4.6.19 Motor Bearing Currents
It is generally recommended that motors of a rating 150 hp [110 kW] or higher operating via adjustable frequency drives should have NDE (Non-Drive
End) insulated bearings installed to eliminate circulating bearing currents due to the physical size of the motor. 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. Although failure due to bearing currents
is low and very dependent on many different items, for security of operation the following are mitigation strategies which can be implemented.
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 adjustable frequency drive for instance by shielded cable which has 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. Apply conductive lubrication
4. Try to ensure the line voltage is balanced to ground. This can be difficult for IT, TT, TN-CS or Grounded leg systems
5. Use an insulated bearing as recommended by the motor manufacturer (note: Motors from reputable manufacturers will typically have these
fitted as standard in motors of this size)
If found to be necessary and after consultation with Danfoss:
6. Lower the IGBT switching frequency
7.
Modify the inverter waveform, 60° AVM vs. SFAVM
8. Install a shaft grounding system or use an isolating coupling between motor and load
9. Use minimum speed settings, if possible.
10. Use a dU/dt or sinus filter
4.6.20 Control Cable Routing
Tie down all control wires to the designated control cable routing as shown in the picture. Remember to connect the shields in a proper way to ensure
optimum electrical immunity.
Serial communication bus connection
Connections are made to the relevant options on the control card. For details, see the relevant serial communication bus instruction. The cable must be
placed in the provided path inside the adjustable frequency drive and tied down together with other control wires (see pictures).
4 How to Install
VLT AQUA Low Harmonic Drive Instruction
Manual
4-52
MG.20.T1.22 - VLT
®
is a registered Danfoss trademark
4
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