Types of Electric Braking Used in DC Motors | Electrical Engineering

In this article we will discuss about the types of electric braking used in dc motors.

Plugging or Counter-Current Braking:

In a dc motor a reversed torque is obtained by reversing the current either in the armature or in the field (not both), Polarity reversal of field winding is rarely used because it results in longer braking time due to relatively large inductance of the field winding in comparison to that can be obtained by polarity reversal of armature winding.

The connections for dc series and shunt motors during normal running and braking conditions are shown in Figs. 1.92 and 1.93 respectively. In case of a dc series motor it should be ensured that the direction of flow of current in the field winding remains unchanged when the current flow in the armature winding is reversed.

In the normal running position the back emf is nearly equal to applied voltage and opposite in direction, so that a small voltage acts across the armature circuit to drive the normal current through a small resistance of the motor. In the plugging position, the induced emf acts in the direction of applied voltage, therefore, at the instant of switching the motor to the plugging position, twice of supply voltage acts across the machine circuit and heavy current would flow (about twice the current drawn by the stationary motor on normal rated voltage). Hence to avoid flow of heavy current and limit it to the safer value, it is necessary that switching performing the plugging operation may also re-insert starting resistance and some additional resistance in series with the armature circuit of the motor.

Figures 1.94 (a) and 1.94 (b) explain the plugging operation of the dc shunt and series motors respectively on a quadrantal diagram.

AB and EF represent the speed-torque characteristics of the motor in normal and reverse direction of rotation without any external resistance, while CL and KG represent the same with external resistances respectively. A is the initial operating point with load, speed N and load torque T_L and the external resistance zero.

When plugging is resorted, the motor continues to run in the normal direction but develops torque in reverse direction and point G is the operating point (on characteristic KHG, as resistance is inserted here). The speed falls till it reaches point H (zero) and at this stage supply should be switched off, failing which the motor attains speed in reverse direction under motoring action and operates at point E on switching out external resistance.

Dynamic (or Rheostatic) Braking:

1. Rheostatic Braking With DC Shunt Motors:

The armature of a dc shunt motor is disconnected from the supply and connected across a braking resistance, keeping the field connected to the supply, as shown in Fig. 1.95 (b). The machine then acts as a generator loaded with a resistance. Since the field current flows as before but the armature current in the reversed direction, therefore, a retarding torque is produced. As the motor speed falls during rheostatic braking, the retarding effect decreases.

In order to maintain the retarding torque, some sections of the braking resistance may be then cut out of circuit. If, during rheostatic braking, supply fails, the magnetic flux of the motor will collapse, and there will be no braking effect. This drawback is overcome by providing a series winding in the armature circuit which is connected during the braking period only. The braking effect can be controlled by varying the braking resistance R.

2. Rheostatic Braking With DC Series Motors:

In the case of a series wound direct current motor, when it is disconnected from the supply and is connected across a braking resistance, it is essential that the connections of the field winding are changed over. If this is not done the current through the field will also be reversed, and no braking will occur. To avoid this, the field connections are so changed that the current through the field winding will flow in the same direction as before. The resistance inserted in the circuit must be less than the critical resistance, otherwise the generator will not be self-exciting.

Rheostatic braking of a dc series motor can also be affected by feeding its field winding from an independent source and connecting a resistance across the armature terminals i.e., by reconnecting it as a separately excited machine, as shown in Fig. 1.96 (c). An additional resistance is also connected in series with the field winding to limit the field current to a safe value. Since the machine operates as a separately excited generator, the braking characteristics will be same as those for dc shunt motor during dynamic braking.

Dynamic braking with self-excitation is normally not used as the field of the machine may build up suddenly and cause the braking torque to jump to high value and the driven unit may experience sudden jerks.

3. Rheostatic Braking With DC Compound Motors:

For direct current compound wound motors rheostatic braking is performed in the same way as for dc shunt wound motors.

The relation between the braking torque and speed for direct current motors can be determined as-

Electric barking torque =

Since barking current, I = E/R, where E is the induced emf and R is the total resistance in the motor circuit.

In case of series wound motors since flux φ varies with the variation in current the value of torque therefore, can only be determined graphically from the magnetisation curve.

In case of shunt wound motors since flux remains constant, the electric braking torque, therefore, for any given value of R is directly proportional to speed.

In actual practice, in order to have maximum braking torque resistance in the armature circuit is so reduced with speed that armature current and, therefore, braking torque remains constant till external resistance becomes zero. The braking torque falls linearly with the speed and becomes zero at zero speed. In case of dc series motors with self- excitation speed-torque curve is not linear because magnetic flux changes with the change in armature current which changes with the variations in speed.

In practice rheostatic braking is preferred for non-reversing drives as this simplifies the drive circuit. For reversing drives, plugging is preferred for braking and starting of the motor in the opposite direction in a short time.

Potentiometric Lowering Circuit. Such a circuit is shown in Fig. 1.97 which is just the same as shown in Fig. 1.93 except that the armature carrying reverse current is supplied from a potential divider of which field winding forms a part (not across full supply voltage).

Regenerative Braking:

1. Regenerative Braking with DC Shunt Motors:

Regenerative braking is an inherent characteristic of shunt wound motors and does not require any changing of connections.

