Breaking of dc motor

In this article we will discuss about the breaking of dc motor. we will also learn that how to stop a dc motor

Breaking of D.C Motor

Breaking is used for stopping for a motor and its load. Mechanical braking and electric breaking are used. In case of mechanical breaking it is difficult to achieve a smooth stop because it depends on the condition of the breaking surface and on operator skills. The electric breaking may be done for various reasons such as.

Main reason of breaking of dc motor

1. To extend the brake power of the mechanical brakes.
2. Increase the life of mechanical breaks
3. To generate the electrical power and improve the energy efficiency.
4. in emergencies to stop the machine instantly.
5. in production process by reducing the stopping time.
Electric breaking is simple. The electric motor can be made to work as a generator by suitable terminal conditions and absorb mechanical energy. This converted mechanical power is dissipated / used for electrical network.

Breaking can be broadly classified into various

1. Dynamic breaking
2. Regenerative breaking
3. Reverse voltage breaking or plugging

1. Dynamic breaking

In dynamic breaking the motor is disconnected from the supply and connected to a dynamic breaking resistance. In figure. This is done by changing the switch from position one to  two . 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 breaking resistance. The direction of this current is opposite to the one which was flowing before change in 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 breaking mode is as shown in figure. For a particular value. The positive torque corresponds to the motoring operation. Here the machine behaves as a self-excited generator. Below a certain speed the self-excitation collapses and the braking action becomes zero.

2.Regenerative breaking

This braking is commonly used in electric trains. This braking is applied when the load on the motor has overhauling characteristics as in downgrade motion of an electric train. Regenerating occurs when back EMF> voltage. When the overhauling load acts as a prime mover and so drives the machine as a generator.  Hence, armature current changes, so armature torque is reversed and speed falls until Back EMF < voltage. During slowing down of the motor, power is returned to the line which may be used for supplying another train on an upgrade.

3. Reverse voltage breaking or plugging

In this method, connections to the armature terminals are reversed so that motor tends to run in the opposite direction as shown in figger.  Due to the reversal of armature connections. Applied voltage V and back EMF start acting in the same direction around the circuit. In order to limit the armature current to a reasonable value. It is necessary to insert a resistor in the circuit while reversing armature connections.
This breaking is commonly used in controlling elevators; rolling mills etc. here the connection to the armature terminals is reversed so that motor tends to run in the opposite direction. Because of this reversal, both applied voltage and back EMF start acting in the same direction.  A resister is inserted I the circuit to limit the armature current to exceed the limit.