Question regarding voltage and current changes during inverter vector control
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Dear teachers, the frequency converter I am using is an AB Rockwell PowerFlex 70, with a control mode of FVC Vector (magnetic field-oriented vector control). The motor is a Siemens 11KW, grade 4 model, and an encoder is installed at its end to provide feedback on speed. A fault occurred during production: as the production line was coming to a stop, there was a speed reduction process, and when the speed dropped to about 60% of the normal operating level, the frequency converter stopped working, indicating an overcurrent fault. It could not be restarted, and the fault was attributed to the loss of signal from the encoder. The load on the production line is similar to that of a conveyor belt or winch under traction; it remains stable during normal operation, and decreases slightly during the deceleration process, but still stays at around 90%. I personally think it is caused by the increased current when the inverter operates at low frequencies, but since I am not an electrical engineer, I don’t fully understand the underlying principles. I would like to ask about the changes in voltage and current during low-frequency operation under vector control. Note: High precision is required for operation, and speed control needs to be sensitive; therefore, vector control is used.
In vector control mode, the inverter performs calculations based on the preset motor parameters in order to precisely control parameters such as the motor’s speed and torque. At low-frequency operation, the inductance of the motor is high and the current changes slowly; therefore, an increased output voltage is required to maintain the current necessary for the motor to operate. This could be one of the reasons for the problem you are experiencing. Additionally, it is also possible that changes in the motor’s load caused the current to exceed the inverter’s rated value, thereby leading to a fault. It is recommended to check whether the rated parameters of the inverter and the motor are compatible, and to adjust these parameters appropriately to ensure the stable operation of the system. Additionally, when troubleshooting, the impact of the encoder also needs to be considered. The encoder provides an accurate speed feedback signal; a failure in it can affect the control accuracy of the frequency converter. It is recommended to check the connection and status of the encoder, and carry out any necessary repairs or replacements. .