March 14th, 2016, Published in Articles: EE Publishers, Articles: Energize, Articles: Vector, Featured: Energize

by Mike Rycroft, features editor, EE Publishers

 

Rail transport is coming back into favour for the transportation of goods over long distances. This article takes a look at the role that electric motors play in this sector of the transport industry.

Two types of locomotive are in use: the pure electric, which takes power from an overhead medium voltage catenary wire, and is limited in use to long distance runs, and the diesel engine driven locomotive which can be used for all types of operation, including shunting.  This article covers the diesel locomotive.

The name “diesel locomotive” is misleading, as the traction power is provided by electric motors driving the wheels directly, and the electricity to power the motors is generated by an alternator driven by a diesel engine. The use of a diesel engine frees the locomotive from being coupled to an external electricity supply, and the use of electric motors and drives allows control over the traction capabilities of the locomotive which would not be possible with a direct drive from the diesel engine.

Locomotive traction motors

Motors can be mounted in a number of different configurations:

• Truck or bogie control: A single motor drives all the wheels on the truck or bogie, typically four wheels per motor.
• Axle control: The motor drives both wheels on a single axle. This is the most common configuration (see Fig. 1).
• Wheel control: Each wheel is driven by its own motor. This allows the maximum degree of control over the locomotive, but is not often used.

Three types of motor are used in locomotives:

• DC motors.
• AC motors with variable frequency drives.
• AC permanent magnet motors.

The primary requirements for a locomotive motor are that it must be possible to vary and control the speed, and it must be able to supply the start-up and accelerating torque. Early locomotives used DC motors as these were the only type of large motor which could provide speed control and the required torque. AC motors ran at a fixed speed and could therefore not be used in this application. DC motors have several disadvantages which will be discussed later.

The development of variable frequency drives for large AC synchronous motors changed the situation and today most locomotives use this type of motor and drive combination. Large permanent magnet (PM) motors have appeared on the market and have several advantages over synchronous AC wound-stator motors for traction applications. A number of manufacturers use PM motors in their locomotives.

DC traction motors

DC motors are used in a series-wound configuration and speed is controlled by switching series resistance in and out of the circuit. In early applications, resistance was controlled manually by the driver, but relay systems were later installed to do this automatically. At start up the maximum current flows through the motor, providing maximum torque. As the motor speeds up, back-EMF reduces the current and the torque, and series resistance is switched out step-by-step to maintain the required torque until full speed is attained. Resistance switching gives a step-type change in torque, and hence acceleration. Relay systems were replaced by electronic controls in later systems to give smoother acceleration and deceleration characteristics. Systems in use today commonly use separately excited DC motors and thyristor controls for both field excitation and main supply voltage. DC motors are still favoured for applications that require constant start-stop operation under heavy load.

Series-wound DC motors have the disadvantage that, if wheel-slip occurs in a standard DC drive, there is the tendency for the traction motor to speed up and run away, even to the point of mechanical failure if the power is not reduced quickly. As the wheel slippage increases, the coefficient (µ) of friction also drops rapidly to a level of 0,10 or less, and because all the motors are connected together, the load to the entire locomotive must be reduced. Therefore, maximum adhesion is obtained by operating at a level with a comfortable margin of safety below the theoretical maximum. More modern DC systems incorporate a wheel-slip control which senses the beginning of a slip and automatically modulates the power to retain control. This allows the locomotive to operate safely at a point closer to its theoretical maximum [4].

AC VFD motors: induction motors

The replacement of DC motors by AC motors was made possible by the development of high power electronic devices used in variable frequency drives (VFDs). VFDs make speed and torque control possible to an extent greater than with DC motors, and allowed more control functions to be implemented. AC traction motors have replaced DC motors in many traction applications. The motors used are induction or asynchronous motors designed to have characteristics suitable for traction. The speed and torque of the motor are controlled by varying the frequency, voltage and current applied to the stator coils. Motors for a typical locomotive would be in the range 400 to 600 kW.

