield on the
casing must be put to earth before entering the terminal board.
3- Even if not essential, it is also good practice to put the shielded cable in the input power line, in a way to
prevent irradiated interference are taken outside the cable.
The network input power cables and the inverter output cables must not be placed in the same duct and
must remain as far away as possible from each other (at least 50 cm.).
4- The power cables (input and output) and the inverter command cables must be as far away as possible
and not be parallel, even if shielded; if the cables should cross, they must be positioned in a way to form
a 90° angle.
5- Independently from the building general earth connection, the motor casing MUST be connected to the
cable shield and the yellow/green earth wire, which is inside the shielded cable.
6- The inverter emits irradiated interference; consequently this can be captured and taken outside the
control board by the cables, in particular the flexible cables which irradiate them into the run
compartment.
If you want to avoid this problem, for the command connections between board logic and inverter,
shielded wires must be used with the shield connected to earth on both sides. It is not allowed to use
shielded cables with the shield not connected to earth, as in this case the interference is greater than the
non-shielded cable.
Any free or non-used multi-core cable wire, must be connected to earth on both sides.
7- Any cable, both command and external connection for shaft and cabin, must never be near or parallel to the
power cable, even if shielded; if they must be parallel due to necessity, theymust be in distinct metal ducts.
8- All earth connections must be as short and wide as possible.
Solution (a) (copper braid) is preferable to solution (b) (wire).
9- To prevent undesired interventions of the RCD, it is good practice to:
•Make the power connection as short as possible
•Use suitable residual current devices (type A or B - 300mA)
•Decrease (where possible) the carrier frequency of the inverter: in fact, the lower the frequency, the
louder engine noise, but the lower the leakage currents to earth and the EMC interference; the motor
windings are less stressed.
