AC Traction vs DC Traction

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.



AC traction for locomotives is a major improvement over the old DC systems. The primary advantages of AC traction are adhesion levels up to 100% greater than DC and much higher reliability and reduced maintenance requirements of AC traction motors.

The tractive effort of a locomotive (whether AC or DC) is defined by the equations:

Tractive effort = Weight on drivers x Adhesion

Adhesion = Coefficient of friction x Locomotive adhesion variable

The friction coefficient between wheel and rail is usually in the range of .40 to .45 for relatively clean, dry rail in reasonable condition and is essentially the same for all locomotives. The locomotive adhesion variable represents the ability of the locomotive to convert the available friction into usable friction at the wheel rail interface. It varies dramatically from about .45 for old DC units to about .90 for modern AC units. This variable incorporates many factors including electrical design, control systems, truck type and wheel conditions.

First generation DC locomotives such as SW1200s, GP9s, SD40s, and GE center cabs typically have adhesion levels of 18% to 20%. More modern units with adhesion control such as SD60s and Dash 8s have adhesion levels of 25% to 27%. The newer AC traction units such as the SD80MAC and the C44AC are usually rated at 37% to 39% adhesion. Thus, the newer locomotives have about twice the adhesion of the older units and the Class I railroads are, in fact, typically replacing two older units with a single new AC unit.

There are three primary reasons that AC traction offers so much more adhesion. First, in a standard DC drive, if wheel slip occurs, there is a tendency for the traction motor to speed up and run away, even to the point of mechanical failure if the load is not quickly reduced. As the wheel slippage increases, the coefficient 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 in order to retain control. This allows the locomotive to operate safely at a point closer to its theoretical maximum.

The AC system, however, operates in a very different fashion. The variable frequency drive creates a rotating magnetic field which spins about 1% faster than the motor is turning. Since the rotor cannot exceed the field speed, any wheel slip is minimal (less than 1%) and is quickly detected by the drive which instantly reduces load to the axle.

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.

The third way that AC traction provides improved adhesion is through weight transfer compensation. When a locomotive is pulling 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, then in a DC system where the motors are fed from a common source, the tractive effort will be determined by the lightest axle. Thus, in effect, the equivalent locomotive weight is reduced by about 20%. With an AC system, however, the drive is able to compensate for the weight transfer. When the lead axle goes light, the AC drive system will reduce power to that axle and apply more power to the rear axle without incurring wheelspin.

The combination of eliminating wheel slip and compensating for weight transfer gives the AC traction system an adhesion of 37% to 39% versus the 18% to 20% of the older DC systems. Therefore, a locomotive with AC traction can provide the same tractive effort as a DC locomotive weighing twice as much or can give twice as much tractive effort for the same weight.

GE and EMD added AC traction to their mainline units and were then able to replace two older DC units with one new AC locomotive. Republic locomotive took a different approach and decided to make a lighter, less costly unit for industrial switching. The DC powered SW9/SW1200, produced in large quantities from 1951 to 1965 and used for heavy yard switching as well as branch line service, was taken as the performance standard. At 230,000 to 240,000 pounds these units are typically rated at about 40,000 pounds tractive effort continuous (somewhat higher intermittent but limited by traction motors and generators). The AC traction RX500 at 144,000 pounds and a conservative 35% adhesion level is rated at 50,400 pounds tractive effort continuous.

With AC traction, it is also important to consider braking. As with traction, braking is a function of weight on drivers. Therefore, when using standard friction braking (tread brakes) the braking capability of the locomotive (excluding train braking) is proportional to the locomotive weight. With AC traction, however, the braking can be much higher because the drive system in braking acts just like the drive does in traction thus eliminating wheel slip. The drive converts the motors to generating mode (dynamic braking) and the electricity produced is dissipated in the braking resistors. Thus the motors are slowing the locomotive without using the air brakes. Again, the adhesion levels are much higher so the locomotive can again be significantly lighter for the same amount of braking. The dynamic braking in AC traction locomotives also allows full braking down to zero speed, unlike DC dynamic braking.

All in all, the AC traction locomotive offers about twice the amount of adhesion as a DC unit. Therefore, a modern lightweight AC locomotive such as the RX500 can provide as much or more tractive effort than an old style DC unit like the SW1200 which weighs 60% more.