Preventive measures of traction motor over-voltage

Update:11 Oct 2021

Due to the different causes of over-voltage, the countermeasures adopted are different. For over-voltage during parking, if there is no special requirement for the stopping time or position, then the method of extending the inverter deceleration time or free stop can be adopted. To solve the problem; the so-called free stop means that the inverter disconnects the main switching device and allows the traction motor to coast to stop.
If there are certain requirements for the parking time or parking position, the DC braking (DC braking) function can be used. The DC braking function is to decelerate the traction motor to a certain frequency, and then pass DC power into the stator winding of the motor to form a static magnetic field. The motor rotor winding cuts this magnetic field to generate a braking torque, which turns the kinetic energy of the load into electrical energy and is consumed in the motor rotor circuit in the form of heat. Therefore, this kind of braking is also called dynamic braking. The process of DC braking actually includes two processes, regenerative braking and dynamic braking. The efficiency of this braking method is only 30-60% of regenerative braking, and the braking torque is small. Since consuming energy in the motor will overheat the traction motor, the braking time should not be too long. Moreover, the start frequency, braking time and braking voltage of DC braking are all set manually, and cannot be adjusted automatically according to the level of regenerative voltage. Therefore, DC braking cannot be used for the overvoltage generated in normal operation, but can only be used for Braking when parking.
For the over-voltage caused by excessive GD2 (flywheel torque) of the load when decelerating (turning from high speed to low speed, but not stopping), the method of appropriately extending the deceleration time can be taken to solve the problem. In fact, this method also uses the principle of regenerative braking. Extending the deceleration time is only to control the charging speed of the inverter by the regenerative voltage of the load, so that the 20% regenerative braking capacity of the inverter itself can be reasonably used. As for those loads that make the traction motor in a regenerative state due to external forces (including potential energy decentralization), because they are normally operating in a braking state, the regenerative energy is too high and cannot be consumed by the inverter itself, so it is impossible to use DC braking Or extend the deceleration time.

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