Motor And Inverter Protection By Limiting The Voltage - Siemens SINAMICS G120 Operating Instructions Manual

Advanced commissioning

6.24 Motor and inverter protection by limiting the voltage

6.24 Motor and inverter protection by limiting the voltage
What causes an excessively high voltage?
To drive the load, an electric motor converts electrical energy into mechanical energy. If the
motor is driven by its load, e.g. due to the load moment of inertia when braking, then the energy
flow reverses: The motor temporarily operates as generator, and converts mechanical energy
into electrical energy. In inverter operation, the electrical energy flows from the motor into the
inverter. If the inverter cannot output the electrical energy supplied by the motor, e.g. to a
braking resistor, then the inverter stores the energy in its DC link capacitance. As a
consequence, the DC link voltage Vdc in the inverter is higher.
An excessively high DC link voltage damages the inverter and also the motor. As a
consequence, the inverter monitors its DC link voltage - and when necessary switches off the
motor and outputs fault "DC link overvoltage".
Protecting the motor and inverter against overvoltage
Figure 6-61
The Vdc_max control extends the motor ramp-down time when braking. The motor then only
feeds so much energy into the inverter to cover the losses in the inverter. The DC link voltage
remains in the permissible range.
The Vdc_max control is not suitable for applications where the motor is in continuous
regenerative operation, e.g. as is the case for cranes and centrifuges.
322
Simplified representation of the Vdc_max control
Electrically braking the motor (Page 302)
Converter with the CU240B-2 and CU240E-2 Control Units
Operating Instructions, 01/2017, FW V4.7 SP6, A5E34259001B AD