How do traction motors work

In an effort to reduce weight, gear boxes used to direct power from the motor to the wheels were replaced by placing the wheels directly onto the motor's center shaft. With the motor working as its own axle, the locomotive's chassis as well as the tracks were given a longer life due, in part, to the reduction of weight being applied to both components.

By placing specially designed resistors in the motors, the speed control is much easier than the early traction motor models in which the engineer was continually forced to manually reduce the amount of current being sent to the motors in order to maintain a steady speed. Lori Kilchermann. Please enter the following code:. Login: Forgot password?

This is key operated and keys are only issued to qualified drivers or maintainers. It also means that, in our example, lights can only be switched on or off from one end at a time. The same principle, using contactors or relays, is applied to all other systems on the train - driving controls, braking control, heaters, doors, air conditioning, public address etc.

Of course, current for the equipment on each vehicle, as in this case, lighting, comes from a separate source - the auxiliary supplies - in the form of a battery, an alternator, an inverter or a power train line. How, one might ask, does one ensure that a number of locomotives or EMUs or DMUs for that matter , coupled together to work in multiple, perhaps facing in different directions, will all respond to the driver's command to go in the same direction, say forward, from the cab where he is sitting?

How do you prevent one locomotive taking off in the wrong direction? Well, it's built in to the wiring and it's simple, as shown in Figure 9.

Figure 9: A diagram showing the wiring schematic required to allow locomotives or multiple units working in the same train to operate in forward and reverse regardless of the direction of the driving positions. Each power unit whether it be a locomotive or EMU has a forward wire and a reverse wire connected to a "Forward and Reverse" switch of one sort or another in the cab.

Looking at Unit 1, if the driver selects "Forward", the forward wire in red is energised and the "forward relay" the arrow shows the direction of movement obtained for each relay is energised to make the locomotive move in the forward direction. To ensure the correct direction is achieved by a second locomotive Unit 2 that is coupled to the first, the forward and reverse wires are crossed over in the jumper cable.

If the second locomotive faces in the opposite direction to the first, its reverse wire shown in black here will be energised to make the loco run in the same direction as its partner.

To make sure this always happens, all multiple unit control jumpers have their forward and reverse wires crossed. But, you might ask, what if the locos both face in the same direction? You don't need the crossed wires in the jumper.

The crossed wires in the jumper will make the second loco go the opposite way. No, that's been solved too. So that the same jumper with the crossed wires can be used anywhere, the forward and reverse wires are also crossed ON each locomotive, only at one end, usually near the jumper socket.

Now, no matter which way round the locos are coupled to each other, and in what order, the forward command will always make all units drive in the same direction and the reverse command will make all units drive in the opposite direction.

One final point. The jumper heads are designed so that they can only be inserted one way into the coupler socket on the locomotive, rather like a USB plug on a computer. Modern systems use single wires or even fibre optic cables for controls. The system is sometimes referred to as "multiplexing", where a number of control signals are sent along a single wire. Some administrations require hard wired controls for safety systems like train braking but diverse programming can be used to make this redundant.

Although series-parallel connecting was common for resistance controlled DC traction motor control, it has been replaced for new equipment by electronic control.

However, there are many series-parallel systems still in use around the world. See Figure 2 above. Field Weakening. The DC motor can be made to run faster than the basic "balancing speed" achieved whilst in the full parallel configuration without any resistance in circuit. This is done by "field shunting". An additional circuit is provided in the motor field to weaken the current flowing through the field.

The weakening is achieved by placing a resistance in parallel with the field. This has the effect of forcing the armature to speed up to restore the balance between its magnetic filed and that being produced in the field coils. It makes the train go faster. Various stages of field weakening can be employed, according to the design of the motor and the intended purpose. Some locomotives used as many as six steps of field weakening. Since the DC motor and a DC generator are virtually the same machine mechanically, it was immediately realised that a train could use its motors to act as generators and that this would provide some braking effect if a suitable way could be found to dispose of the energy.

The idea formed that if the power could be returned to the source, other trains could use it. Trains were designed therefore, which could return current, generated during braking, to the supply system for use by other trains. Various schemes were tried over many years with more or less success but it was not until the adoption of modern electronics that reliable schemes have been available.

The major drawback with the regenerative braking system is that the line is not always able to accept the regenerated current. Some railways had substations fitted with giant resistors to absorb regenerated current not used by trains but this was a complex and not always reliable solution.

As each train already had resistors, it was a logical step to use these to dispose of the generated current. The result was rheostatic braking.

When the driver calls for brake, the power circuit connections to the motors are changed from their power configuration to a brake configuration and the resistors inserted into the motor circuit. As the motor generated energy is dispersed in the resistors and the train speed slows, the resistors are switched out in steps, just as they are during acceleration.

Rheostatic braking on a DC motored train can be continued down to less than 20 mph when the friction brakes are used to bring the train to a stop. Before the advent of power electronics, there were some attempts to combine the two forms of what we now call "dynamic braking" so that the generated current would go to the power supply overhead line or third rail if it could be absorbed by other trains but diverted to on-board resistors if not.

