8.5.4 Motor InsulationFor motor cable lengths ≤, the maximum cable length listed in the GeneralSpecifications tables the following motor insulation ratings are recom-mended because the peak voltage can be up to twice the DC link voltage,2.8 times the AC line voltage, due to transmission line effects in the motorcable. If a motor has lower insulation rating, it is recommended to use adu/dt or sine-wave filter.Nominal AC Line Voltage Motor InsulationUN ≤ 420 V Standard U LL = 1300 V420 V < UN ≤ 500 V Reinforced ULL = 1600 V500 V < UN ≤ 600 V Reinforced ULL = 1800 V600 V < UN ≤ 690 V Reinforced ULL = 2000 V8.5.5 Motor Bearing CurrentsAll motors installed with AutomationDrive FC 302 125 hp [90 kW] or higher power drives should have NDE (Non-Drive End) insulated bearings installedto eliminate circulating bearing currents. To minimize DE (Drive End) bearing and shaft currents proper grounding of the drive, motor, driven machine,and motor to the driven machine is required.Standard Mitigation Strategies:1. Use an insulated bearing2. Apply rigorous installation procedures- Ensure the motor and load motor are aligned- Strictly follow the EMC Installation guideline- Reinforce the PE so the high frequency impedance is lower in the PE than the input power leads.- Provide a good high frequency connection between the motor and the adjustable frequency drive for instance by shielded cable whichhas a 360° connection in the motor and the adjustable frequency drive.- Make sure that the impedance from adjustable frequency drive to building ground is lower that the grounding impedance of the machine.This can be difficult for pumps.- Make a direct ground connection between the motor and load motor.3. Lower the IGBT switching frequency4. Modify the inverter waveform, 60° AVM vs. SFAVM5. Install a shaft grounding system or use an isolating coupling.6. Apply conductive lubrication7. Use minimum speed settings, if possible.8. Try to ensure the line voltage is balanced to ground. This can be difficult for IT, TT, TN-CS or Grounded leg systems9. Use a dU/dt or sinus filter8.6 Control Cables and Terminals8.6.1 Access to Control TerminalsAll terminals to the control cables are located underneath the terminal cover on the front of the adjustable frequency drive. Remove the terminal coverby means of a screwdriver (see figure).8 Electrical Installation FC 300 Design Guide8-32 MG.33.BC.22 - VLT® is a registered Danfoss trademark8
