SME AC-M1, Manual

Page: 47 / 62

7 EMC Suggestions
7.1 General overview on EMC
Electromagnetic compatibility (EMC) encompasses two areas: emissions, i.e. the ability to
work without causing electromagnetic disturbances to the nearing devices, and immunity,
i.e. the ability to work in the presence of RF energy.
7.2 EM emissions
Signals with high frequency content can produce significant emissions if connected to a
large enough radiating area (created by long wires spaced far apart).
Also the contactor and motor drivers can emit significant disturbances, because their
outputs are pulse width modulated square waves that are rich in harmonics (however, if a
contactor supply is not modulated, its emission will be zero).
The best way to minimize this kind of emission is to make the wires from the controller to
the contactor or motor as short as possible and place, if possible, each current near its
return (i.e.: bundle contactor wires with coil return and bundle motor wires separately).
Another good solution is to put the controller, the wires, the motors and the contactor in a
shielded box, especially if very low emissions are required.
Emissions can also couple to battery supply leads and throttle circuit wires outside the
box, so ferrite beads near the controller may also be required on these unshielded wires in
some applications.
It is best to keep the noisy signals as far as possible from sensitive wires.
7.3 Immunity to EM disturbances
I
mmunity is generally achieved by preventing the external electromagnetic disturbance
from coupling into sensitive circuitry.
The RF energy can get into the controller circuitry via conducted paths and radiated paths.
Conducted paths are created by the wires connected to the controller.
They act as antennas and the amount of RF energy coupled into them is proportional to
their length.
The RF voltages and currents induced in each wire are applied to the controller pin to
which the wire is connected. SME controllers include bypass capacitors on the printed
circuit board’s wires to reduce the impact of this source of noise on the internal circuitry,
but in some applications an additional filtering in the form of ferrite beads might also be
required.
Radiated paths are created when the controller circuitry is immersed in an external field.
This radiation may couple with the traces on the board and generate various kinds of
malfunctions. If radiated disturbance is an issue, a good solution is to increase the
distance between the controller and the possible sources of disturbance or to shield the
controller by placing a metal enclosure around it.
If a shield is required, holes should be added for ventilation purposes. In this case, using
PRELIMINARY VERSION 47