In this article we will discuss about:- 1. Introduction to Braking 2. Methods of Applying Braking 3. Systems 4. Mechanical Considerations 5. Control Equipment.
Introduction to Braking:
Electrical and mechanical, both types of braking are used in electric traction. In electric braking the braking energy is converted into electrical energy instead of converting it into heat energy at the break shoes and either dissipated in the resistances mounted on the vehicle or returned to the supply system. Electric braking reduces the wear of the brake shoes and wheel tyres considerably and gives higher rate of braking retardation, thus brings the vehicle quickly to rest and shortens the running time to a considerable extent.
In case regenerative braking is employed, the braking energy is returned to the supply system thereby a considerable saving is affected in the running cost, higher speeds are possible while going down the gradients as more braking power is available and heavier trains can be propelled down the gradients with safety and speed without dividing it into sections. Electric braking cannot replace the ordinary mechanical brakes, as the vehicle cannot be held stationary by it.
The desirable requirements of a braking system are given below:
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1. The braking system should be simple, robust, quick, and reliable in action.
2. Maintenance needs should be minimum and braking system must be easy for driver to control and operate.
3. The system should apply brakes simultaneously over all the vehicles.
4. Normal service application of brakes should be very gradual and smooth so as to avoid damage to the goods and discomfort to the passengers.
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5. The braking force applied to each axle should be proportional to the axle load so as to obtain uniform deceleration.
6. In case of emergency braking, safety consideration is the main consideration. As such retardation rate would be maximum consistent with the safety, so as to make unfailing halt in the minimum possible distance.
7. Kinetic energy of the train should as far as possible be storable during braking which could subsequently be used during acceleration of the train.
8. The braking system should be inexhaustible.
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9. There should be automatic slack adjustment for constant piston stroke as a result of wear on the rim and the brake blocks in the case of mechanical braking.
The requirements of a braking system on a main line locomotive differ from those of motor coaches—the former usually needs braking to hold the train at the steady speed on a long down grade, whereas with the latter braking is primarily required for stopping the train. Rheostatic braking is employed in both the cases but regenerative braking is mainly confined to main line locomotives due to the complications involved in providing regenerative braking with dc series motors.
Methods of Applying Electric Braking:
There are three methods of applying electric braking namely: 1. Plugging 2. Rheostatic or Dynamic Braking 3. Regenerative Braking.
1. Plugging:
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This is the simplest type of braking. In this method of braking the torque of the motor is reversed, which brings the motor and its driven machine to standstill.
2. Rheostatic or Dynamic Braking:
In this method of braking, the motor is disconnected from the supply and operated as a generator driven by the kinetic energy of the rotating parts of the motor and its driven machines. Thus the kinetic energy of rotation is converted into electrical energy, which is dissipated in the external resistance connected across the motor at the braking instant.
In traction work where two or more motors are employed, they are put in parallel across a resistance for braking, as shown in Fig. 14.1, as the series connection would produce too high voltage. During the braking period the motors are driven as generators owing to the kinetic energy of the train and the electrical energy so generated is dissipated in the form of heat in the resistances connected across them. An equaliser connection, as shown in Fig. 14.1 (a), is used in order to ensure that the two machines share the load equally.
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If the equaliser connection were not done, the machine that would build up first would send a large current through the other in the opposite direction, causing it to excite with reversed voltage and so the two machines would be short circuited on themselves. A large braking torque would be developed in this way, but it would be quite uncontrollable and the currents would be dangerously large. An alternative method of avoiding this situation is cross-connection of the two machines, as illustrated in Fig. 14.1 (b).
In case the voltage of one of the machines becomes larger than that of the other, the first machine will cause flow of a higher current through the field winding of the second machine. As a result the second machine will have higher voltage while the field current in the first machine due to second one is smaller and hence it will cause lower voltage across first machine. Thus, an automatic compensation of the unbalancing is achieved.
The second method is advantageous to the first one as if the direction of rotation of the machine armatures reverses (may be on account of run back due to upgradient), the machines will fail to excite with equaliser connection and, therefore, no braking effect will be produced which may prove fatal for the passengers whereas with cross-connection the machines will build up in series and being short circuited on themselves will provide an emergency braking and the train will not be allowed to run back on the gradient.
