Articles

AC versus DC Why does the railroad industry give such high acclaim to AC traction Systems? Perhaps the simplest way to answer such a question is to compare the power generating circuits on both DC and AC locomotives.     System schmatic.   On conventional DC locomotives, a diesel engine is used to turn a generator. This generator then produces power which is passed on to traction motors to drive the wheels. On most of today's existing locomotives, the generator is actually a three phase AC alternator whose output is internally rectified to DC. This DC power is the energy used to drive the wheels as mentioned above.   With AC traction locomotives, the same base principle is used. A diesel engine drives an alternator and the output is rectified to DC. This DC output (referred to as DC Link) is passed on to inverters which convert the power back into an AC form which can be used by the AC traction motors. The DC and AC sides of the circuit are in a sense decoupled by the inverter. This means that operating efficiencies of the generator and motors can be chosen independently. In other words, the generator can operate at some certain throttle position at which it is most efficient while the motors operate at some lower power which is most efficient for them. It is entirely possible to operate the generator at its maximum 2,600 VDC output while operating the motors at zero volts. This is the first benefit of AC traction systems. Let's take a look now at some of the more tangible benefits of AC traction systems.   Nearly all of these advantages are a direct result of the AC traction motor. The motor used is a squirrel cage, 4 pole, three phase, AC induction motor. The four poles referred to are magnetic poles. The motor actually has 12 physical poles on the stator (3 phases per magnetic pole). The advantages gained by using the AC motor are more pulling power, elimination of stall burns, low maintenance requirements, and the elimination of short time ratings.   Another advantage to the AC traction system is that an AC induction motor is in a sense "self-correcting". That is, the wheels will tend not to slip dramatically. This characteristic is inherent in the concept of an induction motor. With an induction motor, a magnetic field is set up inside the motor which rotates at some speed. The rotor will tend to want to "catch up" with this rotating field. The more the rotor lags behind the field, the harder it gets "pulled". The more the rotor catches up to the field, the less it gets pulled. If the rotor actually begins to turn faster than the field, the rotor is retarded by the field. (This is the case in Dynamic Braking operation.) so, we see now how the rotor will tend not to exceed the rotation speed of the field. Taking into account the fact that under most conditions, field frequency does not exceed rotor frequency by more than 1 Hz, it becomes clear how the induction motor when used in this system is self-correcting.   A prime example of this characteristic of the motor occurred when one of the prototype units encountered a slipped motor pinion. The detection software failed to recognize the faulty motor condition. Since a slipped pinion means that there is no longer a stable mechanical connection to the axle, the rotor of a DC motor would almost instantly spin out of control until self-destruction. But with an AC motor, the rotor will just coast along at the same speed as the rotating field. Since the maximum continuous speed of the AC motor is 4,000 RPM, and the inverters will not create a field that rotates faster than 3,600 RPM, the motor is well within its non-destructive operating range. In fact, when the slipped pinion happened, it went undetected for more than a day even though system engineers were riding the locomotive at the time. The software bug that missed detecting this fault was soon found and corrected.   When a wheel slip occurs on a DC powered locomotive, the main Generator excitation is reduced in order to reduce the generator output. This reduction limits power into the slipping motor. The drawback here is that all motors, not just the slipping motor, see a power reduction. This can be seen easily by looking at the schematic. In order for power reduction to occur on EMD-built AC locomotives, all wheels within a truck must encounter slipping conditions! For the unusual case when simultaneous slips within a truck do occur, power is reduced to motors in that truck only due to the VSI per truck design of the RADIALAC system. The other truck continues to work at its non-slipping power level. This can be seen in the drawing.   Internally, an AC induction traction motor contains no electrical connections for commutation. Commutation is carried out through the principle of electro-magnetic induction, as the name suggests. Since no commutators or brushes exist within the motor, stall burns are no longer possible and short time ratings have been eliminated. Again, continuous stall condition operation may not be possible with other manufacturer's locomotives due to fundamental system design differences.   Lets take this opportunity to clarify the elimination of short time ratings. As with any electrical machine, these AC traction motors can overheat. However, unlike earlier motors, these have temperature sensors built into the field windings that provide feedback into each inverter computer. If the temperature of a motor gets too high, the inverter computer automatically reduces power to that truck. The operator does not have to worry about overheating motors on these AC traction units as they will protect themselves.   How long will the motors run under full tractive effort before the inverter reduces power to prevent an overheat? This question was answered quite by accident during an SD60MAC demonstration on Burlington Northern. Three SD60MACs were assembled to pull a 17,000 ton coal train up a steep grade in southern Colorado. Five SD40-2s normally pull this 30 mile climb at an average speed of 15 MPH. The test consist reached and consistently held 12 MPH until one of the SD60MACs experienced difficulties and unloaded. Train speed fell to 6.3 MPG and tractive effort per unit rose to the maximum of 175,000 pounds. These numbers held for nearly and hour until the motors approached overheat. The inverters, as expected, automatically reduced power gracefully, and over the span of 20 additional minutes train speed fell to an eventual stall only 300 feet short of cresting the grade. What made this exhibition of producing full (not continuous) tractive effort for such an extended period even more astonishing is that it happened in 85 degree F ambient air temperature at roughly 4,000 feet above sea level!   Maintenance requirements on the AC motors are virtually non-existent. Brush maintenance is no longer required and commutators need not be stoned simply because the components do not exist in an AC motor. Also, traction motor flashovers cannot occur, meaning a dramatic reduction in the amount of ground relay trips, hence less road failures. This means great advantages in high speed operation. The AC motor is an entirely sealed case with no inspection doors. In fact, visual inspection of the motor itself, connections in cabling, and gearcase oil level are the only 122 day maintenance items.   Several factors combine to give an AC motor more "pulling power" than its DC counterparts. First, an AC motor produces higher starting torques providing superior performance "out of the hole". Second, AC motors are rated higher in terms of peak RPM meaning the gear ratio at the bullgear/pinion interface can be altered to provide higher torques overall without sacrificing ground speed. Finally, an AC motor can squeeze the same amount of horsepower into a remarkably smaller package than its DC counterpart. On modern DC units, larger motors occupy much of the available space in the truck. This makes maintenance of the DC motors difficult. Since comparable AC motors are smaller in size, higher horsepower can now be achieved with more reasonably sized motors.   From an operational standpoint, a major variation must be noted under low speed, heavy pull operation. AC motors on EMD locomotives do not require the high currents for low speed pulling that a DC motor or other manufacturer's AC locomotives do. Consequently, much less heat builds in the AC motor on an EMD AC locomotive at low speeds, virtually eliminating short time ratings. Third, because brushes and commutators do not exist in an AC motor, stall burns cannot occur. Last, loading times when moving the reverser from center position to forward or reverse will be longer as "flux" or magnetism must build up in the motors before operation can continue. Loading times when changing throttle positions, however, will not be slower, in fact they may even be faster than on a conventional DC traction unit.   © 2005 William C. Slim       http://www.okthepk.ca