Limiting The Maximum Dc Link Voltage - Siemens sinamics G120 Operating Instructions Manual

Advanced commissioning
6.6 Protection functions
6.6.5

Limiting the maximum DC link voltage

How does the motor generate overvoltage?
An induction motor operates as a generator if it is driven by the connected load. A generator
converts mechanical energy into electrical energy. The electrical energy flows back into the
inverter. If the inverter cannot dissipate the electrical energy, for example in a braking
resistor, then the DC-link voltage Vdc increases in the inverter.
Above a critical DC-link voltage both the inverter and the motor will be damaged. Before the
voltage can reach critical levels, however, the inverter switches the motor off with the fault
message "DC-link overvoltage".
Protecting the motor and inverter against overvoltage
The Vdc_max control avoids an excessively high DC-link voltage, assuming that the
application permits this. The Vdc_max control increases the ramp-down time of the motor
when braking, so that the motor feeds back only as little energy to the inverter as is covered
by the losses in the inverter.
The Vdc_max control is not suitable for applications where the motor is continuously in the
generator mode. This includes, for example, cranes or applications involving braking large
moments of inertia.
The Vdc_max control can only be used with the PM240, PM240-2 and PM340 Power
Modules. The Vdc_max control is not required if you use a braking resistor.
PM250 Power Modules feed back regenerative energy into the line supply. Therefore, the
Vdc_max control is not required for a PM250 Power Module.
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Electrically braking the motor (Page 271)
Operating Instructions, 01/2016, FW V4.7 SP6, A5E31759476B AE
Converter with CU250S-2 Control Unit