Process Locomotive AITMC and AITMC-PT
 

Many of the process locomotives use a patented automatic traction motor control system which provides superior traction and reliability for locomotive with DC traction motors.  This is protected by U.S. Patent No. 6012011 dated January 4, 2000.  The basic system is referred to as AITMC, automatic individual traction motor control.  The deluxe system is referred to as AITMC-PT, automatic individual traction motor control with power trim.  
The word individual means that each traction motor is controlled independently of the other traction motors.  If a wheel slip begins to occur on any axle the control system for that axle will take corrective action to control that traction motor in such a way as to correct the slip problem.  At the same time it will apply sand in the direction of travel for a preset period of time.   This is done totally automatically without any input from the operator or engineer.  The AITMC and AITMC-PT systems automatically control each traction motor individually to prevent wheel slippage and to provide a higher overall average adhesion factor.   
The process locomotives have a higher level of reliability than standard locomotives.  AITMC is one of the main reasons for this.  On a standard four or six axle locomotive if a traction motor fails it requires shutting down the locomotive until maintenance personnel can arrive to replace the motor or disconnect it and wire around it.  This can that several hours or in some remote locations several days.  With the AITMC system the PLC drops out the problem circuit and the locomotive continues to run with the remaining traction motors.  No down time.  No lost production.  It just requires dropping back on the maximum number of railcars that the locomotive can move.  This will prevent overheating of the remaining traction motors under starting conditions and long heavy hauls.  
With AITMC the average adhesion factor is approximately 30% v. 25% for standard locomotives.   With AITMC-PT the average adhesion factor is increased to 32%.  The average adhesion factor is what is important not the adhesion factor for any one axle.  When a wheel slips on a standard locomotive power is cut off to all the traction motors and then back on.  Tractive power goes from 100% to zero and back to 100%.  With AITMC its goes from 100% to 75% and back to 100%.  This makes a big difference in the average adhesion factor and therefore in the average tractive effort produced.
The difference between AITMC and AITMC-PT is simple.  AITMC works like a standard locomotive in that it cuts power in the event of a wheel slip.  AITMC-PT doesn't cut the power but rather calculates how much power can be used without slippage and cuts the power back a little below that number.  With this method less momentum is lost and there is less strain on the traction motor. 
AITMC systems have been in service since 1996 and have proven to be reliable and popular.  Over 70% of the process locomotives that we have placed in service since 1996 have the AITMC system. 
Process locomotives are available in several sizes: 80, 100, 135, 144, 200, and 210 tons   The most popular size for in plant switching are the 135 ton units.  A 135 ton standard locomotive with a average adhesion of 25% will produce 67,500 lbs. of tractive effort.  With AITMC the same locomotive will produce 81,000 lbs. of tractive effort.  With AITMC-PT the same locomotive will produce 86,400 lbs. of tractive effort.
An advantage to the AITMC design is that it works with standard off the shelf DC traction motors.  DC traction motors are much cheaper and more available than AC traction motors.  The vast majority of the locomotives in service today use DC traction motors.  The AC motors are expensive and usually have long lead times when a replacement is necessary.  Typically a rebuilt DC tractive motor can be obtained within 24 to 72 hours.  Often AC traction motors take many weeks for delivery.  AITMC does not use inverters like AC systems.  Inverters can be quite expensive in the current ranges required for locomotive power.
The AITMC system isn't claimed to be better than the elaborate AC traction control systems.  It isn't designed to operate at high main line rail speeds.  It is designed for in plant switching and short line service.  The point is that AITMC is a lot less expensive than most AC traction control systems.  It brings state of the art technology down to a price that is affordable to industrial plants and short lines that otherwise wouldn't be able to justify this type equipment. 
 

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