Advanced commissioning
6.6 Protection functions
Converter with the CU240B-2 and CU240E-2 Control Units
274 Operating Instructions, 01/2016, FW V4.7 SP6, A5E34259001B AC
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.
Electrically braking the motor (Page 283)
The Vdc_max control is only possible with the PM230, 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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