In this article we will discuss about:- 1. Introduction to Regenerative Braking 2. Difficulties Encountered in Regenerative Braking 3. Calculations of Energy Returned during Regeneration 4. Advantages and Disadvantages.
Introduction to Regenerative Braking:
Regenerative braking is an inherent characteristic of dc shunt wound motors and does not require any change of connections. DC shunt motors are electrically stable also i.e., braking torque is independent of line voltage fluctuations.
The dc series motors cannot be used readily for regenerative braking and some modification of shunt winding or separate excitation of the series field at low voltage is necessary to be employed to enable the motor to act as a generator. One method of obtaining regenerative braking with series motors is French method.
Inherently a dc series generator has no stability of voltage, and therefore, an external means is required. In Fig. 14.2 (a) a separate motor driven exciter has a separately excited field winding F1 and also another field winding F2 connected in the main motor circuit in such a fashion that the field created by it opposes the field created by separately excited field winding F1 during regeneration. The stabilising resistance R is employed to prevent current surges when the tram crosses from one section of the supply to another, and to compensate for variable line voltage.
In case the line voltage falls, regenerated current will tend to increase resulting in strengthening of field F2 which will weaken the field F1 and, therefore, reduction in emf generated by the exciter. Thus the field of the traction motors will be weakened resulting in reduction of emfs generated by the traction motors operating as dc series generators. Thus compensation for a decrease in the line voltage is automatically provided.
The arrangement shown in Fig. 14.2 (b) has the exciter armature connected in the circuit of the field windings of traction motors and the stabilising resistance. The current through the stabilising resistance is the sum of the exciter current and the regenerated current. The voltage of the exciter circuit can be regulated by either varying its field strength or manipulating resistances in series with the armature.
In case the line voltage falls the regenerated current will tend to increase resulting in increase in voltage drop across the stabilising resistance and, therefore, reduction in the voltage available in the exciter armature circuit causing a reduction in the excitation current of the traction motors operating as dc series generators. This reduces the emf’s generated and thus compensation is provided automatically.
Regenerative braking is an inherent characteristic of an induction motor since an induction motor operates as an induction (non-synchronous) generator when run at speeds above synchronous speed and it feeds power back to the supply line. The machine, however, is not self-exciting as a generator and is required to be connected to a system supplied from synchronous generators. This system supplies the excitation and determines the frequency at which the induction generator operates. Torque-speed characteristics of an induction machine are shown in Fig. 14.3.
From the torque-speed characteristics of an induction machine it is obvious that without any extra resistance in the rotor circuit, there is only a slight variation of speed with the torque whereas with the external resistance inserted in the rotor circuit speed increases for a particular braking torque. Thus we see that with no extra resistance in the rotor circuit the speed during braking remains almost constant and independent of the gradient and the weight of the train.
This is a great advantage with the induction motor when used for traction. But if increased speeds are necessary with light loads, these can be obtained by inserting external resistance in the rotor circuit. It is advantageous on mountain railways. It returns about 20% of the total energy on certain railway run and saves a great deal of brake shoe wear.
Difficulties Encountered in Regenerative Braking:
Regenerative braking with ac series motors is more difficult than that with the dc series motor.
The difficulties encountered are as follows:
ADVERTISEMENTS:
During the regeneration period, the machine should not operate as a self-excited generator. Generally the high power factor is the main consideration in order to achieve a reasonable braking torque. Circuit for regeneration at high power factor usually suffers from self-excitation, and those which are inherently stable and free from self-excitation usually operate at a poor power factor.
For regenerative braking the regenerated power should have the same frequency as that of the main supply. This necessitates the energizing of the motor field winding from the ac mains. The regenerated current must be in phase opposition to the applied voltage and also the flux ɸ, so that power may be supplied back into the supply system.
The arrangement to provide the above conditions is illustrated in Fig. 14.4.
ADVERTISEMENTS:
In another satisfactory arrangement one of the traction motors is used as a generator to supply current to excite the fields of the remaining motors. As the speed of the locomotive falls, the voltage of the motors can be controlled by increasing the excitation current. This is accomplished by increasing the voltage on the exciting motor by employing another transformer tap. Further flexibility can be obtained by changing the tap from the generating motor (Fig. 14.5).
Another scheme known as Behn Eschenburg scheme of regenerative braking is shown in Fig. 14.6 (a). In this case an auxiliary transformer is used to excite the exciting winding of the traction motor. The armature of the traction motor is connected to the main transformer through tap changer.
ADVERTISEMENTS:
In between the motor armature and tap changer is inserted a choking coil or iron-cored reactor in series, as shown in Fig. 14.6 (a). Commutating pole winding C in series with a resistor R is shunted with another iron-cored reactor in order to obtain the correct (leading) phase of the commutating flux.