In the case of diesel-electric locomotives, dynamic braking is restricted to the rheostatic type. Racks of resistors can often be seen on the roofs of heavy-haul locomotives for which dynamic braking is a big advantage on long downhill grades where speed must be maintained at a restricted level for long periods.

To understand the principles of modern traction power control systems, it is worth a look at the basics of DC and AC circuitry. DC is direct current - it travels in one direction only along a conductor. AC is alternating current - so called because it changes direction, flowing first one way along the conductor, then the other.

It does this very rapidly. The number of times it changes direction per second is called the frequency and is measured in Hertz Hz. It used to be called cycles per second, in case you've read of this in historical papers. In a diagrammatic representation, the two types of current appear as shown in the diagram above left. From a transmission point of view, AC is better than DC because it can be distributed at high voltages over a small size conductor wire, whereas DC needs a large, heavy wire or, on many DC railways, an extra rail.

DC also needs more frequent feeder substations than AC - the ratio for a railway averages at about 8 to 1. It varies widely from one application to another but this gives a rough idea. Over the hundred years or so since the introduction of electric traction on railways, the rule has generally been that AC is used for longer distances and main lines and DC for shorter, suburban or metro lines. DC gets up to volts, while AC uses 15, - 50, volts.

Until recently, DC motors have been the preferred type for railways because their characteristics were just right for the job. They were easy to control too. For this reason, even trains powered from AC supplies were usually equipped with DC motors.

This diagram above shows a simplified schematic for a 25 kV AC electric locomotive used in the UK from the late s. The 25 kV AC is collected by the pantograph and passed to the transformer. The transformer is needed to step down the voltage to a level which can be managed by the traction motors. The level of current applied to the motors is controlled by a "tap changer", which switches in more sections of the transformer to increase the voltage passing through to the motors.

It works in the same way as the resistance controllers used in DC traction, where the resistance contactors are controlled by a camshaft operating under the driver's commands. Before being passed to the motors, the AC has to be changed to DC by passing it through a rectifier.

For the last 30 years, rectifiers have used diodes and their derivatives, the continuing development of which has led to the present, state-of-the-art AC traction systems. A diode is a device with no moving parts, known as a semi-conductor, which allows current to flow through it in one direction only. It will block any current which tries to flow in the opposite direction. Four diodes arranged in a bridge configuration, as shown below, use this property to convert AC into DC or to "rectify" it.

It is called a "bridge rectifier". Diodes quickly became popular for railway applications because they represent a low maintenance option. They first appeared in the late s when diode rectifiers were introduced on 25 kV AC electric locomotives.

The thyristor is a development of the diode. It acts like a diode in that it allows current to flow in only one direction but differs from the diode in that it will only permit the current to flow after it has been switched on or "gated". Once it has been gated and the current is flowing, the only way it can be turned off is to send current in the opposite direction. This cancels the original gating command.

It's simple to achieve on an AC locomotive because the current switches its direction during each cycle. With this development, controllable rectifiers became possible and tap changers quickly became history. A thyristor controlled version of the 25 kV AC electric locomotive traction system looks like the diagram here on the left. A tapping is taken off the transformer for each DC motor and each has its own controlling thyristors and diodes. The AC from the transformer is rectified to DC by chopping the cycles, so to speak, so that they appear in the raw as half cycles of AC as shown on the left.

In reality, a smoothing circuit is added to remove most of the "ripple" and provide a more constant power flow as shown in the diagram left. Meanwhile, the power level for the motor is controlled by varying the point in each rectified cycle at which the thyristors are fired. The later in the cycle the thyristor is gated, the lower the current available to the motor. As the gating is advanced, so the amount of current increases until the thyristors are "on" for the full cycle. This form of control is known as "phase angle control".

In more recent thyristor control systems, the motors themselves are wired differently from the old standard DC arrangement. The armatures and fields are no longer wired in series, they are wired separately - separate excitement, or SEPEX. Each field has its own thyristor, which is used to control the individual fields more precisely.

Since the motors are separately excited, the acceleration sequence is carried out in two stages. In the first stage, the armature is fed current by its thyristors until it reaches the full voltage. In the second stage, the field thyristors are used to weaken the field current, forcing the motor to speed up to compensate.

This technique is known as field weakening and was already used in pre-electronic applications. A big advantage of SEPEX is that wheel slip can be detected and corrected quickly, instead of the traditional method of either letting the wheels spin until the driver noticed or using a wheel slip relay to switch off the circuit and then restart it.

The traditional resistance control of DC motors wastes current because it is drawn from the line overhead or third rail and only some is used to accelerate the train to mph when, at last, full voltage is applied. The remainder is consumed in the resistances. Immediately thyristors were shown to work for AC traction, everyone began looking for a way to use them on DC systems. The problem was how to switch the thyristor off once it had been fired, in other words, how to get the reverse voltage to operate on an essentially one-way DC circuit.