Rheostatic braking cannot be employed with 3-phase induction motors. In case of ac series motors the rheostatic braking is obtained by operating the machines as generators excited from the supply or as self-excited dc generators supplying power to resistance load. In the former case the fields are energised at low voltage from a suitable tapping on the main transformer while in the latter case the fields of the motors are excited from one of the motors acting as a series generator and in this case dc will be generated in rotors of the motors and the kinetic energy of the rotors will be dissipated as dc power in the loading resistors.
3. Regenerative Braking:
In the previous two methods of electric braking namely plugging and rheostatic braking, stored energy of the rotating parts of the motor and its driven machines is wasted whilst in plugging extra energy is drawn during the braking period and wasted. In the regenerative braking the motors remain connected to the supply and return the braking energy to the supply.
For regenerative braking it is necessary that traction motors must generate power at a voltage higher than the supply voltage and at a reasonable constant voltage.
Mechanical Regenerative Braking:
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. Thus coasting may be considered as a form of mechanical regenerative braking.
The saving in energy consumption can be considerably affected by increasing the coasting but it will reduce the schedule speed. If, however, the original schedule speed is to be maintained, an increase in the acceleration is required which is possible with large motors. Thus the saving in energy consumption may be neutralised by the increased train weight and the additional cost of the equipment.
With modern city and suburban traffic conditions, the coasting period is about 20 to 50 per cent of the total running period and consequently a large percentage of kinetic energy has to be wasted in brakes. One of the way of tackling this problem based on the principle of mechanical regenerative braking is adoption of graded track systems, as shown in Fig. 14.9. It facilitates the saving of energy consumption to about 25%.
In the graded track system the stations are situated at a certain height above level track, slope of track while leaving station is 1 in 30 and while approaching station is 1 in 60. In this system the kinetic energy of the train instead of being wasted in brakes is utilised and stored as potential energy as the train has to climb up a slope of 1 in 60 while approaching the station. This potential energy is utilised in moving the train down gradient and imparting acceleration to it.
By using this system small motors can be used for driving the train and specific energy consumption can be reduced to 75 per cent of that when the level track was employed. This system can be employed only in underground railways because on surface railway difficulties will be experienced in the construction of the required track.
Systems of Braking:
1. Mechanical Braking:
For stopping the train and holding it stationary, in addition to electrical brakes, mechanical brakes are also required. The modern tendency is to employ regenerative braking down to a speed of 16 km per hour, then rheostatic braking down to about 6.5 km per hour, and finally mechanical braking to a standstill. Mechanical brakes are usually applied by brake block shoes pressed with force against the tread of the wheels. Braking force is caused by the movement of the piston which is transmitted to the brake blocks through a system of levers.
Mechanical brakes are of two types namely the compressed air brakes and vacuum brakes, the former is extensively used in electric railways whereas the latter is used in steam railways. The compressed air brakes are little advantageous to the vacuum brakes as compressed air can conveniently be stored up and released for quick action whereas in the vacuum brakes the necessary vacuum is to be created by the pump. However, this drawback is overcome by use of vacuum reservoir.
a. Compressed Air Brakes:
The compressed air brake system consists of a reservoir of compressed air, a brake cylinder, compressed air pipe, a valve, springs, piston and a piston rod, as shown in Fig. 14.10. The piston is connected to brake shoe through piston rod and levers. The brakes are kept in the ‘off’ position by means of springs provided for this purpose. While applying brakes, compressed air is allowed to enter into the brake cylinder through the air pipe and valve, which presses the piston against the force of springs.
The force with which the brakes are applied depends upon the quantity of compressed air allowed to enter into the cylinder. A valve placed at the inlet of brake cylinder controls the flow of air into it. The brakes are released by exhausting the air. The piston along with the brakes return to its original position under the influence of spring force on exhausting the air. The compressed air is used at about 5.5 kg/cm2 pressure. The compressed air brakes are extensively employed on electric railways.
b. Vacuum Brakes:
The vacuum brake system consists of a vertical cylinder with a piston inside, the piston is an easy fit in the cylinder and is provided with rolling rubber ring. The piston is connected to the piston rod which operates the braking arrangement through a system of levers. Vacuum is always maintained on the top and the under-side of the piston so that in normal conditions the piston rests at the bottom of the cylinder.