The current flowing in the exciting winding, If will lag behind the voltage V at the tap changer approximately by 90° and the emf generated in the armature of the motor, E will be in phase with the exciting current If. The voltage equal to phasor difference of voltage V and emf E will act across the reactor I. The armature current la lags behind the voltage acting across the reactor I (represented by phasor OD) by roughly 90°.
For a given voltage across the tap changer the braking torque produced will be proportional to Ia cos Ψ and the power returned to the supply mains will be proportional to Ia cos α if losses are neglected.
From the phasor diagram we conclude that, for constant excitation:
(i)The braking torque is approximately constant at all speeds.
(ii) Volt-amperes absorbed by reactor I are more than those generated by the armature.
(iii) The power factor of the generated power is about 0.7, which is considered as low power factor. But this is not considered a serious drawback as the arrangement provides simplicity of operation, reliability, stability and free from self-excitation.
Calculations of Energy Returned during Regeneration:
When the train is accelerated up to a certain speed, it acquires energy, known as kinetic energy, corresponding to that speed (KE = ½ mv2). While coasting a part of this stored energy is utilised in propelling the train against frictional and other resistances to motion and, therefore, the speed falls. Under ideal conditions (no resistance to motion of the train) the speed of the train would have not decreased.
Similarly while the train going down the gradient or moving on level track, the speed remaining the same or reduced, this stored energy can be converted into electrical energy and returned back to the lines.
The amount of energy returned to the line depends upon:
(i) The initial and final speeds during regenerative braking
(ii) The train resistance and gradient of the track also in case the train is moving down the gradient and
(iii) Efficiency of the system.
Let the initial and final speeds of the train be V1 and V2 kmph respectively. The kinetic energy stored in the train at a speed of V1 kmph
Similarly kinetic energy at speed
V2 = 0.01072 WeV22 watt-hours Energy available during regeneration
= 0.01072 We (V,2 – V22) watt-hours
= 0.01072 We (V12 – V22) watt-hours …(14.1)
Some of the energy is lost to overcome the resistance to motion and the losses in the traction system including traction motors.
Energy lost to overcome the resistance to motion
W x r x S x 1,000/3,600 watt-hours = 0.2778 WrS Wh …(14.2)
where r is the specific resistance in newton/tonne.
While going down the gradient in the hilly track service, energy is supplied as tractive effort due to the gradient and energy is added up to the energy available during regeneration. Energy available due to motion down the gradient.
98.1GW x S x 1,000/3,600 = 27.25 GSW …(14.3)
Hence total energy available during regeneration = [0.01072We (V12 – V22) + 27.25 GSW – 0.2778 WrS] watt-hours …(14.4)
Taking ƞ as the efficiency of the system, Energy returned to the line
= [0.01072Wt, (V12 – V22) + 27.25 GSW – 0.2778WrS] x ƞ watt-hours …(14.5)
Advantages and Disadvantages of Regenerative Braking:
Advantages:
1. A part of energy is returned to the supply system, so energy consumption for the run is considerably (about 20 to 30 per cent) reduced thereby affecting a considerable saving in the operating cost.
2. The wear of the brake shoes and wheel tyres is reduced to considerable extent, therefore, their life is increased and replacement cost is reduced.
3. Higher value of braking retardation is obtained so that the vehicle can be brought to rest quickly and running time is considerably reduced.
4. Small amount of brake dust is produced when the mechanical brakes are applied.
5. Higher speeds are possible while going down the gradients because the high braking retardation can be obtained with regenerative braking.
6. Propulsion of heavier trains on gradients is possible without dividing them into sections with speed and safety.
Disadvantages:
1. Additional equipment is required for control of regeneration and for protection of equipment and machines, hence initial as well as maintenance cost is increased.
2. The dc machines required in case of regenerative braking are of large size and cost more than those ordinarily employed, therefore, the weight of the locomotive and thus the required mechanical strength and cost increase.
3. Owing to recuperated energy the operation of the substations becomes complicated and difficult.
4. In case of substations employing mercury-arc rectifiers for conversion purpose, additional equipment is required either to deal with regenerated energy separately or to change one or more of the ordinary rectifiers over to inverted operation. No such difficulty is experienced in case of substations employing rotary convertors or motor- generator sets for converting purpose.
5. Regenerative braking is employed down to a speed of 16 kmph, then rheostatic braking to about 6.5 kmph and then mechanical braking is required to bring the locomotive to rest.
In most of the cases, however, and especially with motor-coach trains the increased cost of the train equipment and the additional features required in order to obtain regenerative braking, combined with the increase in the maintenance cost of the electrical equipment, may entirely offset the economics in the energy consumption and the other items.
For tramways, trolley buses, the regenerative braking is not recommended as it will unnecessarily increase the initial cost as well as increase the operating problems. Generally regenerative braking is desirable and necessary for the service lines having long gradients exceeding 0.6%.