It is done by adding a "resonant circuit" using an inductor and a capacitor to force current to flow in the opposite direction to normal. This has the effect of switching off the thyristor, or "commutating" it. It is shown as part of the complete DC thyristor control circuit diagram Figure The sand dramatically increases the traction of the drive wheels. The train has an electronic traction-control system that automatically starts the sand sprayers when the wheels slip or when the engineer makes an emergency stop.

The system can also reduce the power of any traction motor whose wheels are slipping. The giant two-stroke, turbocharged V and electrical generator provide the huge amount of power needed to pull heavy loads at high speeds.

The engine alone weighs over 30, pounds 13, kg , and the generator weighs 17, pounds 8, kg. We'll talk more about the engine and generator later. The cab of the locomotive rides on its own suspension system, which helps isolate the engineer from bumps.

The seats have a suspension system as well. Inside the cab there are two seats: one for the engineer and one for the fireman. The engineer has easy access to all of the locomotive's controls; the fireman has just a radio and a brake control. Also inside the car, right in the nose of the locomotive, is a toilet. The trucks are the complete assembly of two axles with wheels, traction motors, gearing, suspension and brakes. We'll discuss these components later.

The head-end power unit consists of another big diesel engine, this time a four-stroke, twin-turbocharged Caterpillar V The engine itself is more powerful than the engine in almost any semi-truck. It drives a generator that provides volt, 3-phase AC power for the rest of the train.

This engine and generator provide over kW of electrical power to the rest of the train, to be used by the electric air conditioners, lights and kitchen facilities. By using a completely separate engine and generator for these systems, the train can keep the passengers comfortable even if the main engine fails.

It also decreases the load on the main engine. This huge tank in the underbelly of the locomotive holds 2, gallons 8, L of diesel fuel. The fuel tank is compartmentalized, so if any compartment is damaged or starts to leak, pumps can remove the fuel from that compartment. The locomotive operates on a nominal volt electrical system. The locomotive has eight 8-volt batteries , each weighing over pounds kg. These batteries provide the power needed to start the engine it has a huge starter motor , as well as to run the electronics in the locomotive.

Once the main engine is running, an alternator supplies power to the electronics and the batteries. The "" means that each cylinder in this turbocharged, two-stroke, diesel V has a displacement of cubic inches That's more than double the size of most of the biggest gasoline V-8 car engines -- and we're only talking about one of the 12 cylinders in this 3,hp engine.

So why two-stroke? Even though this engine is huge, if it operated on the four-stroke diesel cycle, like most smaller diesel engines do, it would only make about half the power. This is because with the two-stroke cycle, there are twice as many combustion events which produce the power per revolution. It turns out that the diesel two-stoke engine is really much more elegant and efficient than the two-stroke gasoline engine.

You might be thinking, if this engine is about 24 times the size of a big V-8 car engine, and uses a two-stroke instead of a four-stroke cycle, why does it only make about 10 times the power? The reason is that this engine is designed to produce 3, hp continuously, and it lasts for decades. If you continuously ran the engine in your car at full power, you'd be lucky if it lasted a week.

This giant engine is hooked up to an equally impressive generator. It is about 6 feet 1. At peak power, this generator makes enough electricity to power a neighborhood of about 1, houses! So where does all this power go?

It goes into four, massive electric motors located in the trucks. The trucks are the heaviest things on the train -- each one weighs 37, pounds 16, kg. The trucks do several jobs. They support the weight of the locomotive. They provide the propulsion, the suspensions and the braking. As you can imagine, they are tremendous structures. The traction motors provide propulsion power to the wheels. There is one on each axle. Each motor drives a small gear, which meshes with a larger gear on the axle shaft.

This provides the gear reduction that allows the motor to drive the train at speeds of up to mph. The trucks also provide the suspension for the locomotive. The weight of the locomotive rests on a big, round bearing , which allows the trucks to pivot so the train can make a turn. Below the pivot is a huge leaf spring that rests on a platform. The platform is suspended by four, giant metal links , which connect to the truck assembly. These links allow the locomotive to swing from side to side.

The weight of the locomotive rests on the leaf springs , which compress when it passes over a bump. This isolates the body of the locomotive from the bump. The links allow the trucks to move from side to side with fluctuations in the track.

The track is not perfectly straight, and at high speeds, the small variations in the track would make for a rough ride if the trucks could not swing laterally. The system also keeps the amount of weight on each rail relatively equal, reducing wear on the tracks and wheels.

Braking is provided by a mechanism that is similar to a car drum brake. An air-powered piston pushes a pad against the outer surface of the train wheel. In conjunction with the mechanical brakes, the locomotive has dynamic braking. In this mode, each of the four traction motors acts like a generator, using the wheels of the train to apply torque to the motors and generate electrical current. The torque that the wheels apply to turn the motors slows the train down instead of the motors turning the wheels, the wheels turn the motors.

The current generated up to amps is routed into a giant resistive mesh that turns that current into heat. A cooling fan sucks air through the mesh and blows it out the top of the locomotive -- effectively the world's most powerful hair dryer.

On the rear truck there is also a hand brake -- yes, even trains need hand brakes. Since the brakes are air powered, they can only function while the compressor is running.