10 Ultrasound Controller Manual
4. Electric Noise
Ground Connection and Sensor Cable
All sensor cables must be shielded. The unshielded part of the
sensor cable should be maximum 40mm. The sensor cable may not
be extended.
Best connection to ground is achieved by using grounded cable
glands on the sensor cables. Only if the sensor cable is not grounded
via the cable gland, is it recommended to connect terminal 16 on the
D72, DP72, G72, GP72, GS72 or terminal 19 on D128, G128 to
ground, near the transmitter as shown in the picture.
Electric noise, with frequency around 2 MHz, entering the sensor
cable can cause measurement problems and is better to be minimized by using shielded cables
and by not routing the sensor cables along high voltage cables connected to a variable frequency
drive.
Noise from Variable Frequency Drives
Variable frequency drives produces noise which exist mainly in the cable connecting the variable
frequency drive to the motor. It is good if this cable is shielded and not routed along the sensor
cables. The variable frequency drive also produces ground current noise, caused by noise
travelling capacitively to ground inside the motor. To reduce
such ground current, a common mode filter can be used. The
most simple common mode filter is a ferrite clamp, clamped on
the three wires coming out of the variable frequency drive. An
even more efficient filter is a toroid. It consist of a ferrite toroid
with the three wires, connected to the 3-phase output of the
variable frequency drive, threaded through the toroid as shown
in the picture.
The filter in the picture is very good and is made of two ferrite
toroids. The three wires are wound three times through the
toroid. The ground wire do not go through the toroid.
Ferroxcube toroid TN25/15/10-3F3 is suitable for wires up to
2,5mm
2
and it is shown in the picture. Alternatively, EPCOS
toroid B64290L616X830 can be used. If the wires do not fit, a
larger toroid can be used. Ferroxcube toroid of material 3F3 or
EPCOS toroid of material N30 or Amidon toroid of material 77
are suitable for filtering frequencies around 2MHz.
5. Ex-installation with ex-barriers
Ex-barriers are not shielded so if ex-barriers are
used, Ex-barrier shields must be inserted on
each side of the group of EX-barriers connected
to each Ultrasound Controller (see picture). The
ex-barrier connects the sensor cable shield to
ground which makes it possible for ground
currents to enter. Because of this, minimizing
ground currents is very important especially
when EX-barriers are used.
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