Motor Bearing Currents - Danfoss VLT AutomationDrive FC 300 Design Manual

Electrical Installation
Frame Power Voltage 1 cable
Size Size [V] [m]
[kW]
A1, A2, 0.37-0.7 400 150
A5 5
500 150
A2, A5 1.1-1.5 400 150
500 150
A2, A5 2.2-4 400 150
500 150
A3, A5 5.5-7.5 400 150
500 150
B1, B2, 11-75 400 150
B3, B4,
500 150
C1, C2,
C3, C4
Table 7.35 Max. Cable Length for Each Parallel Cable, Depending
on Quantity of Parallel Cables.
Problems may arise at start and at low RPM values if motor
sizes are widely different because small motors' relatively
high ohmic resistance in the stator calls for a higher
voltage at start and at low RPM values.
The electronic thermal relay (ETR) of the frequency
converter cannot be used as motor protection for the
individual motor of systems with parallel-connected
motors. Provide further motor protection by e.g.
thermistors in each motor or individual thermal relays.
(Circuit breakers are not suitable as protection).
7.4.4 Motor Insulation
For motor cable lengths ≤ the maximum cable length
listed in 4.2 General Specifications, the following motor
insulation ratings are recommended 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.
Nominal Mains Voltage Motor Insulation
≤ 420 V Standard U
U
N
420V < U ≤ 500 V Reinforced U
N
500V < U ≤ 600 V Reinforced U
N
600V < U ≤ 690 V Reinforced U
N
Table 7.36 Motor Insulation
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®
VLT AutomationDrive FC 300 Design Guide, 0.25-75 kW
2 cables 3 cables 4 cables
[m] [m] [m]
45 8 6
7 4 3
45 20 8
45 5 4
45 20 11
45 20 6
45 20 11
45 20 11
75 50 37
75 50 37
= 1300V
LL
= 1600V
LL
= 1800V
LL
= 2000V
LL
®
MG33BE02 - VLT is a registered Danfoss trademark

7.4.5 Motor Bearing Currents

All motors installed with FC 302 90 kW or higher power
frequency converter 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 frequency converter,
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
7 7
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