AC vs DC traction motors - what's the difference?

AC TRACTION

The AC (alternating current) Drive, also known as Variable Frequency Drive, has been the standard in industry for many years. While it has been used in locomotives for over two decades (especially in Europe), it has only been recently that the price of the drives has allowed them to be used in most of the new diesel-electric locomotives in the United States. The AC drive works by converting the traction alternator output to DC (direct current) and reconverting it to a variable frequency AC which powers AC traction motors. Because AC motors operate at approximately the frequency of the current, the drives must adjust the frequency so that the motors can have a speed range of zero to maximum rpm.

Next, the DC locomotive typically has a number of throttle settings with a set power level for each one. While this sytem is simple and effective, it does not produce a constant motor torque since power is the product of torque and speed. Therefore, the tractive effort varies significantly for each throttle setting depending on speed, making it impossible to obtain maximum adhesion.
The AC locomotive, however, can control to a specific motor torque level allowing the tractive effort to be essentially constant at the higher range of available adhesion. Ths fast acting wheel slip control can counteract any wheel slip so that the torque level can be set close to the upper limits.

Introduction

AC vs DC traction motors which one? In engineering terms, every device, invention, or product comes with its merits and demerits. In the locomotive industry, the pivotal role of locomotive traction motors results in constant development in this technology. In this article, we made a comprehensive comparison on AC vs DC traction motors

Initially, this industry followed the development path through the use of DC traction motors. The GP9, SD40, and SW1200 are famous examples of first-generation DC motors. After development stages in the adhesion levels, the SD60 and Dash 8 series conquered the industry for quite a while.

However, the adhesion levels were still far from optimal, and that is where the development and economization of AC traction motors technology came into play.

Tractive Effort=Weight on Driver Motors ×Adhesion

Where,

Adhesion=Coefficient of Friction ×Locomotive Adhesion Value

The locomotive adhesion value is in basic terms, the efficiency of a locomotive in converting available friction to utilizable friction at the rail and wheel contact point. In other words, the ability of a locomotive to convert the friction present between the wheel and the rail into useful friction that is required for the locomotive to move is the locomotive adhesion value. The Greater this value is, the greater the efficiency of traction between the track and locomotive. This value ranges from 0.45 to 0.90 for DC and AC motors, respectively. The coefficient of friction’s value is generally considered between 0.4-0.45 for most scenarios i.e. dry and clean conditions for a locomotive’s operation.

Having discussed all the basics, let us now dive into the main topic i.e. the core differences between AC traction vs. DC traction motors in terms of functionality and much more!

Further Details of AC Traction vs. DC Traction Motors and Their Comparison

On the other hand, the braking system in locomotives utilizing AC motors is not dependent only on the locomotive weight. This is because, during braking, the locomotive can shift to a generative model (dynamic braking). During productive mode, the electricity that is being produced by the AC traction motors using the rotation of the wheels as the energy source is in turn used to reduce the wheel rotation causing the locomotive to slow down. The braking resistors dissipate the energy received from the AC motors during braking.

source :

https://supcorailway.com/ac-vs-dc-traction-motors/

How the improvement in the design of a traction motor pinion by SUPCO will prolong the pinion life span and reduce the maintenance costs of different types of traction motors.
When it comes to traction motor pinion gears, there is always a slight sliding or slipping on both sides of pitch point over tooth curvature. This sliding causes pitting and damages the traction motor pinion teeth, which as a consequence produces torsional vibration when rotating the gearwheel, and damaging the traction motor. In this paper, SUPCO demonstrates the root cause of such damages and proposes the method for how to improve the traction motor pinions. Contact SUPCO for more information on traction motors.

An Introduction to Locomotive traction motor Pinion :

Transmission of rotational force from one axle to the other has a long history by using pulley and belt. This is still in use for many applications. However, when there is a high amplitude of force then due to tangential traction between belt and pulley, and limited fraction counterforce, slipping between locomotive traction motor pinion gear will be the main issue. Clip 1. Therefore, an alternative means of transmission such as spur gear is to be utilized.

As the case is in locomotives, where the traction motors are driving the locomotive’s axle through axle-gears. Spur gear transmission transfers force from locomotive traction motor’s pinion to axle gear as Normal force to the gear’s teeth compare to tangential force between pully and belt, so direct push will be applied by meshing 2 gears. See Clip 2

Replacing pullies by meshing gears, and string by engaging pinion and axle gear’s teeth. One should study the correct section of engaging teeth to have a constant velocity ratio while these 2 teeth are touching and passing one over another, see Fig 3, and video clip 2

traction motor pinion Section

The experiment on video clip 5 shows how such curve called as involute can be generated. To trace this curve, we locate a pen at point P on this string to draw the tooth profile for each pulley. The planes where these two profiles are drawing on, is bond to the surface of their own pulley, as a rigid part turning together. For better understanding the 2 profiles generated, here on this clip are shown again on Fig 8 of the next.