While applying brakes air at atmospheric pressure is gradually admitted beneath the piston through air pipe and valve the piston moves up and applies the brakes. The force with which the brakes are applied depends upon the rapidity with which the vacuum is destroyed.
The brakes are released by recreating the vacuum. The vacuum is recreated by drawing air from lower side of the piston through the pipe. The piston falls to bottom of the cylinder on account of its own weight.
Vacuum brakes are applied with steam locomotives.
2. Hydraulic Brakes:
This system consists of an assembly similar to a hydraulic coupling, the rotor being keyed to the axle and the stator being keyed to the bogie frame. The circulation of fluid (water or oil) through the assembly by the rotor vanes produces a braking torque. The brake is applied by filling the coupling by means of a pump.
3. Magnetic Track Brakes:
It comprises a bipolar electromagnet with elongated pole faces a short distance apart (about 6 mm) and along with track. The body of the electromagnet is made of cast steel, the pole faces are made of soft steel and can be easily replaced and the winding is enclosed in a water tight metal case. When the line voltage is impressed on these coils, the poles are strongly attracted to the rails and produce a retarding effect due partly to the magnetic pull (which increases the effective weight of the coils) and partly to mechanical drag.
The flux created due to flow of current is perpendicular to the pole faces and track and force of attraction between the track and magnet is given as –
F = B2A/2 x 4π x 10–7 newtons …(14.6)
Where, B is flux density in Wb/m2 and A is the area of the pole face in square metres. The braking force or the drag = μF where μ. is the coefficient of friction. Magnetic brakes are usually employed on trolley buses and cars.
4. Electromechanical Drum Brakes:
In this type the brake drum is mounted on the motor shaft. The brake shoe is pressed against it by springs. The brakes are released by means of a solenoid energised from a battery. This type of brake is usually employed on trolley buses and cars.
5. Eddy Current Brakes:
Eddy current brakes are of two types viz., linear type and rotary type. In the linear type brakes, shoe carrying the excitation current acts as a primary and the rail itself forms the secondary element of the system. During braking, traction motors are made to operate as generators and supply power to the shoe of the eddy current brakes. In the rotary system, the rotor, which acts as a secondary member is attached to the motor shaft and the dc excitation is supplied to the primary member which is fixed in space.
The torque developed due to induced currents in the rotor will tend to reduce the relative speed between the rotor and the stationary field. Thus braking effect will come in action. The braking torque can be varied by the varying the dc excitation of the primary member (stator).
The main advantage of eddy current braking is the same as that of the disc brake and is used satisfactorily in conjunction with the friction brakes for bringing the vehicle to rest.
Mechanical Considerations for Braking System:
The main considerations in connection with the locomotive are the wheel arrangement and general disposition of equipment in order to have good riding properties and the transmission of power from motors to the driving wheels.
Wheel Arrangement:
The maximum tractive effort which can be exerted by a locomotive without slipping of the driving wheels depends upon the coefficient of adhesion (which is about 0.15 to 0.25) and the weight on the driving wheels. Hence for propulsion of train, certain maximum weight on the driving wheels is essential. The maximum permissible weight on the driving wheels depends on the number of driving axles and the strength of the track, bridges etc. (which is between 15 and 30 tonnes). The minimum number of driving axles required, for a certain tractive effort F, given coefficient of adhesion and maximum permissible weight per driving axle.
= F/µ x maximum permissible weight per driving axle …(14.7)
For example, if the maximum tractive effort required is 30,000 kg, coefficient of adhesion is 0.2 and maximum allowable weight per driving axle is 19 tonnes, then number of driving axles required = [30,000/(0.2 x 10000 x 19)] = 8.
Such a train will be hauld by two locomotives each having 4 driving axles making 8 in all.
In case of dc locomotives since the electrical equipment is lighter in weight, so whole of the locomotive weight is placed on the driving axles so that required tractive effort may be obtained.
In case of ac locomotives, since the electrical equipment is comparatively heavy, so the required tractive effort can be obtained by placing only a portion of the weight on the driving wheels and supporting the rest on trailing axles. The riding qualities of high speed locomotives are improved by the trailing axles, which are often used on dc locomotives for this purpose.
