GE AF-650 GP Design Manual page 36

3.5 General Aspects of EMC
3.5.1 General Aspects of EMC Emissions
Electrical interference is usually conducted at frequences in the range 150 kHz to 30 MHz. Airborne interference from the drive system in the range 30 MHz to 1
GHz is generated from the inverter, motor cable, and the motor.
As shown in the illustration below, capacitive currents in the motor cable coupled with a high dV/dt from the motor voltage generate leakage currents.
The use of a screened motor cable increases the leakage current (see illustration below) because screened cables have higher capacitance to earth than
unscreened cables. If the leakage current is not filtered, it will cause greater interference on the mains in the radio frequency range below approximately 5 MHz.
Since the leakage current (I ) is carried back to the unit through the screen (I
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motor cable according to the below figure.
The screen reduces the radiated interference but increases the low-frequency interference on the mains. The motor cable screen must be connected to the
frequency converter enclosure as well as on the motor enclosure. This is best done by using integrated screen clamps so as to avoid twisted screen ends (pigtails).
These increase the screen impedance at higher frequencies, which reduces the screen effect and increases the leakage current (I
If a screened cable is used for networknetwork, relay, control cable, signal interface and brake, the screen must be mounted on the enclosure at both ends. In
some situations, however, it will be necessary to break the screen to avoid current loops.
If the screen is to be placed on a mounting plate for the frequency converter, the mounting plate must be made of metal, because the screen currents have to
be conveyed back to the unit. Moreover, ensure good electrical contact from the mounting plate through the mounting screws to the frequency converter chassis.
When unscreened cables are used, some emission requirements are not complied with, although the immunity requirements are observed.
In order to reduce the interference level from the entire system (unit + installation), make motor and brake cables as short as possible. Avoid placing cables with
a sensitive signal level alongside motor and brake cables. Radio interference higher than 50 MHz (airborne) is especially generated by the control electronics.
), there will in principle only be a small electro-magnetic field (I
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AF-650 GP Design Guide
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) from the screened
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).
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