Motor Insulation; Motor Bearing Currents - Danfoss VLT FC 102 Series Design Manual

Electrical Installation
Motor
U V W
2 2 2
U V W
1 1 1
FC
96 97 98
Motor
U V W
2 2 2
U V W
1 1 1
FC
96 97 98
Figure 6.42 Motor Rotation Check Steps

6.4.5 Motor Insulation

For motor cable lengths ≤ the maximum cable length
listed in chapter 9 General Specifications and Trouble-
shooting, the motor insulation ratings listed in Table 6.36
are recommended. If a motor has lower insulation rating, it
is recommended to use a dU/dt or sine-wave filter.
Nominal AC Line Voltage [V] Motor Insulation [V]
≤ 420
U
N
420 V < U ≤ 500
N
500 V < U ≤ 600
N
600 V < U ≤ 690
N
Table 6.36 Motor Insulation
MG11BC22
Design Guide
Standard U = 1300
LL
Reinforced U = 1600
LL
Reinforced U = 1800
LL
Reinforced U = 2000
LL
Danfoss A/S © Rev. 06/2014 All rights reserved.

6.4.6 Motor Bearing Currents

All motors installed with FC 102 90 kW or higher power
adjustable frequency drive 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 adjustable
frequency 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
2a Ensure the motor and load motor are
aligned.
2b Strictly follow the EMC Installation
guideline.
2c Reinforce the PE so the high frequency
impedance is lower in the PE than the
input power leads.
2d 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.
2e 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.
2f Make a direct ground 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.
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