Drive:
There are two methods of driving the electric train. These are individual drive and collective drive. In individual drive separate motor of smaller size is employed for each driving axle whereas in collective drive a single large sized motor is employed for driving all the axles through connecting rods as in steam locomotives. The collective drive has the following advantages and disadvantages over individual drive.
Advantages:
1. The wheels when coupled have got less tendency to slip.
2. The motor can be mounted well up in main body of the locomotive, therefore, the centre of gravity is raised thereby reducing the wear on track.
3. The motor for collective drive need not be paid so much attention in designing with regard to dimensions as much that for individual drive.
Disadvantages:
Collective drive is not suitable for high speed locomotives because of stresses and vibrations in the connecting rods at high speeds.
From the above it can be concluded that individual drive is advantageous only for high-speed locomotives whereas for low-speed locomotives, which are usually employed for passenger and goods trains, collective drive is preferred.
Transmission of Drive:
The electric locomotives are specifically designed to have springs between the driving axles and the main body and it is desirable to have as much of locomotive weight spring- borne as possible. This reduces not only vibrations in the locomotive but also reduces substantially the damage to the track owing to hammer blows.
In case the motors are fixed on the locomotive or bogie frame in order to have maximum locomotive weight spring-borne, there will be a relative motion between the motor armature and the driving axle owing to compression and extension of the springs. Hence flexible drive between the two is required, which is one of the most difficult problems of the electric locomotive.
There are several methods of transmitting power from the armatures of the electric motors to the driving wheels and only a few will be described here:
1. Gearless Drive:
(a) Direct Drive:
The simplest way is to employ bipolar motors, whose armatures are mounted directly on the driving axles with the field attached to the frame of the locomotive, as shown in Fig. 14.13. The pole pieces of the motors are approximately flat so that the armature can move relatively to them without affecting the operation.
The size of motor armature is limited by the diameter of driving wheels. In case driving wheels of large diameters are employed, the speed of the motor will have to be limited to lower one. Because of these facts and relatively large unsprung weight, poor utilization of material and the low centre of gravity of the locomotives the applications of this method are limited.
(b) Direct Quill Drive:
The quill is a hollow shaft which surrounds the driving axle and is connected to it by springs or some other flexible device, as shown in Fig. 14.14. The armature of the motor is the directly mounted on the quill and, therefore, it is entirely spring-borne. Thus the unsprung weight is reduced to a minimum and motor of normal design can be employed. The speed and size of armature are still limited by the diameter of the driving wheels.
One class of devices for obtaining a flexible connection between the quill flange and the driving wheel uses springs; other manufacturers avoid the use of springs and possible troubles owing to breakages by employing a link motion.
2. Geared Drive:
The size of the motor is reduced with the increase in speed for a given output, so the gear drive is necessary in order to reduce the size of the motor for given output by running it at a higher speed. The gearing ratio usually employed is 3 to 5:1.
(a) Nose-Suspension Geared Drive:
The most common type of geared drive is nose suspended motor shown in Fig. 14.15. In nose suspension the motor is partly supported from the driving axles and partly between springs which are put on the frame of the vehicle. More than 50 per cent of the total weight of the motor and gear is thus spring-borne. The motor is coupled to the driving axle by means of gear, one being on the same axle and the other on the shaft of the motor armature.
Because of axle support the space available is short and limits the size of the motor to about 337.5 kW for normal gauge vehicles. The method is usually employed for motor coaches and tramways. Because of the relatively large unsprung weight this system is not used on large locomotives. This limitation is also imposed by the available space which will not accommodate motor of larger size.
(b) Geared Quill Drive:
Geared quill drive is employed when the motor is to be placed higher in the frame than is possible with the nose suspension so that a motor of larger size (output) can be employed, centre of gravity can be raised and when unsprung weight is to be reduced. In this case quill surrounds the driving axle, as in direct quill drive and gear is mounted on the quill instead of motor armature.
3. Brown Boveri Individual Drive:
In this drive, flexibility is achieved by providing a special link arrangement between the gearwheel and the driving wheel, as illustrated in Fig. 14.16. This arrangement can be employed with considerable success on both high- and low-speed locomotives.