Permanent magnet synchronous motor (PMSM)

The PMSM is a 3-phase AC synchronous motor with the usual squirrel cage or induction construction replaced by magnets fixed in the rotor. The rotor is propelled by a rotating magnetic field realised through 3-phase AC supply to the stator winding. The rotor will rotate in synchronism with the rotating field produced by the stator. The motor requires a complex control system but it can be up to 25% smaller than a conventional 3-phase motor for the same power rating. The design also gives lower operating temperatures so that rotor cooling is not needed and the stator is a sealed unit with integral liquid cooling. A number of different types of train have been equipped with permanent magnet motors. The reduced size is particularly attractive for low floor vehicles where hub motors can be an effective way of providing traction in a compact bogie. The development of motor design and the associated control systems continues and it is certain that the permanent magnet motor will be seen on more railways in the future [3]. Control systems are capable of controlling both the torque and speed of the motor, which gives a wide range of operation suitable for traction.

The PM motor is claimed to offer higher start-up torque than AC asynchronous or induction motors used in locomotives, giving the ability to drive the axle directly as opposed to the gear drive used with other motors. This reduces weight and increases efficiency. The PMSM uses a specially designed inverter/controller to take advantage of the motor’s characteristics.

Mounting a traction motor directly on the wheel has been a goal since electric motors were first used in locomotives. PMSMs with high specific torque are likely to make this possible in the future. Prototype models have been developed successfully. Axial flux designs have been produced for industry and it may be possible to use this configuration on locomotives.

Alternators

Alternators are used in both DC and AC driven locomotives to produce the required electric power. A typical alternator would be a brushless 3-phase synchronous type. The alternator is driven directly by the diesel engine, and so operates over a range of speeds and with a consequent varying output frequency. This is possibly the prime reason for the use of rectifier/inverter combinations. There is no reason why the alternator has to operate at a fixed frequency, as it does not directly drive frequency dependent devices. The frequency range of operation can also be chosen to suit the rectification process, with a typical output being at 75 Hz 3-phase when running at full engine speed. In many alternators, the rectifier assembly is attached to the alternator frame and is supplied as a unit matched to the alternator output. This allows the alternator to be used for both AC and DC traction applications.

Rectifier converters

All forms of traction motor require a DC supply, either directly in the case of the DC motor, or indirectly, via the VFD in the case of the AC motor. In many modern motors, the rectifier unit is supplied as part of the alternator and matched to the machine characteristics.

Locomotive controls

Early controls were based on speed only. Modern developments take many other factors into account to maximise the tractive power of locomotives, for the same drive motor size. One of the main factors affecting the tractive power is wheel-slip or creep, and most modern control systems are designed to control the amount of slip between the wheel and the rail.

Adhesion and slip in locomotives

The locomotive is driven by the contact between the wheel and the rail. This is a metal-to-metal contact and the force transmitted depends on the adhesion coefficient and the locomotive adhesion weight. The adhesion coefficient refers to the amount of a locomotive’s weight on its driving wheels, which can be converted into tractive effort.

Wheel slipping occurs when tractive effort exceeds adhesive weight. Adhesive weight is defined as the force that can be exerted by a wheel without slipping or sliding. Slip occurs when the circumferential velocity exceeds the linear velocity of the wheel on the rail.

Adhesive weight = µ_adhesion x weight    (1)

The adhesion coefficient depends on the slip velocity, conditions of the rail’s surface, train velocity and temperature in the contact area. Of all the parameters which can influence the adhesion coefficient, only the train’s velocity and slip velocity can be changed and controlled. Because the train’s velocity is typically maintained at the required value, only the slip velocity can be controlled [1]. Characteristics of wheels differ slightly and coupled drive wheels will exhibit a certain amount of slip.

Even when operating at optimal conditions, there will be a certain percentage of slip between the wheels and the rails. The aim of the modern control system is to maximise the adhesion coefficient by limiting or controlling the amount of slip on the wheels. This is done by controlling the motors driving a multi-axle truck or unit. Tractive effort varies with slip, as shown in Fig. 2, and the aim of the control system is operate the locomotive in its maximum adhesion coefficient zone. There is a difference in the adhesion coefficients achievable with AC and DC motor drives. AC drives give higher starting adhesion coefficients, as well as higher dispatchable adhesion.

There are a number of different systems used to manage slip and optimise the adhesion coefficient. All use some means of comparing the wheel rotation speed with the linear speed of the train and feed appropriate controls to the inverter. The slip is measured by detecting the locomotive speed by Doppler radar (instead of using the rotating wheels) and by comparing it to the motor current to see if the wheel rotation matches the ground speed. If there is disparity between the two, the motor current is adjusted to keep the slip within the “creep” range and to keep the tractive effort at the maximum level possible under the creep conditions [3].

