Nominal Mains Voltage Motor InsulationUN≤420 V Standard ULL=1300 V420 VN≤500 V Reinforced ULL=1600 VTable 3.8 Mains Voltage and Motor Insulation3.3.3.5 Motor Bearing CurrentsAll motors installed with FC 302 90 kW or higher powerdrives should have NDE (Non-Drive End) insulated bearingsinstalled to eliminate circulating bearing currents. Tominimize DE (Drive End) bearing and shaft currents propergrounding of the drive, motor, driven machine, and motorto the driven machine is required.Standard Mitigation Strategies1. Use an insulated bearing2. Apply rigorous installation procedures- Ensure the motor and load motor arealigned- Strictly follow the EMC Installationguideline- Reinforce the PE so the high frequencyimpedance is lower in the PE than theinput power leads- Provide a good high frequencyconnection between the motor and thefrequency converter for instance byscreened cable which has a 360°connection in the motor and thefrequency converter- Make sure that the impedance fromfrequency converter to building groundis lower that the grounding impedanceof the machine. This can be difficult forpumps- Make a direct earth connection betweenthe motor and load motor3. Lower the IGBT switching frequency4. Modify the inverter waveform, 60° AVM vs.SFAVM5. Install a shaft grounding system or use anisolating coupling6. Apply conductive lubrication7. Use minimum speed settings if possible8. Try to ensure the line voltage is balanced toground. This can be difficult for IT, TT, TN-CS orGrounded leg systems9. Use a dU/dt or sinus filter3.3.4 Extreme Running ConditionsShort Circuit (Motor Phase – Phase)The frequency converter is protected against short circuitswith current measurement in each of the three motorphases or in the DC link. A short circuit between twooutput phases causes an overcurrent in the inverter. Theinverter is turned off individually when the short circuitcurrent exceeds the permitted value (Alarm 16 Trip Lock).To protect the frequency converter against a short circuitat the load sharing and brake outputs, see the designguidelines.Switching on the OutputSwitching on the output between the motor and thefrequency converter is fully permitted. No damage to thefrequency converter can occur by switching on the output.However, fault messages can appear.Motor-generated Over-voltageThe voltage in the intermediate circuit is increased whenthe motor acts as a generator, in the following cases:1. The load drives the motor (at constant outputfrequency from the frequency converter), that is,the load generates energy.2. During deceleration, ("ramp-down") if themoment of inertia is high, the friction is low, andthe ramp-down time is too short for the energyto be dissipated as a loss in the frequencyconverter, the motor, and the installation.3. Incorrect slip compensation setting can causehigher DC link voltage.4. Back-EMF from PM motor operation. Whencoasted at high rpm, the PM motor back-EMF canpotentially exceed the maximum voltagetolerance of the frequency converter and causedamage. The frequency converter is designed toprevent the occurrence of back EMF: the value of4-19 Max Output Frequency is automaticallylimited based on an internal calculation based onthe value of 1-40 Back EMF at 1000 RPM,1-25 Motor Nominal Speed and 1-39 Motor Poles.When motor overspeed is possible (for example,due to excessive windmilling effects), then abrake resistor is recommended.NOTEThe frequency converter must be equipped with a breakchopper.When possible, the control unit may attempt to correct theramp (2-17 Over-voltage Control.The inverter turns off to protect the transistors and theintermediate circuit capacitors when a certain voltage levelis reached.System Integration VLT® Decentral Drive FCD 30234 MG04H102 - VLT® is a registered Danfoss trademark33
