Danfoss VLT FC 360 Design Manual

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Contents

Danfoss VLT FC 360 Design Manual Manual pdf 34 page image

2.6 General Aspects of EMC2.6.1 General Aspects of EMC EmissionsElectrical interference is usually conducted at frequencies in the range 150 kHz to 30 MHz. Airborne interference from thefrequency converter system in the range 30 MHz to 1G Hz is generated from the inverter, motor cable and the motor.Capacitive currents in the motor cable coupled with a high dU/dt from the motor voltage generate leakage currents.The use of a screened motor cable increases the leakage current (see Illustration 2.17) because screened cables have highercapacitance to earth than unscreened cables. If the leakage current is not filtered, it will cause greater interference on themains in the radio frequency range below approximately 5 MHz. Since the leakage current (I1) is carried back to the unitthrough the screen (I3), there will only be a small electro-magnetic field (I4) from the screened motor cable.The screen reduces the radiated interference but increases the low-frequency interference on the mains. The motor cablescreen must be connected to the frequency converter enclosure as well as the motor enclosure. This is best done by usingintegrated screen clamps so as to avoid twisted screen ends (pigtails). These increase the screen impedance at higherfrequencies, which reduces the screen effect and increases the leakage current (I4).If a screened cable is used for fieldbus, relay, control cable, signal interface and brake, the screen must be mounted on theenclosure at both ends. In some situations, however, it will be necessary to break the screen to avoid current loops.12zzzL1L2L3PEUVWCSI2I1I3I4C S CS CSCSI4CSz PE3 4 5 6175ZA062.12Illustration 2.17 EMC Emission1 Earth wire2 Screen3 AC mains supply4 Frequency converter5 Screened motor cable6 MotorTable 2.10 Legend to Illustration 2.17If 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 themounting plate through the mounting screws to the frequency converter chassis.When unscreened cables are used, some emission requirements are not complied with, although the immunityrequirements are observed.To reduce the interference level from the entire system (unit + installation), make motor and brake cables as short aspossible. Avoid placing cables with a sensitive signal level alongside motor and brake cables. Radio interference higher than50 MHz (airborne) is especially generated by the control electronics.Product Overview VLT® AutomationDrive FC 360 Design Guide32 MG06B202 - VLT® is a registered Danfoss trademark22