DC injection braking

When DC brake is called in drive technology, a method of braking are decelerated at the three-phase motors by means of direct current. So that the braking can be by means of direct current, the stator winding must be connected from the network and connected to a DC voltage source. Since the braking effect of the DC braking strong loss just before the stop, the braking process must be stopped with a mechanical brake to a stop.

Function

When the stator is connected to the DC voltage source, a direct current flows in the winding. This direct current is based on a DC magnetic field. This dc field induced in the rotating rotor in it a voltage. Because the rotor winding is short-circuited, a current in it is made to flow. Due to the low resistance of the rotor small induced voltages are already sufficient to generate a large current in the rotor. This current produces a magnetic field in the rotor. By this magnetic field, a force is exerted on the current-carrying conductor of the rotor which is directed according to the Lenz's law, so that the rotor is braked. Characterized the speed of the motor then drops. At the same time decreasing speed also decreases the frequency of the induced voltage in the rotor and hence the inductive reactance of the rotor. Characterized now prevails the resistance of the rotor. Due to the now predominantly ohmic resistance, the braking effect will increase with decreasing speed. The braking torque at the DC outlet braking strongly covered until just before standstill and decreases when the rotor is zero. After the stop, as opposed to plugging, be no acceleration of the motor in the opposite direction.

Circuit

The DC braking, it is necessary that there are two or three connected in series, phases of the stator winding can be supplied with direct current. The best braking effect is achieved when the stator winding is connected in star or delta either. The connection of the DC voltage is then each of two terminals of the stator winding. In the star connection can be either open or closed using a star configuration. The open star connection string will not be used in the closed star connection two strands are joined together. For generating the DC voltage, a transformer is used, the AC voltage is rectified by a bridge rectifier. So that the DC voltage can be connected to the stator winding, the supply voltage of the motor needs to be turned off earlier. Thus, the rectifier when switching from AC to DC voltage can not be destroyed by regeneratively generated AC voltages of the stator winding, the voltage level about a particular voltage relay is monitored. So that the windings will not be excessively heated by the DC, the DC power source is again turned off when the brake operation is completed.

Height of the DC voltage

Since only the ohmic resistance of the stator windings is effective for the direct current, the magnitude of the DC voltage used must be substantially less than the rms value of the rated motor voltage. The DC voltage may be about ten to fifteen percent of rated AC voltage. However, the amount of required DC voltage is directed not only on the nominal voltage of the motor, but also according to the rated motor power. For motors with high motor power lower voltage values ​​are needed with the same nominal voltage of the motor as for motors with low motor power. For a mean torque, which roughly corresponds to the rated motor torque, a direct current is required, which corresponds to about 2.5 times the rated motor current.

Application

The DC braking can be in slip ring motors and three-phase motors with squirrel cage rotor apply. With slip -ring motors can the speed corresponding to the associated brake torque influence by Läufervorwiderständen. The flywheel energy of the machine and also the stator supplied electrical power is converted into heat in the rotor. This caused the engine is heated during braking. For this reason, this braking method is not suitable for braking large flywheels. For the periodic braking of this method is not suitable.

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