Dynamic braking for various machines are as follows:

Shunt machine

• In dynamic breaking the motor is disconnected from the supply and connected to a dynamic braking resistance RDB. In and Fig this is done by changing the switch from position 1 to 2.

• The supply to the field should not be removed. Due to the rotation of the armature during motoring mode and due to the inertia, the armature continues to rotate.

• An emf is induced due to the presence of the field and the rotation. This voltage drives a current through the braking resistance. The direction of this current is opposite to the one which was flowing before change in the connection.

• Therefore, torque developed also gets reversed. The machine acts like a brake. The torque speed characteristics separate by excited shunt of the machine under dynamic braking mode is as shown in Fig. 49(b) for a particular value of RDB. The positive torque corresponds to the motoring operation.

• Fig. 50 shows the dynamic braking of a shunt excited motor and the corresponding torque-speed curve. Here the machine behaves as a self-excited generator. Below a certain speed the self-excitation collapses and the braking action becomes Zero.

Series machine

• In the case of a series machine the excitation current becomes zero as soon as the armature is disconnected from the mains and hence the induced emf also vanishes.

• In order to achieve dynamic braking the series field must be isolated and connected to a low voltage high current source to provide the field.

• Rather, the motor is made to work like a separately excited machine.

• When several machines are available at any spot, as in railway locomotives, dynamic braking is feasible.

• Series connection of all the series fields with parallel connection of all the armatures connected across a single dynamic braking resistor is used in that case.

Compound generators

• In the case of compound machine, the situation is like in a shunt machine. A separately excited shunt field and the armature connected across the braking resistance are used.

• A cumulatively connected motor becomes differentially compounded generator and the braking torque generated comes down.

• It is therefore necessary to reverse the series field if large braking torques is desired.