(a) Collective Drive with Connecting Rod:
Common method of collective drive is to drive with coupled wheels by means of a connecting rod attached to a crank mounted on the motor shaft or on a “jack” shaft geared to the motor shaft. The connecting rod is fixed in such a way as to remain nearly horizontal as in the steam locomotive. Such an arrangement eliminates the chances of any relative motion between the motor shaft and the driving wheels. Such an arrangement is illustrated in Fig. 14.17(a).
(b) Collective Drive With Scotch Yoke:
In this drive also, known as Scotch Yoke, or Kando type of drive, a triangular frame work having two sides projecting from the apex is used. The apex of the triangular framework is attached to the coupling rod and the other two corners are attached to the cranks mounted on two motor shafts. Vertical play is allowed in the bearings to secure the required flexibility between motor shaft and driving wheels. Such an arrangement is shown in Fig. 14.17 (b).
Control Equipment Required for Braking:
The control equipment is required for:
(i) Connecting the motor or motors to the supply mains without taking excessive current at start and provide smooth acceleration without causing sudden shock so as to avoid damage to the coupling
(ii) Adjusting the speed as per requirement, and
(iii) Providing rheostatic or regenerative braking.
The operation may be by hand in which the closing of the accelerating contactors is governed by the position of the controller handle and so the rate of acceleration remains completely under the control of driver or semi-automatic in which the position of the controller handle governs the number of contactors which will close, but the sequence of operation and the time interval between their individual operations are governed by time-lag devices, as in fully automatic equipment.
The hand-operation is preferred in locomotives, where there is a probability of divergences in the hauling loads and possibility of slipping of wheels. Semi-automatic operation is sometimes employed for suburban services in order to ensure uniform acceleration with a minimum energy consumption.
Plain drum controllers of the type are employed by the trams and small industrial locomotives but in railway work, where the currents to be dealt are too high for this type of equipment, master controller system is employed. In master controller system the contact instead of being made or broken by hand are closed or opened by the switches known as contactors operated by the solenoids.
The master controller is usually equipped with ‘dead man’s handle’ the function of which is to stop the train in case the driver faints or becomes incapacitated. This consists of a contact attached to the knob of the controller handle. The driver is to hold it always while the train is running. As soon as the dead man’s handle is released, circuit of the operating solenoid will get opened. This will lead to the opening of the motor circuit and bringing the train to a stop by automatic application of brakes.
There are various methods for closing and opening of the contactors by the master controller and the methods, usually employed are:
(i) All electric method
(ii) Electropneumatic method and
(iii) Cam shaft method.
i. All Electric Method:
In this method the contactors are operated (closed or opened) by the solenoids energised from the master controller. The main disadvantages of this method is that the wide fluctuations in the supply voltage, which are liable to occur, may cause uncertainty of action of the contactors. The difficulty however, can be overcome by employing a special low-voltage generator to provide a supply at 100 V or lower for the operating circuits.
ii. Electropneumatic Method:
In this method, contactors are operated by the compressed air. The valves, which admit the air into the operating cylinders, are operated electrically by the solenoids energised from the master controller.
This system is preferred to all electric operation system because:
a. The contactor groups are more compact and robust.
b. Each valve magnet requires only a few watts for operation whereas the solenoids in all electric operation system may require from 100 to 200 watts.
c. A low voltage control supply can be employed, which simplifies the interlocking and auxiliary contacts on the contactors and their circuits, reduces the size of the master controller and gives reliability to the whole of the control apparatus and circuits.
As electric trains are almost universally equipped with compressed air brakes, the supply of air for electropneumatic operation is, therefore, always available.
iii. Cam Shaft Drive:
In this method the contactors are closed or opened directly by cams, which are mounted on a cam shaft driven by an electric or pneumatic motor. The electrical connections are very simple, therefore, this method is widely used. The position of the master controller handle governs the angle through which the cam shaft is turned and therefore, fixes the number of contactors, which are closed.
This method has also the advantages of:
(i) Less wear and tear
(ii) Light in weight
(iii) Easy maintenance and
(iv) Occupying less space.
Multiple Unit Control:
For city and suburban services motor coach units are usually employed. Each motor coach unit consists of one motor coach and two or three trailing coaches for passengers. Motor coach carries driving electric motors and other controlling equipment. The number of motor coaches employed at different hours of the day are different, which depends upon the traffic to be handled at the various hours.
