Electric Braking of DC Motors
A motor that is running may be brought to rest through mechanical braking or Electronic brakes . The mechanical braking process is carried out via break shoes that are mechanically constructed. Therefore, the ease of mechanical braking is dependent on the physical and surface condition of the brakes. The brakes are smooth the motor’s braking can be accomplished through electronic brakes .
It is the electric braking system of the DC motor is comprised of three kinds: (i) Dynamic or rheostatic brakes, (ii) Plugging or reverse current braking, and (iii) Regenerative beaking.
(i) Dynamic or rheostatic brakes:
In the case of DC Shunt motors the armature is removed from the source and an in turn, a rheostat (variable resistance) is connected to it. The field winding is connected to the supply. The armature will now be driven by inertia, and thus the machine is acting as generator. Therefore, the machine will supply the current to the rheostat , and heat will be dissipated in the direction of I 2.R. The effect of braking is controlled by changing the resistance that is connected to the armour.
If it is a DC series motor, the motor is disconnected from the source and the field connections switched while a rheostat attached in series. Field connections have been reversed to ensure that the current flowing through the field winding will flow in same direction as before.
(ii) plugging, or reverse current brakes:
In this way, connections to armature are reversed and thus motors tend to run in the opposite direction. Because of the reversed armature’s terminals, the applied voltage V and the back emf Eb begins to act exactly in that direction, therefore the total armature voltage is greater than. To reduce this armature current a variable resistor is inserted to the armature. The same is true for the shunt and series wound techniques.
Plugging can provide greater braking force in comparison to the rheostatic brake. This technique is typically employed in the control of machines, elevators and printing presses, among others.
(iii) Regenerative braking:
Regenerative brake is utilized when loads on the motor have an extremely high amount of energy (e.g on electric trains). When the voltage applied for the motor decreased to less than the back Eb’s emf, the the armature current Ia is reversed, and the the torque of the armature is reversed. This causes a decrease in speed. When the generated emf is higher than voltage applied (machine acts as an DC generator) the power will be transferred to the line. This process is known as regeneration. As the speed decreases, returning to emf Eb will also decrease until it is lower than the voltage applied and the direction of the armature current changes to the opposite direction of Eb.