Existing railroad locomotives are typically powered by a diesel engine which utilizes an alternator to deliver electric power to traction motors which in turn power the drive wheels of the locomotive. The power to the traction motors is typically provided by a single chopper for DC traction motors or a single inverter for AC traction motors. One of the present inventors has disclosed a method and apparatus for controlling power provided to DC traction motors by furnishing an individual chopper circuit for each traction motor in U.S. Pat. No. 6,812,656 which is incorporated herein by reference. In this invention, independently controllable pulse width modulated power pulses are sequentially sent each motor. This patent discloses the practice of power reduction to individual motors to eliminate non-synchronous wheel slip.
As described in U.S. Pat. No. 6,208,097, when a locomotive accelerates, the traction motors apply torque to the driving axles which is converted to tractive effort of the wheels on the rails. When braking, an air brake system and often the motors themselves, may be used to apply a braking force on the rails. Maximum tractive or braking effort is achieved if each of the driving axles is rotating such that its actual tangential speed is slightly higher while accelerating or slightly lower when braking than the true ground speed of the locomotive. If adhesion is reduced or lost, some or all of the driving wheels may experience slip while accelerating or skid while braking. Wheel slip or wheel skid can lead to accelerated wheel wear, rail damage, high mechanical stresses in the drive components of the propulsion system, and an undesirable decrease of the desire tractive or braking effort.
Various methods of detection of wheel slip and wheel skid are known and are discussed, for example, in U.S. Pat. Nos. 5,610,819, 6,208,097 and 6,012,011. These methods include measurement of traction motor current, traction motor rpm and the use of tachometers on the driving axles.
As noted in U.S. Pat. No. 6,012,011, when wheel-slip occurs, the traction motors continue to develop torque further exacerbating the slip and the wheel speed must be reduced to correct this runaway condition. Typically, once wheel slip is detected, power is reduced to all the wheels, regardless of how many of the driving wheels are actually experiencing slip. Several techniques have been used in an attempt to control wheel-slip on railroad locomotives such as:                reducing the power output to all driving wheels when wheel-slip is detected on any axle until the wheel-slip stops        applying an abrader material to the rails, such as sand, to increase adhesion.        application of friction brakes on the wheels that are slipping to slow the wheels.        when several locomotives are located at the front of a train and wheel-slip is detected on the leading locomotive, it can be stopped by reducing the power of only this locomotive.        
While there is substantial prior art on detection of wheel slip conditions on individual wheels or axles, there is little prior art on means of controlling wheel slip by controlling individual wheels or axles. Johnson, in U.S. Pat. No. 6,012,011, discloses a traction control system for detecting and remedying wheel-slippage on individual wheels or axles. His system monitors the speed of each of the traction motors used to drive the wheels of a locomotive. If the speed of a particular traction motor indicates that the wheels are slipping, power is totally removed from that particular traction motor. While this method is an improvement in the art, independently turning traction motors on or off, even for brief periods, can still result in significant problems. For example, the power removed from a particular traction motor may be redistributed to the other motors until the diesel engine/electric generator prime power source is able to adjust to the new load. This power added to the other traction motors can, in turn, lead to wheel slippage on these other drive wheels, especially if they, as is often the case, are themselves near the threshold of slippage. Further, an abrupt change in power to a traction motor can have the same negative effects as an abrupt change in power to all the motors and may include accelerated wheel wear, rail damage, high mechanical stresses in the drive components of the propulsion system, and an undesirable decrease of tractive (or braking) effort.
Thus, there remains a need for a more precise control of individual traction motor power for better management of synchronous and non-synchronous wheel slip and wheel skid. A more precise control of individual traction motor power particularly during non-synchronous wheel slip and wheel skid can lead to strategies for better predicting and preempting wheel slip and skid and for modifying adhesion characteristics of the rails to inhibit the onset of conditions that lead to wheel slip and skid.