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March 30, 2021

AC vs DC traction motors

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


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.


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


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.

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Posted by      Isobel Flynn at 4:47 AM MDT


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