Another control which will give improved adhesion is weight transfer compensation. When a locomotive pulls a load, weight tends to transfer from the front axle to the rear axle of each truck. At maximum tractive effort, the weight on the lead axle may be reduced by about 20%. Since the tractive effort is proportional to the weight on drivers, the tractive effort will be determined by the lightest axle in a system where the motors are fed from a common source. So, in effect, the equivalent locomotive weight is reduced by about 20%. With an axle control system, however, the drive can compensate for the weight transfer. When the lead axle goes light, the drive system will reduce power to that axle and apply more power to the rear axle without incurring wheel-spin.

Inverters and control systems

The inverter, which is really the motor drive or VFD unit, supplies AC of varying frequency and current to the motors. Initially, a single inverter supplied all the motors but the latest technology tends to go for one inverter per motor. This has the advantage of reducing the inverter size and allowing for individual motor control. Most large locomotives use a one motor-per-axle configuration so each converter controls an axle and wheel pair.

There are some variations in how the inverters are configured. Some manufacturers rely on one inverter per truck, while others use one inverter per axle. Both systems have their merits. The truck control system links the axles within each truck in parallel, ensuring wheel-slip control is maximised among the axles equally. Parallel control also means a more even wheel wear between axles. However, if one inverter (i.e. one truck) fails, then the unit is only able to produce 50% of its tractive effort. One inverter per axle is more complicated, but the view is that individual axle control can provide the best tractive effort. If an inverter fails, the tractive effort for that axle is lost, but full tractive effort is still available through the other five inverters (for a six-axle unit). By controlling each axle individually, keeping wheel diameters closely matched for optimum performance is no longer necessary [4].

Dynamic braking

In dynamic braking systems the motors are run as generators and the current generated is fed to rheostats or variable resistors mounted on the locomotive chassis. The braking force is controlled by varying the resistance of the rheostat. The power required to brake or slow a locomotive is the same as that required to accelerate it and the rheostats are therefore required to dissipate a large amount of energy and are generally forced-air cooled. In more recent developments, the current generated has been used to charge storage batteries or ultra-capacitors, the stored energy being used to assist in re-accelerating the locomotive.

Upgrade possibilities

Locomotives are a long-term investment and, in Africa, there are many units which are over 20 years old, using older technology and controls. Fortunately, it is possible to upgrade the control systems on older locomotives to achieve improved performance and extend the locomotive’s life span without replacing major drive components. Improvements of up to 25% in tractive effort and in all-weather or dispatchable adhesion of up to 26% have been claimed. This could reduce the number of units needed to haul high loads.

Energy storage or supercapacitors for start-up boost

Super and ultracapacitors are used in some locomotives to provide the additional power required during start-up. This allows smaller engines and alternators to be used. Capacitors can store energy from regenerative braking, which would otherwise be dissipated in resistors or other devices.

The most South African locomotive

The GE Evolution series of locomotives being built in South Africa are six-axle locomotives (two groups of three at the front and rear, all axles driven) using AC individual-axle traction-control technology that enables greater hauling power by reducing slippage on start-ups, inclines and during suboptimal track conditions. This technology ensures optimum performance, less wasted energy and reduces maintenance costs and associated down-time substantially during the locomotive’s life span, when compared to older DC and other AC technology traction systems. The locomotive features sophisticated operator controls which improve diagnostics and simplify operation. The consolidated control architecture of the Evolution series locomotive makes it easier to upgrade software and download data. “Smart” displays eliminate several add-on black boxes in favour of a computer and display combination, which enhances both reliability and operator ergonomics.

References

[1]    P Pichlík and J Zděnek: “Overview of slip control methods used in locomotives”, Transactions on electrical engineering, Vol. 3 (2014), No. 2, www.transoneleng.org/2014/20142c.pdf
[2]    RTWP: “Electronic power for trains”,www.railway-technical.com/tract-02.shtml
[3]    RTWP: “Diesel locomotive technology”,www.railway-technical.com/diesel.shtml
[4]    Republic locomotive: “AC traction vs DC traction”, www.republiclocomotive.com/ac_traction_vs_dc_traction.html
[5]    Railelectrica: “Selection of suspension arrangement of traction motors”, www.railelectrica.com/traction-motor/selection-of-suspension-arrangement-of-traction-motors-a-right-approach-2/

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