Here is an experiment on ‘Electric Locomotive’ especially written for school and college students.
Aim of the Experiment:
Study of an electric locomotive
Objective:
To visit the railways repair shop at a nearby station, for study of electric locomotive.
Theory:
Electric railways are widespread throughout the world and there are many different varieties but all are based on either direct current or alternating current. Electric locomotive benefits from high efficiency of electric motors, often above 90%.
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Additional efficiency can be gained from regenerative braking which allows kinetic energy to be recovered during braking to put some power back on line. Newer electric locomotive use ac motor inverter drives systems that provide for regenerative braking.
The chief disadvantages of electrification are the cost of overhead power lines, substations and control systems.
In an electric locomotive various types of power supplied are used:
1. Rechargeable energy storage systems, as battery.
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2. A stationary source, such as a third rail or overhead wire.
The distinguishing design features of electric locomotives are:
(i) The type of electric power used, either alternating current or direct current.
(ii) The method of store (batteries, ultra capacitors) or collecting electric power transmission.
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The earliest systems used direct current and locomotives typically run relatively at low voltage (750 to 3000 volts) and the currents involved are large in order to transmit sufficient power. Then alternating motors were developed ac high voltages are then used because this allows the use of low currents, transmission losses are proportional to the square of the current. Thus high power can be conducted over long distances on lighter and cheaper wires. Transformers in the locomotives transform this power to low voltage, high current for the motors.
A similar high voltage, low current system could not be employed with direct current locomotives because there is no early way for dc to do the voltage/current transformation so efficiently achieved by ac transformers. Nowadays ac traction system used three phase current rather than single phase of household use. Today electric locomotive have invariably three phase induction motors. These poly phase motor machines are powered from G.T.O. inverters. The cost of electric devices in modern locomotive can be reduced by 50% of the total cost of the vehicle.
Electric traction allows the use of regenerative braking in which the motors are used as brakes and become generators that transform the motion of the train into electrical power that can feed back into the lines. This system is particularly advantageous in mountainous operations, as descending locomotives can produces a large portion of power required for ascending lines.
The original electrification used a sliding shoe in overhead channel. Now it was replaced with third rail system in which shoe rode underneath or on top of a small rail parallel to the main track above ground level.
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However, railways generally tend to prefer overhead lines, often called ‘catenaries’ after the support system used is hold the wire parallel to the ground.
Three collection methods are possible:
1. Trolley Pole:
A long flexible pole, engages the line with a wheel or shoe.
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2. Bow Collector:
A frame holds a long collecting rod against the wire.
3. Pantograph:
A hinged frame that holds the collecting shoes against the wire in a fixed geometry. Of the three, the pantograph method is best suited for high speed operation.
The following points describe the two systems and they are applied to different railways, this also includes various control systems used on electric locomotives and trains:
1. Asynchronous Motor:
Modern traction motor type using three phase AC electrical supply and now the favoured design for modern train traction systems can be used on DC and AC electrified railways with suitable control electronics and on diesel-electric locomotives.
2. Axle Brush:
The means by which the powers supply circuit is completed with the substation once power has been drawn on the locomotive. Current collected from the overhead line or third rail is returned via the axle brush and one of the running rails.
3. Battery:
All trains are provided with a battery to provide start up current and for supplying essential circuits, such as emergency lighting in, when the line supply fails. The battery is usually connected across the DC control supply circuit.
4. Bucholz Relay:
A device inserted in the oil cooling circuits of electric locomotive transformers to detect low oil pressure. In this event the relay trips out the power system. Often a source of spurious circuit breaker trips if not carefully calibrated.
5. Camshaft:
Most DC electric traction power circuits use a camshaft to open or close the contactors controlling the resistances of the traction motor power circuit. The camshaft is driven by an electric motor or pneumatic cylinder.
