4.1.19 Motor Bearing Currents
It is generally recommended that motors of a rating 110kW or higher
operating via Variable Frequency Drives should have NDE (Non-Drive
End) insulated bearings installed to eliminate circulating bearing cur-
rents 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 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.
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 man-
ufacturer (note: Motors from reputable manufacturers will typ-
ically 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
4.1.20 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.
Fieldbus connection
Connections are made to the relevant options on the control card. For
details see the relevant fieldbus instruction. The cable must be placed
in the provided path inside the frequency converter and tied down to-
gether with other control wires (see illustrations).
176F
A246.10
U/T1 96
-DC 88
R/L1 91
S/L2 92
T/L3 93
+DC 89
V/T2 97
W/13
Illustration 4.16: Control card wiring path for the D3. Con-
trol card wiring for the D1, D2, D4, E1 and E2 use the
same path.
VLT HVAC Drive High Power Operating In-
structions
4 Electrical Installation
MG.11.F3.02 - VLT
®
is a registered Danfoss trademark
71
4