Chalanges in designing Traction Motor Pinion Gears

Since both Involute sections are generated simultaneously at common point P, they constitute the shape of the 2 engaging tooth, having contact at pitch point P. This is the point where the tooth on driving gear or pinion is touching the opposite tooth on driven gear installed over locomotive’s axle. Within a short period of time, when the 2 opposite teeth are meshed, this touching or pitch point is tracing these 2 wedding profiles.

In fact the touching point P is

traction motor pinion Section view
Fig 6

moving along 3 different paths depending on the plane in which the pen on point p is sketching on, Fig 6.

1)- Lower involute generated by red pulley .

2)- Upper involute generated by Green pulley.

3)- The straight line tangent to both pulleys is called line of action or pressure line .

Clip 5 on previous page , shows how these profiles are generating

The 2 wedding involutes shown on Fig 6 always make a contact at point P, a direct push is applied and a steady motion with constant velocity is transferred. As mentioned earlier point P is moving with a constant speed along Pressure Line tangent to the base circle of both gears, exactly as it is the case in pulleys and belt mechanism, so the velocity ratio of these 2 gears is equal to the ratio of their Base Circle Diameters. Although the contact point is moving with constant speed along Pressure Line, but since it moves on different speeds over the wedding involutes, therefore it is our main concern as the root cause of gears wear.

At the first stage, the involute generated by axle gear growing faster before reaching pitch point C (See Fig 6, and Fig 7 A ) so the slides passing the second involute generated by traction motor pinion gear, at the single point where the tangent point of the 2 Pitch circles, these two involutes have the same speed, so there is no sliding. Then at the third stage, after passing the pitch point, the involute generated by pinion grows faster and slides passing over the involute produced by axle gear. So there is always sliding in both sides of the pitch point. Consequently, the optimum tooth curve is the small segment at 2 sides of the pitch point, called Addendum and Dedendum sections. Even small, but sliding always happens in 2 opposite directions around pitch point in locomotive traction motor pinion gear, during tooth engagement.

About SUPCO

SUPCO is a company that is producing and exporting Traction Motor Pinion Gears. It includes experts from different branches of Locomotive designing to establish a potential atmosphere for competition in order to create the most qualified Locomotive Traction Motors. Our company is a highly dependable compound for the best North American producers, which offers a long-term relationship and gives after-sale services to customers. To see our products, more information on traction motor pinion visit the Traction Motor Products page on our website. Also, check other articles on our blog for different types of traction motors and details of AC vs DC traction motors.

source : 

https://supcorailway.com/locomotive-pinions-emd-9556211/

Traction motor

Traction motor is considered as one of the most important parts of the propulsion system of Propelled trains and locomotives. In other words, electric motors act as train muscles to provide the tensile force required to move the train.depending on the type of feeding, the traction motors are divided into AC or DC categories. traction motors were traditionally DC but the development of modern power and control electronics has led to the introduction of 3-phase AC motors. Thanks to improvements in Electronic science and control algorithm, AC motors are widely used in Rail industry for their lower maintenance costs and higher reliability. Also their more sophisticated control system leads to higher tractive force of rail vehicle. Taam Locomotive Arya is a leading company providing technical services to rail industry.

Our drives and traction motors operate all over the world: More than 90,000 motors are in use in a variety of rail vehicles, from trams to high-speed trains to powerful locomotives. We match our traction motors and drives perfectly to one another and to the project-specific requirements. Our expertise in manufacturing and high product quality is certified to ISO/TS 22163 (IRIS International Railway Industry Standard) and Q1 (highest quality rating from Deutsche Bahn). We can manufacture motors and drives in a multitude of countries to meet our customers’ needs, and our service network covers the world.

Persuasive in every detail

Traction motors from Siemens Mobility stand for reliability, longevity, and energy-efficiency. We use our long track record of experience in development, manufacturing, and operation to configure the optimal solution to your requirements.

A smoother ride with our motors
Traction motors and drives from Siemens Mobility are in use in many rail vehicles around the world – for good reason.

High quality, optimal reliability
Our Traction motors and drives harness all the advantages of digitalization. While customer requirements are the starting point, our experts also incorporate field data into their work to create a digital twin. The electrical design is then optimized for performance, robustness, and thermal characteristics using modern analytics. The results of simulations are verified in our own test center and in real system checks on the vehicle and the intelligence gained flows back into future development. Quality and longevity are assured by comprehensive testing of 100 percent of the components during manufacturing. In addition to testing the individual component, the compatibility of the entire system of gears, motors, converters, controls, and transformers is also checked in our system test center.

Traction motors and drives for all rail applications
Our drive system portfolio covers the entire spectrum of rail applications. Here are a few examples from the multitude of possible combinations of traction motors and gears.

Locomotive Traction Motor are the major contributors to maintenance expenses for railways and locomotive owners worldwide.

source :

https://supcorailway.com/categories/traction-motors/