The cams on the shaft are arranged to ensure that the contactors open and close in the correct sequence. It is controlled by commands from the driver’s cab and regulated by the fall of current in the motor circuit as each section of resistance is cut out in steps. The sound of this camshaft stepping can be head under many older (pre electronics) trains as they accelerate.
6. Cannon Box:
Sleeve used to mount a traction motor on axle in electric power bogies and sometimes including an axle brush.
7. Chopper Control:
A development in electric traction control which eliminates the need for power resistors by causing the voltage to the traction motors to be switched on and off (chopped) very rapidly during acceleration. It is accomplished by the use of thyristors and will give up to 20% improvement in efficiency over conventional resistance control.
8. Circuit Breaker:
An electric train is almost always provided with some sort of circuit breaker to isolate the power supply when there is a fault, or for maintenance. On AC systems they are usually on the roof near the pantograph. There are two types the air blast circuit breaker and the vacuum circuit breaker. The air or vacuum part is used to extinguish the arc which occurs as the two tips of the circuit breaker are opened.
9. Contactor:
Similar to a relay in that it is a remotely operated switch used to control a higher power local circuit. The difference is that contactors normally latch or locks closed and have to be opened by a separate action. A lighting contactor will have two, low voltage operating coils, one to “set” the contactor closed to switch on the lights; the other to “trip” off the lights.
10. Converter:
Solid state electronic system is used for converting alternating current to direct current or vice versa. Where an AC supply has to be converted to DC it is called a rectifier and where DC is converted to AC it is called an inverter.
11. Cooling Fans:
To keep the thyristors and other electronic power systems cool, the interior of a modern locomotive is equipped with an air management system, electronically controlled to keep all systems operating at the correct temperature. The fans are powered by an auxiliary inverter producing 3-phase AC at about 400 volts.
12. Creep Control:
A form of electronically monitored acceleration control used very effectively on some modern drive systems which permits a certain degree of wheel slip to develop under maximum power application. A locomotive can develop maximum slow speed tractive effort if its wheels are turning between 5% and 15% faster than actually required by the train speed.
13. DC Link:
Used on modern electronic power systems between the single phase rectifier and the 3-phase inverter. It is easier to convert the single phase AC from the overhead line to 3-phase required for the motors by rectifying it to DC and then inverting the DC in 3- phase AC.
14. Dynamic Braking:
A train braking system using the traction motors of the power vehicle(s) to act as generators which provide the braking effort. The power generated during braking is dissipated either as heat through on-board resistors (rheostatic braking) or by return to the traction supply line (regenerative braking). Most regenerative systems include on board resistors to allow rheostatic braking if the traction supply system is not receptive. The choice is automatically selected by the traction control system.
15. Grid:
Train or locomotive mounted expanded steel resistor used to absorb excess electrical energy during motor or braking power control. Often seen on the roofs of diesel electric locomotives where they are used to dissipate heat during dynamic braking.
16. Ground Relay:
An electrical relay provided in diesel and electric traction systems to protect the equipment against damage from earths and so-called “grounds”. The result of such a relay operating is usually a shut-down of the electrical drive. Also sometimes call an Earth Fault Relay.
17. GTO Thyristor:
(Gate Turn Off Thyristor) A thyristor which does not requires a commutation (reverse flow circuit) circuit to switch it off.
18. IGBT:
Most recent power electronics development, it is replacing the GTO thyristor as it is smaller and requires less current to operate the switching sequences.
19. Inverter:
Electronic power device mounted on trains to provide alternating current from direct current. Popular nowadays for DC railways to allow three phase drive or for auxiliary supplies which need an AC supply.
20. Jerk Limit:
A means by which starting is smoothed by adjusting the rate of acceleration of a train by limiting the initial acceleration rate upon starting. It could be described as limiting the initial rate of change of acceleration. Also used in the dynamic braking.
