3.4.16 Motor Bearing Currents
Motors with a rating 110 kW or higher combined with
adjustable frequency drives are best with NDE (Non-Drive
End) insulated bearings to eliminate circulating bearing
currents caused by motor size. To minimize DE (Drive End)
bearing and shaft currents, proper grounding is required
for:
•
Adjustable frequency drive
•
Motor
•
Motor-driven machine
•
Motor to the driven machine
Although failure due to bearing currents is infrequent, use
the following strategies to reduce the likelihood:
•
Use an insulated bearing
•
Apply rigorous installation procedures
•
Ensure that 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
•
Ensure that the impedance from adjustable
frequency drive to building ground is lower than
the grounding impedance of the machine. Make
a direct ground connection between the motor
and load motor.
•
Apply conductive lubrication
•
Try to ensure that the line voltage is balanced to
ground.
•
Use an insulated bearing as recommended by the
motor manufacturer (note: motors from reputable
manufacturers typically have insulated bearings
as standard in motors of this size).
If found to be necessary and after consultation with
Danfoss:
•
Lower the IGBT switching frequency
•
Modify the inverter waveform, 60° AVM vs.
SFAVM
•
Install a shaft grounding system or use an
isolating coupling between motor and load
•
Use minimum speed settings, if possible.
•
Use a dU/dt or sinus filter
The electronic thermal relay in the adjustable frequency
drive has received UL-approval for single motor protection,
when
parameter 1-90 Motor Thermal Protection
is set for
ETR Trip
and
1-24 Motor Current
is set to the rated motor
current (see the motor nameplate).
For thermal motor protection, it is also possible to use the
MCB 112 PTC thermistor card option. This card provides an
ATEX certificate to protect motors in explosion hazard
areas, Zone 1/21 and Zone 2/22. When
parameter 1-90 Motor Thermal Protection
is set to [20] ATEX
ETR is combined with the use of MCB 112, it is possible to
control an Ex-e motor in explosion hazardous areas.
Consult the Programming Guide for details on how to set
up the adjustable frequency drive for safe operation of Ex-
e motors.
3.4.17 Control Cable Routing
Tie down all control wires to the designated control cable
routing as shown in
,
, and
. 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 communi-
cation bus instructions. The cable must be placed in the
provided path inside the adjustable frequency drive and
tied down together with other control wires (see
and
Installation
Instruction Manual
MG37A222
Danfoss A/S © Rev. 2014-02-07 All rights reserved.
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Summary of Contents for VLT AutomationDrive FC 302
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