Let us consider a shunt motor running in normal condition. Now if due to, say, hauling load, speed increases above normal, due to increase in speed the induced emf will increase and if it exceeds the line voltage, the machine will start supplying current to the line, thus have tendency to prevent any further increase in speed.

Similarly if the field current is increased so that motor emf exceeds the line voltage and it may start supplying current to the line, the motor will slow down to the speed corresponding to this new value of field current.

Regenerative braking can be easily applied to dc shunt motors, particularly in cases where it is required to hold a load at a certain speed for instance lowering a hoist, it is not,
however, possible to obtain regenerative braking down to very low speeds, since a sufficient increase in field current cannot be obtained.

Before bringing the motor to rest, the field excitation is increased to the permissible maximum value, as a result of which the speed of the motor is reduced to the minimum value and the kinetic energy released from the motor is fed back to the supply.

Since the motor is required to operate with a weak field at rated condition, the armature has to be designed to carry a large current to produce the rated torque and hence motor will be larger in size, poor in efficiency and costlier.

DC Shunt Motor in Hoisting Mechanism:

Consider a dc shunt motor (or a separately excited motor) used in hoisting mechanism. When it is switched on for lowering a load, the torque developed by the motor and that due to load torque act in unison and, therefore, accelerates the motor. With the increase in speed, induced emf in the armature (i.e., back emf, E) increases till E = applied voltage V.

At this moment the speed becomes the ideal no-load speed, the armature current and, therefore, the developed electromagnetic torque is zero, so that the downward motion of the hoist is sustained only by the downward moving load. When the speed exceeds the ideal no-load speed, induced emf E exceeds the applied voltage V and, therefore, armature current becomes negative. The drive, then acts as a generator and provides the braking torque. Under such condition-

The speed-torque characteristics of motor during motoring and braking operation are shown in Fig. 1.98. From Fig. 1.98 it is obvious that braking or regeneration can occur only at speeds greater than the ideal no-load speed of the motor. From Fig. 1.98 it can also be noted that higher the resistance of the armature circuit, to develop a particular braking torque, higher is the speed required.

2. Regenerative Braking with DC Series Motors:

The dc series motors cannot be used for regenerative braking in an ordinary way. Since the reversal of armature current necessary to produce regeneration would cause a reversal of field, therefore, series field connections must be reversed. But even if the field connections are reversed at the exact moment, this method would still be useless. Because at the instant of reversal, the emf induced in the motor will be small, so current will flow through the field in wrong direction, which will reverse the field and cause the motor emf to help the supply voltage. This will result in short circuit of supply. Due to these complications this method is not used for common industrial purposes. Regenerative braking is, however, used with series motors for traction either by modification of windings or by supplying the machines with separate excitation.

One method of obtaining regenerative braking with series motors is the French method. If there is a single series motor as in case of a trolley buses, tramways, it is provided with a main series field winding and auxiliary field windings connected in parallel with the main series field winding as shown in Fig. 1.99 (a).

During regeneration (braking period) the auxiliary field windings are put in series with each other and are switched across the supply, as shown in Fig. 1.99 (b). The machine acts as a compound generator slightly differentially compounded. Such an arrangement is quite stable.

Any change in line voltage causes a change in excitation which produces a corresponding change in the induced emf of the machine so that inherent compensation is provided. For example, if the line voltage increases beyond the emf of the generator the increased voltage across the generator’s field will send a large exciting current through it causing the emf of the generator to rise. The reversal of this will happen when the line voltage decreases.

If there are several motors, we do not require any auxil­iary winding. During normal running the motors are connected in parallel with the field winding connected in series with their respective armatures, as shown in Fig. 1.100(a).

But during regeneration the motors are connected, as shown in Fig. 1.100 (b), i.e., all armatures are connected in parallel and series field windings of all motors but one are connected in series and placed across the supply. Suitable resistance is also connected in series with the series field windings, as illustrated in the Fig. 1.100 (b).

Another alternative method of regenerative braking, which obviates the loss of power in the resistances in the field circuit, is by using a separate exciter for controlling the excitation of the field windings during regeneration. The exciter may either be axle driven or driven by a motor operated from the auxiliary supply. Many devices have been used to secure compensation against variations in line voltage, most of which make use of some form of differential winding on the exciter. One such method is illustrated in Fig. 1.101 (a) in which the exciter is differentially compounded.

The shunt field winding of the exciter is separately excited from an auxiliary supply the series field winding is connected in the main-motor circuit in such a way that it opposes the sepa­rately excited winding during regeneration. It can be seen that a rise in line voltage during regeneration, which will tend to reduce the regenerated current, will strengthen the exciter field and counteract the change. The stabilising re­sistance R also assists in this action.

Another method used for regenerative braking is depicted in Fig. 1.101 (b). Here the exciter armatures along with the field windings of the series motors are connected across the stabilizing resistance in the main circuit. The current through the stabilizing resistance is the sum of the exciter current and the regenerated current. Any increase in regenerated current, due to fall in line voltage, causes larger voltage drop in the stabilizing resistance and, therefore, less current through the exciter circuit, again counter-acting the effect of line voltage variations.

3. Regenerative Braking With DC Compound Motors:

Compound wound motors can be re-generatively retarded by an hauling load only if their shunt field is stronger than their series field.