21. Line Breaker:
Electro-mechanical switch in a traction motor power circuit used to activate or disable the circuit. It is normally closed to start the train and remains closed all the time power is required. It is opened by a command from the driving controller, no volts detected, overload detected and (were required) wheel spin or slide detected. It is linked to the overload and no-volt control circuits so that it actually functions as a protective circuit breaker.
22. Master Controller:
Drivers power control device located in the cab. The driver moves the handle of the master controller to apply or reduce power to the locomotive or train.
23. Motor Blowers:
Traction motors on electric locomotives get very hot and, to keep their temperature at a reasonable level for long periods of hard work, they are usually fitted with electric fans called motor blowers. On a modern locomotive, they are powered by an auxiliary 3-phase AC supply 400 volts supplied by an auxiliary inverter.
24. Notching Relay:
A DC motor power relay which detect the rise and fall of current in the circuit and inhibits the operation of the resistance contactors during the acceleration sequence of automatically controlled motors. The relay operates a contactor stepping circuit so that during acceleration of the motor, when the current falls, the relay detects the fall and calls for the next step of resistance to be switched out of the circuit.
25. No-Volt Relay:
A power circuit relay which detected if power was lost for any reason and made sure that the control sequence was returned to the starting point before power could be re-applied.
26. Overload Relay:
A power circuit relay which detected excessive currents in the circuit and switched off the power to avoid damage to the motors.
27. Rectifier:
A converter consists of thyristors and diodes which is used to convert AC to DC. A modern locomotive will usually have at least two, one for the power circuits and one or more for the auxiliary circuits.
28. Relay:
A remotely controlled switch which uses a low voltage control circuit. It will close (or open) a switch in a local circuit, usually of higher power.
29. Resistance Control:
Method of fractions motor, control formerly almost universal on DC electric railways whereby the power to the motors was gradually increased from start up by removing resistances from the power circuit in steps. Originally this step control was done manually but it was later automatic, a relay in the circuit monitoring the rise and fall of current as the steps were removed. Many examples of this system still exist but new builds now use solid state control with power electronics.
30. Shoe-Gear:
Equipment carried by a train and used for current collection on track mounted (third rail) power supply systems. Shoe-gear is usually mounted on the bogies close to the third rail. It is often equipped with devices to enable it to be retracted if required to isolate the car or on-board system which it supplies.
31. Synchronous Motor:
Traction motor where the field coils are mounted on the drive shaft and the armature coils in the housing, the inverse of normal practice. This is a single-phase machine controlled by simple inverter, now superseded by the asynchronous motor.
32. Tap Changer:
Camshaft operated set of switched used on AC electric locomotives to control the voltage taken off the main transformer for traction motor power, Superseded by thyristor control.
33. Transformer:
A set of winding with a magnetic core used to step down or step up a voltage from one level to another. The voltage differences are determined by the proportion of windings on the input side compared with the proportion on the output side. An essential requirement for locomotives and trains using AC powers, where the line voltage has to be stepped down before use on the train.
34. Wheel Spin:
On a steam locomotive, the driver must reduce the steam admission to the cylinders by easing closed (or partially closed) the throttle/regulator when he hears the wheels start to spin. On diesel or electric locomotives, the current drawn by individual or groups of traction motors are compared-the motor (or group) which draws proportionally less amps than the others is deemed to be in state of slip and the power is reduced. Some systems-EMD Super Series for one-measure known wheel speed against ground speed as registered on a Doppler radar. Many locomotives additionally use sand, which is applied to the wheel/ rail contact point to improve adhesion—this is either controlled automatically or manually by the driver.
35. Wheel Spin Relay (WSR):
A relay in older traction motor control circuits used to detect wheel spin or slide by measuring the current levels in a pair of motors on a bogie and comparing them. The idea is to prevent motor damage by preventing an over speeding motor causing an unacceptable rise in current tin the other motor of the pair. If detected, the imbalance causes the control circuits to open the line breakers and reset the power control to the start position like a “no-volt” relay.
