Danfoss FC 102 Design Manual

Also see for VLT HVAC Drive FC 102: Operating instructions Service guide Programming guide Installation guide Installation guide

Contents

2.9 General Aspects of EMCElectrical 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 1 GHz is generated from the inverter, motor cable, and the motor.As shown in Illustration 2.32, capacitance in the motor cable coupled with a high dU/dt from the motor voltage generateleakage currents.The use of a screened motor cable increases the leakage current (see Illustration 2.32) because screened cables have highercapacitance to earth than unscreened cables. If the leakage current is not filtered, it causes 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 is in principle only a small electro-magnetic field (I4) from the screened motor cable accordingto Illustration 2.32.The screen reduces the radiated interference, but increases the low-frequency interference on the mains. Connect the motorcable screen to the frequency converter enclosure as well as on the motor enclosure. This is best done by using integratedscreen clamps so as to avoid twisted screen ends (pigtails). Pigtails increase the screen impedance at higher frequencies,which reduces the screen effect and increases the leakage current (I4).If a screened cable is used for relay, control cable, signal interface and brake, mount the screen on the enclosure at bothends. In some situations, however, it is necessary to break the screen to avoid current loops.12zzzL1L2L3PEUVWCSI2I1I3I4C S CS CSCSI4CSz PE3 4 5 6175ZA062.12Illustration 2.32 Situation that Generates Leakage Currents1 Earth wire 4 Frequency converter2 Screen 5 Screened motor cable3 AC mains supply 6 MotorTable 2.16 Legend to Illustration 2.32If the screen is to be placed on a mounting plate for the frequency converter, the mounting plate must be made of metal,to convey the screen currents back to the unit. Moreover, ensure good electrical contact from the mounting plate throughthe mounting screws to the frequency converter chassis.When unscreened cables are used, some emission requirements are not complied with, although most 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. See for more information on EMC.Introduction to VLT® HVAC D... Design GuideMG11BC02 Danfoss A/S © Rev. 06/2014 All rights reserved. 412 2