Dynamic or capacitive braking of electrical machine

Dynamic braking is a process in which kinetic energy of rotor is dissipated in external resistor as heat energy. It is an electrical braking process used in many industrial applications. Dynamic braking allows sudden stop of electrical motor without mechanical wear and tear.

In dynamic braking, stator has no supply at the time of braking. However, rotor is rotating due to inertia. In synchronous motor, rotor has a supply even when stator is disconnected from supply. In permanent magnet synchronous motor, rotor is a permanent magnet. In case of induction motor, rotor is very likely to have a residual magnetism. Therefore, in major types
of motors, there is source of magnetic flux in rotor. When stator has no supply and rotor is rotating, this flux induces voltage in stator.In other words, motor is acting as generator and rotors kinetic energy is now transferred to stator as electrical energy. If we can just arrange a system that dissipates this energy, we actually can devise a dynamic braking.
Now look at the following figure.

Fig: Dynamic braking

If we disconnect stator from main supply by using switch set-1 and simultaneously apply capacitor bank across stator using switch set-2, we can store energy which comes from rotor magnetic field. Now if we connect a three-phase resistor bank across stator, this energy will be dissipated. Hence rotor loses its kinetic energy more rapidly and brake is experienced. This process is also called capacitive braking since capacitor is used in braking process. Capacitive braking is useful and low cost. Nevertheless, the capacitive braking process described here has a drawback. We get some energy but we are not using it, rather we dissipate it. A more efficient method can be thought by utilizing this energy and is called regenerative braking. My next post will be on regenerative braking. I will post the link here when I will be done. Meanwhile, you may read the following posts about reason of DC injection in electrical braking and  a simple explanation of induction machine principle. You may also look at Google provided links, but be aware of clicking on them, they may or may not be relevant. 

You may go through the following posts also.
1. Regenerative braking of electrical machines
2. Simple explanation of induction machine principle
3. Capacitive or dynamic braking of electrical machine