In my posting “Let’s have some fun” http://clevelandtrains.blogspot.com/2009/10/now-lets-have-some-fun.html traction effort and adhesion is mentioned briefly. Adhesion and traction effort is what it takes to get tonnage moving over grades, but there are wheel slip control systems to maximize both. Some of the wheel slip systems out there are EMD’s EM2000 and Super Series, Wabtec Q-Tron, GE BrightStar and ZTR BOA/Nexsys II. These microprocessor systems can also monitor the health of traction motors, engine, blowers, alternator, water temperature and air compressor to just name a few components.
In order to maximize traction effort and adhesion during heavy tonnage situations and or wet rail conditions, the wheel slip systems works to reduce the wheels from slipping when below 1.5 mph. This allows starting the train smoothly and not breaking a knuckle or pulling a drawbar. The system controls slippage by reducing power to traction motors (or excitation from the generator on older DC-DC locomotives). Reducing power on the slipping traction motor(s) allows them to establish a footing or bite on the rail to get moving. Above 1.5mph, the wheel slip system turns in a creep control system. Creep control allows each traction motor to develop maximum traction effort by rotating 1-2mph faster than ground speed. The wheel is allowed to spin for a few seconds until it grips the rail. Once it finds a grip after this higher speed rotation, it is at the maximum adhesion point. Wheel creep can happen at any speed and is controlled by the wheel slip system. There is a high pitched sound, similar to wheel slippage, when a wheel is creeping.
Somewhere, I have a short video of a GP38-2 creeping to share.