Motor Insulation; Motor Bearing Currents - Danfoss FC 300 Design Manual

FC 300 Design Guide

8.5.4 Motor Insulation

For motor cable lengths ≤ the maximum cable length listed in the General
Specifications 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 mains voltage, due to transmission line effects in the motor
cable. If a motor has lower insulation rating it recommended to use a du/
dt or sine wave filter.

8.5.5 Motor Bearing Currents

All motors installed with AutomationDrive FC 302 90 kW or higher power drives should have NDE (Non-Drive End) insulated bearings installed to 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 bearing
2. 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 frequency converter for instance by screened cable which has
a 360° connection in the motor and the frequency converter
- Make sure that the impedance from frequency converter to building ground is lower that the grounding impedance of the machine.
This can be difficult for pumps
- Make a direct earth connection between the motor and load motor
3. Lower the IGBT switching frequency
4. Modify the inverter waveform, 60° AVM vs. SFAVM
5. Install a shaft grounding system or use an isolating coupling
6. Apply conductive lubrication
7. 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 systems
9. Use a dU/dt or sinus filter
Nominal Mains Voltage
≤ 420 V
U
N
420 V < U
500 V < U
600 V < U
MG.33.BC.02 - VLT ® is a registered Danfoss trademark
8 Electrical Installation
Motor Insulation
Standard U = 1300 V
LL
≤ 500 V
Reinforced U
N LL
≤ 600 V Reinforced U
N LL
≤ 690 V
Reinforced U
N LL
= 1600 V
= 1800 V
= 2000 V
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