33
VLT
®
Series 3500
Electrical Noise
In general, electrical noise can be divided into two forms:
wire-borne noise (EMI) and radiating noise (RFI).
As shown in the sketch below, the capacitance of the
motor wires combined with a high dV/dt from the motor
voltage generates noise.
The use of shielded motor cables increases the noise
current (I
1
) as indicated in the figure below. This is
because shielded wires have a greater capacitance than
unshielded wires. If the noise current is not filtered, there
will be more noise on the AC line in the radio noise
range, below approximately 5 MHz. Since the noise
current (I
1
) is taken back to the unit through the shield (I
3
),
only a small electromagnetic field will in principle be
generated from the shielded motor wires. See the figure
below.
The shield reduces the radiating noise, but increases the
low-frequency noise on the AC line. With an EMI filter, the
noise level on the AC line will be reduced to about the
same level for shielded and unshielded wires alike.
The motor wiring shield must be connected in the
enclosure of the drive as well as at the motor. The best
way of doing this is by using shield brackets to avoid
“pigtail” shield ends. Even short “pigtails” increase the
shield’s impedance at higher frequencies, which reduces
the shield’s effect and increases the noise current.
If a shielded cable is used for serial communication and
signal interface, the shield must be connected at both
ends. In certain situations, however, it will be necessary
to break the shield to avoid current loops.
In cases where the shield is to be connected to a
mounting plate, the mounting plate must be made of
metal, because the shield currents are to be taken back
to the unit. It is also important to ensure good, electric
contact from the mounting plate through the mounting
screws to the chassis of the VLT Series 3500.
It is generally easier and less complicated to use
unshielded motor wires than shielded cables. If
unshielded wires are used, the RFI will be greater. Since
the strength of the radiated signal decreases with
distance from the signal source, radiated noise is
generally not a problem.
In order to reduce the noise level from the total system
(drive + installation) to the greatest possible extent, it is
important to make the motor wiring as short as possible.
Incoming power and control wiring should never be run in
the same conduit or raceway as motor wiring.
