Motor Bearing Currents; Extreme Running Conditions - Danfoss VLT Decentral Drive FCD 302 Design Manual

System Integration
Nominal Mains Voltage
≤420 V
U
N
420 V<U ≤500 V
N
Table 3.8 Mains Voltage and Motor Insulation
3 3

3.3.3.5 Motor Bearing Currents

All motors installed with FC 302 90 kW or higher power
drives 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 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
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-
-
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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
34
®
VLT
Motor Insulation
Standard U =1300 V
LL
Reinforced U =1600 V
LL
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
MG04H102 - VLT
Decentral Drive FCD 302

3.3.4 Extreme Running Conditions

Short Circuit (Motor Phase – Phase)
The frequency converter is protected against short circuits
with current measurement in each of the three motor
phases or in the DC link. A short circuit between two
output phases causes an overcurrent in the inverter. The
inverter is turned off individually when the short circuit
current exceeds the permitted value (Alarm 16 Trip Lock).
To protect the frequency converter against a short circuit
at the load sharing and brake outputs, see the design
guidelines.
Switching on the Output
Switching on the output between the motor and the
frequency converter is fully permitted. No damage to the
frequency converter can occur by switching on the output.
However, fault messages can appear.
Motor-generated Over-voltage
The voltage in the intermediate circuit is increased when
the motor acts as a generator, in the following cases:
1. The load drives the motor (at constant output
frequency from the frequency converter), that is,
the load generates energy.
2. During deceleration, ("ramp-down") if the
moment of inertia is high, the friction is low, and
the ramp-down time is too short for the energy
to be dissipated as a loss in the frequency
converter, the motor, and the installation.
3. Incorrect slip compensation setting can cause
higher DC link voltage.
4. Back-EMF from PM motor operation. When
coasted at high rpm, the PM motor back-EMF can
potentially exceed the maximum voltage
tolerance of the frequency converter and cause
damage. The frequency converter is designed to
prevent the occurrence of back EMF: the value of
4-19 Max Output Frequency is automatically
limited based on an internal calculation based on
the value of 1-40 Back EMF at 1000 RPM,
1-25 Motor Nominal Speed and 1-39 Motor Poles.
When motor overspeed is possible (for example,
due to excessive windmilling effects), then a
brake resistor is recommended.
NOTE
The frequency converter must be equipped with a break
chopper.
When possible, the control unit may attempt to correct the
ramp (2-17 Over-voltage Control.
The inverter turns off to protect the transistors and the
intermediate circuit capacitors when a certain voltage level
is reached.
®
is a registered Danfoss trademark