In peak traffic hours number of motor coach units aye combined to clear of the traffic. The units are so combined that they can easily be split up and run individually during the light traffic hours. The major coach unit (having number of motor coach units combined) thus formed must be capable of control from a single point. This is achieved in the multiple unit control.
In multiple unit control method each motor coach unit consists of two or more traction motors provided with a master controller, series-parallel controller (with rheostat and reverser), control-circuit multi-core cables (control bus line) with coupler sockets, accelerating relay; also a motor-generator set, battery or tapped line connected resistor (potentiometer) if the control circuit is to be supplied at low voltage.
The trailer coach comprises master controller (when required), control-circuit multi-core cables with coupler sockets. Each motor coach is also equipped with overcurrent relay with driver’s control switch, isolating switch for master controller, cut-out switch and fuses for isolating the control circuit of the motor controller from the control bus line.
All these individual coach units are put in parallel and are controlled from a single point by means of a master controller. Control circuits of various coach units are put in parallel by means of multi-core cables known as coupling cables. There being more than one master controller, it becomes necessary to incorporate interlocks so that only one master controller can be operated at a time.
Multiple unit control for three motor coach units is shown in Fig. 14.18. The controller of any one unit can be employed for master control over the whole train. If control of unit no. 1 is operated to start the motor, it is seen from Fig. 14.18 that relay 1 of all units are energised, whole of the starting resistance is placed in series with the motors and all the motors start simultaneously.
In next step some of the starting resistance will be taken out of the circuits of all the motors by controller of unit no. 1. In the same way other operations will be carried out by controller of unit no. 1 which will be same and simultaneous for each unit.
Auxiliary Equipment:
Auxiliary equipment, in addition to main traction motors, required to be installed on the electric locomotives are:
i. Motor-Generator Set:
The motor-generator set is of the two bearing type and consists of a series motor and a shunt generator. The speed at light load is limited by the ventilating fan and the voltage of the generator is controlled by an automatic regulator. It is to supply lighting, control system and other power circuits at low voltage from 30 V to 100 volts. The set must be of special design so that fluctuation of line voltage do not have any effect on the low voltage supply.
ii. Battery:
Provision of batteries is very important as some source of energy is required to raise the pantograph, to operate the air blast circuit breaker, to run the auxiliary compressor. Arno-converter provides cabin lighting. Batteries may be lead type or alkaline type. The voltage of lead type battery is 2 volts/cell. Lead acid batteries require more attention and maintenance, cannot be left in discharged conditions, and cannot withstand high rate of charge and discharge without reducing their life.
The voltage of alkaline battery is 1.2 V per cell. Alkaline cells can be left in discharged conditions and can withstand the short-circuit conditions even without any effect on their life. The battery is either charged by a separate rectifier or from a dc generator. The capacity of battery depends on the vehicle and may be between 175 and 375 AH at the 5 hours. The batteries are usually connected in parallel with the motor-generator set.
iii. Air Compressor:
The compressor is usually of two cylinder type and is directly connected to the motor. With compressors for motor coaches, a moderate speed motor is employed so that the weight is reduced and the compressor is driven through double-helical spur gearing.
Automatic starting and stopping of the motor to maintain the air pressure within desired limits is achieved by employing a pressure operated switch, the moving contact of which is actuated, through a system of levers and a toggle mechanism by the air pressure on a spring loaded diaphragm. It is for supplying compressed air for operating brakes, pantograph operating gear and electropneumatic control gear. At 1,500 V or below the compressor motor can be supplied directly from the line, otherwise, it is usually fed from the auxiliary generator.
iv. Heating Equipment:
It is required for heating the locomotive and train if desired. This is supplied directly from the line up to 3,000 V. Where an electric locomotive has to haul steam-heated rolling stock an electrically heated boiler is often provided.
v. Pantograph Operating Equipment:
Raising and lowering of pantograph is usually carried out by compressed air and suitable switches for controlling the solenoid-operated valves of the air supply are required to be provided.
vi. Traction Motor Blowers:
These are required on locomotives when forced ventilated motors are used. In some cases a single blower is used which delivers air at a low pressure into a central duct built into the under frame of the locomotive body from which it is distributed to the motors. In other cases with large frame mounted motors, each motor is provided with a separate blower. The blowers are then mounted on the motor frames and two or more blowers are coupled together and are driven by a single motor.