….let's share our railways knowledge

Addressing an age old problem of wheel slipping and stalling of trains

By on December 9, 2015

Wheel slipping and stalling of freight trains is one of the major hurdles in running trains during a certain period of monsoon and dew during the early hours of winter. Such happening is accepted as seasonal with stereotype repetition of age-old instruction and action plan circulated every year in June-July. The instructions do not carry much meaning with the introduction of step-less torque and creep control. Now let us look at how to train management unfolds in the next decade.

What is wheel slipping and stalling?

While negotiating an upgradient, there is every possibility wherein the applied tractive effort may exceed the adhesive weight. In such a situation, the wheel rotates faster than in linear motion. The traction control arrangement is such that the applied tractive effort is reduced to stop wheel slip which has a detrimental effect on the rail. With reduced tractive effort, there is every chance that the train may come to a stop if negotiating a gradient. If so, it becomes difficult to start the train because the tractive effort requirement will now be more than that when it was in motion. This leads to stalling of the train requiring assisting engine to clear the block section.

Wheel Slip Control System

Wheel slips or skids when the tractive and braking effort on the wheel exceeds the adhesive weight. The wheel may skid or slip, resulting in damages to the rail or wheel, as the case may be. The conventional locomotive type WAG7 and WAP4 measure the difference in traction motor current, and if it exceeds more than a prescribed limit, autoregression of the tap changer takes place. This reduces the tractive effort and prevents the wheel slip. The reduction of tractive effort is normally much higher than the loss of adhesion, developing a probability of the train coming to a stop on the gradient.

With the introduction of thyristor, GTO, and IGBT base power converter, it could be possible to introduce micro-control of tractive effort and creep. This has helped optimize the application of tractive effort as per available adhesion, and the chances of the train coming to a stop on the gradient reduced considerably.

Coefficient of Adhesion

Adhesion is the property between two bodies, and there is a limit to the maximum value it can be harnessed. Adhesion is different from friction. When a tractive effort is applied, and the motion starts, the linear speed is slightly lower than the rotational speed, called creep speed. The creep increases with the increase in the tractive effort up to a limit; after that, creep speed increases without any increase in its ability to transmit additional tractive effort. The curve below explains the characteristics of the rail-wheel adhesion when subjected to tractive effort. (Read more at https://www.railelectrica.com/wp-admin/post.php?post=227&action=edit

  • The maximum adhesion coefficient during dry and clean rail at any time is at the top of this curve. This top reduces with reduced adhesion coefficient during inclement weather or impurities on the rail table.
  • Measuring creep and micro-control of tractive effort helps to attain the tractive effort at the top of this curve. This helps in the optimization of the available adhesion to effective use. This is possible with a three-phase and separately excited DC motor working as a traction motor.
  • The use of sanding will only help in improving adhesion when it drops considerably during inclement weather.
    Individual control of the traction motor helps to reduce the tractive effort of only one motor.
  • Wheel slip control in a different class of locomotive is as follows:
    • WAG7/WAP4: Individual control of Series DC traction motor with tractive effort reduced in steps and no creep control. However, the provision of control of tractive effort of individual motors helps in isolation when defective and working the train up to its destination with five motors up the destination.
    • WAG9/WAP5: Control of a three-phase motor in one block and having creep control. Reference speed is determined through an algorithm. The Tractive effort of the three motors is reduced simultaneously when the tendency of wheel slip is detected. But with the introduction of an IGBT device-based converter, each axle has its inverter and control, the loss of tractive effort is less than GTO-based locomotive.
    • 12000 HP: Twin locos with eight numbers of three phases induction motor having creep control of two motors in one block. With eight axles providing higher adhesive weight and creep control, the tendency of wheel slip will reduce.
    • 9000 HP: 6 nos. of three-phase traction motor having a separate Main converter for each motor along with creep control will reduce the chances of wheel slip. The reference speed is taken from the rearmost axle having the least probability of slip due to higher adhesive weight arising due to weight transfer.
  • Sanding improves adhesion during inclement weather. The capacity of the sandbox is limited and manual sanding shall not be resorted to by the loco pilot. The sanding shall come into service automatically when the sharp acceleration of the wheel is detected, which is a sign of loss of adhesion. This will reduce the need for a frequent sand filling.
  • The quality of sand is an important issue for improving adhesion when required. The need for sanding will reduce considerably when automatic sanding is an integral part of the design and manual sanding is not permitted as a routine. Indian Railway shall consider improving the quality of sand used for the purpose
    • washed to remove fine dust,
    • higher content of silica
    • dried and stored
    • sandbox water sealed
    • and air intake fully dried.
  • WAG9 (IGBT), 9000 and 12000 HP locomotives offer an excellent opportunity to overcome and minimize wheel slipping and stalling. The new class also calls for the revised driving habit of not using manual sanding.

Attacking Speed on Gradient

The attacking speed of the train at the start of the gradient plays an important role in its ability to negotiate the gradient comfortably. As per the simple equation of motion, we know v2+u2=2gh; and without traction and rolling only, the train will climb to a distance of v2/(2g) or will travel a linear distance of [v2/(2g)]*(1/G) where G is gradient, i.e., vertical distance traveled divided by horizontal distance. As per the theoretical calculations considering the train resistance involving factors a, b, and c, 1/3rd of the energy is lost in overcoming train resistances. Therefore, empirically, the climbing distance for different speeds and gradients can be calculated as per the formula:  Distance = (1/3)[v2/(2g)]*(1/G) and costing distance table given below to understand the potential of attaching speed.
Capture

The attacking speed is generally of the order of 50-55 Kmph. This is mainly due to the restrictions imposed on the loaded stock when the carrying capacity of the wagon increased.
Indian Railway has upgraded the track to cater to a 25 T axle load on a dedicated freight corridor and feeder route. In case IR decides to upgrade the 3-4 Km stretch at the beginning of the upgradient permitting the speed to 75-80 Kmph ( and even 90-100 Kmph), negotiating the train on an up gradient even when adhesion is limited to 20% for an exit speed of 30 kmph on a gradient of 5 km will be very easy. The expenditure involved will only be around 5 Crs per such section.
This is a much better option than regular debates and discussions at various forums but with an almost negligible long-term solution. All discussion finally boils down to overpowering; therefore, this will certainly benefit until such a mission of overpowering is achieved.   In any case, overpowering is a much costlier proposal than upgrading the track to a 25 T axle load before the start of such critical sections.

5347141956_ac37aef6b1_o
Raising attacking speed at the start of the vulnerable section  will help IR coming out of the age old problem of wheel slipping and stalling meaningfully.

 

Post a Comment

Your email address will not be published. Required fields are marked *

There is 1 Comment

  1. K.Raman says:

    Well said! It is a pity that few people understand the mechanics of traction.

Top