Investigation of Braking Modes in the Electric Drive with Direct Torque Control System

The paper is concerned with braking modes of the automatic AC electric drive with the asynchronous motor supplied from the frequency converter with the DC link with an autonomous voltage inverter and the Direct Torque Control (DTC) system. Such electric drives do not support the mode of power regeneration into the ac supply mains. If in the operating practice braking modes occur relatively rarely, then braking resistors are applied, which are connected to the DC link by means of a switch. During braking, the voltage in the direct current buses increases due to the capacitor charge. This switch opens and connects the braking resistor to the direct current buses when the voltage in these buses achieves a certain value during braking, and when the voltage decreases, the switch closes. The paper describes the investigation of electric drive operation with the DTC system in braking modes in order to study the converter operating mode with the braking resistance and to estimate the currents and voltages in the converter DC link for different electrical circuit parameters providing the motor braking. A model of an automatic AC electric drive with an asynchronous motor supplied from a frequency converter with a DC link with an autonomous voltage inverter and the Direct Torque Control (DTC) system was developed. It was shown that the decrease in the size of the braking resistor significantly increases the maximum current through the braking resistor, however, the integral value of current through the resistor does not change. The voltage in the DC link increases when the resistance value of the brake resistor decreases. It was shown that at a certain resistance value of the braking resistor, the capacitor voltage in the DC link will vary within the set points of closing and opening of the switch controlling the braking resistor.