Electric Braking of D.C. Motors

       An electric motor can be brought to rest quickly by using usual mechanical brakes or electric brakes. With the mechanical brakes it is not possible to achieve a smooth stop and also linings, levers and mechanical arrangement is necessary. It also depends on the skill of the operator.
       As against this electric braking is easy, reliable which brings motor to rest very quickly. Three types of electric braking methods are available i) Rheostatic or dynamic braking ii) Plugging and iii) Regenerative braking. Though motor is brought to rest electrically, to maintain its state of rest a mechanical brake is must. Let us see these methods as applied to shunt and series motor.
1.1 Electric Braking of D.C. Shunt Motor
       (a) Rheostatic or Dynamic Braking :
      In this method, the armature is simply disconnected from the supply and connected to a resistance, while the field remains connected to the supply as shown in Fig. 1(a) and (b).
Fig. 1

       All the kinetic energy of the moving mass is converted to electrical energy which is dissipated in the variable resistance connected. The magnitude of the braking torque can be controlled by varying the value of the resistance R. The disadvantages of this method is that in case of failure of electric supply, this method is ineffective.
        (b) Plugging :
       This involves the reversing of the armature connections of the motor. We have seen that if the direction of current through armature or field is reversed. In case of plugging, motor is subjected to the reversely directed torque. Under such condition the back e.m.f. and the voltage act in the same direction, so the current flowing in the circuit is produced by a voltage V + E i.e. approximately 2 V. To limit high inrush of current an additional resistance R is connected. In this method, motor tries to accelerate in the other direction, after coming to rest. So some auxiliary device is required to cut-off the supply as soon as motor comes to rest. In case of failure of supply this method is ineffective. This method gives more braking torque that the rheostatic braking. The method applied to a shunt motor is shown in the Fig. 2(a) and (b).
Fig. 2

       The method is generally used in braking elevators, rolling mills, machine tools etc.
       (c) Regenerative braking :
       In this method, instead of being disconnected from the supply, it remains connected and returns the braking energy to the line. Consider a shunt motor running as shown in Fig. 3(a). Suppose the load causes the speed to be increased above normal, the field current remaining the same then the back e.m.f. becomes greater than the supply voltage (E > V).
Fig. 3

       The current gets reversed and power will be supplied to the line, tending to prevent any further increase in the speed. Alternatively same effect can be obtained by increasing the field current where motor quickly slowed down to the speed corresponding to the new value of the field current. Due to reversal of direction of armature current as E > V, armature torque is reversed and speed falls until E becomes less than V.
       The method is used while lowering the cage of a hoist or the downgrade motion of an electric train.
1.2 Electric Braking of D.C. Series Motor
       (a) Rheostatic or dynamic braking :
The basic principle of disconnecting the armature from the supply remains the same. The direction of the current through the armature reverse while braking. The motor runs as a generator. In case of series motor care should be taken to see that the direction of current through field does not change. This is shown in Fig. 4(a) and (b).
Fig. 4

       (b) Plugging :
       The basic principle of reversing the armature connection remains same. Similar to the shunt motor, the resistance R added can be controlled to control the magnitude of the braking torque. The principle is illustrated in the Fig. 5(a) and (b).
Fig. 5

       (c) Regenerative braking ;
       In case of a d.c. series motor, increase in excitation results decrease in speed. As such it is not possible to get e.m.f. more than voltage. It is not possible to make field current more than the armature current. Hence regeneration braking with series motor is not possible. But can be used with traction motors with some special arrangements.

Conditions necessary for regenerative braking :
1. In generators, the generator voltage is more than the terminal voltage hence armature supplies current to the load. In motors, the generated voltage E is less than applied voltage V hence current flows from supply to the armature. Hence for regenerative braking it is necessary that motor must function a a generator such that generated voltage is more than supply voltage and at the same time its speed goes on decreasing. This is possible by controlling excitation level.
2. The braking system must have mechanical stability. This means that in case if speed of the motor increases due to powerful overhauling forces then braking system must apply more and more braking torque to maintain stabilit.
3. The braking system must have electrical stability. This means that effectiveness of braking system should not get disturbed due to the fluctuations in the line voltage.
4. If the regenerative energy becomes more than its demand then such a surplus energy should be wasted by generating station equipment otherwise the regenerative braking will not be effective.
       It is mentioned that that the series motor are not suitable for regenerative braking as these motors are electrically unstable when used for regenerative braking. Consider that the line voltage is increased due to the fluctuations while using series motor for regenerative braking. Due to this, difference between E and V decreases. Hence regenerative current decreases. The same current flow through series field hence flux and hence induced e.m.f. further reduces. This further reduces regenerative current and cumulatively the E and V become equal and there can not be any regenerative action. Similarly if the line voltage increases, the regenerative current becomes excessively high.
Note : The flow of disturbance current through the series field causes the electrical instability for the series motors when used for regenerative braking.
       Hence while using series motors for regenerative braking the fields must be excited separately and use stabilizing circuits.
       The Fig. 6 shows the arrangement preferred for the series motors when used for for regenerative braking.
Fig. 6  Series motor used for regenerative braking
      The armature of all the motors are either connected in series or parallel while the fields are connected in series. The resistance R1 and R are the protective resistances.
Methods of using series motors for regenerative braking
       The stabilizing action can be achieved by,
1. Using differentially excited exciter.
2. Connecting circuit in parallel with the stabilizing resistance which is connected in series with the main circuit.
       The Fig. 6 shows the use of differentially excited exciter. The armatures are connected in series while fields of all the series motors are connected in series.
       The exciter is driven by the separate series motor which is connected to a mechanical load as well. If due to the fluctuations there is decrease in the line voltage then the current surge is induced from motors to the line. This current flows through the differential winding of the exciter. Due to its differential nature it demagnetises its field. This reduces the excitation of field coils of main motors reducing the generated voltage. Thus the difference between Eand V is maintained constant. This maintains electrical stability.
       The Fig 7 shows the use of stabilizing resistance to obtain electrical stability.
The entire field circuit is connected in parallel with the stabilizing resistance causing more drop across it. The exciter is the series with the stabilizing resistance and hence will supply less current to the field coils of main motors, due to increased drop across stabilizing resistance. Due to this the generated voltage decreases. Thus the difference between and voltage decreases. Thus the difference between and is maintained constant, maintaining electrical stability.
Fig. 7 Use of stabilizing resistance

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  2. Regenerative braking is used where, load on the motor has very high inertia (e.g in electric trains). When applied voltage to the motor is reduced to less than back emf Eb, obviously armature current Ia will get reversed, and hence armature torque is reversed. Thus speed falls.
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