In the AC speed control system using the frequency converter, the motor speed reduction is achieved by reducing the inverter output frequency. When the motor is required to decelerate at a faster speed than the free deceleration, the rate of decrease of the output frequency of the inverter can be increased, and the output speed corresponding to the output speed is lower than the actual speed of the motor, and the motor is regeneratively braked.
Rail transit resistance mainly includes urban rail transit braking resistors, high speed railway braking resistors, locomotive braking resistors, grounding resistors, pre-charging resistors and filter resistors. When the urban rail transit is braking, the traction motor works in the power generation state, and the energy is fed back to the grid. When this part of the energy cannot be completely absorbed by other vehicles or electrical equipment, the grid voltage will rise.
In this case, the asynchronous motor will become an asynchronous generator, and the mechanical energy of the load will be converted to electrical energy and fed back to the frequency converter. However, when the above-mentioned feeding energy is too large, the overvoltage protection circuit of the inverter itself will operate and cut off the output of the inverter, so that the motor is in a free deceleration state, but the purpose of rapid deceleration cannot be achieved. In order to avoid the above phenomenon, the above energy is self-consumption in other parts of the DC intermediate circuit without causing a voltage rise, and a regenerative braking circuit is usually employed in the voltage type inverter.
Power resistors mainly include neutral grounding resistance, neutral point grounding device, HVDC filter resistor, SVC filter resistor, high voltage start resistor and pre-charge resistor.
In the regenerative braking circuit, when the voltage of the self-flow intermediate circuit rises to a certain value, the brake transistor will be turned on, so that the DC voltage is discharged through the braking resistor, and the energy fed back to the DC circuit is in the form of heat energy Use up.