SD70MAC
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Builder :
General Motors Electro-Motive Division
Electricity :
AC
Length :
74'
Width :
10' 5"
Height :
15' 8"
Weight :
207 1/2 tons
Cylinders :
16
Wheel Diam :
42"
Fuel Tank :
5800 gal
Max Speed :
75 mph
Horsepower :
4000 hp
     

Until 1994, all commercially built North American diesel-electric locomotives used conventional DC traction motors. DC traction was well understood, developed, and straightforward. Although DC traction was the prevalant technology, it has several inherent limitations to locomotive performance, the railroads' desire for greater power and better motor relaibility pushed the devlopment of three-phase AC traction technology.

The railroad pushing the most for the alternative traction was Burlington Northern. By the 1990's, 30 percent of BN traffic base came from coal, and the traffic was still growing. However BN's traditional fleet of EMD SD40-2's and GE C30-7's were approaching retirement. AC traction offered significantly greater tractive effort that would allow BN to use fewer locomotives to move equivalent tonnage. Specifically, superior adhesion afforded by three 4,000 hp SD70MACs could effectively do the same work as five of the older models.

The SD70MAC's greatest strength is at starting and lower speeds. Where a SD40-2 produces just 87,150 pounds of continuous tractive feffort, a SD70MAC can develop 137,000 pounds. Another advantage is that DC motors cannot operate at full power at lower speeds due to overheating, AC motors can operate indefinately at full power at low speeds without risking any damage to the engine.

In addition to greater tractive effort, AC traction motors offer other advantages, including more effective dynamic braking and longer motor life. These advantages reduce operational costs and potentially save money. But these savings have a price. An SD70MAC costs around 25% more than a comparable EMD DC traction locomotive. For this reason, AC traction is not necessarily the best option in all situations. A railroad needs to weigh greater unit cost against savings provided by improved performance and lower maintenance.

Initially BN ordered 350 units, the first making its debut on January 10, 1944. The success of the SD70MAC led BN and its successor, BNSF, to place large repeat orders. CSX and Conrail also received some, although their fleets were much smaller. When Conrail broke up, all of its SD70MACs went to CSX. Since mixing AC and DC diminishes the benefits of AC usage, BNSF has made a concerted effort to keep its SD70MACs in matched sets for dedicated services.