Jan 1, 2015

Electric Braking / Dynamic Braking of DC Motor

Electric Braking

Occasionally it is necessary to stop a DC Motor rapidly. This might be required in the event of crisis if the motor is continuously used for repeated operations. The DC motor and its load might be taken to rest by means of either mechanical braking or Electric Braking. In mechanical braking, the motor is stopped owed to the friction amongst the movable parts and brake shoe of the motor. But this mechanical braking has more disadvantages.
In electric braking, the motor kinetic energy is transformed into electrical energy which is degenerate in a resistance as heat or alternatively, it is returned to the supply source. The main advantage of electric braking is that decreases the wear and tear of mechanical brakes and reduces the stoppage timing extensively due to high braking retardation. In electric braking, there are three methods used for DC shunt motor and DC series motor. They are:

  1. Dynamic breaking or Rheostat breaking
  2. Plugging
  3. Regenerative braking

Dynamic braking

The fig shows that the dynamic or rheostat braking of a DC shunt motor. In this the running motor armature is cut off from the source and it’s connected across an adjustable resistance. But the field winding of the motor is left connected from the supply. When the armature slowing down, rotates in a strong magnetic field and therefore works as a generator and sending a high current through resistance R. This reasons the energy influenced by the revolving armature to be dissipated rapidly in the form of heat in the resistance. Because of this the motor is taken to standstill immediately.

By varying the adjustable resistance R the braking torque can be controlled. The motor speed reduces once the value of resistance R decreases, and then the breaking torque might be controlled at a large value. When the motor speed is low, the braking torque becomes low and the motor attains to stop because of friction. Generally this dynamic braking type is used widely in connection with the control of hoists and elevators. Also this type is used where the motor should be stopped, started and reversed repeatedly.

Let us see how the baking torque of motor depends upon the speed.
Let us see how the baking torque of motor depends upon the speed.
Referring to the figure-b above;
                              Armature Current Ia = Eb / (R+Ra)
                                                               = (K1.NΦ) / (R+Ra) (∵  Eb∝N)
                                Braking Torque TB = K2IaΦ
                                                                = K2Φ (K1.NΦ / R+Ra)
                                                                = K32Where,
K2 and K3 are constant
For Shunt Motor, Φ is constant.
Therefore Braking torque TB∝N
i.e. Braking torque decrease the motor speed is also decrease.


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